Why did MCP pick JSON-RPC 2.0 as its wire format instead of REST or gRPC?

**JSON-RPC 2.0** is a small, text-based remote procedure call protocol where every message is a JSON object. The full spec is short enough to memorize. A request has four fields: `jsonrpc: "2.0"` (the version string), `method` (a string naming the operation), `params` (the arguments), and `id` (a string or number the receiver echoes back in the response so the sender can match it). A response has `jsonrpc: "2.0"`, the matching `id`, and either `result` (success) or `error` (failure with a numeric code and message). A notification looks like a request but drops the `id`, meaning no reply is expected. MCP picked JSON-RPC 2.0 as its wire format rather than inventing a new one. Every MCP operation is a JSON-RPC method. `initialize` is a request. `tools/list` is a request. `tools/call` is a request. `notifications/initialized` is a notification. The split of responsibilities is clean. JSON-RPC defines the message envelope, the request/response/notification pattern, and the standard error codes. MCP defines what methods exist, what each one does, what params and results look like, the initialize then negotiate lifecycle, and the security model. MCP reused a well-established wire format and focused its original work on the AI-specific concerns.

**JSON-RPC 2.0** is a lightweight RPC protocol where every message is a JSON object with a fixed shape: `jsonrpc: "2.0"`, a `method` string, a `params` payload, and (for requests) an `id` for correlating responses. Responses carry the matching `id` plus a `result` on success or an `error` object on failure. Notifications omit the `id` and signal that no reply is expected. MCP chose JSON-RPC 2.0 because it is transport-independent, human-readable, trivially implementable in any language, and already provides clean semantics for requests, responses, and one-way signals. This let the MCP designers skip wire-format design entirely and focus on the AI-specific contract. The method set MCP layers on top includes `initialize`, `tools/list`, `tools/call`, `resources/list`, `resources/read`, `prompts/list`, `prompts/get`, `ping`, and notifications like `notifications/initialized`, `notifications/tools/list_changed`, and `notifications/progress`. The slash-based naming is convention; JSON-RPC imposes no naming rules. JSON-RPC supplies the envelope, the `id` correlation mechanism, the success/error/notification semantics, and the numeric error codes (`-32601` method not found, `-32602` invalid params). MCP supplies the methods, the lifecycle (initialize first, then capabilities exchange, then operate), the content-block result shape, and the security model. JSON-RPC also gives MCP transport independence for free: the same messages flow over stdio for local subprocess servers and over streamable HTTP for remote servers without changing the protocol. The practical line for interviews: MCP is JSON-RPC 2.0 plus a defined method set plus a lifecycle plus capability negotiation plus transports. The wire format is borrowed; the AI-specific contract is what MCP contributes.