# Tools: tools/list + tools/call + error split (2025-11-25)

## Metadata

| Field | Value |
|---|---|
| Institute | Molodetz Institute - Research & Retrieval Division |
| Subject | mcp-client-support |
| Document type | Source Dossier |
| Tier | 1 |
| Source title | Tools (MCP Specification v2025-11-25) |
| Source URL | https://modelcontextprotocol.io/specification/2025-11-25/server/tools |
| Source publisher / author | Model Context Protocol authors (Anthropic-donated open project; site docs) |
| Source type | standard |
| Source date (published) | 2025-11-25 (version stamp; no other publish date stated) |
| Date researched | 2026-10-18 |
| Researched by | tai (agent), model: Claude Sonnet 5, on behalf of retoor |
| Research method | gather-subagent direct page fetch, parent synthesis from evidence pack |
| Confidence | High |
| Status | Active |

## Summary

This page defines the server tools feature: capability declaration with optional `listChanged`, paginated `tools/list`, and `tools/call` with `name`/`arguments` params returning `content` plus `isError`. Its central point for clients is the two-track error model: protocol errors are standard JSON-RPC errors (for example unknown tools), while tool execution failures arrive as successful results with `isError: true`. It also lists content types (text, image, audio, resource_link, embedded resource, optional structuredContent) and client security guidance.

## Key findings

- "Servers that support tools MUST declare the `tools` capability", with optional `listChanged`.
- `tools/list` "supports pagination" (`cursor` param, `nextCursor` in result).
- `tools/call` params: `{"name": …, "arguments": {…}}`; result: `{"content": […], "isError": false}`.
- "Tools use two error reporting mechanisms: 1. Protocol Errors: Standard JSON-RPC errors… Unknown tools… 2. Tool Execution Errors: Reported in tool results with `isError: true`"; "Clients SHOULD provide tool execution errors to language models."
- Protocol-error example: `{"code": -32602, "message": "Unknown tool: invalid_tool_name"}`.
- Unstructured content types: `text`, `image` (base64 plus `mimeType`), `audio`, `resource_link`, embedded `resource`; plus optional `structuredContent` object validated against `outputSchema` when present.
- Client security guidance: "Implement timeouts for tool calls", "Validate tool results before passing to LLM", "treat tool annotations as untrusted."

## Pros / strengths

- The JSON-RPC-error vs `isError: true` split is stated normatively with examples, which is exactly the distinction a client must get right to avoid misrouting tool failures as transport faults.
- Request and result shapes are given literally (`name`/`arguments`, `content`/`isError`, `cursor`/`nextCursor`), so the data plane is implementable without inference.
- Security guidance is client-actionable (timeouts, result validation, untrusted annotations) rather than vague.

## Cons / limitations / caveats

- No explicit author names or publish dates appear on the page; the date above is the version stamp in the URL, per the pack's Uncertainty notes.
- Content-type breadth (image, audio, resource_link, embedded resource, structuredContent with outputSchema validation) means a minimal client must decide up front how much result rendering to support versus passing through.
- `listChanged` notification handling and `outputSchema` validation are extra client work beyond the two-method core, not covered in depth here.

## Relevance to tai

This is the entire tools data plane for a minimal client: two methods, a pagination loop, and the critical JSON-RPC-error vs `isError: true` distinction. Combined with the lifecycle handshake, this page plus transports is nearly the whole minimal-client spec.

## Changelog

| Date | Change |
|---|---|
| 2026-10-18 | Initial dossier created. |