JSON-RPC 2.0 Messages

The problem

MCP (Model Context Protocol) is the standard way for an application that uses a language model to talk to programs that offer tools. All its messages are JSON-RPC 2.0 objects: JSON (JavaScript Object Notation) text that asks another program to run a function (a remote procedure call) or answers such a request. Before anything else, a program has to recognize what kind of message it received. You will write that classifier.

The kinds of message

classify(msg) takes a parsed message (any JSON value: a dict, a list, a number...) and returns (kind, error_code):

request, ("request", None): a dict with "jsonrpc": "2.0", a string "method", and an "id" that is a string or an integer. MCP forbids a null id, and a bool is not an integer here. "params", if present, must be an object (a dict).
notification, ("notification", None): the same, but with no "id" key at all. Nobody answers a notification.
response, ("response", None): "jsonrpc": "2.0", a valid "id", and exactly one of "result" (an object) or "error" (an object with an integer "code" and a string "message"), and no "method". An error response may have "id": null when the id of the request could not be read.
invalid, ("invalid", -32600): anything else, including batches (lists): MCP removed JSON-RPC batching in revision 2025-06-18.

Example.

message kind

{"jsonrpc": "2.0", "id": 1, "method": "tools/list"} request

{"jsonrpc": "2.0", "method": "notifications/progress"} notification

{"jsonrpc": "2.0", "id": 1, "result": {}} response

{"jsonrpc": "2.0", "id": null, "error": {"code": -32700, "message": "Parse error"}} response

{"jsonrpc": "2.0", "id": null, "method": "x"} invalid: a request's id cannot be null

{"jsonrpc": "2.0", "id": true, "method": "x"} invalid: true is not an integer

{"jsonrpc": "2.0", "id": 1, "result": {}, "error": {...}} invalid: both result and error

a list containing one request invalid: a batch

message	kind
`{"jsonrpc": "2.0", "id": 1, "method": "tools/list"}`	request
`{"jsonrpc": "2.0", "method": "notifications/progress"}`	notification
`{"jsonrpc": "2.0", "id": 1, "result": {}}`	response
`{"jsonrpc": "2.0", "id": null, "error": {"code": -32700, "message": "Parse error"}}`	response
`{"jsonrpc": "2.0", "id": null, "method": "x"}`	invalid: a request's id cannot be `null`
`{"jsonrpc": "2.0", "id": true, "method": "x"}`	invalid: `true` is not an integer
`{"jsonrpc": "2.0", "id": 1, "result": {}, "error": {...}}`	invalid: both `result` and `error`
a list containing one request	invalid: a batch

Errors

JSON-RPC reserves standard error codes, and error_response(id, code, message) builds the error response for them: {"jsonrpc": "2.0", "id": id, "error": {"code": code, "message": message}}.

Example. error_response(1, -32601, "Method not found") is {"jsonrpc": "2.0", "id": 1, "error": {"code": -32601, "message": "Method not found"}}.

What to implement

classify(msg): the kind and error code above.
error_response(id, code, message): the error response dict.
The constants PARSE_ERROR = -32700, INVALID_REQUEST = -32600, METHOD_NOT_FOUND = -32601, INVALID_PARAMS = -32602, INTERNAL_ERROR = -32603.

Requirements

An error response may have "id": None when the request's id could not be read; accept it.