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Give your agent tools with MCP (Python) · Challenge 03 · 中文版

Validation and error isolation

Checkpoint l0310 ptsgated by test suiteClaude Code 2.x (protocol: MCP 2026-07-28)

Arguments are untrusted input: validate before execution, and never let a handler crash the transport.

Doing this with your agent? One sentence starts the whole course:

Read https://flypython.com/skills/flypython/SKILL.md and start the FlyPython course `mcp-server-in-python`.

Validation and error isolation

Objective

Arguments are untrusted input: validate before execution, and never let a handler crash the transport.

The lesson

Two boundaries make a tool server safe: validate required arguments against the declared input schema (missing arguments become structured results, not exceptions), and wrap every handler so its failures return isError instead of breaking the connection. 03-missing-argument.json shows the first boundary on the wire.

Exercise

  • Trace each validation test to the contract line it pins
  • Ask: what happens if a handler raises InputRequired? (Next lesson.)

Checkpoint

The lesson’s commands run, and you can answer these in your own words (the agent asks; you answer — that is the gate):

  1. Which command did this lesson run, and what did it decide?
  2. What failed at the start, and why — in your own words?
  3. Before trusting a similar change next time, what would you check first?

Expected evidence

The command transcript and your answers.

Hints

Stuck? Open one at a time.

Hint 1 — What this checkpoint tests

python verify.py starter — MCPServer with register_tool and handle_request: validation, dispatch, and error isolation all green.

Hint 2 — Tools are untrusted

Missing required argument → isError content 'Missing required argument: <arg>'; unknown tool → 'Tool not found'; handler exceptions → isError 'Error: <e>'. A tool failing must never crash the transport.

Hint 3 — Return shapes

tools/list yields result.tools entries with name/description/inputSchema; tools/call yields result.content of type text — get the envelope exact, the suite compares structure.

Submit

Done? Record it.

The 8-character code verify.py progress printed for this checkpoint — from the browser, or straight from your agent.

Youbrowser

Submit the claim code

Your agentauto-submit

Let the agent solve and submit

Hand it this checkpoint (copy button on hover) — it solves, verifies, and submits on its own:

Work on the FlyPython challenge "Give your agent tools with MCP (Python)" (course id course-mcp-tools), checkpoint l03.
Machine-readable brief: https://flypython.com/api/challenges/mcp-server-in-python
Open the course folder and read TASK.md first — it is the contract.
Rules: smallest change, no new dependencies, never edit tests/ or solution/.
Check with python verify.py until its gates pass, then report each claim code to me.
Target: solve only checkpoint l03, and submit it as soon as it passes.

With a token from your agent page (env FLYPYTHON_TOKEN), it records the result directly:

curl -X POST https://flypython.com/api/claims \
  -H "Authorization: Bearer $FLYPYTHON_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"claims":[{"course":"mcp-server-in-python","checkpoint":"l03","code":"<8-char code>"}]}'

Or install the full FlyPython agent skill once and skip the paste.

Files

The starter and verifier

Solving runs on your machine — your agent fetches the course files itself via /api/challenges/mcp-server-in-python/files; you download nothing by hand. Prefer reading the source? Browse the folder on GitHub ↗