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).

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).
• 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[]>, 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? }. 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.
• 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.
• ToolCallPresentation / ToolResultPresentation — provider-neutral shapes 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.

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:

• presentCall(args): ToolCallPresentation | undefined — the PENDING state: a human-readable title (always-visible label), 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 shell command as a string, NOT the whole args object), an optional content (UI content shown alongside the title/card — e.g. a bash description as a text block above the terminal card), and an optional terminal (a neutral { cwd? } asking a capable UI to render this call as a TERMINAL, e.g. for bash).
• presentResult(args, result): ToolResultPresentation | undefined — the COMPLETED state, given the same args and the { content, isError } result: an optional replacement title, reformatted content (e.g. wrap command output in a fenced ```console block — a UI-only affordance that must NOT appear in the model-facing execute result), and an optional terminal (the { output?, exitCode?, signal? } for a terminal-rendered call). The ToolTerminal shape is provider-neutral; a UI bridge (the ACP bridge) maps it to a terminal card (with an exit-status pill) and a UI that can't ignores it and uses content.

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. 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 shapes are provider-neutral — the ACP bridge (dsh-acp) maps them to ACP tool_call/tool_call_update wire fields, and dsh-tool-bash is the reference implementation.

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}` }]
  },
  // The command is the readable title; the description rides as a content block.
  presentCall: args => ({ title: args.command, kind: 'execute', rawInput: args.command, content: [{ type: 'text', text: args.description }] }),
  // Wrap the output as a console block for the UI (not in the model-facing result).
  presentResult: (_args, result) => {
    const block = result.content.length === 1 ? result.content[0] : undefined
    if (block === undefined || block.type !== 'text') return undefined
    return { content: [{ type: 'text', text: '```console\n' + block.text + '\n```' }] }
  },
})

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.