diff --git a/docs/module-graph.md b/docs/module-graph.md
index e4d0b3367d..8ac71b11cc 100644
--- a/docs/module-graph.md
+++ b/docs/module-graph.md
@@ -179,6 +179,7 @@ flowchart TD
   pkg_workflow --> pkg_agent
   pkg_workflow --> pkg_brand
   pkg_workflow --> pkg_llm
+  pkg_workflow --> pkg_session
   pkg_tools --> pkg_agent
   pkg_tools --> pkg_code_runtime
   pkg_tools --> pkg_llm
@@ -215,6 +216,7 @@ flowchart TD
   pkg_subagent --> pkg_agent
   pkg_subagent --> pkg_llm
   pkg_subagent --> pkg_scope
+  pkg_subagent --> pkg_session
   pkg_subagent --> pkg_tools
   pkg_tool_web --> pkg_llm
   pkg_tool_web --> pkg_system_prompt
@@ -265,6 +267,7 @@ flowchart TD
   pkg_agent_core --> pkg_tools
   pkg_subagent_acp --> pkg_agent
   pkg_subagent_acp --> pkg_llm
+  pkg_subagent_acp --> pkg_session
   pkg_subagent_acp --> pkg_subagent
   pkg_subagent_acp --> pkg_subagent_subprocess
   pkg_subagent_inprocess --> pkg_agent
@@ -285,6 +288,7 @@ flowchart TD
   pkg_hooks_claude --> pkg_tools
   pkg_subagent_mock --> pkg_agent
   pkg_subagent_mock --> pkg_llm
+  pkg_subagent_mock --> pkg_session
   pkg_subagent_mock --> pkg_subagent
   pkg_jsonrpc --> pkg_agent
   pkg_jsonrpc --> pkg_llm
@@ -362,14 +366,14 @@ flowchart TD
 | [`invariants`](../packages/support/invariants) | `support` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`scope`](../packages/core/scope), [`session`](../packages/core/session) |
 | [`user-approval`](../packages/ui/user-approval) | `ui` | [`agent`](../packages/core/agent), [`brand`](../packages/util/brand), [`llm`](../packages/llm/llm), [`scope`](../packages/core/scope), [`session`](../packages/core/session), [`system-prompt`](../packages/core/system-prompt) |
 | [`user-interaction`](../packages/ui/user-interaction) | `ui` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm) |
-| [`workflow`](../packages/workflow/workflow) | `workflow` | [`agent`](../packages/core/agent), [`brand`](../packages/util/brand), [`llm`](../packages/llm/llm) |
+| [`workflow`](../packages/workflow/workflow) | `workflow` | [`agent`](../packages/core/agent), [`brand`](../packages/util/brand), [`llm`](../packages/llm/llm), [`session`](../packages/core/session) |
 | [`tools`](../packages/core/tools) | `core` | [`agent`](../packages/core/agent), [`code-runtime`](../packages/code-runtime/code-runtime), [`llm`](../packages/llm/llm), [`scope`](../packages/core/scope), [`session`](../packages/core/session), [`system-prompt`](../packages/core/system-prompt), [`user-approval`](../packages/ui/user-approval) |
 | [`bash-sandbox`](../packages/bash/bash-sandbox) | `bash` | [`bash`](../packages/bash/bash), [`bash-local`](../packages/bash/bash-local), [`sandbox`](../packages/sandbox/sandbox) |
 | [`agent-loop`](../packages/core/agent-loop) | `core` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`scope`](../packages/core/scope), [`session`](../packages/core/session), [`session-persistence`](../packages/session-persistence/session-persistence), [`system-prompt`](../packages/core/system-prompt), [`tools`](../packages/core/tools) |
 | [`tool-bash`](../packages/bash/tool-bash) | `bash` | [`agent`](../packages/core/agent), [`bash`](../packages/bash/bash), [`llm`](../packages/llm/llm), [`sandbox`](../packages/sandbox/sandbox), [`system-prompt`](../packages/core/system-prompt), [`tools`](../packages/core/tools), [`user-approval`](../packages/ui/user-approval) |
 | [`tool-fs`](../packages/fs/tool-fs) | `fs` | [`fs`](../packages/fs/fs), [`llm`](../packages/llm/llm), [`session`](../packages/core/session), [`system-prompt`](../packages/core/system-prompt), [`tools`](../packages/core/tools) |
 | [`tool-skill`](../packages/skill/tool-skill) | `skill` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`skill`](../packages/skill/skill), [`tools`](../packages/core/tools) |
-| [`subagent`](../packages/subagent/subagent) | `subagent` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`scope`](../packages/core/scope), [`tools`](../packages/core/tools) |
+| [`subagent`](../packages/subagent/subagent) | `subagent` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`scope`](../packages/core/scope), [`session`](../packages/core/session), [`tools`](../packages/core/tools) |
 | [`tool-web`](../packages/web/tool-web) | `web` | [`llm`](../packages/llm/llm), [`system-prompt`](../packages/core/system-prompt), [`tools`](../packages/core/tools), [`web`](../packages/web/web) |
 | [`timeout-policy`](../packages/timeout/timeout-policy) | `timeout` | [`llm`](../packages/llm/llm), [`timeout`](../packages/util/timeout), [`tools`](../packages/core/tools) |
 | [`tool-todo`](../packages/todo/tool-todo) | `todo` | [`agent`](../packages/core/agent), [`session`](../packages/core/session), [`tools`](../packages/core/tools) |
@@ -380,11 +384,11 @@ flowchart TD
 | [`repeat-tool-guard`](../packages/guard/repeat-tool-guard) | `guard` | [`agent`](../packages/core/agent), [`tools`](../packages/core/tools) |
 | [`tool-workflow`](../packages/workflow/tool-workflow) | `workflow` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`system-prompt`](../packages/core/system-prompt), [`tools`](../packages/core/tools), [`workflow`](../packages/workflow/workflow) |
 | [`agent-core`](../packages/core/agent-core) | `core` | [`agent`](../packages/core/agent), [`agent-loop`](../packages/core/agent-loop), [`invariants`](../packages/support/invariants), [`llm`](../packages/llm/llm), [`session`](../packages/core/session), [`skill`](../packages/skill/skill), [`skill-local`](../packages/skill/skill-local), [`system-prompt`](../packages/core/system-prompt), [`tool-bash`](../packages/bash/tool-bash), [`tool-skill`](../packages/skill/tool-skill), [`tools`](../packages/core/tools) |
-| [`subagent-acp`](../packages/subagent/subagent-acp) | `subagent` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`subagent`](../packages/subagent/subagent), [`subagent-subprocess`](../packages/subagent/subagent-subprocess) |
+| [`subagent-acp`](../packages/subagent/subagent-acp) | `subagent` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`session`](../packages/core/session), [`subagent`](../packages/subagent/subagent), [`subagent-subprocess`](../packages/subagent/subagent-subprocess) |
 | [`subagent-inprocess`](../packages/subagent/subagent-inprocess) | `subagent` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`session`](../packages/core/session), [`subagent`](../packages/subagent/subagent), [`system-prompt`](../packages/core/system-prompt), [`tools`](../packages/core/tools) |
 | [`tool-subagent`](../packages/subagent/tool-subagent) | `subagent` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`subagent`](../packages/subagent/subagent), [`tools`](../packages/core/tools) |
 | [`hooks-claude`](../packages/hooks/hooks-claude) | `hooks` | [`agent`](../packages/core/agent), [`hook-protocol`](../packages/hooks/hook-protocol), [`llm`](../packages/llm/llm), [`session`](../packages/core/session), [`subagent`](../packages/subagent/subagent), [`tools`](../packages/core/tools) |
-| [`subagent-mock`](../packages/support/subagent-mock) | `support` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`subagent`](../packages/subagent/subagent) |
+| [`subagent-mock`](../packages/support/subagent-mock) | `support` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`session`](../packages/core/session), [`subagent`](../packages/subagent/subagent) |
 | [`jsonrpc`](../packages/ui/jsonrpc) | `ui` | [`agent`](../packages/core/agent), [`llm`](../packages/llm/llm), [`llm-deepseek`](../packages/llm/llm-deepseek), [`session`](../packages/core/session), [`subagent`](../packages/subagent/subagent) |
 | [`workflow-workerthread`](../packages/workflow/workflow-workerthread) | `workflow` | [`agent`](../packages/core/agent), [`brand`](../packages/util/brand), [`llm`](../packages/llm/llm), [`session`](../packages/core/session), [`subagent`](../packages/subagent/subagent), [`tools`](../packages/core/tools), [`workflow`](../packages/workflow/workflow) |
 | [`subagent-fork`](../packages/subagent/subagent-fork) | `subagent` | [`agent`](../packages/core/agent), [`session`](../packages/core/session), [`subagent`](../packages/subagent/subagent), [`subagent-inprocess`](../packages/subagent/subagent-inprocess) |
diff --git a/docs/rfc/INDEX.md b/docs/rfc/INDEX.md
index a5334a63e1..5ed8c6ae6d 100644
--- a/docs/rfc/INDEX.md
+++ b/docs/rfc/INDEX.md
@@ -17,7 +17,6 @@ Generated by `pnpm run gen-rfc-index` from the RFC tree — never edit by hand;
 
 | Title | First proposed |
 |---|---|
-| [Unify the agent id and the session id](proposed/simplification/2026-06-20-unify-agent-and-session-id.md) | 2026-06-20 |
 | [Prune dead public and result surface](proposed/simplification/2026-07-04-prune-dead-core-spine-surface.md) | 2026-07-04 |
 | [Drop unconsumed skill provider events](proposed/simplification/2026-07-12-drop-unconsumed-skill-provider-events.md) | 2026-07-12 |
 | [Prune unused web seam fields](proposed/simplification/2026-07-12-prune-unused-web-seam-fields.md) | 2026-07-12 |
@@ -87,6 +86,7 @@ Generated by `pnpm run gen-rfc-index` from the RFC tree — never edit by hand;
 | [Prune dead methods from the persistence seam](implemented/simplification/2026-06-20-prune-dead-seam-methods.md) | 2026-06-20 |
 | [Keep one public stop primitive](implemented/simplification/2026-06-20-public-agent-stop-surface.md) | 2026-06-20 |
 | [Stop mirroring durable boundaries as agent events](implemented/simplification/2026-06-20-remove-agent-boundary-mirror-events.md) | 2026-06-20 |
+| [Unify the agent id and the session id](implemented/simplification/2026-06-20-unify-agent-and-session-id.md) | 2026-06-20 |
 | [Split the filesystem seam — provider text mutations plus the `dsh-fs-policy` plugin](implemented/simplification/2026-06-26-fsspec-style-fs-seam.md) | 2026-06-26 |
 | [Stop mirroring the token stream as an agent event](implemented/simplification/2026-07-02-remove-stream-chunk-mirror.md) | 2026-07-02 |
 | [Drop the `image` content block until a path can honor it](implemented/simplification/2026-07-04-drop-image-content-block.md) | 2026-07-04 |
diff --git a/docs/rfc/implemented/architecture/2026-06-20-branded-ids.md b/docs/rfc/implemented/architecture/2026-06-20-branded-ids.md
index 5fe8224644..4fe7f58168 100644
--- a/docs/rfc/implemented/architecture/2026-06-20-branded-ids.md
+++ b/docs/rfc/implemented/architecture/2026-06-20-branded-ids.md
@@ -8,7 +8,7 @@ The harness brands `CallId` (`packages/llm/llm/src/brand.ts`) and the shared age
 
 **Gap 1 — unbranded cross-boundary IDs in the bash seam.** The background-task id is a plain `string`: `BashTask.id: string` (`packages/bash/bash/src/types.ts`), carried as `string` through the whole executor seam (`BashExecutor.get`/`ownerOf`/`readOutput`/`kill(id: string)` in `packages/bash/bash/src/index.ts`) and validated/passed as `string` by the model-facing tools (`validateTaskId`, `assertTaskAccess`, the `task_id` schema arg in `packages/bash/tool-bash/src/index.ts`). It is generated by a per-executor counter — `` `bash-${this.nextTaskId++}` `` in `packages/bash/bash-local/src/index.ts` — which gives it **exactly the same `name-N` shape as `SessionId`'s default** (`` `session-${++counter}` `` in `packages/core/session/src/index.ts`). A bash task id and a session id are trivially swappable at a call site and the compiler says nothing. This is the headline case the user asked about, and it is a model-facing id (the model passes `task_id` back to `bash_output`/`bash_kill`), so a confusion here is reachable from untrusted input.
 
-The bash **owner token** is the related sub-case: `BashExecRequest.owner?: string` and `BashExecSpec.owner: string | undefined` (`packages/bash/bash/src/types.ts`) are documented as a deliberately *opaque* isolation key, but in every live caller the value IS the owning agent's `session.header.id` (`callerToken = (exec) => exec.agent?.session.header.id` in `packages/bash/tool-bash/src/index.ts`) — i.e. a `SessionId` wearing a `string` disguise. It is compared for access control (`owner !== callerToken(exec)`), so a mismatched-but-well-typed string here is a cross-session isolation bug the type system currently cannot catch. This is the same `session.header.id`-as-owner alias that the [unify-the-agent-id-and-the-session-id](../../proposed/simplification/2026-06-20-unify-agent-and-session-id.md) proposal calls the "bash owner-token alias hole".
+The bash **owner token** is the related sub-case: `BashExecRequest.owner?: string` and `BashExecSpec.owner: string | undefined` (`packages/bash/bash/src/types.ts`) are documented as a deliberately *opaque* isolation key, but in every live caller the value IS the owning agent's `session.header.id` (`callerToken = (exec) => exec.agent?.session.header.id` in `packages/bash/tool-bash/src/index.ts`) — i.e. a `SessionId` wearing a `string` disguise. It is compared for access control (`owner !== callerToken(exec)`), so a mismatched-but-well-typed string here is a cross-session isolation bug the type system currently cannot catch. This is the same `session.header.id`-as-owner alias covered by the [unified agent/session identity decision](../simplification/2026-06-20-unify-agent-and-session-id.md).
 
 **Gap 2 — brand erosion at the seams of the *already-branded* IDs.** Even `CallId` and `SessionId` decay back to bare `string` at exactly the places confusion is most likely: registry/store key types and public method params. Representative sites include the session store, the agent registry (both keyed by the shared `SessionId`), `ToolPresenter`'s call-id map, ACP's session-id records and loading set, and the persistence coordinator. A brand that is dropped at a collection key buys nothing on lookups — the value of the existing brands is partly unrealized.
 
@@ -62,6 +62,6 @@ The landed invariants: `BashTaskId` and `OwnerToken` are defined in `dsh-bash` a
 
 ## Consequences
 
-- **Mechanical churn across two surfaces.** Propagating brands touches the bash seam (interface + impl + consumer) and the ACP session-id surface plus the persistence coordinator. The churn is broad but low-severity: a missed site is a compile error, not a silent bug. The change is observably type-only — no snapshot or e2e behavioral diff. It sits next to the [unify-the-agent-id-and-the-session-id](../../proposed/simplification/2026-06-20-unify-agent-and-session-id.md) proposal (both touch the session-id / owner-token boundary); if that proposal lands, `OwnerToken` still stays distinct from the unified id for the decoupling reason above.
+- **Mechanical churn across two surfaces.** Propagating brands touches the bash seam (interface + impl + consumer) and the ACP session-id surface plus the persistence coordinator. The churn is broad but low-severity: a missed site is a compile error, not a silent bug. The change is observably type-only — no snapshot or e2e behavioral diff. It sits next to the [unified agent/session identity decision](../simplification/2026-06-20-unify-agent-and-session-id.md) because both touch the session-id / owner-token boundary; `OwnerToken` stays distinct from the unified id for the decoupling reason above.
 - **Brands do not validate.** A brand is a confusability guard, not a correctness proof: a *wrong* session id that is still a well-formed string passes the type checker exactly as before. This RFC does not close that gap (see Out of scope) — it only stops the *category* error of passing the wrong *kind* of id.
 - **The "where to stop" line stays a judgment call.** Branding `BashTaskId` but not `ToolName`, `OwnerToken` but not `ModelId`, is a taste call about which strings "could plausibly be confused." Reasonable reviewers may want more or fewer; the policy in `brand.ts` is the tie-breaker, and this RFC errs toward the ids that are model-facing or used for access control.
diff --git a/docs/rfc/implemented/simplification/2026-06-20-unify-agent-and-session-id.md b/docs/rfc/implemented/simplification/2026-06-20-unify-agent-and-session-id.md
new file mode 100644
index 0000000000..be1052830a
--- /dev/null
+++ b/docs/rfc/implemented/simplification/2026-06-20-unify-agent-and-session-id.md
@@ -0,0 +1,38 @@
+# RFC: Unify the agent id and the session id
+
+Status: implemented
+
+## Problem
+
+The agent factory previously carried two ids for each live agent/session pair: `agentId`, the `AgentRegistry` routing handle, and `sessionId`, the event-sourced/persisted-log identity. `CreateAgentOptions` took both; `ResumeAgentOptions` took `agentId` plus `resumeSessionId`; in-process subagents minted two independent UUIDs despite recording lineage separately.
+
+ACP already used the same value for both identities. Where they diverged, stdio kept `labelBySession` solely to recover an agent label from session events, and hooks exposed both values for authors to reconcile. No production path reattached one live agent object to several sessions or drove one session through several agent ids.
+
+The [agent-scope runtime](../../implemented/architecture/2026-07-12-agent-scope-runtime-design.md) had no reservation side tables: create and resume used one `AgentCreationTransaction`, and agent/session entries used the same final-entry collision rule. Separate ids therefore did not duplicate asynchronous liveness, rollback, or quiescence machinery. Identity unification was only an API and representation simplification: it deleted one caller-supplied id, one UUID per in-process child, and the remaining translation paths without changing the transaction lifecycle.
+
+Session itself repeated the same fact as `Session.id` and `Session.header.id`. Construction rejected a header whose id differed, so the aliases were constrained equal; the durable boundary nevertheless had to validate the duplicate, and production consumers chose between its two homes.
+
+## Decision
+
+An agent's registry id equals its session id. `CreateAgentOptions` accepts one `sessionId` used for both final registry entries; resume registers the agent under `resumeSessionId`; in-process and ACP subagent creation use the child session id; and `Session.id` derives from `header.id`. The existing creation transaction, final-entry collision checks, and exact-entry detach semantics remain; maps and fields whose sole job was translating between the ids are gone.
+
+The config-driven path keeps `agents[].id` as a stable configuration label, not a live routing identity. A fresh start mints the combined id `${label}-session-${randomUUID()}` so durable restarts do not collide; `resumeSessionId` instead supplies the exact combined identity to load and register. Logs may use the stable label while all live and durable lookups use the one `SessionId`.
+
+`agent/created` and `agent/disposed` remain. They are paired publication lifecycle events, not identity aliases; any later consumer-free removal needs its own proposal after a fresh search.
+
+## Alternatives considered
+
+**Keep separate routing and log identities.** A stable configured label plus a fresh durable conversation is useful, but it does not require two live identities: the label can remain configuration/display metadata while the combined per-run `SessionId` owns routing and persistence. Keeping two ids would preserve translation maps and permit impossible pairings without adding lifecycle capability.
+
+## Verification
+
+- Agent create/resume and subagent creation carry one identity, and `Session` stores it in one place.
+- The creation transaction retains final-entry collision, exact-entry detach, rollback, and quiescence coverage without identity-specific lifecycle state.
+- ACP, stdio, hooks, bash ownership, persistence, and lineage use the shared `SessionId` directly. The ACP subagent backend uses the child server's returned session id as its run id.
+- The config-driven resume-or-create policy is explicit and covered across a durable restart.
+- A production listener search kept `agent/created`/`agent/disposed` and their publication semantics.
+- Typecheck, coverage, snapshots, doc-sync, module-graph verification, build, and hygiene pass.
+
+## Consequences
+
+This forecloses latent multi-session-actor and session-handoff designs and makes persisted client-chosen session identity the registry identity. If separate routing identity becomes a real requirement, it needs an explicit lifecycle design rather than an unconstrained caller-supplied pair.
diff --git a/docs/rfc/proposed/simplification/2026-06-20-unify-agent-and-session-id.md b/docs/rfc/proposed/simplification/2026-06-20-unify-agent-and-session-id.md
deleted file mode 100644
index 6e3eea1515..0000000000
--- a/docs/rfc/proposed/simplification/2026-06-20-unify-agent-and-session-id.md
+++ /dev/null
@@ -1,38 +0,0 @@
-# RFC: Unify the agent id and the session id
-
-Status: proposed
-
-## Problem
-
-The agent factory carries two ids for each live agent/session pair: `agentId`, the `AgentRegistry` routing handle, and `sessionId`, the event-sourced/persisted-log identity. `CreateAgentOptions` takes both; `ResumeAgentOptions` takes `agentId` plus `resumeSessionId`; in-process subagents mint two independent UUIDs despite recording lineage separately.
-
-ACP already uses the same value for both identities. Where they diverge, stdio keeps `labelBySession` solely to recover an agent label from session events, and hooks expose both values for authors to reconcile. No production path reattaches one live agent object to several sessions or drives one session through several agent ids.
-
-The [agent-scope runtime](../../implemented/architecture/2026-07-12-agent-scope-runtime-design.md) has no reservation side tables: create and resume use one `AgentCreationTransaction`, and agent/session entries use the same final-entry collision rule. Separate ids therefore do not duplicate asynchronous liveness, rollback, or quiescence machinery. Identity unification is only an API and representation simplification: it deletes one caller-supplied id, one UUID per in-process child, and the remaining translation paths without changing the transaction lifecycle.
-
-Session itself repeats the same fact as `Session.id` and `Session.header.id`. Construction rejects a header whose id differs, so the aliases are constrained equal; the durable boundary must nevertheless validate the duplicate, and production consumers choose between its two homes.
-
-## Proposal
-
-Make an agent's registry id equal its session id. `CreateAgentOptions` accepts one id used for both final registry entries; resume registers the agent under the resumed session id; subagent creation mints one combined id; Session keeps one identity home by deriving `id` from `header.id` or removing the alias. Keep the existing creation transaction, final-entry collision checks, and exact-entry detach semantics; remove only maps and fields whose sole job is translating between the ids.
-
-The config-driven path keeps `agents[].id` as a stable configuration label, not a live routing identity. A fresh start mints the combined id `${label}-session-${randomUUID()}` so durable restarts do not collide; `resumeSessionId` instead supplies the exact combined identity to load and register. Logs may use the stable label while all live and durable lookups use the one `SessionId`.
-
-`agent/created` and `agent/disposed` remain outside this proposal. They are paired publication lifecycle events, not identity aliases; any later consumer-free removal needs its own proposal after a fresh search.
-
-## Alternatives considered
-
-**Keep separate routing and log identities.** A stable configured label plus a fresh durable conversation is useful, but it does not require two live identities: the label can remain configuration/display metadata while the combined per-run `SessionId` owns routing and persistence. Keeping two ids would preserve translation maps and permit impossible pairings without adding lifecycle capability.
-
-## Acceptance criteria
-
-- Agent create/resume and subagent creation carry one identity; `Session` stores it in one place.
-- The existing creation transaction keeps final-entry collision, exact-entry detach, rollback, and quiescence guarantees without adding identity-specific lifecycle state.
-- ACP, stdio, hooks, bash ownership, persistence, and lineage need no agent/session id translation.
-- The config-driven resume-or-create policy is explicit and covered across a durable restart.
-- `agent/created`/`agent/disposed` are removed only if a post-change production search finds no listener; otherwise they and their publication semantics stay.
-- Typecheck, coverage, snapshots, doc-sync, module-graph verification, build, and hygiene pass.
-
-## Risks
-
-This forecloses latent multi-session-actor and session-handoff designs, makes persisted client-chosen session identity the registry identity, and touches every factory fixture. If separate routing identity becomes a real requirement, it needs an explicit lifecycle design rather than an unconstrained caller-supplied pair.
diff --git a/packages/subagent/subagent-acp/package.json b/packages/subagent/subagent-acp/package.json
index e73d861a79..4c34ebe3fc 100644
--- a/packages/subagent/subagent-acp/package.json
+++ b/packages/subagent/subagent-acp/package.json
@@ -24,6 +24,7 @@
   "peerDependencies": {
     "@deepseek-ai/dsh-agent": "^0.0.1",
     "@deepseek-ai/dsh-llm": "^0.0.1",
+    "@deepseek-ai/dsh-session": "^0.0.1",
     "@deepseek-ai/dsh-subagent": "^0.0.1",
     "@deepseek-ai/dsh-subagent-subprocess": "^0.0.1",
     "cordis": "^4.0.0-rc.6"
@@ -35,6 +36,7 @@
   "devDependencies": {
     "@deepseek-ai/dsh-agent": "workspace:^",
     "@deepseek-ai/dsh-llm": "workspace:^",
+    "@deepseek-ai/dsh-session": "workspace:^",
     "@deepseek-ai/dsh-subagent": "workspace:^",
     "@deepseek-ai/dsh-subagent-subprocess": "workspace:^",
     "@cordisjs/plugin-loader": "^1.0.0-rc.4",
diff --git a/packages/subagent/subagent-acp/src/run.ts b/packages/subagent/subagent-acp/src/run.ts
index 1e82ef85b0..1cc220204f 100644
--- a/packages/subagent/subagent-acp/src/run.ts
+++ b/packages/subagent/subagent-acp/src/run.ts
@@ -23,7 +23,6 @@
  */
 
 import { spawn } from 'node:child_process'
-import { randomUUID } from 'node:crypto'
 import { Readable, Writable } from 'node:stream'
 import {
   ClientSideConnection,
@@ -190,8 +189,6 @@ function toError(value: unknown): Error {
  * @returns the ready run handle for the child subprocess.
  */
 export async function startAcpRun(request: SubagentStartRequest, spec: AcpRunSpec): Promise<SubagentRun> {
-  const id = SessionId(randomUUID())
-
   if (request.signal.aborted) throw new Error('subagent request was aborted before the ACP child started')
 
   // Spawn the child ACP agent. stdin = ACP request channel, stdout = ACP
@@ -260,7 +257,7 @@ export async function startAcpRun(request: SubagentStartRequest, spec: AcpRunSpe
     ),
   )
 
-  let sessionId: string | undefined
+  let sessionId: SessionId | undefined
   // Resolves when a cancel is requested, so `result` can settle `aborted` even
   // if the child never cooperates with `session/cancel` (it ignores the notify,
   // or the prompt wedges). The result path races this against the ACP drive: the
@@ -309,7 +306,7 @@ export async function startAcpRun(request: SubagentStartRequest, spec: AcpRunSpe
           clientCapabilities: {},
         })
         const session = await conn.newSession({ cwd: spec.cwd, mcpServers: [] })
-        sessionId = session.sessionId
+        sessionId = SessionId(session.sessionId)
         if (flags.cancelled) throw new Error('subagent cancelled before the ACP session started')
       })(),
       spawnFailed.then((err): never => { throw err }),
@@ -321,6 +318,11 @@ export async function startAcpRun(request: SubagentStartRequest, spec: AcpRunSpe
     if (flags.cancelled) throw new Error('subagent request was aborted before the ACP child started')
     throw toError(error)
   }
+  // The startup race can fulfill only after newSession assigned the id; this
+  // guard keeps that cross-closure invariant explicit for TypeScript.
+  /* v8 ignore next */
+  if (sessionId === undefined) throw new Error('ACP child published without a session id')
+  const runId = sessionId
 
   const result: Promise<SubagentResult> = (async (): Promise<SubagentResult> => {
     try {
@@ -332,7 +334,7 @@ export async function startAcpRun(request: SubagentStartRequest, spec: AcpRunSpe
       // succeeds, transport/process failure rejects the in-flight prompt RPC.
       const prompt = async (): Promise<SubagentResult> => {
         // The startup phase cannot fulfill without assigning the session id.
-        const promptResult = await conn.prompt({ sessionId: sessionId as string, prompt: toAcpPrompt(request.prompt) })
+        const promptResult = await conn.prompt({ sessionId: runId, prompt: toAcpPrompt(request.prompt) })
         return { output: collectOutput(), stopReason: acpStopReason(promptResult.stopReason) }
       }
       return await Promise.race([
@@ -366,7 +368,7 @@ export async function startAcpRun(request: SubagentStartRequest, spec: AcpRunSpe
 
   let disposal: Promise<void> | undefined
   return {
-    id,
+    id: runId,
     result,
     dispose(): Promise<void> {
       if (disposal !== undefined) return disposal
diff --git a/packages/subagent/subagent-acp/tests/mock-acp-server.ts b/packages/subagent/subagent-acp/tests/mock-acp-server.ts
index fb200f3505..5145941526 100644
--- a/packages/subagent/subagent-acp/tests/mock-acp-server.ts
+++ b/packages/subagent/subagent-acp/tests/mock-acp-server.ts
@@ -99,7 +99,7 @@ function makeAgent(conn: AgentSideConnection): Agent {
         writeFileSync(NEWSESSION_GATE.ready, 'at-newSession')
         while (!existsSync(NEWSESSION_GATE.go)) await new Promise(r => setTimeout(r, 10))
       }
-      return { sessionId: randomUUID() }
+      return { sessionId: process.env.MOCK_SESSION_ID ?? randomUUID() }
     },
     authenticate(_params: AuthenticateRequest): Promise<void> {
       // No auth methods advertised; nothing to do.
diff --git a/packages/subagent/subagent-acp/tests/subagent-acp.spec.ts b/packages/subagent/subagent-acp/tests/subagent-acp.spec.ts
index eed3cfe1ed..115d5c3e97 100644
--- a/packages/subagent/subagent-acp/tests/subagent-acp.spec.ts
+++ b/packages/subagent/subagent-acp/tests/subagent-acp.spec.ts
@@ -122,8 +122,9 @@ describe('buildChildEnv', () => {
 
 describe('dsh-subagent-acp', () => {
   it('drives a child process to completion and returns its streamed output', async () => {
-    const ctx = await setup({ MOCK_TEXT: 'hello from acp child', MOCK_STOP: 'end_turn' })
+    const ctx = await setup({ MOCK_TEXT: 'hello from acp child', MOCK_STOP: 'end_turn', MOCK_SESSION_ID: 'acp-child-session' })
     const run = await ctx.subagents.start('acp', request('do X'))
+    expect(run.id).toBe('acp-child-session')
     const result = await run.result
     expect(result.stopReason).toBe('completed')
     expect(text(result.output)).toBe('hello from acp child')
diff --git a/packages/subagent/subagent/package.json b/packages/subagent/subagent/package.json
index eb0dbf8da0..cf1ebf9485 100644
--- a/packages/subagent/subagent/package.json
+++ b/packages/subagent/subagent/package.json
@@ -25,6 +25,7 @@
     "@deepseek-ai/dsh-agent": "^0.0.1",
     "@deepseek-ai/dsh-llm": "^0.0.1",
     "@deepseek-ai/dsh-scope": "^0.0.1",
+    "@deepseek-ai/dsh-session": "^0.0.1",
     "@deepseek-ai/dsh-tools": "^0.0.1",
     "cordis": "^4.0.0-rc.6"
   },
@@ -32,6 +33,7 @@
     "@deepseek-ai/dsh-agent": "workspace:^",
     "@deepseek-ai/dsh-llm": "workspace:^",
     "@deepseek-ai/dsh-scope": "workspace:^",
+    "@deepseek-ai/dsh-session": "workspace:^",
     "@deepseek-ai/dsh-tools": "workspace:^",
     "cordis": "^4.0.0-rc.6"
   }
diff --git a/packages/support/subagent-mock/package.json b/packages/support/subagent-mock/package.json
index 8980cc35d2..528691daef 100644
--- a/packages/support/subagent-mock/package.json
+++ b/packages/support/subagent-mock/package.json
@@ -24,6 +24,7 @@
   "peerDependencies": {
     "@deepseek-ai/dsh-agent": "^0.0.1",
     "@deepseek-ai/dsh-llm": "^0.0.1",
+    "@deepseek-ai/dsh-session": "^0.0.1",
     "@deepseek-ai/dsh-subagent": "^0.0.1",
     "cordis": "^4.0.0-rc.6"
   },
@@ -33,6 +34,7 @@
   "devDependencies": {
     "@deepseek-ai/dsh-agent": "workspace:^",
     "@deepseek-ai/dsh-llm": "workspace:^",
+    "@deepseek-ai/dsh-session": "workspace:^",
     "@deepseek-ai/dsh-subagent": "workspace:^",
     "@cordisjs/plugin-loader": "^1.0.0-rc.4",
     "cordis": "^4.0.0-rc.6"
diff --git a/packages/workflow/workflow/package.json b/packages/workflow/workflow/package.json
index a6c004d6d0..4d79d672a1 100644
--- a/packages/workflow/workflow/package.json
+++ b/packages/workflow/workflow/package.json
@@ -25,6 +25,7 @@
     "@deepseek-ai/dsh-agent": "^0.0.1",
     "@deepseek-ai/dsh-brand": "^0.0.1",
     "@deepseek-ai/dsh-llm": "^0.0.1",
+    "@deepseek-ai/dsh-session": "^0.0.1",
     "cordis": "^4.0.0-rc.6"
   },
   "devDependencies": {
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index d02b4a9ca6..6214564eae 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -827,6 +827,9 @@ importers:
       '@deepseek-ai/dsh-scope':
         specifier: workspace:^
         version: link:../../core/scope
+      '@deepseek-ai/dsh-session':
+        specifier: workspace:^
+        version: link:../../core/session
       '@deepseek-ai/dsh-tools':
         specifier: workspace:^
         version: link:../../core/tools
@@ -852,6 +855,9 @@ importers:
       '@deepseek-ai/dsh-llm':
         specifier: workspace:^
         version: link:../../llm/llm
+      '@deepseek-ai/dsh-session':
+        specifier: workspace:^
+        version: link:../../core/session
       '@deepseek-ai/dsh-subagent':
         specifier: workspace:^
         version: link:../subagent
@@ -1085,6 +1091,9 @@ importers:
       '@deepseek-ai/dsh-llm':
         specifier: workspace:^
         version: link:../../llm/llm
+      '@deepseek-ai/dsh-session':
+        specifier: workspace:^
+        version: link:../../core/session
       '@deepseek-ai/dsh-subagent':
         specifier: workspace:^
         version: link:../../subagent/subagent