Synthetic API errors were carrying only English display text, so desktop chat could not distinguish our compatibility guidance from raw upstream failures and recovery logic depended on those English strings.
Add stable business error codes for media, PDF, request-size, prompt-length, and auto-mode errors. Desktop chat now resolves those codes through locale keys and suppresses the stale English fallback for known business errors, while normalizeMessagesForAPI still supports legacy text matching for old transcripts.
Constraint: Raw upstream provider details must remain available for diagnostics but should not drive localized business UX.
Rejected: Localize src/services/api/errors.ts strings directly | that would make persisted transcript text and media recovery depend on the active UI language.
Confidence: high
Scope-risk: moderate
Directive: Add new user-facing synthetic API errors with businessErrorCode values before localizing their display text.
Tested: bun test src/services/api/errors.test.ts tests/mediaRecoveryAndEstimation.test.ts src/server/__tests__/ws-memory-events.test.ts
Tested: cd desktop && bun run test -- run src/stores/chatStore.test.ts src/components/chat/MessageList.test.tsx
Tested: bun run check:server
Tested: bun run check:desktop
Not-tested: Manual desktop screenshot smoke for the rendered Chinese error card
Unsupported image rejections from text-only compatible providers should not
poison the session, and low-trust multimodal usage spikes should not make
context indicators report a full window.
Constraint: Third-party Anthropic-compatible providers may report encoded media bytes as usage tokens.
Rejected: Trust all provider usage uniformly | third-party media responses can pin context to 100% incorrectly.
Confidence: high
Scope-risk: moderate
Directive: Do not remove the media-aware fallback without checking text-only provider recovery and desktop context indicators.
Tested: bun run check:server
Tested: focused media/context regression suite
