feat(models): add Sonnet 5 and Fable 5 with fallback

[dimakis]

Summary

• Add claude-sonnet-5 and claude-fable-5 to model picker (server, mobile, desktop)
• Fable thinking config: always-on, omit param (explicit disabled returns 400)
• Add FALLBACK_MODEL (claude-opus-4-6): if selected model fails with 404/access error, auto-retry with fallback and notify user
• Add Sonnet 5 to Vertex model map
• Tests for Fable and Sonnet 5 thinking resolution

Test plan

• model-spec.test.ts — 13/13 passing (added Fable + Sonnet 5 cases)
• TypeScript compiles clean (npx tsc --noEmit)
• Manual: select Sonnet 5 from picker, verify session starts on Vertex
• Manual: select Fable 5, verify fallback message when on Vertex (no direct API key)

🤖 Generated with Claude Code

[dimakis]

Centaur Review

Found 6 issue(s) (1 critical) (4 warning).

server/chat.ts

The model fallback logic has a likely bug in the finally block firing a spurious session-end event during fallback, fragile error-message matching that could misclassify non-model errors, and no test coverage for the new retry path.

• 🔴 bugs (L1130): The finally block unconditionally fires _onSessionChange?.(clientId, 'end', session.sessionId). When the fallback path is taken (line 1118 return startChat(...)), the finally executes while the fallback session is already in-flight — emitting a spurious 'end' event for clientId. If _onSessionChange triggers orchestrator/task-board cleanup, this could kill or orphan the fallback session. The fallback branch should skip the finally cleanup (e.g., set a flag like didFallback = true and guard the finally body). [fixable]
• 🟡 unsafe_assumptions (L1091): isModelError matches on generic substrings like 'not found' and '404'. Any SDK error whose message happens to contain these words (e.g., "Conversation not found", "File not found", a 404 from an unrelated HTTP call) will trigger the fallback path and silently retry with a different model. Consider matching on more specific patterns (e.g., 'model_not_found', API error codes/types) or checking the error's structured fields instead of free-text search. [fixable]
• 🟡 missing_tests (L1105): The model fallback logic (30+ new lines of error handling, recursive startChat call, worktree cleanup on fallback) has no test coverage. The existing model-spec.test.ts only covers parseModelSpec and resolveThinking. The fallback path has subtle interactions (cleanup ordering, registry state, finally block) that would benefit from at least a unit test verifying: (1) fallback is triggered for model errors, (2) fallback is NOT triggered for non-model errors, (3) fallback is skipped when already on the fallback model. [fixable]
• 🟡 bugs (L1099): The 'No conversation found' check (existing code, line 1099) no longer falls through to the cleanup at lines 1126-1128 correctly in all cases. Before this PR, the if/else had two branches; now it has three. The expired-session branch still correctly falls through to the post-chain cleanup. However, note that canFallback could be true even when options.resume is set and the error is "No conversation found" — the substring 'not found' matches both isModelError and the resume check. The resume check wins because it's first, but this is fragile; a future reorder could cause resume failures to trigger model fallback instead. [fixable]

packages/harness/src/providers/anthropic-vertex.ts

The model fallback logic has a likely bug in the finally block firing a spurious session-end event during fallback, fragile error-message matching that could misclassify non-model errors, and no test coverage for the new retry path.

• 🟡 regressions (L19): claude-fable-5 is added to AVAILABLE_MODELS and both frontend dropdowns but is missing from VERTEX_MODEL_MAP. When CLAUDE_CODE_USE_VERTEX=1, selecting Fable 5 falls through to the raw model string which may not resolve on Vertex AI. The description says "direct API only" — if Fable is intentionally Vertex-incompatible, consider filtering it out of the model picker when Vertex is the active provider, or documenting the omission. [fixable]

frontend/src/pages/ChatView.tsx

The model fallback logic has a likely bug in the finally block firing a spurious session-end event during fallback, fragile error-message matching that could misclassify non-model errors, and no test coverage for the new retry path.

• 🔵 style (L226): The model options are hardcoded in both ChatView.tsx and DesktopChatView.tsx, duplicating the AVAILABLE_MODELS array in server/chat.ts. The server already exposes config via GET /api/config. Consider either importing a shared list from @mitzo/protocol or rendering options from AVAILABLE_MODELS to avoid maintaining three copies that can drift. [fixable]

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Reviewed at 1ff9cfa