deepseek-harness/docs/cookbook/adding-a-tool.md

Cookbook: adding a tool

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How to give the model a new capability. Reference implementations: examples/echo-agent/src/echo-tool.ts (minimal) and packages/bash/tool-bash (production-grade, three-package seam).

The minimal shape

import { readFile } from 'node:fs/promises'
import type { Context } from 'cordis'
import { defineTool } from '@deepseek-ai/dsh-tools'

export const name = 'my-tool'
export const inject = ['tools']

export function apply(ctx: Context) {
  ctx.tools.register(defineTool({
    name: 'read_file',
    description: 'Read a file from disk.',          // what the model sees
    parameters: {
      path: { type: 'string', required: true, description: 'Absolute path' },
      limit: { type: 'number' },                     // optional by default
    },
    async execute(args, exec) {
      // args is TYPED from the schema: { path: string; limit?: number }
      // exec carries immutable identity + token; signal is the operational field
      return [{ type: 'text', text: await readFile(args.path, 'utf8') }]
    },
  }))
}

Registration is effect-based: disposing the plugin fiber unregisters the tool (write the HMR test). Schemas flow into the system-prompt assembly automatically.

Rules of the execute() contract

• Args are validated for you. defineTool validates the model-generated arguments against the SchemaSpec before execute runs (type, required keys, enum membership, nested objects/arrays — runtime arg validation), so inside execute the args already match InferArgs. You still hand-check value constraints the DSL can't express (non-empty strings, positive numbers, cross-field rules); throw a descriptive Error for those. Raw JSON-Schema tools registered directly (MCP) are NOT validated by the harness — they validate their own input.
• Registration borrows your readonly definition. A typed same-process contribution is not a serialization boundary; do not mutate its schema or replace callbacks after registration. schemas() materializes only the explicit model-facing projection. To hot-swap a tool, dispose its owning effect and register the replacement; mutable state inside the callback's closure remains ordinary plugin state.
• Execution identity is protected. The registry materializes arguments as detached lossless JSON in one recursive pass, freezes that value before policy starts, and assigns an opaque exec.token; callId, name, arguments, agent, token, and an optional enclosing-transport parent token stay immutable through dispatch. parent is identity-only and exposes no live outer execution. Treat args as readonly input. An around-dispatch wrapper may add, replace, or remove only exec.signal to impose cancellation or a deadline.
• Throwing or returning non-JSON data means isError. The registry catches throws and materializes the final result before observers run. A malformed or non-JSON result becomes { isError: true }, preventing a live success that cannot be logged. Throw for infrastructure failures; report domain failures in result text when the model must interpret them.
• Honor exec.signal. Cancel in-flight work when it fires.
• Attach durable card data with meta (optional). execute may return { content, meta } instead of a bare ContentBlock[] — meta is a JSON-serializable payload the core treats as opaque, persisted on the tool/result event and handed back to your presentResult (so a card that needs more than args, like write/edit's applied-hunk diff, survives a session replay). Keep UI-only data here, never in the model-facing content.
• Use exec.agent for async notifications. agent.inject(content, {source: {kind: 'plugin', plugin: '<name>'}}) appends durable context the NEXT model request sees — it is not a wake-up (an idle agent stays idle). Guard against disposed agents (try/catch).

Long-running work

Gate run_in_background with producer config, reject a pre-aborted call, then register through ctx.tasks.start({ kind, label, owner: exec.agent, run }). The runtime validates ownership and control-surface availability before run() starts work, then supplies the id, session fence, generic control tools, notices, and owner cleanup.

The producer supplies synchronous cancel, non-rejecting done that settles after resource cleanup, and optional consuming readOutput with bounded-output formatting. Once the id is returned, use a task-owned cancellation signal rather than exec.signal. See the background task runtime RFC and dsh-tool-bash for a stream producer.

Execution policy and observation

Prefer not to build deployment policy into the tool. Use tools/pre-execute for extensible allow/deny/ask policy (the permission-gate example), ctx.tools.guard() for a final monotonic deny that later listeners cannot undo, tools/execute to wrap core dispatch with a deadline/retry/metrics scope, tools/post-execute to transform or attach model-facing context, and tools/result to observe the immutable normalized outcome without changing it. A sandboxing implementation can also sit behind the tool's executor capability seam; the exact contracts are in the dsh-tools README.

Code Mode reaches your tool for free

In Code Mode, every visible registered tool is available as await tools.<name>(args) without extra integration. The SDK derives parameters from the same JSON Schema, and calls re-enter the normal execution pipeline. Write descriptions as model-facing API docs; non-text result blocks become placeholders in programs.

How your tool renders in an editor (ACP presentation)

Your tool's execute returns model-facing content; its editor card is a separate, optional concern you declare with two pure display methods on the defineTool options. Design this alongside execute, not after — an editor (Zed, over the ACP bridge) shows the card, and a tool with no presentation falls back to a bland generic card (title = tool name, raw args as input).

Both methods return a card-tagged render intent — pick the card kind that matches what your tool does:

• presentCall(args) → a ToolCallView (the PENDING card):
  • { card: 'generic', title, kind?, rawInput?, content?, locations? } — the default. Set kind for an icon (read/search/…); set locations: [{ path, line? }] for any file your tool touches so a capable editor follows along / jumps to it.
  • { card: 'terminal', title, description?, cwd? } — your call IS a shell command. title is the command, description renders above the terminal card. (tool-bash.)
  • { card: 'diff', title, diffs, locations? } — your call creates or modifies a file. diffs: [{ path, oldText, newText }] (oldText: null for a new file) renders as an inline diff card. (tool-fs write/edit.)
• presentResult(args, { content, isError, meta? }) returns the completed card:
  • generic supplies an optional title and content.
  • terminal supplies raw output and optional exit metadata; the bridge renders the capability-specific or fenced fallback view.
  • diff supplies applied hunks, often carried in persisted result.meta so replay reproduces them. Mutation tools keep a diff result because an ACP update replaces the pending card's content.

Hard rules (they bite if broken):

• Purity. These run on live streaming AND on session-log REPLAY, so they must be pure functions of args (+ the result) — NO I/O, NO reading session state, NO clock/random. A diff is derived from the args (write uses oldText: null because a call-time presenter has no prior file content); the BRIDGE, not the tool, fills the session cwd and relativizes a display-path title. If you find yourself wanting the file's old content or the working directory inside presentCall, stop — that belongs on the bridge or a future result-event shape, not the presenter.
• UI-only formatting stays out of the model result. A fenced ```console block, a diff, a relativized path — none of these may appear in what execute returns to the model; they live only in the presentation. (A terminal result view carries RAW output; the bridge adds the fences.)
• defineTool soft-validates the display path. A malformed/older logged arg shape makes the wrapper return undefined (a generic fallback) rather than throw — display must never crash a replay.

The neutral vocabulary lives in dsh-tools (never import an ACP type into a tool); the ACP bridge maps each card to the wire. The design and the why are in the render-intent-union RFC; dsh-tool-fs (generic/diff) and dsh-tool-bash (terminal) are the reference implementations.

Tests every tool needs

Cover argument rejection, every result shape, and HMR disposal. For a side-effecting tool, drive the real tool through the agent loop with a scripted MockAdapter and assert its tool/call and tool/result session events. For an editor card, assert the exact presentCall and presentResult views and add an ACP snapshot through the real bridge; a terminal card's scenario sets terminalOutput: true to exercise the capable-client path.