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Chapter 7: Walkthrough: Copilot Scaffolds the Safe Parts

Chapter 7 hero image

Runner: talk/demos/demo1.ps1

This is Chapter 5's idea made concrete. In a few minutes, GitHub Copilot can scaffold a new tool for the Hawker agent, add a guardrail, and write the tests. Follow along step by step.

Step by step

1. Scaffold a typed tool. Prompt Copilot for a function with a typed signature, input validation, and a Pydantic return type:

Add find_stalls(location: str, cuisine: str | None = None) -> list[Stall]
that calls the Maps API tool. Use the Stall pydantic model from models.py.
Validate location is non-empty. Type the return. Raise InvalidInput on bad input.

Result lives in tools/menu_index.py and models.py.

2. Add a guardrail (policy as configuration). Wrap the tool with the governance decorator so the policy is loaded from YAML, not hardcoded:

from merlions.governance import govern, load_policy

@govern(load_policy("hawker"))
def find_stalls(location: str, cuisine: str | None = None) -> list[Stall]:
    ...

The policy (policies/hawker.yaml) sets the allowlist, blocked patterns, a call limit, and require_citation. Your security team can change it without a code change or deployment.

3. Generate the unhappy-path tests first. Ask Copilot for empty input, malicious input (an API key in an argument), tool failure with retry, and a happy path. See tests/test_find_stalls.py. Run them:

pytest tests/test_find_stalls.py -v   # all green

The lesson

Copilot didn't write the agent. It wrote the scaffolding so you could focus on agent behaviour.

Notice the order: unhappy paths first. That is what catches the bugs that ship to production, including the fail-closed assertion that an ambiguous policy match denies the call.

Key terms

  • @govern decorator: wraps a tool so every call is checked against the policy and audit-logged. See governance.py.
  • Fail-closed test: a test that proves the system denies when uncertain.
  • Mock: a stand-in for an external API so tests are fast and deterministic.

Do this next

  1. Run the walkthrough yourself: ./talk/demos/demo1.ps1 (no API keys, no network).
  2. Recreate the flow on one of your own tools: signature → validation → @govern → unhappy-path tests.
  3. Make "unhappy paths first" your default prompt to Copilot.

📺 Build 2026 grounding: LTG405 (Better tests, faster) and ODSP912 (Build agentic testing systems to validate AI-generated code).