deepseek-harness/packages/core/tools/README.md

dsh-tools

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). The registry also owns HOW its tools are presented to the model — its mode config selects native function calling, Code Mode, or both.

Service: ToolRegistry (ctx key: tools)

Config

tools:
  mode: native   # native (default) | code | both

native contributes every registered tool as a wire function definition — the default, byte-for-byte the pre-config behavior. code contributes exactly ONE wire tool, run_code, plus the generated tools:sdk prompt section (see Code Mode). both contributes every native definition AND run_code + the SDK section. Non-native modes require a loaded ctx.codeRuntime with language: 'typescript'; a missing or mismatched runtime rejects every prompt assembly with an actionable error, and a systemPrompt.toolOrder naming tools the mode no longer contributes rejects the assembly the same way.

Public API

• 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). The shipped tools' schemas are catalogued in docs/tool-catalog.md, generated by booting each tool plugin and harvesting this method (see the tool-schema-catalog RFC).
• ctx.tools.execute(exec: ToolExecution): Promise<ToolExecutionResult> Execute one tool call through the tools/pre-execute → dispatch → tools/post-execute pipeline.

Injected services

SystemPrompt — the registry automatically feeds its tool schemas into the system-prompt assembly via ctx.systemPrompt.tools().

Events

Event	Mode	Purpose
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[] | { content: ContentBlock[]; meta? }> (the bare array is the model-facing content; the object form additionally attaches an opaque, JSON-serializable meta presentation payload persisted on the tool/result event and handed back to presentResult), 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?, additionalContext?, meta? }. 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/results in the step. meta is the tool's opaque presentation payload from a successful execute (the object return form); the loop forwards it onto the tool/result session event for result-card rendering.
• 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.
• ToolCallView / ToolResultView — provider-neutral card-tagged render intents 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.
• 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).
• MCP servers: one plugin per server, discover tools, call ctx.tools.register() with the server's schemas.

Typed tool parameter schemas

First-party plugin authors can use the defineTool() helper (exported from this package) for typed tool parameter schemas:

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

declare const ctx: Context

ctx.tools.register(defineTool({
  name: 'read_file',
  description: 'Read a file from disk.',
  parameters: {
    path: { type: 'string', required: true, description: 'Absolute file path' },
    offset: { type: 'number' },
    limit: { type: 'number' },
  },
  async execute(args, exec) {
    // args is typed: { path: string; offset?: number; limit?: number }
    const text = await readFile(args.path, 'utf8')
    return [{ type: 'text', text }]
  },
}))

The helper converts the author-facing SchemaSpec (with required: true as a per-property boolean) to standard JSON Schema for the wire format. Raw JSON-Schema tool definitions (from MCP servers) are still accepted by the registry directly.

A defineTool tool also validates the model-generated arguments against its SchemaSpec before execute runs (validateArgs). The model's JSON is untrusted — InferArgs<S> is a compile-time claim, not a runtime guarantee — so on a mismatch (missing required key, wrong primitive, bad enum member, nested violation) the tool throws a ToolArgsError (code: 'INVALID_ARGS'); the registry turns it into an isError result whose text lists the violations, which the model sees and self-corrects from. Validation mirrors the JSON Schema conversion exactly: extra keys are allowed, default is not applied, and an object/array prop without properties/items only type-checks. Raw-registered tools (MCP) are not validated by the harness — they validate their own input.

See defineTool, validateArgs, ToolArgsError, SchemaSpec, InferArgs, and schemaSpecToJsonSchema in the public API for details.

Structured-output schema subset

A separate vocabulary for callers that DEMAND a machine-readable value from an agent — the subagent seam's SubagentStartRequest.outputSchema (and, by extension, a workflow's agent({ schema })). Unlike SchemaSpec (the author-facing DSL for tool parameters), a StructuredOutputSchema is an object-rooted raw JSON Schema subset as data: it travels verbatim to the model as a forced tool's parameters, and the produced value is validated against it.

The subset is deliberately narrow and REJECTS LOUD outside it — accepting a keyword the validator doesn't enforce would validate less than the schema promises (accepted-then-ignored). Supported: single-string type (object/array/string/number/integer/boolean/null; type arrays rejected), properties/required/additionalProperties (boolean; every required key must be declared), items, scalar-only enum/const; annotations (description/title/default/examples) are ignored but must still be JSON data. assertSupportedOutputSchema(schema) throws OutputSchemaError (code: 'UNSUPPORTED_SCHEMA', listing every violation) for anything else; validateStructuredValue(schema, value) returns path-qualified violations (empty = valid, total — never throws).

Tool-owned UI presentation

A tool owns how ITS calls render in a UI (an editor's tool-call card, a CLI log line) — a UI plugin must NOT special-case tool names. A ToolDefinition may declare two optional, pure, display-only methods that return a card-tagged render intent (a discriminated union — a tool declares its card kind once and a UI bridge switches on card):

• presentCall(args): ToolCallView | undefined — the PENDING state, one of:
  • { card: 'generic', title, kind?, rawInput?, content?, locations? } — the default card: a human-readable title, an optional kind (read/edit/execute/… for icon/treatment, default other), an optional rawInput (the salient input to show in a detail view — e.g. a background task id, NOT the whole args object), optional content (extra UI content blocks), and optional locations ({ path, line? }[] — files this call reads/modifies, so a capable UI can follow along; the ACP bridge forwards them as tool_call.locations).
  • { card: 'terminal', title, description?, cwd? } — a shell command: a capable UI renders a terminal card (the title is the command, description renders above it, cwd heads it); an incapable UI falls back to a generic execute card.
  • { card: 'diff', title, diffs, locations? } — a file create/modify: a capable UI renders an inline diff card from diffs ({ path, oldText, newText }[]; oldText: null for a new file). Used by write/edit.
• presentResult(args, result): ToolResultView | undefined — the COMPLETED state, given the same args and the { content, isError, meta? } result, one of:
  • { card: 'generic', title?, content? } — an optional replacement title and reformatted content.
  • { card: 'terminal', title?, output?, exitCode?, signal? } — a terminal run's captured output and exit status. A capable UI shows an exit-status pill; an incapable UI gets a fenced ```console fallback the BRIDGE derives from output (the tool does not encode the fences).
  • { card: 'diff', title?, diffs } — a completed file mutation as an inline diff. diffs is FileDiff[] — typically the applied hunks with surrounding context computed from the before/after content, or a whole-file diff (oldText: null) when there is no before-image (a file create). Used by write/edit; a tool_call_update.content replaces the call's content, so a mutation tool returns this even when it duplicates the call-time snippet (else the result text would clobber the pending diff).

Returning undefined (or omitting a method) tells a UI to fall back to a generic presentation (title = tool name, raw args as input, raw result content). Both methods must be pure and side-effect-free: a UI may call them during live streaming AND during a session-log replay, so they depend only on their arguments. result.meta is the tool's own optional presentation payload (opaque unknown, JSON-serializable), attached by execute (see below) and persisted on the tool/result event, so a presentResult reading it stays replay-deterministic (the same meta is read back from the log). With defineTool, args is the typed InferArgs<S> shape; the helper soft-validates before calling (a malformed/older logged arg shape yields undefined rather than throwing, since display must never crash a replay). The views are provider-neutral — the ACP bridge (dsh-acp) maps each card to ACP tool_call/tool_call_update wire fields (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. See the render-intent-union RFC (docs/rfc/implemented/architecture/2026-07-02-tool-render-intent-union.md) and the applied-hunk-diffs RFC (docs/rfc/implemented/architecture/2026-07-02-result-time-applied-hunk-diffs.md); dsh-tool-bash (terminal) and dsh-tool-fs (diff/generic) are the reference implementations.

import { defineTool } from '@deepseek-ai/dsh-tools'

const bash = defineTool({
  name: 'bash',
  description: 'Run a shell command.',
  parameters: {
    command: { type: 'string', required: true, description: 'The command to run.' },
    description: { type: 'string', required: true, description: 'One-line summary shown in the UI.' },
  },
  async execute(args) {
    return [{ type: 'text', text: `ran: ${args.command}` }]
  },
  // A terminal card: the command is the title, the description renders above it.
  presentCall: args => ({ card: 'terminal', title: args.command, description: args.description }),
  // A terminal result: the raw output + exit; the bridge derives the fenced fallback.
  presentResult: (_args, result) => {
    const block = result.content.length === 1 ? result.content[0] : undefined
    if (block === undefined || block.type !== 'text') return undefined
    return { card: 'terminal', output: block.text }
  },
})

Code Mode

Under mode: code (or both) the registry turns the tool surface into a programming API, per the Code Mode RFC: the model writes a TypeScript program (the body of an async function) and passes it to the ONE wire tool run_code; the program runs in ctx.codeRuntime (the code-execution seam — the shipped backend is a worker thread) with one async binding per registered tool (await tools.bash({...})), and ONLY what it prints or returns re-enters the model's context.

• The SDK section (tools:sdk, order 150): a lazy prompt section regenerating, at each assembly, a declare const tools: {...} TypeScript declaration of every registered tool except run_code (exotic names via quoted keys), plus fixed usage instructions. Deterministic — lexicographic tool order, byte-identical text for an unchanged tool set (prefix-cache-friendly). The codegen (jsonSchemaToTs, exported) is TOTAL: constructs outside the defineTool subset degrade to unknown, never throw.
• The dispatch bridge (run_code's execute): every binding call is JSON-normalized BEFORE dispatch (a value that does not survive — BigInt, circulars — rejects that one call, so the dispatched form and the logged form are the same JSON value by construction), serialized through a per-run queue (even Promise.all executes the underlying ctx.tools.execute() calls one at a time in submission order — the tool contract carries no concurrency-safety metadata yet), gated by tools/pre-execute/tools/post-execute like any native call (a deny reaches the program as a binding rejection), and logged as one tool/code-dispatch session event (log-only: deriveMessages() never surfaces it) with the deterministic sub-id <parent>:code:<n>. A failed sub-call REJECTS the program-side promise with the tool's error text — real code error handling, no bespoke envelope. A sub-call's additionalContext is deliberately DROPPED (no safe outlet mid-run without breaking tool-call/result adjacency; deferred until a real hook needs it through Code Mode).
• Settlement discipline: the bridge owns a run-scoped abort that follows the outer signal in and fires when the run settles for any reason, so a budget expiry aborts an in-flight sub-tool instead of orphaning it; the bridge then drains its queue BEFORE returning, so every tool/code-dispatch lands inside the open turn. A failed run throws CodeRunFailedError (code: 'CODE_RUN_FAILED', message = the failure kind + captured logs), which the pipeline converts to a structured isError the model self-corrects from.

The wire collapse is the registry's own contribution (systemPrompt.tools() is mode-aware), so the logged request/header records it for free — under code, the assembled tool list is exactly [run_code], pinned by tests and the snapshot goldens. Try it: pnpm run demo:code-mode (examples/code-agent); pnpm run demo:code-mode acp serves the same mode over ACP instead of the REPL.

What is NOT here (TODO)

• Tool shapes review — when real tools land (e.g. a concurrency-safety hint for parallel execution); phase 1 executes tool calls sequentially.
• Parallel execution — the loop currently iterates tool calls sequentially.