Brief: testing instructions at the tool boundary

Type: types/write-brief.md

Intent

The operator commissioned this revision on 2026-11-08 during the frontier-work reconsideration. Explain what tool mocking makes observable that checking a final artifact alone would miss.

Must keep

• One concrete, generic discovery-only example, with controlled search results and captured attempted writes.
• The limits of isolation and mock fidelity; mocking is not the only testing route.

Must exclude

• Commonplace's tool inventory, search policy, or reasons for removing a search dependency. These are local details, not part of this claim.
• A test-harness design, unsupported fixture sizes, or generic testing advice added merely to expand the note.