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

dsh-tools

Tool registry and execution waterfall. Tool plugins register their schemas and executors; the agent loop executes calls through the tools/execute waterfall.

Service: ToolRegistry (ctx key: tools)

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/tools.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/execute waterfall.

Injected services

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

Events

Event	Mode	Purpose
tools/execute	waterfall	Wrap/veto tool execution (sandbox, permission, hooks, plan mode)
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? }. 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).
• 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.
• The tools/execute waterfall is the single seam for sandbox, permission, hooks, and plan-mode plugins to wrap or veto a call. Listeners receive (exec, next): call next() to proceed, or return a result without calling next() to short-circuit (veto).
• 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.

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[] — the APPLIED hunks with surrounding context (one entry per changed site), computed from the before/after file content, distinct from the call-time whole-snippet diff. Used by write/edit; a tool_call_update.content replaces the call's content, so this supersedes the pending snippet.

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 (JsonValue), 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 }
  },
})

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.