Observed

Source-reviewed: fallback fires as documented for RateLimitError and AuthenticationError, but NotFoundError (404) unconditionally bypasses the entire fallback chain (router.py L5285-86; #21377 stale-bot closed not-planned 2026-06-01, fix PR #21378 still open unmerged as of 2026-06-11); mid-stream fallback injects a default English system prompt over the caller's persona with no disable option (#18229, closed not-planned 2026-03-27); multimodal fallback drops image data (#15803, closed not-planned 2026-01-28); when primary and fallback are both unhealthy the router attempts a nonexistent fallback-for-the-fallback (#17729, closed not-planned 2026-03-17).