Define the MCP stdio client input contract

The accepted input remains: stdio server command, tool name, args. Keep parsing and normalization in functions that do not score, train, or call a model. That split lets a test fail the boundary without blaming the core logic. The user-facing decision stays: connect to a local MCP server, list tools, and invoke one allowlisted tool safely.

const mcp={transport:'stdio', allow:['echo']};
function call(name){ if(!mcp.allow.includes(name)) throw new Error('tool denied'); }
call('echo'); console.log('mcp allowlist ok');

Expected evidence: mcp allowlist ok. If the contract is silent on a bad value, later debugging will look like an algorithm bug when it is really a data bug.

Write one assertion for a neighboring valid input to the MCP stdio client so tightening the boundary does not over-reject. Document field names and types the way a teammate would need them on day two of mcp-in-code—not as comments you plan to delete.

List every field in local stdio server exposing one read-only echo tool and mark each as required, optional, or forbidden. Required fields must fail loudly when missing; optional fields need defaults you can quote in a test; forbidden fields (secrets, raw PII, path escapes) must never be accepted silently. This list is the contract for the MCP stdio client.

Add one sentence about encoding, units, or timezones if relevant to stdio server command, tool name, args. Contracts that ignore units create “correct” programs that still ship wrong decisions when someone tries to connect to a local MCP server, list tools, and invoke one allowlisted tool safely.

Write the error string you want for the most likely bad input. Prefer ValueError('threshold out of range')-style messages over generic invalid input. The contract’s job is to make calling a tool not in the discovered list, or hanging on stderr noise harder to confuse with a model or algorithm bug later.

At the data contract 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 malformed inputs rejected with field-named errors 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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