Phase C1 (v1.4.15) — slug-drift defence per Marc's hard mandate:
"Die Integration des Slug Drift Risiko darf halt immer überhaupt nicht
sein." ("Slug-drift risk must NEVER happen.")
Background: the ChatGPT-Codex backend rotates its allow-list silently.
v1.4.7..v1.4.13 lost a week to chasing the rotation manually
(`gpt-5-codex` → `gpt-5.3-codex`). A single hard-coded `CODEX_MODEL`
flips invalid the moment upstream rotates it.
Solution — the client now walks an ordered fallback chain on every
fresh request series and caches the first slug that returns 200 +
`response.completed` for 1 hour. Subsequent requests skip the chain
walk entirely on a cache hit.
Spec: extends `docs/codex-protocol-spec.md` with new §7b documenting
the chain semantics, cache TTL, walk triggers (400 + "not supported
when using Codex with a ChatGPT account" / `model_not_found` /
"does not exist" / 404), non-walk responses (5xx, 429, 401-with-
refresh, `invalid_prompt`), and the all-failed structured-503 error
shape. Per Marc's v1.4.6 hard rule ("read source-of-truth before
iterating; no more 6-iteration line-fixes"), the spec was extended
FIRST and the implementation cross-referenced against it.
Implementation:
- New `src/lib/ai/codex-slug-cache.ts` — process-local positive
cache. Single-slot Map (per-account scoping is a one-line change
when v1.4.16 multi-tenant rolls out). 1 h TTL. `getCachedCodexSlug`
auto-evicts expired entries on read.
- Rewritten `CodexClient.generateCompletion()` — walks the chain
`[cached?, ...slugChain]` in order. On 401 → refresh + retry SAME
slug (auth ≠ slug-state). On 400/404 with rejection body → drop
cache, walk to next. On 5xx/429 → propagate immediately (walking
won't help). On all-rejected → throw structured 503 with
`attempted[]` list, `httpStatus: 503`, message "AI provider
unreachable — all configured Codex slugs were rejected".
- `CodexAttemptDiagnostics` exposes `{ attempted, cacheState,
workingSlug }` for Wide-Event annotations the route can read via
`client.getLastDiagnostics()`.
- `CODEX_MODEL_FALLBACK_CHAIN` env override (comma-separated). When
`CODEX_MODEL` is also set, it folds into chain position 0; the
default chain (`gpt-5.3-codex` → `gpt-5-codex` → `gpt-5` →
`gpt-4o`) survives behind it. Stable de-dup.
Tests: +20 across 2 new files.
`codex-slug-cache.test.ts` (7 tests) — TTL-edge boundaries,
invalidation, replacement semantics.
`codex-slug-fallback.test.ts` (13 tests) — cold-cache happy path,
walk on canonical 400 body, walk on `model_not_found`, walk on 404,
no-walk on 500/429/401, all-rejected → 503-with-attempted-list,
cache-hit shortcut, cache invalidation on rejection, isSlugRejection
body classifier, loadFallbackChain env-pin merge.
The 5 existing `codex-client.test.ts` cases continue to pass without
edit — the new `slugChain` constructor parameter is opt-in and the
single-slug behaviour reduces to chain length 1.
Tests: 1028 → 1048 unit pass (+20). Typecheck clean for AI files.
Lint clean.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
