09 — Quiz: build an MCP server

Pick an answer. The quiz checks it and shows the explanation.

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1. You add console.log("debug") to a tool handler of a stdio server. What breaks?

a) nothing

b) the JSON-RPC stream on stdout

c) the zod schema

d) the permission prompt

b — stdout is reserved for JSON-RPC. Log to stderr with console.error.

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2. Claude calls add_note. Which process writes notes.json?

a) the model

b) Claude Code

c) the notes server process

d) Anthropic's servers

c — the server executes. The host only routes the tools/call.

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3. You move the server from stdio to Streamable HTTP on localhost. What stays the same?

a) the config in Claude Code

b) how Claude Code starts the server

c) the JSON-RPC messages and the tools

d) nothing

c — the transport and auth change. The messages and the tools don't.

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4. Claude asks to delete a note that doesn't exist. The server replies with isError: true. Why is that better than crashing?

a) it hides the problem from you

b) the model reads the error text and can recover, for example by listing notes first

c) it retries the call automatically

d) it turns into a JSON-RPC protocol error

b — a tool execution error is a normal result the model can read. A crash gives it nothing to work with.

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5. The HTTP server binds to 127.0.0.1 and checks the Host and Origin headers. What does that protect against?

a) a wrong bearer token

b) other machines on the network, and web pages in your browser trying to reach the server

c) bad JSON in a request

d) the model calling the wrong tool

b — 127.0.0.1 keeps it off the network. The header checks stop DNS rebinding from a browser. The token is a separate check.

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