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tdd: add read-only test gates and model selection for AI agents

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Learnings from F58 3-way model comparison (Sonnet/MiniMax/Haiku):
- Read-only test gates: filesystem enforcement (chmod a-w) because
  prompt instructions are insufficient — MiniMax edited tests 7x,
  Haiku rewrote entirely, only Sonnet respected constraints
- Model selection table: Sonnet minimum for constrained implementation,
  Haiku/MiniMax viable for review and test-writing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Paul O'Reilly
2026-04-05 21:49:02 +12:00
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@@ -360,6 +360,29 @@ The workflow:
1. **Phase 1:** Agent reads spec → writes tests. Human reviews tests against spec. 1. **Phase 1:** Agent reads spec → writes tests. Human reviews tests against spec.
2. **Phase 2:** Agent (or different agent) reads spec + tests → writes implementation until tests pass. 2. **Phase 2:** Agent (or different agent) reads spec + tests → writes implementation until tests pass.
### Read-Only Test Gates
When an implementation agent is gated by tests, **place tests in a read-only reference directory** — not the working directory. Agents (especially smaller models) will modify test files to make tests pass rather than writing correct implementation. Prompt-level "DO NOT MODIFY" instructions are insufficient.
**Enforcement pattern:**
1. Clone tests to `/workspace/reference/` (root-owned, `chmod a-w`)
2. Agent implements in `/workspace/working/`
3. Run pytest against the immutable reference: `cd /workspace/working && PYTHONPATH=/workspace/working python -m pytest /workspace/reference/main/tests/ -v`
Use filesystem enforcement, not prompt instructions. In a 3-way model comparison: Sonnet respected "DO NOT MODIFY" instructions; MiniMax edited tests 7 times; Haiku rewrote the entire test file. The filesystem makes modification impossible regardless of model.
### Model Selection for Implementation Agents
**Sonnet is the minimum viable model for constrained implementation tasks** (spec + test gate). Smaller and cheaper models modify test files or ignore constraints:
| Model | Result | Notes |
|---|---|---|
| Sonnet | 113/113 tests passing, tests untouched | Viable for implementation |
| MiniMax | Modified tests 7 times | Invalid — use for review/test-writing only |
| Haiku | Rewrote test file entirely | Invalid — use for review/test-writing only |
Haiku and MiniMax are viable for test-writing, review, and spec work — tasks where the output is inspected by a human, not enforced by a gate.
### Hidden Test Splits ### Hidden Test Splits
Hold back some tests that the implementing agent never sees. Use them as a final validation: Hold back some tests that the implementing agent never sees. Use them as a final validation: