feat(events): interception seams — the typed-Decision surface for hooks

Reshape the agent's interception surface so every seam returns a small, typed
Decision union, and the set covers the hook points a CC/Codex bridge (and a
native plugin) needs. "Native hooks" are not a package — a native hook is just a
cordis plugin on these canonical events; the bridges (a later PR) only translate
an external protocol onto the same surface.

dsh-agent:
- NEW agent/session-start(agent, source) emit (once before turn 1; SessionStartSource
  startup|resume|clear|compact) — a pure notification, seeds context via inject().
- NEW agent/prompt-submit waterfall → PromptDecision (allow, optionally rewriting the
  prompt or attaching additionalContext, or block).
- RESHAPE agent/turn-continuation boolean → ContinuationDecision ({action:'stop'} |
  {action:'continue', reason?}; a continue reason is recorded as next-step steering).
- New HookContext envelope (required source — inject() would mislabel a missing one).

dsh-tools: split the single tools/execute waterfall into tools/pre-execute
(PreToolDecision allow/deny/ask gate) and tools/post-execute (PostToolDecision
accept/block, optionally replacing content or attaching additionalContext). Core
dispatch sits between as plain code; the tool body keeps its inner try/catch so a
thrown tool still reaches post-execute as an isError. ToolExecutionResult gains
additionalContext (ferried to the loop's per-step buffer). Input rewrite is
deliberately NOT offered (a proposed RFC designs it consistently).

dsh-session: new `rejected` TurnEndReason — a turn whose whole prompt batch was
blocked by prompt-submit.

agent-loop firing points: session-start emitted at create (source threaded —
startup for create/fork, resume for resume()); prompt-submit per drained message
with the always-open-turn rule (a fully-blocked batch is a zero-step rejected
turn); the continuation reshape; post-tool additionalContext buffered and appended
after all tool/results (adjacency). ACP codec maps rejected→cancelled.

A worked native-plugin example (interception.spec.ts) proves all four seams compose
end-to-end through the real loop with NO hook/* events (those belong to the bridge
lib). All existing tools/execute + turn-continuation tests migrated. The
tool-subagent abort test now aborts after a microtask so it still exercises the
live onAbort bridge (execute() awaits pre-execute before the body runs).

RFCs: implemented/feature/2026-06-30-interception-seams.md (the reshape) +
proposed/feature/2026-06-30-pre-tool-input-rewrite.md (the deferred rewrite design).

docs/architecture.md

@@ -69,7 +69,7 @@ Swappable capabilities are split into **three packages** so each part evolves in
 
 The LLM seam has the same topology folded differently: `dsh-llm` carries the interface (`LlmAdapter`) AND the consumer surface (`ctx.llm.stream()`), with adapters as implementation packages — there the consumer is the loop itself, not a swappable schema surface. Use the full three-package split when the consumer is independently replaceable; keep interface + consumer together when they are one concern. Don't split preemptively: a capability with one conceivable implementation and one consumer stays one package until proven otherwise.
 
-> **"Capability" — two unrelated meanings.** (1) The *seam pattern* above ("one plugin provides a capability, another needs it") is realized by plain Cordis **services + `inject`**: a provider registers a service (`ctx.bash`, declared in `interface Context`); a consumer declares `inject: ['bash']` and its fiber stays pending until the service exists, tearing down via HMR if it later vanishes. No extra library is needed. (2) `@cordisjs/plugin-capability` is a different axis entirely — a **permission/capability-security** service (named permissions with inheritance/dependency, tested against a session via `ctx.capability.test`). It is a candidate for the deferred permissions/sandbox work (the `tools/execute` veto seam), NOT a mechanism for swapping implementations.
+> **"Capability" — two unrelated meanings.** (1) The *seam pattern* above ("one plugin provides a capability, another needs it") is realized by plain Cordis **services + `inject`**: a provider registers a service (`ctx.bash`, declared in `interface Context`); a consumer declares `inject: ['bash']` and its fiber stays pending until the service exists, tearing down via HMR if it later vanishes. No extra library is needed. (2) `@cordisjs/plugin-capability` is a different axis entirely — a **permission/capability-security** service (named permissions with inheritance/dependency, tested against a session via `ctx.capability.test`). It is a candidate for the deferred permissions/sandbox work (the `tools/pre-execute` deny/ask gate), NOT a mechanism for swapping implementations.
 
 ## The vocabulary (dsh-llm)
 
@@ -102,7 +102,7 @@ Tool schemas are deliberately **part of the assembly**: "what the model is told
 
 `ToolRegistry.register()` takes schema + `execute()`. The registry feeds its schemas into the system-prompt assembly automatically.
 
-`execute()` runs through the **`tools/execute` waterfall** — the single seam where sandbox, permission, hooks, and plan-mode plugins wrap or veto a call. This collapses Claude Code's validate → PreToolUse → permission → execute → PostToolUse pipeline into ordered waterfall listeners.
+`execute()` runs through a **two-waterfall pipeline** — `tools/pre-execute` (the allow/deny/ask gate) → core dispatch → `tools/post-execute` (inspect/replace the result, attach context) — the seams where sandbox, permission, hooks, and plan-mode plugins gate or transform a call. This maps Claude Code's validate → PreToolUse → permission → execute → PostToolUse pipeline onto two ordered waterfalls: `pre-execute` returns a `PreToolDecision` (allow/deny/ask), `post-execute` a `PostToolDecision` (accept/block, optionally replacing content or attaching `additionalContext`). Core dispatch sits between them as plain code, inside `execute`'s outer try/catch, with the tool body's own try/catch preserved so a thrown tool still reaches `post-execute` as an `isError`.
 
 **TODO**: tool shapes get revisited now that real tools exist (the bash suite landed; the `TODO(review)` in dsh-tools is still open) — e.g. a concurrency-safety hint for parallel execution; phase 1 executes tool calls sequentially.
 
@@ -126,11 +126,16 @@ Tool schemas are deliberately **part of the assembly**: "what the model is told
 - **Step**: one model request + its tool executions.
 
 ```
+create agent → emit agent/session-start(source)       ⟵ once, before turn 1 (startup|resume)
 forever:
   wait for queued messages (idle)
   emit agent/status(running)
   TURN (error-contained — a throwing plugin ends the turn, never the loop):
-    drain queued → 'turn/start' → session('user/message'…) → emit agent/turn-start
+    'turn/start'
+    each queued msg: waterfall agent/prompt-submit      ⟵ allow (rewrite/+context) | block
+      allow → session('user/message'…); inject additionalContext
+    every prompt blocked → 'turn/end'(rejected), 0 steps ⟵ zero-step turn, model never called
+    emit agent/turn-start
     STEP loop:
       drain steering (late steering from previous step's listeners)
       session('step/start')                           ⟵ durable step boundary (no agent/* mirror)
@@ -145,15 +150,19 @@ forever:
       msg = waterfall agent/step-result               ⟵ runs BEFORE the log append, so the
       session('assistant/message' {content, usage?})     log records what tool dispatch uses
       each tool-call (sequential, abort-checked between calls):
-        session('tool/call'); ctx.tools.execute()     ⟵ waterfall tools/execute
+        session('tool/call'); ctx.tools.execute()     ⟵ waterfall tools/pre-execute (allow/
+          deny/ask gate) → dispatch → tools/post-execute (accept/block, replace, +context)
           tool execution may append tool-owned session events, e.g. `todo/write`
         session('tool/result')
+      append buffered post-execute additionalContext → session('context/message')(s)
+                                                         ⟵ after ALL tool/results (adjacency)
       drain steering → session('steering/message'); emit agent/steering
       session('step/end')                             ⟵ durable step boundary (no agent/* mirror)
-      cont = waterfall agent/turn-continuation(default = hadToolCalls || steered)
-      steering pending forces cont = true (from continuation listeners OR from
-        step/end session-event listeners — the /goal pattern; hasSteering override)
-      if !cont: break
+      cont = waterfall agent/turn-continuation(default = {action: hadToolCalls||steered
+        ? 'continue' : 'stop'}) → ContinuationDecision
+      a continue's reason is recorded as next-step steering (same turn); steering pending
+        also forces continue (continuation OR step/end listeners — the /goal pattern)
+      if action==stop: break
   session('turn/end'); emit agent/turn-end
   await ctx.parallel('session/flush', session)        ⟵ durability checkpoint (failure
                                                          reported via agent/error, not fatal)
@@ -163,7 +172,7 @@ forever:
 
 Error containment: a throwing `agent/turn-continuation` listener or a broken step ends the **turn** with `turn/end { reason: { kind: 'error', step, message, code? } }` — the failure's step number rides on the durable turn reason (there is no separate session `error` event); live diagnostics fire via `agent/error`. Never the driver loop. An adapter that ends its stream with a `finish {kind:'error'}` or `{kind:'aborted'}` chunk (the in-band error path, for adapters that can't throw mid-stream) is likewise translated into a step error, so the turn ends `error`/`aborted` instead of logging a normal `completed` assistant message. A `cancel()` is honored mid-stream **and** between tool calls; disposal mid-turn ends the turn with reason `disposed` and emits `agent/status('disposed')`.
 
-Turn-end reasons: a turn ends with one `TurnEndReason` — `completed`, `aborted`, `error`, `disposed`, or `max-tokens`. `max-tokens` mirrors the model-call `FinishReason` of the same name (DeepSeek's `length`): a step that hit the output-token ceiling makes the turn end `max-tokens` rather than `completed`, by the rule *any `max-tokens` step in the turn surfaces as `max-tokens`* (a continuation plugin may run further steps after one, but the cut-short fact wins; the `disposed`/`aborted`/`error` outcomes still take precedence). This lets a consumer distinguish a clean stop from a truncated one (the ACP bridge maps it to the `max_tokens` stop reason). `TurnEndReason` is merge-extensible; `refusal` and `max_turn_requests` are the next variants to add when an adapter/loop first emits them.
+Turn-end reasons: a turn ends with one `TurnEndReason` — `completed`, `aborted`, `error`, `disposed`, `max-tokens`, `rejected`, or `interrupted`. `max-tokens` mirrors the model-call `FinishReason` of the same name (DeepSeek's `length`): a step that hit the output-token ceiling makes the turn end `max-tokens` rather than `completed`, by the rule *any `max-tokens` step in the turn surfaces as `max-tokens`* (a continuation plugin may run further steps after one, but the cut-short fact wins; the `disposed`/`aborted`/`error` outcomes still take precedence). `rejected` is a zero-step turn whose entire prompt batch was blocked by an `agent/prompt-submit` hook (the turn still opens and closes balanced; the ACP bridge maps it to `cancelled`). `interrupted` is synthesized by a persistence backend closing a crash-orphaned turn on reload. This lets a consumer distinguish a clean stop from a truncated/blocked one (the ACP bridge maps `max-tokens` to the `max_tokens` stop reason). `TurnEndReason` is merge-extensible; `refusal` and `max_turn_requests` are the next variants to add when an adapter/loop first emits them.
 
 A failure that happens once the turn is already closed has no in-turn position for a turn-end error reason (the turn already ended). So a rejecting `session/flush` (the post-`turn/end` durability checkpoint) and a throwing `agent/turn-end` listener are reported via `agent/error` + the logger only, NOT as a session event; the turn stays balanced and the persistence backend keeps its buffered events for the next flush.
 
@@ -189,7 +198,7 @@ Every MVP feature (including the TODO-marked ones), with the mechanism that impl
 
 | MVP feature | Plugin mechanism |
 |---|---|
-| Hook system (user + project level) | listeners on `agent/request`, `agent/step-result`, `tools/execute`, `agent/turn-continuation`; a hooks plugin bridges config files to shell commands |
+| Hook system (user + project level) | listeners on `agent/session-start`, `agent/prompt-submit`, `agent/request`, `agent/step-result`, `tools/pre-execute`, `tools/post-execute`, `agent/turn-continuation` (each interception waterfall returns a typed Decision); a hooks bridge plugin maps config files / shell commands onto those seams, a native hook plugin uses them directly |
 | `/goal` | force-continue via `agent/turn-continuation` + `steer()` reminders |
 | `/loop` | on `agent/turn-end`, `send()` the next iteration; or force-continue |
 | Dynamic workflow | orchestrator plugin on `agent/turn-end` (or the `step/end` session event) driving `send`/`steer` (+ sub-agents later) |
@@ -200,9 +209,9 @@ Every MVP feature (including the TODO-marked ones), with the mechanism that impl
 | AGENTS.md (subdir, on-touch) + file-change notices | `agent.inject()` from a watcher / tool-result listener |
 | Built-in tools (Read/Write/Edit/Bash/…) | `ctx.tools.register()`; schemas flow into the assembly automatically. **Bash: implemented** — `dsh-bash` (seam) + `dsh-bash-local` (subprocesses) + `dsh-tool-bash` (`bash`/`bash_output`/`bash_kill`, incl. background tasks). **`todo_write`: implemented** — `dsh-tool-todo` writes the whole task list to the session log (`todo/write`), rendered as a stdio checklist / ACP `plan` |
 | ToolSearch / progressive disclosure | wrap `agent/request`, filter `req.tools` |
-| Tool sandbox (landlock / sandbox-exec) | wrap `tools/execute`, or implement a sandboxing `BashExecutor` (the dsh-bash seam) |
-| Permission system / AskUserQuestion | wrap `tools/execute` (veto or ask); register an ask tool |
-| Plan mode | wrap `tools/execute` (deny writes) + `agent/request` (inject mode prompt) |
+| Tool sandbox (landlock / sandbox-exec) | `tools/pre-execute` (deny), or implement a sandboxing `BashExecutor` (the dsh-bash seam) |
+| Permission system / AskUserQuestion | `tools/pre-execute` (deny/ask); register an ask tool |
+| Plan mode | `tools/pre-execute` (deny writes) + `agent/request` (inject mode prompt) |
 | Sub-agents (spawn / fork / steer) | TODO seam on `AgentLoop.create()`; fork = seed Session with parent events; `steer()` on the child handle |
 | MCP | one plugin per server: discover tools → `ctx.tools.register()` |
 | Skills | section + tool registration; `inject()` skill content on invocation |

docs/cookbook/extension-cookbook.md

@@ -8,24 +8,20 @@ A tool registers on `ctx.tools`. The annotated `defineTool` example (typed `exec
 
 ## A hook plugin (permission gate)
 
-A hook wraps the `tools/execute` waterfall to veto or rewrite a call — the seam where sandbox, permission, and plan-mode plugins live.
+A hook returns a typed decision from the `tools/pre-execute` gate to allow or deny a call — the seam where sandbox, permission, and plan-mode plugins live. (A "native hook" is just this: an ordinary cordis plugin on the interception seams, returning typed decisions — no external protocol needed.)
 
 ```ts
 import type { Context } from 'cordis'
-import type { ToolExecution } from '@deepseek-ai/dsh-tools'
+import type { PreToolDecision, ToolExecution } from '@deepseek-ai/dsh-tools'
 
 declare function isAllowed(exec: ToolExecution): Promise<boolean>
 
 export const name = 'permission-gate'
 
 export function apply(ctx: Context) {
-  ctx.on('tools/execute', async (exec, next) => {
+  ctx.on('tools/pre-execute', async (exec, next): Promise<PreToolDecision> => {
     if (!(await isAllowed(exec))) {
-      return {
-        callId: exec.callId,
-        content: [{ type: 'text', text: 'Denied by policy.' }],
-        isError: true,
-      }
+      return { kind: 'deny', reason: 'Denied by policy.' }
     }
     return next()
   })

docs/cordis-catalog/events-and-services.md

@@ -25,7 +25,7 @@ An agent was registered in the AgentRegistry and is ready to receive messages.
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:165`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:228`](../../packages/core/agent/src/types.ts)
 
 #### `agent/disposed` — emit
 
@@ -37,7 +37,7 @@ An agent was disposed and removed from the registry; its fiber and any in-flight
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:171`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:234`](../../packages/core/agent/src/types.ts)
 
 #### `agent/error` — emit
 
@@ -49,7 +49,19 @@ A step or turn errored. The loop reports a failure here (plus the logger) even w
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:245`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:334`](../../packages/core/agent/src/types.ts)
+
+#### `agent/prompt-submit` — waterfall
+
+Waterfall: decide what happens to ONE drained queued message before it becomes a `user/message` — allow (optionally rewriting the prompt bytes or attaching `additionalContext`) or block it. Fires inside the already-open turn, per drained message. Maps onto Claude Code's `UserPromptSubmit` hook. Call `next()` to delegate to the default (allow unchanged), or return a PromptDecision without calling `next()` to short-circuit.
+
+```ts cordis-catalog
+'agent/prompt-submit'(agent: Agent, content: ContentBlock[], source: MessageSource, next: () => Promise<PromptDecision>): Promise<PromptDecision>
+```
+
+Types: [Agent](../core-data-structures/core.md) · [ContentBlock](../core-data-structures/core.md) · [MessageSource](../core-data-structures/core.md)
+
+Source: [`packages/core/agent/src/types.ts:293`](../../packages/core/agent/src/types.ts)
 
 #### `agent/queued` — emit
 
@@ -61,7 +73,7 @@ A message entered the agent's inbox (queued or steering). `source` is the resolv
 
 Types: [Agent](../core-data-structures/core.md) · [ContentBlock](../core-data-structures/core.md) · [MessageSource](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:184`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:247`](../../packages/core/agent/src/types.ts)
 
 #### `agent/request` — waterfall
 
@@ -73,7 +85,19 @@ Waterfall: mutate the fully-assembled GenerateOptions before the model call (hoo
 
 Types: [Agent](../core-data-structures/core.md) · [GenerateOptions](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:214`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:300`](../../packages/core/agent/src/types.ts)
+
+#### `agent/session-start` — emit
+
+The agent's session lifecycle began, fired once before its first turn. `source` says why (SessionStartSource: fresh startup, a resumed persisted session, …). A pure NOTIFICATION (emit, not waterfall): it carries no veto — a session-start listener that wants to seed context does so via `agent.inject()` (a `context/message` the first request sees), not by returning a decision. Cannot block the session from starting; that gap is deliberate (a bridge logs/injects, it does not gate startup).
+
+```ts cordis-catalog
+'agent/session-start'(agent: Agent, source: SessionStartSource): void
+```
+
+Types: [Agent](../core-data-structures/core.md)
+
+Source: [`packages/core/agent/src/types.ts:260`](../../packages/core/agent/src/types.ts)
 
 #### `agent/status` — emit
 
@@ -85,7 +109,7 @@ Agent status changed (`idle` ⇄ `running`, or → `disposed`). Drive lifecycle
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:178`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:241`](../../packages/core/agent/src/types.ts)
 
 #### `agent/steering` — emit
 
@@ -97,7 +121,7 @@ Steering content was injected into a running turn.
 
 Types: [Agent](../core-data-structures/core.md) · [ContentBlock](../core-data-structures/core.md) · [MessageSource](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:239`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:328`](../../packages/core/agent/src/types.ts)
 
 #### `agent/step-result` — waterfall
 
@@ -109,7 +133,7 @@ Waterfall: post-process the assembled assistant Message before tool dispatch (va
 
 Types: [Agent](../core-data-structures/core.md) · [Message](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:220`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:306`](../../packages/core/agent/src/types.ts)
 
 #### `agent/stream-chunk` — emit
 
@@ -121,19 +145,19 @@ A raw StreamChunk arrived from the model (token-level UI/log feed).
 
 Types: [Agent](../core-data-structures/core.md) · [StreamChunk](../core-data-structures/llm-streaming.md)
 
-Source: [`packages/core/agent/src/types.ts:234`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:323`](../../packages/core/agent/src/types.ts)
 
 #### `agent/turn-continuation` — waterfall
 
-Waterfall: override the turn-continuation decision. The default (computed by the loop) is `hadToolCalls || steeringInjected`. Listeners can force-continue (/goal, /loop) or force-stop (budget guards).
+Waterfall: override the turn-continuation decision via a typed ContinuationDecision. The loop's `defaultDecision` is `continue` when the step had tool calls or steering was injected, else `stop`. Listeners force-continue (`/goal`, `/loop` — optionally attaching a `reason` recorded as next-step steering) or force-stop (budget guards). Call `next()` to delegate to the default, or return a decision to override.
 
 ```ts cordis-catalog
-'agent/turn-continuation'(agent: Agent, turn: number, defaultDecision: boolean, next: () => Promise<boolean>): Promise<boolean>
+'agent/turn-continuation'(agent: Agent, turn: number, defaultDecision: ContinuationDecision, next: () => Promise<ContinuationDecision>): Promise<ContinuationDecision>
 ```
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:227`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:316`](../../packages/core/agent/src/types.ts)
 
 #### `agent/turn-end` — emit
 
@@ -145,7 +169,7 @@ A turn ended. `reason` distinguishes a clean stop from a truncated, aborted, fai
 
 Types: [Agent](../core-data-structures/core.md) · [TurnEndReason](../core-data-structures/session.md)
 
-Source: [`packages/core/agent/src/types.ts:205`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:281`](../../packages/core/agent/src/types.ts)
 
 #### `agent/turn-start` — emit
 
@@ -157,7 +181,7 @@ A turn began. `turn` is the 1-based turn number within the session.
 
 Types: [Agent](../core-data-structures/core.md)
 
-Source: [`packages/core/agent/src/types.ts:197`](../../packages/core/agent/src/types.ts)
+Source: [`packages/core/agent/src/types.ts:273`](../../packages/core/agent/src/types.ts)
 
 ### `llm/*`
 
@@ -261,19 +285,31 @@ A tool was registered or unregistered (the available tool set changed).
 'tools/change'(): void
 ```
 
-Source: [`packages/core/tools/src/index.ts:48`](../../packages/core/tools/src/index.ts)
+Source: [`packages/core/tools/src/index.ts:66`](../../packages/core/tools/src/index.ts)
 
-#### `tools/execute` — waterfall
+#### `tools/post-execute` — waterfall
 
-Waterfall around every tool execution — the single seam where sandbox, permission, hook, and plan-mode plugins wrap or veto a call. Listeners receive `(exec, next)`: call `next()` to proceed (possibly around your own logic), or return a ToolExecutionResult without calling `next()` to short-circuit (veto).
+Waterfall AFTER a tool runs — where hook plugins inspect the result and accept it (optionally REPLACING the model-facing content, and/or attaching `additionalContext` for the next request) or block it with corrective `feedback` (Claude Code's `PostToolUse`). Listeners receive `(exec, result, next)`: call `next()` to delegate to the default (accept unchanged), or return a PostToolDecision to override. The core tool dispatch sits between the two waterfalls as plain code, all inside `execute`'s outer try/catch (and the tool body keeps its own inner try/catch, so a thrown tool still reaches `post-execute` as an `isError` result).
 
 ```ts cordis-catalog
-'tools/execute'(this: ToolRegistry, exec: ToolExecution, next: () => Promise<ToolExecutionResult>): Promise<ToolExecutionResult>
+'tools/post-execute'(this: ToolRegistry, exec: ToolExecution, result: ToolExecutionResult, next: () => Promise<PostToolDecision>): Promise<PostToolDecision>
 ```
 
 Types: [ToolExecution](../core-data-structures/tools.md) · [ToolExecutionResult](../core-data-structures/tools.md)
 
-Source: [`packages/core/tools/src/index.ts:43`](../../packages/core/tools/src/index.ts)
+Source: [`packages/core/tools/src/index.ts:61`](../../packages/core/tools/src/index.ts)
+
+#### `tools/pre-execute` — waterfall
+
+Waterfall BEFORE a tool runs — the gate where sandbox, permission, and hook plugins allow or deny a call (Claude Code's `PreToolUse`). Listeners receive `(exec, next)`: call `next()` to delegate to the default (allow), or return a PreToolDecision without calling `next()` to short-circuit. A `deny` skips dispatch and yields an `isError` result; the tool body never runs. Input rewrite is deliberately NOT offered here (see PreToolDecision); `ask` degrades to deny until the permission system lands (`FIXME(permissions)`).
+
+```ts cordis-catalog
+'tools/pre-execute'(this: ToolRegistry, exec: ToolExecution, next: () => Promise<PreToolDecision>): Promise<PreToolDecision>
+```
+
+Types: [ToolExecution](../core-data-structures/tools.md)
+
+Source: [`packages/core/tools/src/index.ts:47`](../../packages/core/tools/src/index.ts)
 
 ## Services
 
@@ -446,7 +482,7 @@ async execute(exec: ToolExecution): Promise<ToolExecutionResult>
 
 Types: [ToolDefinition](../core-data-structures/tools.md) · [ToolExecution](../core-data-structures/tools.md) · [ToolExecutionResult](../core-data-structures/tools.md)
 
-Source: [`packages/core/tools/src/index.ts:277`](../../packages/core/tools/src/index.ts)
+Source: [`packages/core/tools/src/index.ts:341`](../../packages/core/tools/src/index.ts)
 
 ## Inherited tier (cordis core + loader/hmr/timer)
 

docs/core-data-structures/core.md

@@ -306,7 +306,42 @@ interface Agent {
 }
 ```
 
-`AgentStatus` is `'idle' | 'running' | 'disposed'`. `AgentId` is a branded string. `AgentOptions` (`model?`, `systemPrompt?`) is merge-extensible — plugins add creation options by declaration merging. The `agent/*` event taxonomy (lifecycle, turn/step boundaries, the `agent/request`/`agent/step-result`/`agent/turn-continuation` waterfalls) is in [architecture.md § Event taxonomy](../architecture.md#event-taxonomy).
+`AgentStatus` is `'idle' | 'running' | 'disposed'`. `AgentId` is a branded string. `AgentOptions` (`model?`, `systemPrompt?`) is merge-extensible — plugins add creation options by declaration merging. The `agent/*` event taxonomy (lifecycle, turn/step boundaries, the `agent/prompt-submit`/`agent/request`/`agent/step-result`/`agent/turn-continuation` waterfalls) is in [architecture.md § Event taxonomy](../architecture.md#event-taxonomy).
+
+## Interception decisions
+
+Each `agent/*` interception waterfall returns a small, seam-specific typed union — the unified Decision idiom (the tool seams' `PreToolDecision`/`PostToolDecision` in [tools.md](tools.md) follow the same shape). A CC/Codex hook bridge maps its `permissionDecision`/`decision`/`continue`/`additionalContext` fields onto these; a native plugin returns them directly. They share one envelope for model-facing context, `HookContext`, which is `inject()`ed as a `context/message` and so carries a REQUIRED `source` (a missing source would default to `{kind:'user'}` and mislabel plugin context as a user prompt).
+
+Source: [`packages/core/agent/src/types.ts`](../../packages/core/agent/src/types.ts)
+
+```ts type-equiv
+interface HookContext {
+  content: ContentBlock[]
+  source: MessageSource
+}
+```
+
+`agent/prompt-submit` returns a `PromptDecision` (allow a drained queued message — optionally rewriting its `content` or attaching `additionalContext` — or block it; a batch whose every prompt is blocked opens a zero-step turn that ends `rejected`):
+
+```ts type-equiv
+type PromptDecision =
+  | { kind: 'allow'; content?: ContentBlock[]; additionalContext?: HookContext }
+  | { kind: 'block'; reason: string }
+```
+
+`agent/turn-continuation` returns a `ContinuationDecision` (the loop's default is `continue` when the step had tool calls or steering was injected, else `stop`; a `continue` `reason` is recorded as next-step steering in the same turn — the typed `/goal` pattern):
+
+```ts type-equiv
+type ContinuationDecision =
+  | { action: 'stop' }
+  | { action: 'continue'; reason?: HookContext }
+```
+
+`agent/session-start` carries a `SessionStartSource` (why the session lifecycle began; a bridge keys its SessionStart matcher on it):
+
+```ts type-equiv
+type SessionStartSource = 'startup' | 'resume' | 'clear' | 'compact'
+```
 
 ## `ToolDefinition`
 

docs/core-data-structures/session.md

@@ -181,6 +181,16 @@ interface TurnEndReasonMap {
   error: { kind: 'error'; step: number; message: string; code?: string }
   disposed: { kind: 'disposed' }
   'max-tokens': { kind: 'max-tokens' }
+  /**
+   * The turn's entire prompt batch was BLOCKED before any step ran — every
+   * drained queued message was vetoed by an `agent/prompt-submit` listener (a
+   * hook). The turn still opened (so the boundary stays balanced and the block
+   * is a durable in-turn fact), but ran zero steps. `reason` carries the block
+   * message from the vetoing decision. Distinct from `aborted` (a user-driven
+   * cancel) and `error` (a failure): the prompt was rejected by policy, not
+   * interrupted or broken. A UI renders it as "prompt blocked by hook".
+   */
+  rejected: { kind: 'rejected'; reason: string }
   /**
    * The turn never ended on its own: the process crashed mid-turn and a
    * persistence backend later closed the orphaned (open) turn on reload so the
@@ -195,7 +205,7 @@ interface TurnEndReasonMap {
 }
 ```
 
-`max-tokens` mirrors the model-call `FinishReason` of the same name: any `max-tokens` step in a turn makes the whole turn end `max-tokens` (the cut-short fact wins over a later continuation), so a consumer can tell a clean stop from a truncated one. `interrupted` is the one reason no loop emits — it is synthesized by crash recovery (see [persistence.md](persistence.md)). Both maps are merge-extensible.
+`max-tokens` mirrors the model-call `FinishReason` of the same name: any `max-tokens` step in a turn makes the whole turn end `max-tokens` (the cut-short fact wins over a later continuation), so a consumer can tell a clean stop from a truncated one. `rejected` is a zero-step turn whose whole prompt batch an `agent/prompt-submit` hook blocked (the ACP bridge maps it to `cancelled`). `interrupted` is the one reason no loop emits — it is synthesized by crash recovery (see [persistence.md](persistence.md)). Both maps are merge-extensible.
 
 ## The turn-enclosure invariant
 

docs/core-data-structures/tools.md

@@ -73,7 +73,7 @@ type InferArgs<S extends SchemaSpec> = Simplify<
 
 ## Execution: the `tools/execute` waterfall shapes
 
-`ctx.tools.execute()` runs each call through the `tools/execute` waterfall — the single seam where sandbox, permission, hook, and plan-mode plugins wrap or veto. The pending call is a `ToolExecution`; the outcome is a `ToolExecutionResult`.
+`ctx.tools.execute()` runs each call through a two-waterfall pipeline — `tools/pre-execute` (the allow/deny/ask gate) → core dispatch → `tools/post-execute` (inspect/replace the result, attach context) — the seams where sandbox, permission, hook, and plan-mode plugins gate or transform a call. The pending call is a `ToolExecution`; the outcome is a `ToolExecutionResult`.
 
 ```ts type-equiv
 interface ToolExecution {
@@ -98,10 +98,36 @@ interface ToolExecutionResult {
    * text in `content` is always present; this is extra structure for code.
    */
   error?: ToolErrorInfo
+  /**
+   * Extra model-facing context a `tools/post-execute` listener attached for the
+   * NEXT request (Claude Code's PostToolUse `additionalContext`). It is NOT part
+   * of this call's `content` — `content`/`feedback` shape the tool RESULT, but
+   * `additionalContext` is a SEPARATE `context/message`. A step can carry
+   * multiple tool calls, so the loop BUFFERS every call's `additionalContext`
+   * and appends them only AFTER all `tool/result`s for the step, keeping
+   * tool-call/result adjacency intact. Carried on the result purely to ferry it
+   * from `execute()` up to the loop's per-step buffer.
+   */
+  additionalContext?: HookContext
 }
 ```
 
-A waterfall listener receives `(exec, next)`: call `next()` to proceed (possibly around your own logic), or return a `ToolExecutionResult` without calling `next()` to veto. An unregistered tool routes through the same catch as a tool-thrown error, so both failure classes get a structured `{ name, code }` (`ToolNotFoundError` → `UNKNOWN_TOOL`) — the loop records a failed tool call instead of failing the whole turn.
+Each interception waterfall returns a typed **Decision** (the idiom shared with the `agent/*` seams). `tools/pre-execute` listeners receive `(exec, next)` and return a `PreToolDecision`; `tools/post-execute` listeners receive `(exec, result, next)` and return a `PostToolDecision`:
+
+```ts type-equiv
+type PreToolDecision =
+  | { kind: 'allow' }
+  | { kind: 'deny'; reason: string }
+  | { kind: 'ask'; reason?: string }
+```
+
+```ts type-equiv
+type PostToolDecision =
+  | { kind: 'accept'; content?: ContentBlock[]; additionalContext?: HookContext }
+  | { kind: 'block'; feedback: ContentBlock[]; additionalContext?: HookContext }
+```
+
+Call `next()` to delegate to the default (allow / accept-unchanged), or return a decision to short-circuit. A `pre-execute` `deny` (or `ask`, which degrades to deny until the permission system lands) skips dispatch and yields an `isError` result; input rewrite is deliberately NOT offered on `PreToolDecision` (it would desync the pre-execution audit/history/UI from what ran — its own proposed RFC). A `post-execute` `accept` may replace the model-facing `content` (clean, because `tool/result` is logged after `execute()` returns); a `block` turns the call into an `isError` whose content is the corrective `feedback`. Core dispatch sits between the waterfalls as plain code; the tool body keeps its own try/catch so a thrown tool still reaches `post-execute` as an `isError`. An unregistered tool routes through the same catch as a tool-thrown error, so both failure classes get a structured `{ name, code }` (`ToolNotFoundError` → `UNKNOWN_TOOL`) — the loop records a failed tool call instead of failing the whole turn.
 
 ## Tool-presentation UI vocabulary
 

docs/rfc/README.md

@@ -45,6 +45,7 @@ Do NOT write one for a mechanical or local choice (a variable name, a one-file r
 | [Multiplex concurrent ACP sessions over one connection](proposed/feature/2026-06-14-acp-multi-session.md) | 2026-06-14 |
 | [Optional Code Mode — model writes TypeScript against an SDK of all tools](proposed/feature/2026-06-15-optional-code-mode.md) | 2026-06-15 |
 | [Compaction as a capability seam (abstract contract + basic backend)](proposed/feature/2026-06-18-compaction-capability-seam.md) | 2026-06-18 |
+| [Pre-tool input rewrite — a consistent design](proposed/feature/2026-06-30-pre-tool-input-rewrite.md) | 2026-06-30 |
 
 ### Simplification
 
@@ -86,6 +87,7 @@ Do NOT write one for a mechanical or local choice (a variable name, a one-file r
 | [Subagent capability seam](implemented/feature/2026-06-21-subagent-capability-seam.md) | 2026-06-21 |
 | [ACP subagent backend (out-of-process delegation)](implemented/feature/2026-06-22-acp-subagent-backend.md) | 2026-06-22 |
 | [The `todo_write` tool — model task list as event-sourced session state](implemented/feature/2026-06-29-todo-write-tool.md) | 2026-06-29 |
+| [Interception seams — the typed-Decision surface a hook programs against](implemented/feature/2026-06-30-interception-seams.md) | 2026-06-30 |
 
 ### Simplification
 

docs/rfc/implemented/feature/2026-06-30-interception-seams.md

@@ -0,0 +1,45 @@
+# RFC: Interception seams — the typed-Decision surface a hook programs against
+
+Status: implemented (accepted 2026-06-30)
+
+<!-- XXX: legacy ADR/RFC body format, not yet normalized to a unified RFC template. -->
+
+## Context
+
+The harness needs a hooks subsystem: users extend or gate the agent at lifecycle points the way Claude Code (CC) and Codex do. The key reframe driving this design is that **"native hooks" are not a package** — a native hook is just an ordinary Cordis plugin subscribing to the canonical lifecycle events. So the real product is a *powerful, well-typed canonical event surface*; the CC/Codex bridges (a later stack PR) are merely translators that map an external shell-hook protocol onto that same surface. Anything a bridge can do, a plain plugin can do directly — more powerfully (no serialization boundary, full `ctx`, typed returns).
+
+Before this change the interception surface was incomplete and inconsistent for that goal: there was no per-prompt seam (CC's `UserPromptSubmit`), no session-start signal (CC's `SessionStart`), the single `tools/execute` waterfall conflated the pre-gate and post-inspect phases (CC splits `PreToolUse`/`PostToolUse`), and `agent/turn-continuation` returned a bare `boolean` with no room for a force-continue *reason*. The [event-domain-semantics RFC](../architecture/2026-06-30-event-domain-semantics.md) (the stack's first change) pinned down the three-domain rule and the typed-Decision idiom as the interception convention; this RFC builds the actual seams on top of it.
+
+## Decision
+
+Add/​reshape the interception seams so every one returns a small, seam-specific **typed Decision union**, and the set covers the hook points in scope (`session-start`, `prompt-submit`, `pre-tool`, `post-tool`, `stop`-via-continuation).
+
+**New `agent/*` events** (`dsh-agent`):
+- `agent/session-start(agent, source)` — emit, once before turn 1, carrying a `SessionStartSource` (`startup` for a fresh/forked create, `resume` for a reloaded persisted session; `clear`/`compact` reserved). A pure notification — it CANNOT block startup (a deliberate gap: a bridge logs/injects, it does not gate startup). A listener seeds context via `agent.inject()`.
+- `agent/prompt-submit(agent, content, source, next) → PromptDecision` — waterfall, fired per drained queued message inside the open turn, before the `user/message` append. `allow` (optionally rewriting the prompt `content` or attaching `additionalContext`) or `block`.
+
+**Reshaped** `agent/turn-continuation` from `(…, defaultDecision: boolean) → boolean` to `(…, defaultDecision: ContinuationDecision) → ContinuationDecision`. A `{action:'continue', reason?}` may carry model-facing context recorded as next-step steering in the same turn — the typed twin of the existing `/goal` step-end-steer pattern.
+
+**Split** the single `tools/execute` waterfall into `tools/pre-execute` (→ `PreToolDecision` allow/deny/ask gate) and `tools/post-execute` (→ `PostToolDecision` accept/block, optionally replacing content or attaching `additionalContext`). Core dispatch sits between them as plain code inside `ToolRegistry.execute`'s outer try/catch, and the tool body keeps its own inner try/catch so a thrown tool still becomes an `isError` result that `post-execute` listeners can inspect.
+
+**New `TurnEndReason` variant** `rejected` (`dsh-session`): a turn whose entire prompt batch was blocked by `prompt-submit`.
+
+### Three load-bearing loop decisions
+
+1. **Always open the turn first; a fully-blocked batch is a zero-step `rejected` turn.** `prompt-submit` fires AFTER `turn/start`, per message. A batch whose every prompt is blocked does NOT skip the turn — it opens a zero-step turn that closes with `rejected`. This one move resolves three problems at once: (1) turn-enclosure holds (every event has an open turn to live in); (2) `agent/turn-end` fires and the ACP bridge settles normally (mapping `rejected`→`cancelled`) instead of hanging; (3) the block reason is a durable in-turn fact. An `allow`'s `additionalContext` is `inject()`ed into this now-open turn.
+
+2. **Post-tool `additionalContext` is buffered and appended AFTER all `tool/result`s.** `content`/`feedback` shape the result `execute()` returns, but `additionalContext` is a SEPARATE `context/message`, and a single step can carry multiple tool calls. Appending context right after each result would interleave `result(c1) → context → result(c2)` and break tool-call/result adjacency. So `execute()` surfaces `additionalContext` on its `ToolExecutionResult`, and the loop buffers every per-call context for the step and appends them as `context/message`(s) only after every `tool/result` is appended.
+
+3. **A forced `continue` `reason` is enqueued through the steering channel**, so the next step's top-of-loop drain records it as steering for the continued turn — next-*step* steering within the SAME turn, not a next-*turn* prompt (matching the existing `hasSteering` force-continue override).
+
+### Pre-tool INPUT rewrite is DEFERRED (the over-reach signal)
+
+`PreToolDecision` is allow/deny/ask only — **no `arguments` rewrite**. Output replacement (`PostToolDecision.accept.content`) is safe because `tool/result` is logged AFTER execution (one source of truth). Input rewrite is NOT safe today: `assistant/message` (the model-history source) and `tool/call` (the audit record) are both logged BEFORE execution, and live consumers READ `tool/call.arguments` for presentation (the ACP bridge remembers them for `presentResult`; `dsh-tool-bash` derives the title/cwd/terminal-vs-background from them). A rewrite that changed only execution would make the UI show one command while another RAN. Designing that consistently (rewriting the audit + history + presentation as one unit) is a real consistency-design problem CC itself warns is racy — so it gets its own [proposed RFC](../../proposed/feature/2026-06-30-pre-tool-input-rewrite.md), and `TODO(pre-tool-input-rewrite)` anchors it at the loop's pre-execute call site. This does not regress any production consumer (no production `tools/execute` listener mutated `exec.arguments`). The low-level capability to mutate `exec` in a `pre-execute` listener still exists (unadvertised — a test shim uses it to thread a generated id), but it is not a first-class advertised contract.
+
+### What this PR does NOT do
+
+It does **not** declare `hook/*` SessionEvents (the durable hook-invocation log) — those belong to the `dsh-hook-protocol` library (a later stack PR), because a native plugin can already use the typed Decisions without a durable hook log. A worked native-plugin example/test in this PR (`packages/core/agent-loop/tests/interception.spec.ts`) proves all the seams compose end-to-end through the REAL loop with NO `hook/*` involved — the concrete proof that "native hooks are just a plugin". Compaction (`PreCompact`/`PostCompact`), the Notification hook, Codex `PermissionRequest`, the permission/`ask` system, and the Stop loop-guard remain deferred (`FIXME(permissions)` marks the `ask`→deny degrade).
+
+## Consequences
+
+The canonical interception surface is now complete and uniformly typed: a native plugin returns typed decisions directly, and a CC/Codex bridge maps its protocol fields onto the same unions. The loop gained four firing points (session-start emit, prompt-submit waterfall, the post-tool context buffer, the continuation reshape) and the `dsh-tools` registry runs a two-waterfall pipeline; both are documented in [architecture.md](../../../architecture.md) and the package READMEs, and the decision types in [core-data-structures](../../../core-data-structures/core.md#interception-decisions) + [tools.md](../../../core-data-structures/tools.md). All existing `tools/execute` and `turn-continuation` listeners (tests, docs) migrated to the new seams. The ACP bridge maps the new `rejected` reason to `cancelled` (its codec). A pure internal change with no editor-visible transcript shift for the existing scenarios — the new behavior only fires when a hook is registered — so the snapshot goldens are unchanged; a hook-driven snapshot scenario lands with the bridges (the PR that makes a hook observable end-to-end through ACP).

docs/rfc/proposed/feature/2026-06-30-pre-tool-input-rewrite.md

@@ -0,0 +1,39 @@
+# RFC: Pre-tool input rewrite — a consistent design (proposed)
+
+Status: proposed (2026-06-30)
+
+<!-- XXX: legacy ADR/RFC body format, not yet normalized to a unified RFC template. -->
+
+## Context
+
+The [interception-seams RFC](../../implemented/feature/2026-06-30-interception-seams.md) added `tools/pre-execute` returning a `PreToolDecision` (allow/deny/ask) — but deliberately NOT input rewrite (a hook changing a tool call's `arguments` before it runs). Claude Code's `PreToolUse` hook offers an `updatedInput`, so a faithful CC bridge wants the same. This RFC designs that, separately, because doing it consistently is a real problem — not a field to bolt onto the allow decision.
+
+## The problem: three readers of pre-execution arguments
+
+In the loop, a tool call's arguments are committed to the log and read by live consumers BEFORE the tool executes:
+
+1. **`assistant/message`** is appended before tool dispatch — it is the model-history source `deriveMessages()` replays, so it carries the tool-call arguments the model itself emitted.
+2. **`tool/call`** is the durable AUDIT record, appended before `ctx.tools.execute()`.
+3. **Live presentation reads `tool/call.arguments`**: the ACP bridge remembers them and passes them to `presentResult`; `dsh-tool-bash` derives the card title, the rawInput, the cwd, and the terminal-vs-background treatment from them.
+
+So an "input rewrite" that changes ONLY what executes would make the UI show one command while another RAN, and render result state against the wrong arguments — a real inconsistency, not a documentable gap. (The existing low-level capability to mutate `exec.arguments` in a listener has exactly this latent inconsistency; it is unadvertised precisely because of this.)
+
+## Proposed design (sketch — to validate against the code when built)
+
+Treat input rewrite as a consistency unit: when a `pre-execute` hook supplies `updatedInput`, the rewrite must be reflected in ALL three readers, atomically, before execution:
+
+- The `tool/call` audit event records the REWRITTEN arguments (with the original retained in a sidecar field for the audit trail — a hook changed the call, and both the original and the effective arguments are facts worth keeping).
+- The `assistant/message` in derived history must agree with what executed — options to evaluate: rewrite the assistant message's tool-call block in place (changes what the model "sees it said"), or record a separate correction the next request carries. The CC model is that the model sees the rewrite took effect.
+- Presentation (`presentCall`/`presentResult`) reads the rewritten arguments, so the UI shows what actually ran.
+
+The shape would extend `PreToolDecision` with an allow-variant `arguments` (or a dedicated `{kind:'rewrite', arguments}`), and the loop would thread the rewrite through the three readers above rather than only into `ctx.tools.execute()`.
+
+## Why not now
+
+The interception-seams RFC notes input rewrite "fought the code across two review rounds" — the signal AGENTS.md names for an over-reaching change. Shipping allow/deny/ask first keeps the seam honest (no advertised contract that silently desyncs the UI), and a CC/Codex bridge that receives an `updatedInput` logs it and surfaces a faithful-but-degraded warning (like `ask`→deny) until this lands. This RFC is the home for the consistent design; `TODO(pre-tool-input-rewrite)` in the loop's pre-execute call site anchors it.
+
+## Open questions
+
+- Does rewriting the `assistant/message` tool-call block corrupt any provider's expectation on replay, or is a separate correction safer?
+- Should the original arguments be preserved on the `tool/call` event (audit) and, if so, under what field?
+- How does this interact with a future permission `ask` flow (a user approving a rewritten call)?

packages/bash/tool-bash/tests/integration.spec.ts

@@ -123,7 +123,10 @@ describe('bash tool through the agent loop', () => {
       textResponse('Background task finished.'),
     ])
     // The second tool call needs the REAL task id from the first result;
-    // a tools/execute waterfall listener rewrites the scripted arguments.
+    // a tools/pre-execute listener rewrites the scripted arguments. (This uses
+    // the low-level capability to mutate `exec` before dispatch — the
+    // unadvertised mechanism behind a future first-class input-rewrite decision;
+    // here it is a test shim to thread the generated id, not a product feature.)
     let taskId = ''
 
     const ctx = await harness(adapter)
@@ -137,7 +140,7 @@ describe('bash tool through the agent loop', () => {
         if (match) taskId = match[1]!
       }
     })
-    ctx.on('tools/execute', async (exec, next) => {
+    ctx.on('tools/pre-execute', async (exec, next) => {
       if (exec.name === 'bash_output') {
         exec.arguments = { task_id: taskId }
       }

packages/core/agent-loop/README.md

@@ -45,10 +45,14 @@ Agents listed in config are auto-created at startup.
 One invocation of `runLoop()` drives one agent for its whole lifetime:
 
 ```
+create agent → emit agent/session-start(source)   ⟵ once, before turn 1
 forever:
   wait for queued messages (idle)
   TURN (error-contained):
-    drain queued → 'turn/start' → session('user/message')
+    'turn/start'
+    each queued: waterfall agent/prompt-submit → allow (→ session('user/message'),
+      inject additionalContext) | block (drop)
+    if every prompt blocked: 'turn/end'(rejected), no step  ⟵ zero-step turn
     STEP loop:
       drain steering
       assembly = systemPrompt.assemble()
@@ -56,10 +60,14 @@ forever:
       stream llm.stream(request) → session('assistant/chunk')
       message = waterfall agent/step-result
       session('assistant/message')
-      each tool-call: session('tool/call') → tools.execute() → session('tool/result')
+      each tool-call: session('tool/call')
+        → tools.execute() [waterfall tools/pre-execute → dispatch → tools/post-execute]
+        → session('tool/result')
+      append buffered post-execute additionalContext as session('context/message')(s)
       drain steering → session('steering/message')
-      cont = waterfall agent/turn-continuation
-      if !cont: break
+      cont = waterfall agent/turn-continuation → ContinuationDecision
+        ({action:'continue', reason?} records reason as next-step steering)
+      if action==stop (and no pending steering): break
     session('turn/end')
     await session/flush
     re-enqueue leftover steering as queued
@@ -73,9 +81,9 @@ Cancellation: `agent.cancel()` is the single public stop primitive — it clears
 ### What is NOT here
 
 Everything that goes beyond "call the model, run the tools, repeat" belongs to plugins listening on the event taxonomy:
-- Hooks: `agent/request`, `agent/step-result`, `tools/execute`, `agent/turn-continuation`
+- Hooks: `agent/session-start`, `agent/prompt-submit`, `agent/request`, `agent/step-result`, `tools/pre-execute`, `tools/post-execute`, `agent/turn-continuation`
 - Compaction: `agent/request`
-- Sandbox, permission, plan mode: `tools/execute`
+- Sandbox, permission, plan mode: `tools/pre-execute` (deny/ask gate), `tools/post-execute`
 - Sub-agents: TODO seam on `AgentLoop.create()`
 - Persistence: `session/event` + `session/flush`
 - UI: `agent/stream-chunk` + `agent/*` events

packages/core/agent-loop/src/index.ts

@@ -10,7 +10,7 @@
 import { Context, Service } from 'cordis'
 import { randomUUID } from 'node:crypto'
 import z from 'schemastery'
-import type { AgentFactory, AgentHandle, AgentId, AgentOptions, CreateAgentOptions, ResumeAgentOptions } from '@deepseek-ai/dsh-agent'
+import type { AgentFactory, AgentHandle, AgentId, AgentOptions, CreateAgentOptions, ResumeAgentOptions, SessionStartSource } from '@deepseek-ai/dsh-agent'
 import type {} from '@deepseek-ai/dsh-llm'
 import { SessionId } from '@deepseek-ai/dsh-session'
 import type { Session } from '@deepseek-ai/dsh-session'
@@ -129,7 +129,7 @@ export class AgentLoop extends Service implements AgentFactory {
     // session + agent down as one ordered chain, capturing the loop's closing
     // flush). The whole effect is owned by THIS fiber; no AgentHandle is needed.
     const session = this.ctx.sessions.prepare(SessionId(`${id}-session-${randomUUID()}`), { meta: {} })
-    const { agent } = this.start(id, options, session)
+    const { agent } = this.start(id, options, session, 'startup')
     return agent
   }
 
@@ -152,7 +152,9 @@ export class AgentLoop extends Service implements AgentFactory {
       ...options.seed !== undefined ? { seed: options.seed } : {},
       meta: options.meta ?? {},
     })
-    return this.startOwned(options.agentId, options.agentOptions ?? {}, session)
+    // A seeded (forked) create is still a fresh start, NOT a resume — `resume`
+    // is reserved for reloading a PERSISTED session via resume()/resumeWith().
+    return this.startOwned(options.agentId, options.agentOptions ?? {}, session, 'startup')
   }
 
   /**
@@ -224,7 +226,7 @@ export class AgentLoop extends Service implements AgentFactory {
         ...meta.seedLength !== undefined ? { seedLength: meta.seedLength } : {},
       },
     })
-    return this.startOwned(options.agentId, options.agentOptions ?? {}, session)
+    return this.startOwned(options.agentId, options.agentOptions ?? {}, session, 'resume')
   }
 
   /**
@@ -261,14 +263,33 @@ export class AgentLoop extends Service implements AgentFactory {
    * so a throwing `session/created`/`agent/created` listener unwinds the
    * already-yielded disposers instead of leaking.
    *
+   * `source` says why the session began ({@link SessionStartSource}); it is
+   * emitted as `agent/session-start` once, AFTER the agent is registered (so a
+   * listener can resolve the agent via `ctx.agents.get(id)` and `inject()` into
+   * it) and BEFORE the loop starts its first turn. The emit is contained: a
+   * throwing session-start listener must not abort agent construction — it is
+   * logged, and the agent still starts. (Unlike a turn-boundary throw, there is
+   * no open turn here to balance; the durable evidence of a session-start hook
+   * is whatever it `inject()`ed.)
+   *
    * Returns the agent plus the composite effect's disposer (`disposeAgent`).
    */
-  private start(id: AgentId, options: AgentOptions, session: Session): { agent: ReactLoopAgent; disposeAgent: () => Promise<void> } {
+  private start(
+    id: AgentId, options: AgentOptions, session: Session, source: SessionStartSource,
+  ): { agent: ReactLoopAgent; disposeAgent: () => Promise<void> } {
     const agent = new ReactLoopAgent(this.ctx, id, options, session)
     const dispose = this.ctx.effect(function* (this: AgentLoop) {
       yield this.ctx.sessions.enter(session)
       this.ctx.sessions.announce(session)
       yield this.ctx.agents.register(agent)
+      // Fire AFTER register (a listener can ctx.agents.get(id) + inject()) and
+      // BEFORE the loop's first turn. Contained: a throwing listener is logged,
+      // never aborts construction (no open turn to balance here).
+      try {
+        this.ctx.emit('agent/session-start', agent, source)
+      } catch (error: unknown) {
+        this.ctx.logger.warn(`agent "${id}": agent/session-start listener threw: ${String(error)}`)
+      }
       const stop = agent.start()
       // Disposed FIRST (LIFO): request loop stop (sync), then AWAIT the loop's
       // actual exit so its closing flush lands while onAppend (yielded above,
@@ -295,8 +316,8 @@ export class AgentLoop extends Service implements AgentFactory {
    * `AgentHandle.dispose(): Promise<void>` contract (mirrors the ACP `quiesce()`
    * helper).
    */
-  private startOwned(id: AgentId, options: AgentOptions, session: Session): AgentHandle {
-    const { agent, disposeAgent } = this.start(id, options, session)
+  private startOwned(id: AgentId, options: AgentOptions, session: Session, source: SessionStartSource): AgentHandle {
+    const { agent, disposeAgent } = this.start(id, options, session, source)
     let disposing: Promise<void> | undefined
     return { agent, dispose: () => (disposing ??= disposeAgent()) }
   }

packages/core/agent-loop/src/loop.ts

@@ -10,6 +10,7 @@
 import type { Context } from 'cordis'
 import type { FinishReason, GenerateOptions, Message } from '@deepseek-ai/dsh-llm'
 import { BlockAssembler, HarnessError } from '@deepseek-ai/dsh-llm'
+import type { ContinuationDecision, HookContext, PromptDecision } from '@deepseek-ai/dsh-agent'
 import type { Session, TurnEndReason, TurnTrigger } from '@deepseek-ai/dsh-session'
 import { renderPrompt } from '@deepseek-ai/dsh-system-prompt'
 import type {} from '@deepseek-ai/dsh-tools'
@@ -367,15 +368,48 @@ async function runTurn(ctx: Context, agent: ReactLoopAgent, handle: LoopHandle,
     // decides "owed" from the log via isTurnOpen, so even a throwing turn/start
     // listener — append pushes before notifying — still gets its turn/end).
     session.append('turn/start', { turn, trigger })
-    // Record the queued user messages INSIDE the turn (after turn/start), so
-    // every event in the log is turn-enclosed. turn/end is now owed, so a throw
-    // while appending these is caught below and the turn is still closed.
+    // Each drained queued message runs the `agent/prompt-submit` waterfall before
+    // it becomes a `user/message` — a hook can rewrite the prompt or block it.
+    // Recorded INSIDE the turn (after turn/start) so every event is turn-enclosed;
+    // turn/end is now owed, so a throwing prompt-submit listener (the waterfall
+    // throws) is caught below and the turn still closes.
+    let anyAllowed = false
+    // Seeded with a floor (only observable if the batch were empty, which
+    // runTurn never allows — it is called with ≥1 queued message); each `block`
+    // decision carries a required `reason` and overwrites it, so a fully-blocked
+    // batch always reports the last vetoing reason.
+    let lastBlockReason = 'prompt blocked by hook'
     for (const message of queued) {
-      session.append('user/message', { content: message.content, source: message.source }, { surfaceOp: 'append' })
+      const decision = await ctx.waterfall(
+        'agent/prompt-submit', agent, message.content, message.source,
+        () => Promise.resolve<PromptDecision>({ kind: 'allow' }),
+      )
+      if (decision.kind === 'block') {
+        lastBlockReason = decision.reason
+        continue
+      }
+      anyAllowed = true
+      // `allow.content` REPLACES the prompt bytes (a rewrite); absent keeps them.
+      const content = decision.content ?? message.content
+      session.append('user/message', { content, source: message.source }, { surfaceOp: 'append' })
+      // `allow.additionalContext` is a SEPARATE context/message the next request
+      // also sees. The turn is open, so inject() appends it into THIS turn.
+      if (decision.additionalContext) {
+        agent.inject(decision.additionalContext.content, { source: decision.additionalContext.source })
+      }
     }
     ctx.emit('agent/turn-start', agent, turn)
 
     while (true) {
+      // A fully-blocked batch (every prompt vetoed by prompt-submit) opens a
+      // zero-step turn that ends `rejected`: break BEFORE the first step so the
+      // boundary stays balanced (turn/start → turn/end) and the block is a
+      // durable in-turn fact. `anyAllowed` never changes inside the loop, so this
+      // only ever fires on the first iteration.
+      if (!anyAllowed) {
+        reason = { kind: 'rejected', reason: lastBlockReason }
+        break
+      }
       step += 1
 
       // Steering from the previous round's continuation listeners (or
@@ -448,10 +482,10 @@ async function runTurn(ctx: Context, agent: ReactLoopAgent, handle: LoopHandle,
 
       if (closeStep()) break
 
-      const defaultDecision = stepOutcome.hadToolCalls || steered
-      let shouldContinue: boolean
+      const defaultDecision: ContinuationDecision = { action: stepOutcome.hadToolCalls || steered ? 'continue' : 'stop' }
+      let decision: ContinuationDecision
       try {
-        shouldContinue = await ctx.waterfall(
+        decision = await ctx.waterfall(
           'agent/turn-continuation', agent, turn, defaultDecision,
           () => Promise.resolve(defaultDecision),
         )
@@ -461,9 +495,18 @@ async function runTurn(ctx: Context, agent: ReactLoopAgent, handle: LoopHandle,
         break
       }
 
+      // A forced `continue` may carry model-facing context: record it as
+      // next-STEP steering (the steering channel), so the continued turn's next
+      // iteration drains it before its request — the typed twin of the /goal
+      // step/end-steer pattern.
+      if (decision.action === 'continue' && decision.reason) {
+        agent.inbox.steer({ content: decision.reason.content, source: decision.reason.source })
+      }
+      let shouldContinue = decision.action === 'continue'
+
       // Steering from step/end session-event or continuation listeners (the
-      // /goal pattern) demands the model see it — it overrides a negative
-      // decision; the next iteration's drain records it.
+      // /goal pattern) demands the model see it — it overrides a stop decision;
+      // the next iteration's drain records it.
       if (!shouldContinue && agent.inbox.hasSteering) shouldContinue = true
 
       // A cancel that landed during the continuation window — after the step's
@@ -645,6 +688,12 @@ async function runStep(
   // ToolRegistry.execute converts tool failures (including aborts) into
   // isError results, so abort is re-checked around every call here.
   const toolCalls = message.content.filter(block => block.type === 'tool-call')
+  // Per-step buffer of `additionalContext` attached by tools/post-execute
+  // listeners. Appended as context/message(s) only AFTER every tool/result for
+  // the step, so a multi-call step keeps tool-call/result adjacency
+  // (interleaving context between a call's result and the next call's would
+  // break the pairing the next model request relies on).
+  const pendingContext: HookContext[] = []
   for (const call of toolCalls) {
     /* v8 ignore next -- signal.reason always set: cancel()/disposal provide a default */
     if (signal.aborted) throw new Error(String(signal.reason ?? 'aborted'))
@@ -655,6 +704,12 @@ async function runStep(
     } catch {
       parsedArguments = call.arguments
     }
+    // TODO(pre-tool-input-rewrite): tools/pre-execute deliberately cannot rewrite
+    // `arguments` — tool/call (the audit record) and assistant/message (the
+    // model-history source) are logged BEFORE execute, and live consumers (ACP,
+    // tool-bash presentation) read the pre-execution args, so an execution-only
+    // rewrite would desync the UI from what ran. Designing that consistently is
+    // its own proposed RFC (docs/rfc/proposed/feature/…-pre-tool-input-rewrite.md).
     const result = await ctx.tools.execute({
       callId: call.id,
       name: call.name,
@@ -666,7 +721,7 @@ async function runStep(
       turn, step,
       // The correlation id MUST be the loop's authoritative call.id (the
       // model-transcript id that deriveMessages turns into toolCallId), NOT
-      // result.callId — a tools/execute waterfall listener returning a
+      // result.callId — a post-execute waterfall listener returning a
       // mismatched id would otherwise orphan the call↔result pairing in the
       // next model request. A listener-internal id, if ever needed, belongs in
       // a separate diagnostic field, never overloaded onto callId.
@@ -675,6 +730,8 @@ async function runStep(
       isError: result.isError,
       ...result.error ? { error: result.error } : {},
     }, { surfaceOp: 'append', sourceEventSeqs: [callEvent.seq] })
+    // Buffer (don't append yet) any post-execute additionalContext for this call.
+    if (result.additionalContext) pendingContext.push(result.additionalContext)
     // signal CAN flip during the await above (abort() inside a tool);
     // the analyzer can't see through the await boundary.
     /* v8 ignore start -- signal.reason default unreachable: cancel()/disposal always set it */
@@ -683,6 +740,13 @@ async function runStep(
     /* v8 ignore stop */
   }
 
+  // Append buffered post-execute context AFTER every tool/result, preserving
+  // tool-call/result adjacency across the whole batch. inject() appends into the
+  // open turn (a context/message at its chronological position).
+  for (const context of pendingContext) {
+    agent.inject(context.content, { source: context.source })
+  }
+
   return { hadToolCalls: toolCalls.length > 0, finish: assembler.finish }
 }
 

packages/core/agent-loop/tests/cancel.spec.ts

@@ -213,7 +213,7 @@ describe('Agent.cancel()', () => {
       if (subject === agent && !continued) {
         continued = true
         agent.cancel('from continuation')
-        return true // vote to continue — the post-waterfall marker check must override
+        return { action: 'continue' as const } // vote to continue — the post-waterfall marker check must override
       }
       return next()
     })

packages/core/agent-loop/tests/interception.spec.ts

@@ -0,0 +1,446 @@
+import { describe, expect, it } from 'vitest'
+import { Context } from 'cordis'
+import LlmService, { CallId } from '@deepseek-ai/dsh-llm'
+import SessionStore, { type SessionEvent, type TurnEndReason } from '@deepseek-ai/dsh-session'
+import SystemPrompt from '@deepseek-ai/dsh-system-prompt'
+import ToolRegistry, { defineTool, type PostToolDecision, type PreToolDecision } from '@deepseek-ai/dsh-tools'
+import AgentRegistry, {
+  AgentId,
+  type ContinuationDecision,
+  type PromptDecision,
+  type SessionStartSource,
+} from '@deepseek-ai/dsh-agent'
+import AgentLoop, { type ReactLoopAgent } from '@deepseek-ai/dsh-agent-loop'
+import { MockAdapter, textResponse, toolCallResponse } from './mock-adapter.ts'
+
+/**
+ * The interception seams introduced by the hooks taxonomy: `agent/prompt-submit`,
+ * `agent/session-start`, the reshaped `agent/turn-continuation`
+ * ({@link ContinuationDecision}), and the `tools/pre-execute` / `tools/post-execute`
+ * split with `additionalContext` buffering. These verify the canonical event
+ * surface a hook bridge (or a native plugin) programs against, WITHOUT any
+ * external protocol — a native plugin uses the typed decisions directly.
+ */
+
+async function harness(adapter: MockAdapter) {
+  const ctx = new Context()
+  await ctx.plugin(LlmService)
+  await ctx.plugin(SessionStore)
+  await ctx.plugin(SystemPrompt)
+  await ctx.plugin(ToolRegistry)
+  await ctx.plugin(AgentRegistry)
+  await ctx.plugin(AgentLoop, { agents: [] })
+  ctx.llm.registerAdapter(['mock'], adapter)
+  return ctx
+}
+
+function waitForIdle(ctx: Context, agent: ReactLoopAgent): Promise<void> {
+  return new Promise((resolve) => {
+    const dispose = ctx.on('agent/status', (subject, status) => {
+      if (subject === agent && status === 'idle') {
+        dispose()
+        resolve()
+      }
+    })
+  })
+}
+
+function send(agent: ReactLoopAgent, text: string) {
+  agent.send([{ type: 'text', text }])
+}
+
+function events(agent: ReactLoopAgent): SessionEvent[] {
+  return [...agent.session.events]
+}
+
+describe('agent/prompt-submit', () => {
+  it('allow (default via next) records the user/message unchanged', async () => {
+    const adapter = new MockAdapter([textResponse('ok')])
+    const ctx = await harness(adapter)
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+
+    const seen: string[] = []
+    ctx.on('agent/prompt-submit', async (_agent, content, _source, next) => {
+      seen.push(content.map(b => (b.type === 'text' ? b.text : '')).join(''))
+      return next()
+    })
+
+    send(agent, 'hello')
+    await waitForIdle(ctx, agent)
+
+    expect(seen).toEqual(['hello'])
+    const userMsg = events(agent).find(e => e.type === 'user/message')
+    expect(userMsg?.type === 'user/message' && userMsg.data.content).toEqual([{ type: 'text', text: 'hello' }])
+  })
+
+  it('allow with content REWRITES the prompt before it is recorded', async () => {
+    const adapter = new MockAdapter([textResponse('ok')])
+    const ctx = await harness(adapter)
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+
+    ctx.on('agent/prompt-submit', async (): Promise<PromptDecision> =>
+      ({ kind: 'allow', content: [{ type: 'text', text: 'REWRITTEN' }] }))
+
+    send(agent, 'original')
+    await waitForIdle(ctx, agent)
+
+    const userMsg = events(agent).find(e => e.type === 'user/message')
+    expect(userMsg?.type === 'user/message' && userMsg.data.content).toEqual([{ type: 'text', text: 'REWRITTEN' }])
+    // the rewritten prompt is what reached the model
+    expect(JSON.stringify(adapter.requests[0]!.messages)).toContain('REWRITTEN')
+    expect(JSON.stringify(adapter.requests[0]!.messages)).not.toContain('original')
+  })
+
+  it('allow with additionalContext injects a separate context/message into the turn', async () => {
+    const adapter = new MockAdapter([textResponse('ok')])
+    const ctx = await harness(adapter)
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+
+    ctx.on('agent/prompt-submit', async (): Promise<PromptDecision> =>
+      ({
+        kind: 'allow',
+        additionalContext: { content: [{ type: 'text', text: 'extra ctx' }], source: { kind: 'plugin', plugin: 'test' } },
+      }))
+
+    send(agent, 'go')
+    await waitForIdle(ctx, agent)
+
+    const log = events(agent)
+    const userMsg = log.find(e => e.type === 'user/message')
+    const ctxMsg = log.find(e => e.type === 'context/message')
+    expect(userMsg).toBeDefined()
+    expect(ctxMsg?.type === 'context/message' && ctxMsg.data.content).toEqual([{ type: 'text', text: 'extra ctx' }])
+    expect(ctxMsg?.type === 'context/message' && ctxMsg.data.source).toEqual({ kind: 'plugin', plugin: 'test' })
+    // both the prompt and the injected context reach the model
+    const sent = JSON.stringify(adapter.requests[0]!.messages)
+    expect(sent).toContain('extra ctx')
+  })
+
+  it('block drops the (only) prompt → zero-step turn ends rejected, model never called', async () => {
+    const adapter = new MockAdapter([textResponse('should not run')])
+    const ctx = await harness(adapter)
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+
+    ctx.on('agent/prompt-submit', async (): Promise<PromptDecision> =>
+      ({ kind: 'block', reason: 'blocked by policy' }))
+
+    const reasons: TurnEndReason[] = []
+    ctx.on('agent/turn-end', (_a, _t, reason) => void reasons.push(reason))
+
+    send(agent, 'do something')
+    await waitForIdle(ctx, agent)
+
+    // the model was never called
+    expect(adapter.requests).toHaveLength(0)
+    // the turn opened and closed balanced, with no user/message and no step
+    const log = events(agent)
+    expect(log.some(e => e.type === 'turn/start')).toBe(true)
+    expect(log.some(e => e.type === 'turn/end')).toBe(true)
+    expect(log.some(e => e.type === 'user/message')).toBe(false)
+    expect(log.some(e => e.type === 'step/start')).toBe(false)
+    // ended rejected with the block reason
+    expect(reasons).toEqual([{ kind: 'rejected', reason: 'blocked by policy' }])
+    const turnEnd = log.findLast(e => e.type === 'turn/end')
+    expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason).toEqual({ kind: 'rejected', reason: 'blocked by policy' })
+  })
+
+  it('a throwing prompt-submit listener ends the turn balanced (error), loop survives', async () => {
+    const adapter = new MockAdapter([textResponse('after')])
+    const ctx = await harness(adapter)
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+
+    let threw = false
+    ctx.on('agent/prompt-submit', async () => {
+      if (!threw) { threw = true; throw new Error('prompt hook broke') }
+      return { kind: 'allow' as const }
+    })
+    const errors: Error[] = []
+    ctx.on('agent/error', (_a, _t, _s, error) => void errors.push(error))
+
+    send(agent, 'first')
+    await waitForIdle(ctx, agent)
+    expect(errors.map(e => e.message)).toEqual(['prompt hook broke'])
+    // turn balanced
+    const log = events(agent)
+    expect(log.filter(e => e.type === 'turn/start')).toHaveLength(1)
+    expect(log.filter(e => e.type === 'turn/end')).toHaveLength(1)
+
+    // loop survives: a second prompt runs normally
+    send(agent, 'second')
+    await waitForIdle(ctx, agent)
+    expect(adapter.requests.length).toBeGreaterThanOrEqual(1)
+  })
+})
+
+describe('agent/session-start', () => {
+  it('fires once with source "startup" for a fresh create, before the first turn', async () => {
+    const adapter = new MockAdapter([textResponse('ok')])
+    const ctx = await harness(adapter)
+
+    const sources: SessionStartSource[] = []
+    ctx.on('agent/session-start', (_agent, source) => void sources.push(source))
+
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+    // fires synchronously at create, before any turn
+    expect(sources).toEqual(['startup'])
+    expect(events(agent).some(e => e.type === 'turn/start')).toBe(false)
+
+    send(agent, 'go')
+    await waitForIdle(ctx, agent)
+    // still only one session-start
+    expect(sources).toEqual(['startup'])
+  })
+
+  it('a session-start listener can inject context the first request sees', async () => {
+    const adapter = new MockAdapter([textResponse('ok')])
+    const ctx = await harness(adapter)
+
+    ctx.on('agent/session-start', (agent) => {
+      agent.inject([{ type: 'text', text: 'session preamble' }], { source: { kind: 'plugin', plugin: 'test' } })
+    })
+
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+    send(agent, 'go')
+    await waitForIdle(ctx, agent)
+
+    // the injected context reached the model on the first (only) request
+    expect(JSON.stringify(adapter.requests[0]!.messages)).toContain('session preamble')
+    // and is recorded with the plugin source, never mislabeled as a user prompt
+    const ctxMsg = events(agent).find(e => e.type === 'context/message')
+    expect(ctxMsg?.type === 'context/message' && ctxMsg.data.source).toEqual({ kind: 'plugin', plugin: 'test' })
+  })
+
+  it('a throwing session-start listener does not abort agent construction', async () => {
+    const adapter = new MockAdapter([textResponse('ok')])
+    const ctx = await harness(adapter)
+
+    ctx.on('agent/session-start', () => { throw new Error('session-start hook broke') })
+
+    // create must not throw — the listener error is contained/logged
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+    expect(agent.id).toBe(AgentId('a1'))
+
+    // and the agent still runs
+    send(agent, 'go')
+    await waitForIdle(ctx, agent)
+    expect(adapter.requests).toHaveLength(1)
+  })
+})
+
+describe('agent/turn-continuation (ContinuationDecision)', () => {
+  it('a continue decision with a reason records next-step steering in the same turn', async () => {
+    const adapter = new MockAdapter([textResponse('step 1 no tools'), textResponse('step 2')])
+    const ctx = await harness(adapter)
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+
+    let forced = false
+    ctx.on('agent/turn-continuation', async (_agent, _turn, _default, next): Promise<ContinuationDecision> => {
+      if (!forced) {
+        forced = true
+        return { action: 'continue', reason: { content: [{ type: 'text', text: 'keep going on the goal' }], source: { kind: 'plugin', plugin: 'goal' } } }
+      }
+      return next()
+    })
+
+    send(agent, 'go')
+    await waitForIdle(ctx, agent)
+
+    const log = events(agent)
+    // same turn, two steps
+    expect(log.filter(e => e.type === 'turn/start')).toHaveLength(1)
+    expect(log.filter(e => e.type === 'step/start')).toHaveLength(2)
+    // the reason was recorded as steering BEFORE step 2, with its plugin source
+    const steering = log.find(e => e.type === 'steering/message')
+    expect(steering?.type === 'steering/message' && steering.data.content).toEqual([{ type: 'text', text: 'keep going on the goal' }])
+    expect(steering?.type === 'steering/message' && steering.data.source).toEqual({ kind: 'plugin', plugin: 'goal' })
+    // and reached the next request
+    expect(JSON.stringify(adapter.requests[1]!.messages)).toContain('keep going on the goal')
+  })
+
+  it('a stop decision ends the turn even when the step had tool calls', async () => {
+    const adapter = new MockAdapter([toolCallResponse('c1', 'echo', { text: 'hi' })])
+    const ctx = await harness(adapter)
+    ctx.tools.register(defineTool({
+      name: 'echo', description: 'echo', parameters: { text: { type: 'string' } },
+      async execute(args) { return [{ type: 'text', text: String(args.text) }] },
+    }))
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+
+    ctx.on('agent/turn-continuation', async (): Promise<ContinuationDecision> => ({ action: 'stop' }))
+
+    send(agent, 'go')
+    await waitForIdle(ctx, agent)
+
+    // default would have continued (had tool calls), but the stop decision wins
+    expect(adapter.requests).toHaveLength(1)
+    expect(events(agent).some(e => e.type === 'tool/result')).toBe(true)
+  })
+})
+
+describe('tools/post-execute additionalContext buffering across a multi-call step', () => {
+  it('appends each call\'s additionalContext only AFTER all tool/results, preserving adjacency', async () => {
+    // One assistant step with TWO tool calls; the second model response stops.
+    const twoCalls = [
+      { type: 'block-start' as const, index: 0, blockType: 'tool-call' as const },
+      { type: 'block-end' as const, index: 0, block: { type: 'tool-call' as const, id: CallId('c1'), name: 'echo', arguments: '{"text":"a"}' } },
+      { type: 'block-start' as const, index: 1, blockType: 'tool-call' as const },
+      { type: 'block-end' as const, index: 1, block: { type: 'tool-call' as const, id: CallId('c2'), name: 'echo', arguments: '{"text":"b"}' } },
+      { type: 'usage' as const, usage: { inputTokens: 5, outputTokens: 5 } },
+      { type: 'finish' as const, reason: { kind: 'tool-calls' as const } },
+    ]
+    const adapter = new MockAdapter([twoCalls, textResponse('done')])
+    const ctx = await harness(adapter)
+    ctx.tools.register(defineTool({
+      name: 'echo', description: 'echo', parameters: { text: { type: 'string' } },
+      async execute(args) { return [{ type: 'text', text: String(args.text) }] },
+    }))
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+
+    // Each call attaches additionalContext naming itself.
+    ctx.on('tools/post-execute', async (exec, _result): Promise<PostToolDecision> =>
+      ({ kind: 'accept', additionalContext: { content: [{ type: 'text', text: `ctx-${exec.callId}` }], source: { kind: 'plugin', plugin: 'p' } } }))
+
+    send(agent, 'go')
+    await waitForIdle(ctx, agent)
+
+    // Event order in the log: both tool/results, THEN both context/messages —
+    // never interleaved (which would break tool-call/result adjacency).
+    const types = events(agent).map(e => e.type)
+    const firstResult = types.indexOf('tool/result')
+    const lastResult = types.lastIndexOf('tool/result')
+    const firstCtx = types.indexOf('context/message')
+    expect(firstResult).toBeGreaterThanOrEqual(0)
+    expect(lastResult).toBeGreaterThan(firstResult) // two results
+    expect(firstCtx).toBeGreaterThan(lastResult)    // context only after ALL results
+    // both contexts present
+    const ctxTexts = events(agent)
+      .filter(e => e.type === 'context/message')
+      .flatMap(e => (e.type === 'context/message' ? e.data.content : []))
+      .map(b => (b.type === 'text' ? b.text : ''))
+    expect(ctxTexts).toEqual(['ctx-c1', 'ctx-c2'])
+  })
+})
+
+describe('tools/pre-execute gate (native-plugin permission pattern, end-to-end through the loop)', () => {
+  it('deny short-circuits dispatch into an isError result the model sees', async () => {
+    const adapter = new MockAdapter([toolCallResponse('c1', 'danger', {}), textResponse('ok')])
+    const ctx = await harness(adapter)
+    let ran = false
+    ctx.tools.register(defineTool({
+      name: 'danger', description: 'danger', parameters: {},
+      async execute() { ran = true; return [{ type: 'text', text: 'should not run' }] },
+    }))
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+
+    ctx.on('tools/pre-execute', async (exec, next): Promise<PreToolDecision> => {
+      if (exec.name === 'danger') return { kind: 'deny', reason: 'blocked dangerous tool' }
+      return next()
+    })
+
+    send(agent, 'go')
+    await waitForIdle(ctx, agent)
+
+    expect(ran).toBe(false)
+    const result = events(agent).find(e => e.type === 'tool/result')
+    expect(result?.type === 'tool/result' && result.data.isError).toBe(true)
+    expect(result?.type === 'tool/result'
+      && result.data.content.some(b => b.type === 'text' && b.text.includes('blocked dangerous tool'))).toBe(true)
+  })
+})
+
+describe('worked example: a native hook plugin is just a cordis plugin on the seams', () => {
+  // The whole point of the interception taxonomy: a "native hook" needs no
+  // dsh-hook-protocol, no external command, no hook/* log — it is an ordinary
+  // cordis plugin subscribing to the canonical events and returning typed
+  // decisions. This proves all four seams compose end-to-end through the REAL
+  // loop, with NO hook/* SessionEvents involved (those belong to the bridge lib).
+  const NativeGuard = {
+    name: 'native-guard',
+    apply(ctx: Context) {
+      // 1. SessionStart: seed a standing instruction.
+      ctx.on('agent/session-start', (agent, source) => {
+        agent.inject(
+          [{ type: 'text', text: `policy active (started: ${source})` }],
+          { source: { kind: 'plugin', plugin: 'native-guard' } },
+        )
+      })
+      // 2. PromptSubmit: block a forbidden prompt, annotate the rest.
+      ctx.on('agent/prompt-submit', async (_agent, content, _source, next): Promise<PromptDecision> => {
+        const text = content.map(b => (b.type === 'text' ? b.text : '')).join('')
+        if (text.includes('rm -rf')) return { kind: 'block', reason: 'destructive prompt blocked' }
+        return next()
+      })
+      // 3. PreToolUse: deny a dangerous tool by name.
+      ctx.on('tools/pre-execute', async (exec, next): Promise<PreToolDecision> => {
+        if (exec.name === 'danger') return { kind: 'deny', reason: 'danger tool denied' }
+        return next()
+      })
+      // 4. PostToolUse: attach context after a tool runs.
+      ctx.on('tools/post-execute', async (_exec, _result, next): Promise<PostToolDecision> => {
+        const decision = await next()
+        if (decision.kind === 'accept') {
+          return { kind: 'accept', additionalContext: { content: [{ type: 'text', text: 'audited' }], source: { kind: 'plugin', plugin: 'native-guard' } } }
+        }
+        return decision
+      })
+    },
+  }
+
+  it('all four seams fire for a real allowed turn with a tool call', async () => {
+    const adapter = new MockAdapter([toolCallResponse('c1', 'echo', { text: 'hi' }), textResponse('done')])
+    const ctx = await harness(adapter)
+    await ctx.plugin(NativeGuard)
+    ctx.tools.register(defineTool({
+      name: 'echo', description: 'echo', parameters: { text: { type: 'string' } },
+      async execute(args) { return [{ type: 'text', text: String(args.text) }] },
+    }))
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+
+    send(agent, 'please echo hi')
+    await waitForIdle(ctx, agent)
+
+    const log = events(agent)
+    // session-start preamble injected
+    expect(log.some(e => e.type === 'context/message'
+      && e.data.content.some(b => b.type === 'text' && b.text.includes('policy active (started: startup)')))).toBe(true)
+    // prompt allowed → user/message recorded
+    expect(log.some(e => e.type === 'user/message')).toBe(true)
+    // tool ran (echo allowed) and post-execute attached "audited" context
+    expect(log.some(e => e.type === 'tool/result' && !e.data.isError)).toBe(true)
+    expect(log.some(e => e.type === 'context/message'
+      && e.data.content.some(b => b.type === 'text' && b.text === 'audited'))).toBe(true)
+    // NO hook/* events — a native plugin needs none
+    expect(log.some(e => e.type.startsWith('hook/'))).toBe(false)
+  })
+
+  it('the same plugin blocks a destructive prompt → rejected turn, model never called', async () => {
+    const adapter = new MockAdapter([textResponse('should not run')])
+    const ctx = await harness(adapter)
+    await ctx.plugin(NativeGuard)
+    const agent = ctx.agentLoop.create(AgentId('a2'), { model: 'mock' })
+
+    const reasons: TurnEndReason[] = []
+    ctx.on('agent/turn-end', (_a, _t, reason) => void reasons.push(reason))
+
+    send(agent, 'run rm -rf /')
+    await waitForIdle(ctx, agent)
+
+    expect(adapter.requests).toHaveLength(0)
+    expect(reasons).toEqual([{ kind: 'rejected', reason: 'destructive prompt blocked' }])
+  })
+
+  it('HMR-safety: disposing the plugin fiber removes all four listeners', async () => {
+    const adapter = new MockAdapter([textResponse('ok')])
+    const ctx = await harness(adapter)
+    const fiber = await ctx.plugin(NativeGuard)
+    await fiber.dispose()
+
+    // After disposal, a destructive prompt is NOT blocked (the listener is gone).
+    const agent = ctx.agentLoop.create(AgentId('a3'), { model: 'mock' })
+    send(agent, 'run rm -rf /')
+    await waitForIdle(ctx, agent)
+    // the prompt ran (not rejected) — proving the prompt-submit listener was disposed
+    expect(adapter.requests).toHaveLength(1)
+    expect(events(agent).some(e => e.type === 'user/message')).toBe(true)
+  })
+})

packages/core/agent-loop/tests/loop.spec.ts

@@ -277,7 +277,7 @@ describe('agent loop', () => {
     let steps = 0
     ctx.on('session/event', (_session, event) => { if (event.type === 'step/end') steps++ })
     ctx.on('agent/turn-continuation', async (_agent, _turn, _defaultDecision, next) => {
-      if (steps < 3) return true
+      if (steps < 3) return { action: 'continue' as const }
       return next()
     })
 
@@ -300,7 +300,7 @@ describe('agent loop', () => {
     }))
     const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
 
-    ctx.on('agent/turn-continuation', async () => false as const)
+    ctx.on('agent/turn-continuation', async () => ({ action: 'stop' }) as const)
 
     send(agent, 'go')
     await waitForIdle(ctx, agent)
@@ -381,7 +381,7 @@ describe('agent loop', () => {
     // Force exactly one continuation (step 1 → step 2), then defer to default
     // (step 2 is a plain stop with no tool calls → stops).
     ctx.on('agent/turn-continuation', async (_agent, _turn, _defaultDecision, next) => {
-      if (steps < 2) return true
+      if (steps < 2) return { action: 'continue' as const }
       return next()
     })
 

packages/core/agent-loop/tests/resume.spec.ts

@@ -94,6 +94,36 @@ describe('the session-persistence RFC: AgentLoop factory create/resume', () => {
     await ctx2.fiber.dispose()
   })
 
+  it('agent/session-start fires "startup" for createAgent and "resume" for resume()', async () => {
+    // Lifecycle 1: a fresh createAgent emits session-start with source 'startup'.
+    const adapter1 = new MockAdapter([textResponse('a')])
+    const { ctx: ctx1, root } = await persistentHarness(adapter1)
+    const sources1: string[] = []
+    ctx1.on('agent/session-start', (_agent, source) => void sources1.push(source))
+    const a1 = ctx1.agents.create({ agentId: AgentId('s'), sessionId: SessionId('start-sess') }).agent as ReactLoopAgent
+    expect(sources1).toEqual(['startup'])
+    a1.send([{ type: 'text', text: 'q' }], { source: { kind: 'user' } })
+    await waitForIdle(ctx1, a1)
+    await ctx1.fiber.dispose()
+
+    // Lifecycle 2: resuming the persisted session emits session-start 'resume'.
+    const adapter2 = new MockAdapter([textResponse('b')])
+    const ctx2 = new Context()
+    await ctx2.plugin(LlmService)
+    await ctx2.plugin(SessionStore)
+    await ctx2.plugin(SystemPrompt)
+    await ctx2.plugin(ToolRegistry)
+    await ctx2.plugin(AgentRegistry)
+    await ctx2.plugin(AgentLoop, { agents: [] })
+    await ctx2.plugin(SessionPersistenceJsonl, { root })
+    ctx2.llm.registerAdapter(['mock'], adapter2)
+    const sources2: string[] = []
+    ctx2.on('agent/session-start', (_agent, source) => void sources2.push(source))
+    await ctx2.agents.resume({ agentId: AgentId('s'), resumeSessionId: SessionId('start-sess') })
+    expect(sources2).toEqual(['resume'])
+    await ctx2.fiber.dispose()
+  })
+
   it('resume of a forked session preserves the parentSession lineage and seed boundary in the header', async () => {
     // Lifecycle 1: persist a FORKED session (carries parentSession + seedLength
     // in its header) by creating it with a complete-turn seed — the write path

packages/core/agent-loop/tests/review-fixes.spec.ts

@@ -4,7 +4,7 @@ import LlmService, { CallId, MessageSource, StreamChunk } from '@deepseek-ai/dsh
 import SessionStore, { Session, SessionEvent, SessionId, TurnEndReason } from '@deepseek-ai/dsh-session'
 import SystemPrompt from '@deepseek-ai/dsh-system-prompt'
 import ToolRegistry, { defineTool } from '@deepseek-ai/dsh-tools'
-import AgentRegistry, { AgentId } from '@deepseek-ai/dsh-agent'
+import AgentRegistry, { AgentId, type ContinuationDecision } from '@deepseek-ai/dsh-agent'
 import AgentLoop, { ReactLoopAgent } from '@deepseek-ai/dsh-agent-loop'
 import * as Invariants from '@deepseek-ai/dsh-invariants'
 import { MockAdapter, textResponse, toolCallResponse } from './mock-adapter.ts'
@@ -271,12 +271,12 @@ describe('HIGH: plugin exceptions are contained', () => {
     const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
 
     let threwOnce = false
-    ctx.on('agent/turn-continuation', async (): Promise<boolean> => {
+    ctx.on('agent/turn-continuation', async (): Promise<ContinuationDecision> => {
       if (!threwOnce) {
         threwOnce = true
         throw new Error('broken continuation plugin')
       }
-      return false
+      return { action: 'stop' }
     })
 
     const errors: Error[] = []
@@ -1005,7 +1005,7 @@ describe('P1-5: a started turn (and any open step) is always closed on a boundar
 })
 
 describe('P1-7: tool/result is logged under the originating call.id, not result.callId', () => {
-  it('a tools/execute listener returning a mismatched callId cannot orphan the call↔result pairing', async () => {
+  it('the loop records tool/result under the model call.id even when a post-execute listener replaces content', async () => {
     // Model emits a tool-call with id "c1", then a final text turn.
     const adapter = new MockAdapter([
       toolCallResponse('c1', 'echo', { x: 1 }),
@@ -1019,12 +1019,13 @@ describe('P1-7: tool/result is logged under the originating call.id, not result.
       async execute() { return [{ type: 'text', text: 'ok' }] },
     }))
 
-    // A waterfall listener short-circuits with a result carrying the WRONG
-    // callId (a listener-internal/proxy id). The loop must still record the
-    // tool/result under the model's authoritative call.id.
-    ctx.on('tools/execute', (exec) => {
+    // A post-execute listener transforms the result (accept-with-replacement).
+    // The loop must still record the tool/result under the model's authoritative
+    // call.id (the loop ignores result.callId — which the registry always sets to
+    // exec.callId anyway — and uses call.id, the model-transcript id).
+    ctx.on('tools/post-execute', (exec, _result) => {
       expect(exec.callId).toBe(CallId('c1')) // the loop passed the real id in
-      return Promise.resolve({ callId: CallId('wrong-proxy-id'), content: [{ type: 'text', text: 'ok' }], isError: false })
+      return Promise.resolve({ kind: 'accept', content: [{ type: 'text', text: 'ok' }] })
     }, { prepend: true })
 
     const agent = ctx.agentLoop.create(AgentId('a-callid'), { model: 'mock' })

packages/core/agent/README.md

@@ -31,6 +31,7 @@ The full `agent/*` event taxonomy is declared via declaration merging in `dsh-ag
 - `agent/created`, `agent/disposed` — registration/deregistration
 - `agent/status` — idle / running / disposed transition
 - `agent/queued` — message entered inbox (source-resolved, steering flag)
+- `agent/session-start` — the session lifecycle began (once, before turn 1), carrying a `SessionStartSource` (`startup` for a fresh or forked create, `resume` for a reloaded persisted session; `clear`/`compact` reserved). A pure notification — it cannot block startup; a listener seeds context via `agent.inject()` (a `context/message` the first request sees).
 
 #### Turn boundaries (emit)
 
@@ -40,9 +41,14 @@ Step boundaries are NOT mirrored as `agent/*` emits: a consumer that needs per-s
 
 #### Interception seams (waterfall)
 
+Each interception waterfall returns a small, seam-specific typed **Decision** union (the unified idiom across the taxonomy — a CC/Codex bridge maps its `permissionDecision`/`decision`/`continue` fields onto these, a native plugin returns them directly):
+
+- `agent/prompt-submit` — decide what happens to one drained queued message before it becomes a `user/message`: `PromptDecision` = `allow` (optionally rewriting the prompt `content` or attaching `additionalContext`) or `block` (drop it; a batch whose every prompt is blocked opens a zero-step turn that ends `rejected`). Maps onto Claude Code's `UserPromptSubmit`.
 - `agent/request` — mutate `GenerateOptions` before the model call (hooks, compaction, model switching, tool filtering)
 - `agent/step-result` — post-process the assembled assistant message before tool dispatch (validates what the log records)
-- `agent/turn-continuation` — override the continue/stop decision (force-continue /loop, force-stop budget guard)
+- `agent/turn-continuation` — override the continue/stop decision via `ContinuationDecision` = `{action:'stop'}` or `{action:'continue', reason?}` (a `continue` `reason` is recorded as next-step steering in the same turn — the typed `/goal` pattern). Force-continue `/loop`, force-stop budget guard.
+
+Tool interception is the `tools/pre-execute` / `tools/post-execute` pair in [`dsh-tools`](../tools/README.md) (`PreToolDecision` allow/deny/ask, `PostToolDecision` accept/block) — same typed-Decision idiom, owned there because it is the tool registry's seam.
 
 #### Streaming + tool (emit)
 

packages/core/agent/src/types.ts

@@ -34,6 +34,11 @@
  * See `docs/rfc/implemented/architecture/2026-06-11-microkernel-event-taxonomy.md`
  * and the related `docs/rfc/proposed/simplification/2026-06-20-remove-agent-boundary-mirror-events.md`.
  *
+ * The interception waterfalls here (`agent/prompt-submit`, `agent/request`,
+ * `agent/step-result`, `agent/turn-continuation`) each return a typed Decision —
+ * the convention pinned by
+ * `docs/rfc/implemented/feature/2026-06-30-interception-seams.md`.
+ *
  * @module @deepseek-ai/dsh-agent/types
  */
 
@@ -66,6 +71,64 @@ export interface SendOptions {
 
 export type AgentStatus = 'idle' | 'running' | 'disposed'
 
+/**
+ * Model-facing context an interception listener wants the agent to SEE on the
+ * next request — the canonical shape behind every "inject extra context"
+ * decision ({@link PromptDecision}, {@link PostToolDecision},
+ * {@link ContinuationDecision}). It is `agent.inject()`ed as a
+ * `context/message`, so it carries a REQUIRED {@link MessageSource}: `inject()`
+ * defaults a missing source to `{kind:'user'}`, which would MISLABEL plugin
+ * context as a user prompt and corrupt derived history. A bridge sets
+ * `{kind:'plugin', plugin:'…'}`; a native plugin names itself. Required, not
+ * optional — the label is load-bearing, never defaulted here.
+ */
+export interface HookContext {
+  content: ContentBlock[]
+  source: MessageSource
+}
+
+/**
+ * The decision an {@link Agent} `agent/prompt-submit` waterfall listener returns
+ * for ONE drained queued message, before it becomes a `user/message`. Maps onto
+ * the Claude Code `UserPromptSubmit` hook's allow/block + `additionalContext`.
+ *
+ * - `allow` proceeds with the prompt; optional `content` REPLACES the prompt
+ *   bytes (a rewrite), and optional `additionalContext` is `inject()`ed as a
+ *   separate `context/message` the next request also sees.
+ * - `block` drops the prompt entirely; `reason` is the durable record of why.
+ *   A batch whose every prompt is blocked still opens a zero-step turn that ends
+ *   with {@link TurnEndReason} `rejected` (so the boundary stays balanced and a
+ *   UI can render "blocked by hook").
+ */
+export type PromptDecision =
+  | { kind: 'allow'; content?: ContentBlock[]; additionalContext?: HookContext }
+  | { kind: 'block'; reason: string }
+
+/**
+ * The decision an {@link Agent} `agent/turn-continuation` waterfall listener
+ * returns. The loop computes the default (`continue` when the step had tool
+ * calls or steering was injected, else `stop`); listeners override it to
+ * force-continue (`/goal`, `/loop`) or force-stop (budget guards).
+ *
+ * A `continue` may carry a `reason`: model-facing context recorded as next-STEP
+ * steering within the SAME turn (the loop enqueues it through the steering
+ * channel, so the continued turn's next step sees it). This is the typed twin of
+ * the existing "steer from a step/end listener" `/goal` pattern.
+ */
+export type ContinuationDecision =
+  | { action: 'stop' }
+  | { action: 'continue'; reason?: HookContext }
+
+/**
+ * Why an agent's session lifecycle began, carried by `agent/session-start`. A
+ * bridge keys its SessionStart hook's matcher on this (Claude Code's
+ * `startup`/`resume`/`clear`/`compact` source set). `startup` = a fresh create
+ * (including a seeded/forked create — a seed is NOT a resume); `resume` = a
+ * persisted session reloaded via `ctx.agents.resume()`. `clear`/`compact` are
+ * driven by those subsystems (compact = `TODO(compaction)`).
+ */
+export type SessionStartSource = 'startup' | 'resume' | 'clear' | 'compact'
+
 /**
  * The agent handle — the surface every plugin (UI, hooks, orchestrators)
  * programs against. The concrete implementation lives in
@@ -183,6 +246,19 @@ declare module 'cordis' {
      */
     'agent/queued'(agent: Agent, content: ContentBlock[], info: { source: MessageSource; steering: boolean }): void
 
+    // ---- session lifecycle (emit) ----
+    /**
+     * The agent's session lifecycle began, fired once before its first turn.
+     * `source` says why ({@link SessionStartSource}: fresh startup, a resumed
+     * persisted session, …). A pure NOTIFICATION (emit, not waterfall): it
+     * carries no veto — a session-start listener that wants to seed context does
+     * so via `agent.inject()` (a `context/message` the first request sees), not
+     * by returning a decision. Cannot block the session from starting; that gap
+     * is deliberate (a bridge logs/injects, it does not gate startup).
+     * @mode emit
+     */
+    'agent/session-start'(agent: Agent, source: SessionStartSource): void
+
     // ---- turn boundaries (emit) — the live boundary surface ----
     // Step boundaries are NOT mirrored here: a consumer that needs per-step
     // boundaries reads the durable `step/start`/`step/end` session events (the
@@ -205,6 +281,16 @@ declare module 'cordis' {
     'agent/turn-end'(agent: Agent, turn: number, reason: TurnEndReason): void
 
     // ---- interception seams (waterfall) ----
+    /**
+     * Waterfall: decide what happens to ONE drained queued message before it
+     * becomes a `user/message` — allow (optionally rewriting the prompt bytes or
+     * attaching `additionalContext`) or block it. Fires inside the already-open
+     * turn, per drained message. Maps onto Claude Code's `UserPromptSubmit` hook.
+     * Call `next()` to delegate to the default (allow unchanged), or return a
+     * {@link PromptDecision} without calling `next()` to short-circuit.
+     * @mode waterfall
+     */
+    'agent/prompt-submit'(agent: Agent, content: ContentBlock[], source: MessageSource, next: () => Promise<PromptDecision>): Promise<PromptDecision>
     /**
      * Waterfall: mutate the fully-assembled {@link GenerateOptions} before the
      * model call (hooks, compaction, model switching, tool filtering, …). Call
@@ -219,12 +305,15 @@ declare module 'cordis' {
      */
     'agent/step-result'(agent: Agent, turn: number, step: number, message: Message, next: () => Promise<Message>): Promise<Message>
     /**
-     * Waterfall: override the turn-continuation decision. The default
-     * (computed by the loop) is `hadToolCalls || steeringInjected`. Listeners
-     * can force-continue (/goal, /loop) or force-stop (budget guards).
+     * Waterfall: override the turn-continuation decision via a typed
+     * {@link ContinuationDecision}. The loop's `defaultDecision` is `continue`
+     * when the step had tool calls or steering was injected, else `stop`.
+     * Listeners force-continue (`/goal`, `/loop` — optionally attaching a
+     * `reason` recorded as next-step steering) or force-stop (budget guards).
+     * Call `next()` to delegate to the default, or return a decision to override.
      * @mode waterfall
      */
-    'agent/turn-continuation'(agent: Agent, turn: number, defaultDecision: boolean, next: () => Promise<boolean>): Promise<boolean>
+    'agent/turn-continuation'(agent: Agent, turn: number, defaultDecision: ContinuationDecision, next: () => Promise<ContinuationDecision>): Promise<ContinuationDecision>
 
     // ---- streaming + tool notifications (emit) ----
     /**

packages/core/session/src/types.ts

@@ -134,6 +134,16 @@ export interface TurnEndReasonMap {
   error: { kind: 'error'; step: number; message: string; code?: string }
   disposed: { kind: 'disposed' }
   'max-tokens': { kind: 'max-tokens' }
+  /**
+   * The turn's entire prompt batch was BLOCKED before any step ran — every
+   * drained queued message was vetoed by an `agent/prompt-submit` listener (a
+   * hook). The turn still opened (so the boundary stays balanced and the block
+   * is a durable in-turn fact), but ran zero steps. `reason` carries the block
+   * message from the vetoing decision. Distinct from `aborted` (a user-driven
+   * cancel) and `error` (a failure): the prompt was rejected by policy, not
+   * interrupted or broken. A UI renders it as "prompt blocked by hook".
+   */
+  rejected: { kind: 'rejected'; reason: string }
   /**
    * The turn never ended on its own: the process crashed mid-turn and a
    * persistence backend later closed the orphaned (open) turn on reload so the

packages/core/tools/README.md

@@ -1,6 +1,6 @@
 # dsh-tools
 
-Tool registry and execution waterfall. Tool plugins register their schemas and executors; the agent loop executes calls through the `tools/execute` waterfall.
+Tool registry and execution pipeline. Tool plugins register their schemas and executors; the agent loop executes each call through `tools/pre-execute` (the allow/deny gate) → core dispatch → `tools/post-execute` (inspect/replace the result, attach context).
 
 ## Service: `ToolRegistry` (ctx key: `tools`)
 
@@ -9,7 +9,7 @@ Tool registry and execution waterfall. Tool plugins register their schemas and e
 - `ctx.tools.register(definition: ToolDefinition): () => void` Register a tool. Disposed with the calling fiber.
 - `ctx.tools.get(name: string): ToolDefinition | undefined`
 - `ctx.tools.schemas(): ToolSchema[]` Schemas of all registered tools (without the `execute` functions).
-- `ctx.tools.execute(exec: ToolExecution): Promise<ToolExecutionResult>` Execute one tool call through the `tools/execute` waterfall.
+- `ctx.tools.execute(exec: ToolExecution): Promise<ToolExecutionResult>` Execute one tool call through the `tools/pre-execute` → dispatch → `tools/post-execute` pipeline.
 
 ### Injected services
 
@@ -19,20 +19,23 @@ Tool registry and execution waterfall. Tool plugins register their schemas and e
 
 | Event | Mode | Purpose |
 |---|---|---|
-| `tools/execute` | waterfall | Wrap/veto tool execution (sandbox, permission, hooks, plan mode) |
+| `tools/pre-execute` | waterfall | Allow/deny gate BEFORE a tool runs (sandbox, permission, hooks); returns `PreToolDecision` |
+| `tools/post-execute` | waterfall | Inspect/replace the result AFTER a tool runs, attach context; returns `PostToolDecision` |
 | `tools/change` | emit | A tool was registered or unregistered |
 
 ### Key types
 
 - `ToolDefinition` — `ToolSchema` + `execute(args, exec): Promise<ContentBlock[]>`, plus optional `presentCall(args)` / `presentResult(args, result)` for tool-owned UI presentation (see below).
 - `ToolExecution` — one pending tool call: `{ callId, name, arguments, agent?, signal? }`.
-- `ToolExecutionResult` — outcome: `{ callId, content, isError, error? }`. On failure with a `HarnessError`, `error: { name, code }` carries the structured failure class alongside the model-facing text (the loop forwards it onto the `tool/result` session event for retry/sandbox plugins and replay).
+- `ToolExecutionResult` — outcome: `{ callId, content, isError, error?, additionalContext? }`. On failure with a `HarnessError`, `error: { name, code }` carries the structured failure class alongside the model-facing text (the loop forwards it onto the `tool/result` session event for retry/sandbox plugins and replay). `additionalContext` (a `HookContext`) ferries any `tools/post-execute` context up to the loop, which buffers it and appends it as a `context/message` after all `tool/result`s in the step.
+- `PreToolDecision` — `{kind:'allow'}` | `{kind:'deny', reason}` | `{kind:'ask', reason?}`. Input rewrite (changing `arguments`) is deliberately NOT offered (it would desync the pre-execution audit/history/UI from what ran — its own proposed RFC); `ask` degrades to `deny` until the permission system lands.
+- `PostToolDecision` — `{kind:'accept', content?, additionalContext?}` (keep the call successful, optionally replacing the model-facing content) | `{kind:'block', feedback, additionalContext?}` (turn it into an `isError` whose content is the corrective feedback). Output replacement is clean because `tool/result` is logged AFTER `execute()` returns.
 - `ToolCallPresentation` / `ToolResultPresentation` — provider-neutral shapes a tool returns from `presentCall` / `presentResult` to own how a UI renders ITS calls (see "Tool-owned UI presentation").
 
 ### Extension points
 
 - Tool plugins call `ctx.tools.register()` — schemas flow into the assembly automatically.
-- The `tools/execute` waterfall is the single seam for sandbox, permission, hooks, and plan-mode plugins to wrap or veto a call. Listeners receive `(exec, next)`: call `next()` to proceed, or return a result without calling `next()` to short-circuit (veto).
+- `tools/pre-execute` is the allow/deny gate (sandbox, permission, hooks): listeners receive `(exec, next)` and call `next()` to delegate to the default (allow) or return a `PreToolDecision` to short-circuit; a `deny`/`ask` skips dispatch and yields an `isError` result. `tools/post-execute` is the inspect/transform seam: `(exec, result, next)` → a `PostToolDecision` that can replace content, block with feedback, or attach `additionalContext`. Core dispatch sits between them as plain code; the tool body keeps its own try/catch so a thrown tool still reaches `post-execute` as an `isError`. Both follow the typed-Decision idiom shared with the `agent/*` interception seams (see [`dsh-agent`](../agent/README.md)).
 - MCP servers: one plugin per server, discover tools, call `ctx.tools.register()` with the server's schemas.
 
 ### Typed tool parameter schemas

packages/core/tools/src/index.ts

@@ -1,8 +1,9 @@
 /**
- * Tool registry and execution waterfall. Plugins register tools; the registry
+ * Tool registry and execution pipeline. Plugins register tools; the registry
  * feeds schemas into the system prompt, and `execute()` dispatches each call
- * through the `tools/execute` waterfall for sandbox, permission, and hook
- * plugins to wrap or veto.
+ * through `tools/pre-execute` (the allow/deny gate) → core dispatch →
+ * `tools/post-execute` (inspect/replace the result, attach context) for
+ * sandbox, permission, and hook plugins to gate or transform a call.
  *
  * @module @deepseek-ai/dsh-tools
  */
@@ -10,7 +11,7 @@
 import { Context, Service } from 'cordis'
 import type { CallId, ContentBlock, ToolSchema } from '@deepseek-ai/dsh-llm'
 import { HarnessError } from '@deepseek-ai/dsh-llm'
-import type { Agent } from '@deepseek-ai/dsh-agent'
+import type { Agent, HookContext } from '@deepseek-ai/dsh-agent'
 import type {} from '@deepseek-ai/dsh-system-prompt'
 
 export {
@@ -33,14 +34,31 @@ declare module 'cordis' {
 
   interface Events {
     /**
-     * Waterfall around every tool execution — the single seam where sandbox,
-     * permission, hook, and plan-mode plugins wrap or veto a call. Listeners
-     * receive `(exec, next)`: call `next()` to proceed (possibly around your
-     * own logic), or return a {@link ToolExecutionResult} without calling
-     * `next()` to short-circuit (veto).
+     * Waterfall BEFORE a tool runs — the gate where sandbox, permission, and
+     * hook plugins allow or deny a call (Claude Code's `PreToolUse`). Listeners
+     * receive `(exec, next)`: call `next()` to delegate to the default (allow),
+     * or return a {@link PreToolDecision} without calling `next()` to
+     * short-circuit. A `deny` skips dispatch and yields an `isError` result; the
+     * tool body never runs. Input rewrite is deliberately NOT offered here (see
+     * {@link PreToolDecision}); `ask` degrades to deny until the permission
+     * system lands (`FIXME(permissions)`).
      * @mode waterfall
      */
-    'tools/execute'(this: ToolRegistry, exec: ToolExecution, next: () => Promise<ToolExecutionResult>): Promise<ToolExecutionResult>
+    'tools/pre-execute'(this: ToolRegistry, exec: ToolExecution, next: () => Promise<PreToolDecision>): Promise<PreToolDecision>
+    /**
+     * Waterfall AFTER a tool runs — where hook plugins inspect the result and
+     * accept it (optionally REPLACING the model-facing content, and/or attaching
+     * `additionalContext` for the next request) or block it with corrective
+     * `feedback` (Claude Code's `PostToolUse`). Listeners receive
+     * `(exec, result, next)`: call `next()` to delegate to the default (accept
+     * unchanged), or return a {@link PostToolDecision} to override. The core tool
+     * dispatch sits between the two waterfalls as plain code, all inside
+     * `execute`'s outer try/catch (and the tool body keeps its own inner
+     * try/catch, so a thrown tool still reaches `post-execute` as an `isError`
+     * result).
+     * @mode waterfall
+     */
+    'tools/post-execute'(this: ToolRegistry, exec: ToolExecution, result: ToolExecutionResult, next: () => Promise<PostToolDecision>): Promise<PostToolDecision>
     /**
      * A tool was registered or unregistered (the available tool set changed).
      * @mode emit
@@ -247,8 +265,54 @@ export interface ToolExecutionResult {
    * text in `content` is always present; this is extra structure for code.
    */
   error?: ToolErrorInfo
+  /**
+   * Extra model-facing context a `tools/post-execute` listener attached for the
+   * NEXT request (Claude Code's PostToolUse `additionalContext`). It is NOT part
+   * of this call's `content` — `content`/`feedback` shape the tool RESULT, but
+   * `additionalContext` is a SEPARATE `context/message`. A step can carry
+   * multiple tool calls, so the loop BUFFERS every call's `additionalContext`
+   * and appends them only AFTER all `tool/result`s for the step, keeping
+   * tool-call/result adjacency intact. Carried on the result purely to ferry it
+   * from `execute()` up to the loop's per-step buffer.
+   */
+  additionalContext?: HookContext
 }
 
+/**
+ * The decision a `tools/pre-execute` listener returns for one pending call.
+ * Maps onto Claude Code's `PreToolUse` `permissionDecision`.
+ *
+ * - `allow` proceeds to dispatch. (Input rewrite — changing `exec.arguments` —
+ *   is deliberately NOT offered: `tool/call` and `assistant/message` are logged
+ *   BEFORE execution and live consumers, e.g. the ACP bridge and `dsh-tool-bash`
+ *   presentation, read the pre-execution arguments, so an execution-only rewrite
+ *   would desync the UI from what RAN. That consistency redesign is its own
+ *   `proposed` RFC; `TODO(pre-tool-input-rewrite)` anchors it at the call site.)
+ * - `deny` skips dispatch; the loop records an `isError` result carrying `reason`.
+ * - `ask` is the permission-prompt intent; until the permission system exists it
+ *   degrades to `deny` (`FIXME(permissions)`).
+ */
+export type PreToolDecision =
+  | { kind: 'allow' }
+  | { kind: 'deny'; reason: string }
+  | { kind: 'ask'; reason?: string }
+
+/**
+ * The decision a `tools/post-execute` listener returns for one finished call.
+ * Maps onto Claude Code's `PostToolUse` decision.
+ *
+ * - `accept` keeps the call successful; optional `content` REPLACES the
+ *   model-facing result (clean: `tool/result` is logged AFTER `execute()`
+ *   returns, so a replaced result is the single source of truth for both derived
+ *   history and UI). Optional `additionalContext` rides to the next request.
+ * - `block` turns the call into an `isError` result whose content is the
+ *   corrective `feedback` (the model is told the call was rejected and why),
+ *   optionally also attaching `additionalContext`.
+ */
+export type PostToolDecision =
+  | { kind: 'accept'; content?: ContentBlock[]; additionalContext?: HookContext }
+  | { kind: 'block'; feedback: ContentBlock[]; additionalContext?: HookContext }
+
 /**
  * Best-effort human-readable message from an arbitrary thrown value: Error
  * instances use `.message`; non-Error objects with a string `message`
@@ -336,31 +400,90 @@ export class ToolRegistry extends Service {
   }
 
   /**
-   * Execute one tool call through the `tools/execute` waterfall. If the tool is
-   * not registered, the result is an `isError` carrying a `UNKNOWN_TOOL`
-   * structured error. If the tool or a waterfall listener throws, the error is
-   * caught and returned as an `isError` result so the loop records a failed tool
-   * call instead of failing the whole turn; a thrown {@link HarnessError}
+   * Execute one tool call through the `tools/pre-execute` → dispatch →
+   * `tools/post-execute` pipeline. The two waterfalls are the gate (allow/deny)
+   * and the inspect/transform seam; core dispatch sits between them as plain
+   * code. The whole thing is wrapped in one outer try/catch so a throwing
+   * listener (in either waterfall) becomes an `isError` result instead of
+   * failing the turn; the tool body ALSO keeps its own inner try/catch, so a
+   * thrown tool becomes an `isError` result that `post-execute` listeners can
+   * still inspect. If the tool is not registered, the result is an `isError`
+   * carrying a `UNKNOWN_TOOL` structured error. A thrown {@link HarnessError}
    * surfaces its `{ name, code }` on the result.
    */
   async execute(exec: ToolExecution): Promise<ToolExecutionResult> {
     try {
-      return await this.ctx.waterfall(this, 'tools/execute', exec, async (): Promise<ToolExecutionResult> => {
-        try {
-          const tool = this.store.get(exec.name)
-          // Unknown tool routes through the same catch as a tool-thrown error, so
-          // both failure classes get structured `{ name, code }` from one path.
-          if (!tool) throw new ToolNotFoundError(exec.name)
-          const content = await tool.execute(exec.arguments, exec)
-          return { callId: exec.callId, content, isError: false }
-        } catch (error: unknown) {
-          return toolErrorResult(exec.callId, error)
+      // --- Gate: tools/pre-execute. A deny (or an ask, which degrades to deny
+      // until the permission system lands) skips dispatch entirely. ---
+      const decision = await this.ctx.waterfall(
+        this, 'tools/pre-execute', exec,
+        () => Promise.resolve<PreToolDecision>({ kind: 'allow' }),
+      )
+      if (decision.kind !== 'allow') {
+        // deny → isError. ask has no permission UI yet, so degrade to deny
+        // (FIXME(permissions)): a forthcoming permission system turns `ask` into
+        // a real prompt; today it is the conservative "not allowed".
+        const reason = decision.kind === 'deny'
+          ? decision.reason
+          : decision.reason ?? `tool "${exec.name}" requires approval (not yet supported)`
+        const denied: ToolExecutionResult = {
+          callId: exec.callId,
+          content: [{ type: 'text', text: `Error: ${reason}` }],
+          isError: true,
         }
-      })
+        return await this.postExecute(exec, denied)
+      }
+
+      // --- Core dispatch (plain code between the waterfalls). The tool body's
+      // own try/catch turns a throw into an isError result so post-execute can
+      // inspect it; an unknown tool routes through the same catch. ---
+      let result: ToolExecutionResult
+      try {
+        const tool = this.store.get(exec.name)
+        if (!tool) throw new ToolNotFoundError(exec.name)
+        const content = await tool.execute(exec.arguments, exec)
+        result = { callId: exec.callId, content, isError: false }
+      } catch (error: unknown) {
+        result = toolErrorResult(exec.callId, error)
+      }
+
+      return await this.postExecute(exec, result)
     } catch (error: unknown) {
+      // Outer backstop: a throwing pre/post-execute listener (or the waterfall
+      // machinery) becomes an isError result, never a turn failure.
       return toolErrorResult(exec.callId, error)
     }
   }
+
+  /**
+   * Run the `tools/post-execute` waterfall over a dispatched `result` and apply
+   * its {@link PostToolDecision}: `accept` keeps the call successful (replacing
+   * `content` when given), `block` turns it into an `isError` whose content is
+   * the corrective `feedback`. Either decision may attach `additionalContext`,
+   * which is ferried on the returned result for the loop's per-step buffer.
+   * Runs inside `execute`'s outer try/catch (a throwing listener → isError).
+   */
+  private async postExecute(exec: ToolExecution, result: ToolExecutionResult): Promise<ToolExecutionResult> {
+    const decision = await this.ctx.waterfall(
+      this, 'tools/post-execute', exec, result,
+      () => Promise.resolve<PostToolDecision>({ kind: 'accept' }),
+    )
+    const additionalContext = decision.additionalContext
+    if (decision.kind === 'block') {
+      return {
+        callId: result.callId,
+        content: decision.feedback,
+        isError: true,
+        ...additionalContext ? { additionalContext } : {},
+      }
+    }
+    // accept: replace content if supplied, preserve the dispatched isError/error.
+    return {
+      ...result,
+      ...decision.content ? { content: decision.content } : {},
+      ...additionalContext ? { additionalContext } : {},
+    }
+  }
 }
 
 function toolErrorResult(callId: ToolExecution['callId'], error: unknown): ToolExecutionResult {

packages/core/tools/tests/tools.spec.ts

@@ -4,7 +4,7 @@ import { CallId, HarnessError } from '@deepseek-ai/dsh-llm'
 import SystemPrompt from '@deepseek-ai/dsh-system-prompt'
 import ToolRegistry, {
   defineTool, schemaSpecToJsonSchema, validateArgs, ToolArgsError, ToolNotFoundError,
-  type InferArgs, type SchemaSpec, type ToolExecutionResult,
+  type InferArgs, type SchemaSpec, type PreToolDecision, type PostToolDecision,
 } from '@deepseek-ai/dsh-tools'
 
 async function setup() {
@@ -112,53 +112,123 @@ describe('ToolRegistry', () => {
     expect(err.message).toBe('unknown tool "ghost"')
   })
 
-  it('lets tools/execute waterfall listeners veto a call (permission pattern)', async () => {
+  it('lets a tools/pre-execute listener deny a call (permission pattern)', async () => {
     const ctx = await setup()
     ctx.tools.register(echoTool)
 
-    ctx.on('tools/execute', async (exec, next): Promise<ToolExecutionResult> => {
-      if (exec.name === 'echo') {
-        return {
-          callId: exec.callId,
-          content: [{ type: 'text', text: 'denied by policy' }],
-          isError: true,
-        }
-      }
+    ctx.on('tools/pre-execute', async (exec, next): Promise<PreToolDecision> => {
+      if (exec.name === 'echo') return { kind: 'deny', reason: 'denied by policy' }
       return next()
     })
 
     const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'echo', arguments: { text: 'hi' } })
     expect(result.isError).toBe(true)
-    expect(result.content[0]).toMatchObject({ text: 'denied by policy' })
+    expect(result.content[0]).toMatchObject({ text: 'Error: denied by policy' })
   })
 
-  it('composes multiple tools/execute listeners (sandbox-wrap pattern)', async () => {
+  it('an ask decision degrades to deny until the permission system lands', async () => {
+    const ctx = await setup()
+    ctx.tools.register(echoTool)
+
+    ctx.on('tools/pre-execute', async (_exec, _next): Promise<PreToolDecision> =>
+      ({ kind: 'ask', reason: 'needs approval' }))
+
+    const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'echo', arguments: { text: 'hi' } })
+    expect(result.isError).toBe(true)
+    expect(result.content[0]).toMatchObject({ text: 'Error: needs approval' })
+  })
+
+  it('an ask decision with no reason degrades to deny with a default message', async () => {
+    const ctx = await setup()
+    ctx.tools.register(echoTool)
+
+    ctx.on('tools/pre-execute', async (_exec, _next): Promise<PreToolDecision> => ({ kind: 'ask' }))
+
+    const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'echo', arguments: { text: 'hi' } })
+    expect(result.isError).toBe(true)
+    expect(result.content[0]).toMatchObject({ text: 'Error: tool "echo" requires approval (not yet supported)' })
+  })
+
+  it('a tools/post-execute listener can replace the result content (accept) ', async () => {
+    const ctx = await setup()
+    ctx.tools.register(echoTool)
+
+    ctx.on('tools/post-execute', async (_exec, _result, _next): Promise<PostToolDecision> =>
+      ({ kind: 'accept', content: [{ type: 'text', text: 'rewritten' }] }))
+
+    const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'echo', arguments: { text: 'hi' } })
+    expect(result.isError).toBe(false)
+    expect(result.content[0]).toMatchObject({ text: 'rewritten' })
+  })
+
+  it('a tools/post-execute block turns the call into an isError with corrective feedback', async () => {
+    const ctx = await setup()
+    ctx.tools.register(echoTool)
+
+    ctx.on('tools/post-execute', async (_exec, _result, _next): Promise<PostToolDecision> =>
+      ({ kind: 'block', feedback: [{ type: 'text', text: 'output rejected: try again' }] }))
+
+    const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'echo', arguments: { text: 'hi' } })
+    expect(result.isError).toBe(true)
+    expect(result.content[0]).toMatchObject({ text: 'output rejected: try again' })
+  })
+
+  it('a block decision can ALSO attach additionalContext', async () => {
+    const ctx = await setup()
+    ctx.tools.register(echoTool)
+
+    ctx.on('tools/post-execute', async (_exec, _result, _next): Promise<PostToolDecision> =>
+      ({
+        kind: 'block',
+        feedback: [{ type: 'text', text: 'rejected' }],
+        additionalContext: { content: [{ type: 'text', text: 'why it was rejected' }], source: { kind: 'plugin', plugin: 'test' } },
+      }))
+
+    const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'echo', arguments: { text: 'hi' } })
+    expect(result.isError).toBe(true)
+    expect(result.content[0]).toMatchObject({ text: 'rejected' })
+    expect(result.additionalContext).toMatchObject({ content: [{ text: 'why it was rejected' }], source: { kind: 'plugin', plugin: 'test' } })
+  })
+
+  it('a post-execute additionalContext rides on the result for the loop to buffer', async () => {
+    const ctx = await setup()
+    ctx.tools.register(echoTool)
+
+    ctx.on('tools/post-execute', async (_exec, _result, _next): Promise<PostToolDecision> =>
+      ({ kind: 'accept', additionalContext: { content: [{ type: 'text', text: 'fyi' }], source: { kind: 'plugin', plugin: 'test' } } }))
+
+    const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'echo', arguments: { text: 'hi' } })
+    expect(result.additionalContext).toMatchObject({ content: [{ text: 'fyi' }], source: { kind: 'plugin', plugin: 'test' } })
+  })
+
+  it('composes pre + post waterfalls around dispatch (sandbox-wrap pattern)', async () => {
     const ctx = await setup()
     ctx.tools.register(echoTool)
 
     const order: string[] = []
-    ctx.on('tools/execute', async (_exec, next) => {
-      order.push('first:before')
-      const result = await next()
-      order.push('first:after')
-      return result
+    ctx.on('tools/pre-execute', async (_exec, next) => {
+      order.push('pre:before')
+      const decision = await next()
+      order.push('pre:after')
+      return decision
     })
-    ctx.on('tools/execute', async (_exec, next) => {
-      order.push('second:before')
-      const result = await next()
-      order.push('second:after')
-      return result
+    ctx.on('tools/post-execute', async (_exec, _result, next) => {
+      order.push('post:before')
+      const decision = await next()
+      order.push('post:after')
+      return decision
     })
 
     const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'echo', arguments: { text: 'x' } })
     expect(result.isError).toBe(false)
-    expect(order).toEqual(['first:before', 'second:before', 'second:after', 'first:after'])
+    // pre runs fully (gate) before dispatch, then post runs over the result.
+    expect(order).toEqual(['pre:before', 'pre:after', 'post:before', 'post:after'])
   })
 
-  it('returns an isError result when a tools/execute listener throws', async () => {
+  it('returns an isError result when a tools/pre-execute listener throws', async () => {
     const ctx = await setup()
     ctx.tools.register(echoTool)
-    ctx.on('tools/execute', async () => {
+    ctx.on('tools/pre-execute', async () => {
       throw new Error('permission hook broke')
     })
 
@@ -171,10 +241,26 @@ describe('ToolRegistry', () => {
     })
   })
 
-  it('preserves structured error info when a tools/execute listener throws HarnessError', async () => {
+  it('returns an isError result when a tools/post-execute listener throws', async () => {
     const ctx = await setup()
     ctx.tools.register(echoTool)
-    ctx.on('tools/execute', async () => {
+    ctx.on('tools/post-execute', async () => {
+      throw new Error('post hook broke')
+    })
+
+    const result = await ctx.tools.execute({ callId: CallId('c1'), name: 'echo', arguments: { text: 'hi' } })
+
+    expect(result).toEqual({
+      callId: CallId('c1'),
+      content: [{ type: 'text', text: 'Error: post hook broke' }],
+      isError: true,
+    })
+  })
+
+  it('preserves structured error info when a tools/pre-execute listener throws HarnessError', async () => {
+    const ctx = await setup()
+    ctx.tools.register(echoTool)
+    ctx.on('tools/pre-execute', async () => {
       throw new HarnessError('denied', 'DENIED')
     })
 

packages/subagent/tool-subagent/tests/tool-subagent.spec.ts

@@ -276,6 +276,14 @@ describe('dsh-tool-subagent', () => {
 
     const controller = new AbortController()
     const pending = callSubagent(ctx, { description: 'd', prompt: 'p' }, { signal: controller.signal })
+    // Abort AFTER the tool body has had a chance to register its abort listener
+    // (ctx.tools.execute now awaits the tools/pre-execute waterfall before the
+    // body runs, so the listener is not registered synchronously). A few
+    // microtask turns let execute() reach `addEventListener('abort')`, so this
+    // exercises the LIVE onAbort bridge — distinct from the already-aborted
+    // sync path the next test covers.
+    await Promise.resolve()
+    await Promise.resolve()
     controller.abort()
     const result = await pending
     expect(cancelled).toHaveBeenCalledTimes(1)

packages/ui/acp/src/codec.ts

@@ -34,6 +34,10 @@ import type { ContentBlock as AcpContentBlock, StopReason } from '@agentclientpr
  *                 for any non-bridge caller / property test.)
  * - `disposed`  → `cancelled` (the agent was torn down mid-turn — closest to a
  *                 cancellation from the client's perspective)
+ * - `rejected`  → `cancelled` (the prompt was blocked by an `agent/prompt-submit`
+ *                 hook before any step ran — ACP has no "rejected" reason, and a
+ *                 blocked prompt is, from the client's view, the prompt not being
+ *                 carried out; `cancelled` is the closest legal wire reason)
  */
 export function turnEndToStopReason(reason: TurnEndReason): StopReason {
   switch (reason.kind) {
@@ -45,6 +49,8 @@ export function turnEndToStopReason(reason: TurnEndReason): StopReason {
       return 'cancelled'
     case 'disposed':
       return 'cancelled'
+    case 'rejected':
+      return 'cancelled'
     case 'error':
       return 'end_turn'
     // Merge-extensible: an unknown future TurnEndReason kind still has to

packages/ui/acp/tests/codec.spec.ts

@@ -16,6 +16,7 @@ describe('turnEndToStopReason', () => {
     expect(turnEndToStopReason({ kind: 'max-tokens' })).toBe('max_tokens')
     expect(turnEndToStopReason({ kind: 'aborted', reason: 'x' })).toBe('cancelled')
     expect(turnEndToStopReason({ kind: 'disposed' })).toBe('cancelled')
+    expect(turnEndToStopReason({ kind: 'rejected', reason: 'blocked by hook' })).toBe('cancelled')
     expect(turnEndToStopReason({ kind: 'error', step: 1, message: 'boom' })).toBe('end_turn')
   })
 

scripts/type-equiv.manifest.json

@@ -10,6 +10,10 @@
     { "doc": "docs/core-data-structures/core.md", "symbol": "ToolSchema", "source": "packages/llm/llm/src/types.ts" },
     { "doc": "docs/core-data-structures/core.md", "symbol": "SessionEvent", "source": "packages/core/session/src/types.ts" },
     { "doc": "docs/core-data-structures/core.md", "symbol": "Agent", "source": "packages/core/agent/src/types.ts" },
+    { "doc": "docs/core-data-structures/core.md", "symbol": "HookContext", "source": "packages/core/agent/src/types.ts" },
+    { "doc": "docs/core-data-structures/core.md", "symbol": "PromptDecision", "source": "packages/core/agent/src/types.ts" },
+    { "doc": "docs/core-data-structures/core.md", "symbol": "ContinuationDecision", "source": "packages/core/agent/src/types.ts" },
+    { "doc": "docs/core-data-structures/core.md", "symbol": "SessionStartSource", "source": "packages/core/agent/src/types.ts" },
 
     { "doc": "docs/core-data-structures/llm-streaming.md", "symbol": "StreamChunk", "source": "packages/llm/llm/src/types.ts" },
     { "doc": "docs/core-data-structures/llm-streaming.md", "symbol": "TokenUsage", "source": "packages/llm/llm/src/types.ts" },
@@ -34,6 +38,8 @@
     { "doc": "docs/core-data-structures/tools.md", "symbol": "InferArgs", "source": "packages/core/tools/src/schema.ts" },
     { "doc": "docs/core-data-structures/tools.md", "symbol": "ToolExecution", "source": "packages/core/tools/src/index.ts" },
     { "doc": "docs/core-data-structures/tools.md", "symbol": "ToolExecutionResult", "source": "packages/core/tools/src/index.ts" },
+    { "doc": "docs/core-data-structures/tools.md", "symbol": "PreToolDecision", "source": "packages/core/tools/src/index.ts" },
+    { "doc": "docs/core-data-structures/tools.md", "symbol": "PostToolDecision", "source": "packages/core/tools/src/index.ts" },
 
     { "doc": "docs/core-data-structures/bash.md", "symbol": "BashExecRequest", "source": "packages/bash/bash/src/types.ts" },
     { "doc": "docs/core-data-structures/bash.md", "symbol": "BashExecSpec", "source": "packages/bash/bash/src/types.ts" },