What is the wire format of MCP messages? Describe the JSON-RPC 2.0 basics the protocol relies on.

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MCP messages are JSON-RPC 2.0 messages, UTF-8 JSON objects that carry "jsonrpc": "2.0". The transport only decides how they are framed and delivered.

There are three shapes. A request has an id, a method, and optional params, and expects exactly one reply. A response repeats that id with either result or error, never both. A notification has a method but no id, so nothing is sent back.

{"jsonrpc":"2.0","id":1,"method":"tools/call","params":{"name":"get_weather","arguments":{"city":"Oslo"}}}
{"jsonrpc":"2.0","id":1,"result":{"content":[{"type":"text","text":"12C, clear"}]}}

An id is a string or an integer, never null, and a sender never reuses one still awaiting its reply. An error holds an integer code, a message, and optional data. Batching was allowed early on, then removed in the 2025-06-18 revision.

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Likely follow-ups

When should a failed tool call come back as a JSON-RPC error, and when as a normal result? Unknown tools or bad params get a JSON-RPC error. A tool that ran and failed returns a result with isError set, so the model can react.
Give an example of an MCP notification. Why is it a notification and not a request? Names a real one, like notifications/initialized or a list_changed event, and says the sender needs no answer, so there is no id.
A client gives up on a slow request. How does it tell the server, and where does the id come in? Mentions a cancellation notification that carries the original request id, and the race where the reply is already in flight.
Over stdio, how are these messages framed, and what must a server never write to stdout? Newline-delimited JSON with no embedded newlines. Only valid MCP messages go on stdout, and logs belong on stderr.
Why can't an id be null, and what breaks if a sender reuses an id that is still pending? Replies are matched to requests by id alone. A reused id makes the reply ambiguous, and null can't identify a request.

What you can say

MCP messages are JSON-RPC 2.0, so each one is a UTF-8 JSON object with a jsonrpc field set to 2.0, and the transport only decides how it gets framed and delivered.
There are three shapes. A request has an id, a method and optional params, and it expects exactly one reply.
A response repeats that id with either a result or an error, never both, and an error carries an integer code, a message and optional data.
A notification has a method but no id, so nothing comes back, which fits one-way messages like a progress update.
Ids are strings or integers, never null, and a sender never reuses one that is still waiting on its reply.
I'd point out that batching was allowed early on but removed in the 2025-06-18 revision, so don't design around sending arrays of messages.

Weak answers to avoid

Mixes up the message format with the transport Answering with stdio or HTTP details misses the question. The messages are JSON-RPC 2.0 objects, and the transport only frames and delivers them.
Says every message gets a response A notification has a method and no id, and the receiver sends nothing back. Leaving out the third shape shows a shallow read of JSON-RPC.
Lets a response carry both result and error A response holds exactly one of result or error, tied to the request by id. Say that plainly and give the error fields: code, message, optional data.
Says MCP still supports JSON-RPC batching Early revisions allowed batching, but the 2025-06-18 revision removed it. Say that it was removed and in which revision.

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