Build the first working MCP stdio client

Build only what the claim requires: discovered tool schemas are printed; only named tool executes with validated args. Prefer boring, deterministic code over frameworks you cannot yet explain. Run the path twice; identical output on this fixture is a feature, not a lack of creativity.

export async function run(input: unknown, deps: Dependencies) {
  const started = performance.now();
  const validated = deps.parseInput(input);
  const raw = await deps.execute(validated); // use the current @modelcontextprotocol/sdk Client with a StdioClientTransport, then listTools and callTool
  const value = deps.parseOutput(raw);
  return { value, durationMs: Math.round(performance.now() - started) };
}

Expected evidence: The client discovers echo, calls it once, prints “hello”, and closes the transport.. Read each printed intermediate as part of the argument that the path works—not as decoration.

Narrate the journey from raw input to result for a single example from local stdio server exposing one read-only echo tool. If you cannot name an intermediate, the implementation is still too opaque for this lab. Only after this path is solid should you generalize data sources or UI.

While implementing the MCP stdio client, print or log at least three intermediates that map to the claim (discovered tool schemas are printed; only named tool executes with validated args). Good intermediates are values a teammate could recompute with a calculator or diff. Bad intermediates are framework traces you cannot explain.

Re-run with local stdio server exposing one read-only echo tool twice. If the second run differs, either the path is nondeterministic (document the seed) or you have hidden global state—both are lab bugs until named.

Stop adding features once the path supports connect to a local MCP server, list tools, and invoke one allowlisted tool safely. Extra UI, extra tools, or extra models belong in later chapters. The mastery bar for this page is simply: a deterministic end-to-end path with intermediates.

At the implementation stage for mcp-in-code, the job is narrower than finishing a product demo. You are creating one progressive evidence piece about local stdio server exposing one read-only echo tool that later pages inherit without redefining success. Keep that fixture small enough to inspect by hand, keep outputs copy-pasteable as text, and refuse to narrate this baseline as if it were a production SLA: direct function call without MCP discovery.

For this page specifically, success looks like a deterministic path with printed intermediates while still centering the user decision to connect to a local MCP server, list tools, and invoke one allowlisted tool safely. If you cannot point to a file, command, or assertion that proves that for the MCP stdio client, stay on this page instead of advancing.

Glossary: tool · Glossary: structured output · Cheatsheet: production ops signals

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