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

Cookbook: adding a tool

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 { callId, name, arguments, agent?, signal? }
      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.
• Throwing means isError. The registry catches anything execute() throws and returns {isError: true} to the model. Use that for infrastructure failures (bad input, spawn errors, aborts) — but REPORT domain failures in the result text instead (e.g. tool-bash returns [exit code: 9] with isError: false: the model decides what a failing command means).
• 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

Follow tool-bash's background pattern: a run_in_background flag returns a task id immediately; companion tools poll incrementally and kill; completion notices arrive via agent.inject(). Bound buffers and spill full output to disk so nothing is silently lost.

TODO: each tool reimplements this background pattern by hand today. At some point we need a generic long-running-tool layer that handles task ids, incremental polling, kill, and completion notices uniformly.

Permissions / sandboxing

Prefer not to build policy into the tool. The seam is the tools/pre-execute gate (deny/ask — see the permission-gate example in extension-cookbook.md) and the tools/post-execute inspect/transform seam, or a sandboxing implementation behind the tool's executor seam.

Code Mode reaches your tool for free

Under the registry's non-native mode (Code Mode), a registered tool is ALSO callable from a run_code program as await tools.<name>(args) — nothing to add. The generated SDK declares your parameters from the same JSON Schema defineTool emits (constructs outside that subset degrade to unknown), each program call re-enters execute() through both waterfalls, and a failed call rejects the program-side promise with your error text. Two consequences worth designing for: your description and parameter descriptions become JSDoc a model reads while WRITING CODE, and non-text result blocks reach programs as placeholders (text is the lingua franca of the bridge).

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? }) → a ToolResultView (the COMPLETED card): { card: 'generic', title?, content? }, { card: 'terminal', title?, output?, exitCode?, signal? } (the run's captured output + exit — the bridge shows an exit pill and derives a fenced `console fallback for editors without the terminal capability), or { card: 'diff', title?, diffs } (a completed file mutation — the applied hunks computed from the before/after content when there is a before-image, else a whole-file diff for a create; write/edit attach the hunks via the meta channel and read them back here). A mutation tool returns the diff result even when it duplicates the call-time card, because an ACP tool_call_update.content REPLACES the call's content — a non-diff result would clobber the pending diff. result.meta is your tool's own optional presentation payload, attached from execute (see below) and persisted so a replay reproduces the card.

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

Arg-validation rejections, result shaping for every outcome, the HMR disposal test, and — for tools with side effects — an integration spec that drives the tool through the agent loop with a scripted MockAdapter (packages/core/agent-loop/tests/mock-adapter.ts), asserting the tool/call / tool/result session events. If your tool has an editor card, also add: a unit test on presentCall/presentResult asserting the exact view shape, AND — because a unit test proves the shape but not that an editor renders it — a snapshot scenario under examples/acp-agent/tests/snapshots/ that drives the real tool through the ACP bridge and pins the rendered tool_call transcript (the card kind is only verified end-to-end there; see the ACP snapshot-tests RFC). A tool whose card is a terminal needs a scenario whose input.json sets terminalOutput: true to exercise the capable-client _meta path.