matrix: strong-model downgrade gate — SPEC §3.4 component 4 of 5
Pure-Go port of mode.rs::execute's pass5 downgrade gate (Rust
2026-04-26). Adds POST /v1/matrix/downgrade endpoint via matrixd.
The gate captures the pass5 finding: composing matrix corpora into
codereview_lakehouse on a strong model LOST 5/5 head-to-head reps
against matrix-free codereview_isolation on grok-4.1-fast (p=0.031).
Strong models have enough native capacity that bug fingerprints +
adversarial framing + file content carry them; matrix chunks
displace depth-of-analysis.
Logic (matches Rust mode.rs:614-632):
if mode == codereview_lakehouse
&& !forced_mode
&& !LH_FORCE_FULL_ENRICHMENT
&& !is_weak_model(model)
→ flip to codereview_isolation, record downgraded_from
is_weak_model captures the empirical weak-list:
- `:free` suffix or `:free/` infix (OpenRouter free tier)
- qwen3.5:latest, qwen3:latest (local last-resort rungs)
- everything else → strong by default
Tests:
- 3 unit tests in internal/matrix/downgrade_test.go: IsWeakModel
coverage, MaybeDowngrade truth table (5 rows), forced-mode
precedence (forced beats every other bypass)
- scripts/downgrade_smoke.sh: 6 assertions through gateway covering
all 5 truth-table rows + empty-mode 400
14-smoke regression sweep all green (D1-D6, G1, G1P, G2,
storaged_cap, pathway, matrix, relevance, downgrade).
SPEC §3.4 progress: 4 of 5 components shipped (corpus builders,
multi-corpus retrieve+merge, relevance filter, downgrade gate).
Last component is learning-loop integration.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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