Can I batch multiple requests in one message?

JSON-RPC 2.0 supports batching, but most MCP clients and servers process requests one at a time in practice; do not rely on batch support being present.

Is there a maximum message size?

Not at the protocol level. Keep responses well under a few MB in practice so they don't blow out the model's context window or strain the transport.

Should a tool-level failure use a JSON-RPC error or isError: true?

Use isError: true on a normal result for business-logic failures (bad input, downstream failure) so the model can read and react to the message. Reserve JSON-RPC error codes for protocol-level problems.

protocolTechArticle

MCP JSON-RPC 2.0 Message Format

The request, response, notification, and error message shapes MCP uses for every client-server exchange, and the standard error codes.

Quick Answer / TL;DR

Every MCP message is JSON-RPC 2.0: a request carries an id, method, and params and expects a matching response; a notification has no id and expects no response; an error response uses a standard numeric code such as -32602 for invalid params. Getting this shape right is what makes a server interoperable with any MCP client.

Key Takeaways

Requests and responses are correlated by a shared id field; notifications omit id and get no response.
Standard JSON-RPC error codes (-32700 to -32603) cover transport-level failures; tool-specific failures use isError: true in a normal result instead.
Always include jsonrpc: "2.0" — a client can reject a message missing it.

Request, response, and notification shapes

A request expects a response with a matching id. A notification is fire-and-forget: no id, and the server must not reply to it.

json

// Request
{ "jsonrpc": "2.0", "id": 1, "method": "tools/call",
  "params": { "name": "get_weather", "arguments": { "location": "Mumbai" } } }

// Response
{ "jsonrpc": "2.0", "id": 1,
  "result": { "content": [{ "type": "text", "text": "28C, Partly Cloudy" }] } }

// Notification (no id, no response expected)
{ "jsonrpc": "2.0", "method": "notifications/initialized" }

Standard error codes

Reserve these codes for transport and protocol-level failures. A tool that fails for a business reason (bad input, downstream API down) should usually return a normal result with isError: true and a clear text message instead, so the model sees it as content rather than a raw protocol error.

Code	Meaning
-32700	Parse error — malformed JSON
-32600	Invalid request — not a valid JSON-RPC object
-32601	Method not found
-32602	Invalid params
-32603	Internal error