Protocol

The server implements the MCP specification 2025-11-25 over Streamable HTTP. It is stateless and answers every request with plain JSON.

Versions

Accepted	Preferred
`2025-11-25`, `2025-06-18`, `2025-03-26`	`2025-11-25`

No MCP-Protocol-Version header: the server assumes 2025-03-26, as the specification says.
A known version in the header: accepted.
An unknown version in the header: 400, with the accepted versions in error.data.
In initialize, a known requested version is echoed back; an unknown one gets 2025-11-25, and the client decides whether to continue.

Methods

Every method needs a credential, initialize and ping included.

Method	Behaviour
`initialize`	announces `tools`, `resources` and `prompts`, the server info (`doers`, version `2.0.0`) and the instructions text
`notifications/*`	`202`, empty body
`ping`	`{}`
`tools/list`, `tools/call`	the tool reference
`resources/list`, `resources/templates/list`, `resources/read`	the resources
`prompts/list`, `prompts/get`	the prompts
anything else	`-32601`

completion/complete, logging/setLevel and resource subscriptions are not implemented, and the matching capabilities are not announced.

Transport

Request	Response
`POST /mcp`	`application/json`, never `text/event-stream`
`GET /mcp`, `DELETE /mcp`	`405` with `Allow: POST, OPTIONS`
`OPTIONS /mcp`	`204` with CORS headers

The server never issues an Mcp-Session-Id. /mcp and /mcp/ both work. Batches. JSON-RPC batches left the specification in 2025-06-18, but a client that sends no version header is assumed to speak 2025-03-26, where they existed. The server accepts an array without announcing it: it answers with an array, answers 202 to an array of notifications only, and refuses an empty array with 400.

Origin

A request with an Origin header must come from a known web client: doers.sh, claude.ai, claude.com, chatgpt.com, chat.openai.com, platform.openai.com, or the loopback address. Anything else gets 403. Native clients send no Origin and are never affected. This is defence against DNS rebinding; the credential remains the real protection.

Tool results

{
  "content": [
    {
      "type": "text",
      "text": "2 open task(s), 1 overdue, 1 due today.\nPriorities:\n- [0ebb429f-…] Send the Acme renewal quote · 2 days overdue, Priority P3"
    }
  ],
  "structuredContent": { "day": "2026-09-22", "counts": { "open": 2, "…": "…" }, "…": "…" }
}

content[0].text is a compact rendering with the identifiers in it, so a client that ignores structuredContent still has everything it needs to act.
structuredContent is the operation’s full output. Every tool declares its outputSchema.
The text is not a copy of the JSON, as the specification recommends: sending thirty tasks twice would double the tokens on the most frequent calls.

Annotations

Annotation	How it is set
`readOnlyHint`	`true` for tools that change nothing
`destructiveHint`	`true` for tools that remove content or access (33 tools)
`idempotentHint`	`true` unless the matching REST route is a `POST`, with a few declared exceptions
`openWorldHint`	`false` on every tool

openWorldHint is false everywhere, including the calendar tools, which call Google Calendar, and the coach tools that spend a model turn on your Anthropic key. Each tool’s page in the reference says which outside service it calls.

Errors

Situation	Answer
No credential, or a refused one	HTTP `401` with `WWW-Authenticate`
Origin not allowed	HTTP `403`
Unsupported protocol version	HTTP `400`
Body is not JSON	HTTP `400`, `-32700`
Not JSON-RPC 2.0	`-32600`
Unknown method	`-32601`
Unknown tool, resource or prompt; missing prompt argument	`-32602`
Invalid arguments	a result with `isError: true`
The operation refused (not found, role too low, conflict)	a result with `isError: true`

Invalid arguments come back as a tool result rather than -32602 on purpose: a model reads “when: Invalid ISO date” and retries correctly, where a protocol error makes it give up. The specification classes these as tool execution errors.

MCP server

Tool reference

Versions

Methods

Transport

Origin

Tool results

Annotations

Errors

​Versions

​Methods

​Transport

​Origin

​Tool results

​Annotations

​Errors

Versions

Methods

Transport

Origin

Tool results

Annotations

Errors