MCP Protocol Internals: How Well Do You Know the Spec?

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MCP Protocol Internals: How Well Do You Know the Spec?

Five advanced questions on the Model Context Protocol wire format, capability negotiation, initialization lifecycle, Streamable HTTP session management, and tool error handling.

The Model Context Protocol encodes all client-server communication using which message format?

JSON-RPC 2.0 messages, which must be UTF-8 encoded
Protocol Buffers over gRPC, with JSON only as a fallback for browser-based clients
A GraphQL schema defined in the official schema.ts file

During MCP capability negotiation, tools, resources, and prompts are declared by the server. Which of the following is a CLIENT capability?

sampling, which lets servers request LLM completions through the client
completions, which lets servers request argument autocompletion from the client
logging, which lets the client emit structured log messages to servers

A client sends an initialize request and the server responds successfully. According to the spec, what MUST the client send before beginning normal operations?

A tools/list request, to complete the mandatory discovery phase
A notifications/initialized notification
A ping request, to confirm the transport is bidirectional

A client using the Streamable HTTP transport sends a request with its Mcp-Session-Id header and receives HTTP 404 Not Found. What MUST the client do?

Retry the same request with exponential backoff, keeping the same Mcp-Session-Id, because 404 signals a transient routing failure
Send an HTTP DELETE to the MCP endpoint to release the session, then resume with the same session ID
Start a new session by sending a new InitializeRequest without a session ID attached

An MCP tool calls an external API that fails with a rate limit error. According to the spec, how should the server report this to the client?

Return a normal tools/call result containing the error message in content and isError: true
Return a JSON-RPC error object with code -32602 and the API failure message
Return HTTP 500 with an empty body, letting the transport layer signal the failure