# Tools

The tool pipeline of [dsh-tools](../../packages/core/tools). [core.md](core.md) introduces `ToolDefinition` as the one pipeline-authoring type promoted to the spine and `ToolSchema` as the model-facing wire shape. This page owns the full `ToolDefinition`, the typed schema DSL that builds it, the guarded execution shapes, and the UI-presentation vocabulary.

Source: [`packages/core/tools/src/index.ts`](../../packages/core/tools/src/index.ts) · [`packages/core/tools/src/schema.ts`](../../packages/core/tools/src/schema.ts) · [`packages/core/tools/src/presentation.ts`](../../packages/core/tools/src/presentation.ts)

## `ToolDefinition` — a registered tool

A `ToolSchema` (the model-facing fields) plus the `execute` function and optional UI presenters. The registry holds these; the loop dispatches calls through them. The registry's `schemas()` builds the model-facing `ToolSchema[]` by an explicit allowlist — `execute`/`presentCall`/`presentResult` must never leak into a model request.

```ts type-equiv
interface ToolDefinition extends ToolSchema {
  execute(args: unknown, exec: ToolExecution): Promise<ToolExecuteReturn>
  /**
   * Cooperative tool-call timeout budget in milliseconds. Omit for no deadline.
   * Enforced by `@deepseek-ai/dsh-timeout-policy` (a `tools/execute` wrapper); it
   * is NEVER sent to the model — `schemas()` whitelists only name/description/
   * parameters. Declaring it asserts this tool forwards `exec.signal` to a
   * cooperative implementation that can reach quiescence when the signal aborts.
   */
  timeoutMs?: number
  /**
   * Optional: how to present the PENDING state of one call in a UI, derived from
   * the call's `args` (parsed arguments, `unknown` — the tool validates/narrows
   * its own input). Returns a {@link ToolCallView} (a `card`-tagged render intent),
   * or `undefined` (or omit the method) to fall back to a generic presentation
   * (title = tool name, raw args as input). Pure and side-effect-free: a UI may
   * call it during live streaming AND a session-log replay, so it must depend
   * only on `args`.
   */
  presentCall?(args: unknown): ToolCallView | undefined
  /**
   * Optional: how to present the COMPLETED state, given the same `args` and the
   * `result` (`execute`'s content + whether it errored). Returns a
   * {@link ToolResultView}, or `undefined` (or omit the method) to keep the
   * pending title and render the raw result content. Pure and side-effect-free
   * for the same replay reason.
   */
  presentResult?(args: unknown, result: ToolResult): ToolResultView | undefined
}
```

`execute` receives `args: unknown` — a raw `ToolDefinition` validates its own input. First-party tools don't write that by hand; they use `defineTool`, which validates and narrows for them.

## The typed schema DSL

Plugin authors write per-property specs with a boolean `required: true`, and a type-level helper maps the spec to the `execute` argument type — zero casts. The DSL is *machinery that types* `ToolDefinition`; it is intentionally a sub-page detail, not core.

Source: [`packages/core/tools/src/schema.ts`](../../packages/core/tools/src/schema.ts)

```ts type-equiv
interface SchemaProp {
  type: SchemaType
  /** Per-property required flag (NOT the JSON Schema top-level required array). */
  required?: true
  /** Human-readable description, surfaced in the JSON Schema as well. */
  description?: string
  /** Enum of allowed values (strings only). */
  enum?: string[]
  /** Default value. */
  default?: unknown
  /** Nested properties for type: 'object'. */
  properties?: SchemaSpec
  /** Items schema for type: 'array'. */
  items?: SchemaProp
}
```

```ts type-equiv
type SchemaSpec = Record<string, SchemaProp>
```

`SchemaType` is the primitive union `'string' | 'number' | 'boolean' | 'object' | 'array'`. `InferArgs<S>` maps a `SchemaSpec` to the TS argument type — `required: true` props become required keys, everything else genuinely optional:

```ts type-equiv
type InferArgs<S extends SchemaSpec> = Simplify<
  & { [K in RequiredKeys<S>]: InferPropValue<S[K]> }
  & { [K in Exclude<keyof S, RequiredKeys<S>>]?: InferPropValue<S[K]> }
>
```

`defineTool({ name, description, parameters, execute, … })` ties it together: `parameters` is a `SchemaSpec`, `execute(args, exec)` gets `args: InferArgs<typeof parameters>`, and the helper converts the spec to JSON Schema (`schemaSpecToJsonSchema`) for the wire and validates model-generated args (`validateArgs`) before the typed body runs. A mismatch throws `ToolArgsError` (`code: 'INVALID_ARGS'`), which the registry turns into an `isError` result so the model can self-correct. Why a custom DSL and not schemastery: tool parameters need JSON Schema (the LLM wire format), not validation/transformation — the lightweight DSL gives the best authoring DX with the smallest surface.

Registration is a value boundary. `ToolRegistry.register()` validates `ToolDefinition.parameters` as lossless JSON before and after cloning, copies the scalar fields, binds the execute/presentation callbacks once to the original definition as their method receiver, and deep-freezes the stored record. Replacing a callback property on the caller-owned definition later does not change dispatch. `get()`/`visible()` expose only that frozen snapshot, while `schemas()` produces detached projections, so the model-visible and executable views cannot drift through a leaked mutable registry object.

## Execution: extensible waterfalls plus monotonic policy

`ctx.tools.execute()` accepts a caller-owned `ToolExecutionInput`, snapshots it into a pipeline-owned `ToolExecution`, and runs that call through `tools/pre-execute` (the reorderable allow/deny/ask waterfall) → registered monotonic guards → `tools/execute` (around-dispatch wrappers) → `tools/post-execute` (inspect/replace the result) → `tools/result` (the immutable authoritative outcome). The outcome is a `ToolExecutionResult`.

```ts type-equiv
interface ToolExecutionToken {
  readonly [toolExecutionTokenBrand]: true
}
```

```ts type-equiv
interface ToolExecutionInput {
  readonly callId: CallId
  readonly name: string
  /** Parsed JSON arguments (unknown — tools validate their own input). */
  readonly arguments: unknown
  /** The agent on whose behalf the call runs (set by the agent loop). */
  readonly agent?: Agent
  /**
   * Opaque token of the enclosing transport execution, when one exists. Code
   * Mode sets this on SDK sub-dispatches so commit-style observers can wait for
   * the outer `run_code` outcome without receiving its live mutable execution.
   */
  readonly parent?: ToolExecutionToken
  signal?: AbortSignal
}
```

```ts type-equiv
interface ToolExecution extends ToolExecutionInput {
  /** Registry-assigned identity shared with nested calls only as their opaque `parent` token. */
  readonly token: ToolExecutionToken
}
```

`ToolExecutionToken` is a compile-time opaque type and a frozen, property-free object at runtime; identity comparison is its only operation. Before policy runs, `ctx.tools.execute()` requires the caller's `arguments` to be losslessly JSON-serializable, checks again after cloning to contain unstable accessors, assigns a fresh token, and deep-freezes the detached arguments. A cloneable mutable exotic such as `Map` is rejected and normalized to an error before policy. `token`, `callId`, `name`, `arguments`, `agent`, and the optional `parent` token are non-writable throughout all waterfalls, so a listener cannot change which capability or scope was authorized or reach a live enclosing execution; an around-dispatch wrapper may add, replace, or remove only optional `signal`. After the complete pipeline the registry freezes the execution and exposes its stable identity to `tools/result` observers, where the execution remains usable as a `WeakMap` key without mutation races.

A `ToolGuard` is scope-aware final pre-dispatch policy. Its shape deliberately has no allow result: `undefined` preserves the waterfall decision, while a returned reason can only reduce permission, so a later listener cannot undo it.

```ts type-equiv
type ToolGuard = (execution: Readonly<ToolExecution>) => string | undefined
```

```ts type-equiv
interface ToolExecutionResult {
  callId: CallId
  content: ContentBlock[]
  isError: boolean
  /**
   * Set when the call failed with a {@link HarnessError}: machine-routable
   * `{ name, code }` for retry/sandbox plugins and replay. The model-facing
   * 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 tool-private presentation payload from a successful `execute` (the object
   * return form). Threaded onto the `tool/result` session event and back into
   * {@link ToolResult} for `presentResult`. Opaque (`unknown`); absent when the
   * tool attached none or the call failed.
   */
  meta?: unknown
}
```

The registry rebuilds and validates the complete authoritative result after post-policy. Its content, structured error, additional context, and presentation metadata must round-trip losslessly through JSON; a malformed or non-JSON value becomes a JSON-safe `isError` result before `tools/result` observers run, so the live outcome is always safe for the later durable `tool/result` append.

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/execute` wrappers return a `ToolExecutionResult`; `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 / dispatch / accept-unchanged), or return a decision/result 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; a registered `ToolGuard` runs after that waterfall and can impose a final denial. Input rewrite is deliberately NOT offered on `PreToolDecision` because it would desync the pre-execution audit/history/UI from what ran. A `post-execute` `accept` may replace the model-facing `content`; a `block` turns the call into an `isError` whose content is the corrective `feedback`. The awaited `tools/result` notification then receives the frozen execution identity and a deep-frozen result snapshot after every wrapper, post decision, and outer error catch; observers cannot transform the outcome or race each other through payload mutation, and one observer failure neither changes the result nor starves peers. 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.

## The structured-output schema subset

The vocabulary a caller uses to demand a machine-readable result from a subagent (`SubagentStartRequest.outputSchema`, [subagent.md](subagent.md#the-start-request)) or a workflow `agent()` call. It is deliberately NOT full JSON Schema: the schema travels verbatim to the model as a forced tool's `parameters`, and the produced value is validated client-side by `validateStructuredValue` — so every accepted keyword must be one the validator actually enforces, and `assertSupportedOutputSchema` rejects anything else loud (`OutputSchemaError`, listing every violation). Both walkers reason over own enumerable properties only (JSON carries nothing else) and reject non-plain objects (`Date`, `Map`) that would serialize lossily.

```ts type-equiv
type StructuredScalar = string | number | boolean | null
```

```ts type-equiv
type StructuredSchemaType = 'object' | 'array' | 'string' | 'number' | 'integer' | 'boolean' | 'null'
```

```ts type-equiv
interface StructuredSchemaNode {
  type: StructuredSchemaType
  properties?: Record<string, StructuredSchemaNode>
  required?: string[]
  additionalProperties?: boolean
  items?: StructuredSchemaNode
  enum?: StructuredScalar[]
  const?: StructuredScalar
  description?: string
  title?: string
  default?: unknown
  examples?: unknown
}
```

A schema is an object-rooted node (`enum`/`const` are scalar-only; `description`/`title`/`default`/`examples` are annotations, allowed and ignored but still required to be JSON data — they ride the wire):

```ts type-equiv
type StructuredOutputSchema = StructuredSchemaNode & { type: 'object' }
```

## Tool-presentation UI vocabulary

How a tool wants its call shown in a UI (an editor tool-call card, a CLI log line), provider-neutral so a tool describes itself without depending on any client protocol. `presentCall`/`presentResult` return a **`card`-tagged render intent** — a discriminated union a UI bridge switches on:

- `ToolCallView` (pending): `{ card: 'generic', title, kind?, rawInput?, content?, locations? }` (the default card; `locations` is `{ path, line? }[]` files the call reads/modifies, for editor follow-along), `{ card: 'terminal', title, description?, cwd? }` (a shell command → a terminal card), or `{ card: 'diff', title, diffs, locations? }` (a file create/modify → an inline diff card; `diffs` is `{ path, oldText, newText }[]`, `oldText: null` for a new file).
- `ToolResultView` (completed): `{ card: 'generic', title?, content? }`, `{ card: 'terminal', title?, output?, exitCode?, signal? }` (the captured run output + exit; a capable UI shows an exit-status pill, an incapable one gets a fenced ` ```console ` fallback the bridge derives from `output`), or `{ card: 'diff', title?, diffs }` (a completed file mutation → the change to show, typically the applied hunks with context lines computed from the before/after content, or a whole-file diff when there is no before-image — e.g. a file create. A `tool_call_update`'s content REPLACES the call's content, so a mutation tool returns this even when it duplicates the call-time snippet, to keep the result from clobbering the diff with result text).

`ToolCallKind` (`'read' | 'edit' | 'delete' | 'move' | 'search' | 'execute' | 'fetch' | 'other'`) picks an icon on a generic card. `FileLocation` (`{ path, line? }`) and `FileDiff` (`{ path, oldText, newText }`) are the shared file-card vocabulary. The design is pinned in [the render-intent-union RFC](../rfc/implemented/architecture/2026-07-02-tool-render-intent-union.md); the ACP bridge maps a `diff` card to a `{ type: 'diff' }` content block, a `terminal` card to the `_meta` terminal convention, and relativizes a file card's title against the session cwd.

The full presentation field docs live in [`packages/core/tools/src/presentation.ts`](../../packages/core/tools/src/presentation.ts). The bash tool's own schemas (`bash`/`bash_output`/`bash_kill`) and the executor they drive are on [bash.md](bash.md).
