Homework — Lesson 09

Task 1 — Per-user write

Refactor the write tool (added in Lesson 08) so it stores notes under the authenticated user instead of the global NOTES alias. Use get_current_user(ctx) to identify the caller, and write into USER_NOTES[user][title]. Update export's helper if needed so it returns the same per-user bucket. The tool signature still takes a WriteArgs model and a Context; only the body changes.

Pay attention to two subtleties:

• The overwrite=False check has to look at the per-user bucket, not the global one — otherwise Alice writing a note named "hello" blocks Bob from creating his own note with the same title.
• The NOTES alias still exists for compatibility with the read tools from earlier lessons. You can either redirect reads through get_current_user too, or leave them as-is and add a # TODO for Lesson 10. Either is acceptable; document your choice in a comment.

✓ Self-check: run the server with PKB_USER=alice in one terminal and PKB_USER=bob in another. Each user writes a different note titled "hello". Calling export() as Alice returns only her notes; calling it as Bob returns only his. There is no cross-talk in either direction.

Task 2 — Secret hygiene audit

Add a hypothetical pkb.translate(id, lang) tool that translates a stored note via a fake third-party translation API. The API key lives in os.environ["PKB_TRANSLATE_KEY"]. Your tool calls a helper _call_translate_api(text, lang) (you can leave this as a one-line stub; the unit test will mock it). When the helper raises, your error formatter must strip the API key from the error message before returning it through McpError.

Then write a unit test test_no_key_leak (in mcp-servers/sample-server/python/tests/) that:

1. Sets PKB_TRANSLATE_KEY=secret-token-xyz via monkeypatch.setenv.
2. Monkey-patches _call_translate_api to raise an exception whose message includes the URL and key, e.g. RuntimeError("GET https://api.example/?key=secret-token-xyz failed").
3. Calls translate(id="hello", lang="es") and expects an McpError to be raised.
4. Asserts the literal string "secret-token-xyz" is not in the resulting error message (or any field on the ErrorData).

✓ Self-check: pytest tests/test_no_key_leak.py -q passes, and adding a deliberate bug (e.g. message=f"translate failed: {e}" with no redaction) makes the test fail with a clear diff.

Task 3 — Lock down export

The export tool currently returns every note the authenticated user owns the moment it's called. For a destructive or sensitive disclosure that's too eager — the LLM should have to say it means it. Add a confirm_export: bool = Field(False) argument to the tool. When confirm_export=False (the default), the tool returns the plain string "set confirm_export=true to proceed" and does not touch USER_NOTES. When confirm_export=True, it behaves exactly as before.

Use a plain return string rather than raising McpError. Errors are for protocol violations; “did you really mean it?” is a soft prompt to the model, not a failure mode.

✓ Self-check: export() (no args) returns "set confirm_export=true to proceed". export(confirm_export=True) returns the dict of the current user's notes. The Inspector or your test client shows both responses without any error frames in between.