Complete the JSON-RPC 2.0 request structure used by MCP

Attempt it

A JSON-RPC 2.0 request must include four fields: the version field `` set to "2.0", the `` field naming the operation (e.g. `tools/call`), an `` for correlation (omitted in notifications), and `` carrying the operation arguments.

Quick reference

TL;DR

Every MCP request is a JSON-RPC 2.0 envelope: jsonrpc set to "2.0", a method name, params for arguments, and an id for correlation that notifications omit.

Memory aid

Sign in for the mnemonic that makes this stick. See Pro pricing.

Easy to grasp

Think of mailing a letter that needs a reply. You write it on standard letterhead so the recipient knows the format, that is the jsonrpc "2.0" field. You say what you want done, that is the method. You enclose the details, that is params. And you stamp a tracking number so the reply can be matched back to your letter, that is the id. If you do not need a reply, like dropping off a notice, you skip the tracking number entirely. The recipient mails back a matching letter carrying either a result or an error, stamped with the same tracking number so you know which request it answers. MCP just sends these letters between the host app and the tool server.

Key concepts

JSON-RPC 2.0 Wire protocol shape Notifications vs requests

Quick scan · 2–4 min

Everything important, quickly.

3 min: name the four request fields, contrast request vs response vs notification, explain id correlation, and cover the result or error rule.

Code example

json

// request (expects a response)
{ "jsonrpc": "2.0", "id": 7, "method": "tools/call",
  "params": { "name": "search", "arguments": { "q": "mcp" } } }

// success response
{ "jsonrpc": "2.0", "id": 7, "result": { "content": [] } }

// error response
{ "jsonrpc": "2.0", "id": 7, "error": { "code": -32602, "message": "Invalid params" } }

// notification (no id, no response)
{ "jsonrpc": "2.0", "method": "notifications/progress",
  "params": { "progressToken": 7, "progress": 0.5 } }

Side-by-side

Field	Request	Response	Notification
jsonrpc	"2.0" (required)	"2.0" (required)	"2.0" (required)
method	Required, names op	Absent	Required, names event
params	Arguments object	Absent	Event payload
id	Required, correlates	Echoes request id	Omitted (the signal)
result / error	Absent	Exactly one present	Absent

Real-world examples

Real products, models, and research that use this idea.

Claude Code and Claude Desktop exchange these exact JSON-RPC envelopes with every registered MCP server over stdio at session start.
The mcp-inspector debugging tool prints raw request and response objects so you can see the jsonrpc, method, params, and id fields on the wire.
Cursor and Zed both speak the same JSON-RPC 2.0 framing, which is why one MCP server runs unchanged across all three hosts.

Interviewer follow-ups

What an interviewer would ask next. Try answering before peeking at the approach.

QHow does a client correlate responses when several requests are in flight at once?

Sign in or go Pro to see the approach for this follow-up.

QWhy does JSON-RPC make result and error mutually exclusive instead of allowing both?

Sign in or go Pro to see the approach for this follow-up.

QWhat changes about the envelope across stdio, SSE, and streamable HTTP transports?

Sign in or go Pro to see the approach for this follow-up.

60-second cheat sheet

The night-before-the-interview bullets. Scan these on the way to the call.

Sign in for 5 unlocks/day or go Pro for unlimited revision sheets. See Pro pricing.

References

Primary sources. Skim if you want the original framing.