fix(ai-gateway): fall back from Fable and Opus 5 to Opus 4.8

[chrarnoldus]

Summary

• pin the Fable fallback to Claude Opus 4.8 instead of the mutable current-Opus model
• make Claude Opus 5 fall back to Claude Opus 4.8 on OpenRouter and Vercel
• detect Opus 5 explicitly so future current Opus versions do not inherit this fallback automatically

Testing

• pnpm --filter web exec jest --runInBand src/lib/ai-gateway/providers/apply-provider-specific-logic.test.ts
• pnpm --filter web run typecheck
• pnpm exec oxlint --config .oxlintrc.json apps/web/src/lib/ai-gateway/providers/anthropic.constants.ts apps/web/src/lib/ai-gateway/providers/apply-provider-specific-logic.ts apps/web/src/lib/ai-gateway/providers/apply-provider-specific-logic.test.ts

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Executive Summary

Reviewed the incremental refactor that replaces the exact-match Opus 5 check with an explicit isOpus5Model substring helper; the change is correct, matches the existing isFableModel/isClaudeModel detection pattern, and the updated tests correctly verify no fallback for Opus 4.8, Opus 6, and non-Anthropic models.

Files Reviewed (3 files)

• apps/web/src/lib/ai-gateway/providers/anthropic.constants.ts
• apps/web/src/lib/ai-gateway/providers/apply-provider-specific-logic.ts
• apps/web/src/lib/ai-gateway/providers/apply-provider-specific-logic.test.ts

Previous Review Summary (commit 0569660)

Current summary above is authoritative. Previous snapshots are kept for context only.

Previous review (commit 0569660)

Status: No Issues Found | Recommendation: Merge

Executive Summary

Reviewed the Opus 4.8 fallback change across the constants, fallback logic, and test file; the implementation is correct, tests cover both providers and both trigger models, and no bugs, security issues, or breaking changes were found.

Files Reviewed (3 files)

• apps/web/src/lib/ai-gateway/providers/anthropic.constants.ts
• apps/web/src/lib/ai-gateway/providers/apply-provider-specific-logic.ts
• apps/web/src/lib/ai-gateway/providers/apply-provider-specific-logic.test.ts

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_{Reviewed by claude-sonnet-5 · Input: 26 · Output: 4.1K · Cached: 711.1K}

_{Review guidance: REVIEW.md from base branch main}