Pinned Fable 5 silently downgrades to Opus 4.8 mid-turn

Bug: pinned model (Fable 5) silently downgrades to Opus 4.8 mid-turn

Summary

I explicitly select Fable 5 in the model picker, but during a running turn the active model silently switches to Opus 4.8. The pin does not hold — the downgrade happens mid-work, not at my request.

Steps to reproduce

1. Open the model picker (/model), tap Fable 5.
2. Start a normal multi-step turn (tool use, subagents, longer reasoning).
3. Mid-turn, the active model changes to Opus 4.8 on its own.

Expected

A pinned model stays pinned for the whole turn. If a fallback is unavoidable (capacity/rate limit/overage), it should be surfaced explicitly, and ideally fall back within the same generation rather than to an older Opus tier.

Actual

Fable 5 → Opus 4.8 downgrade occurs silently during the turn.

What I ruled out (config is clean)

• ~/.claude/settings.json → "model": "claude-fable-5[1m]" (the intended default).
• No per-project pin: in ~/.claude.json, the edyvo project entry has model: null.
• No active opus-4-8 pin anywhere — claude-opus-4-8 appears only in clientDataCacheSlots.* (the list of available models) and in a GrowthBook feature config (tengu_review_bughunter_config.model = claude-opus-4-7), not as a selected model.
• effortLevel: "low", permissions.defaultMode: "auto", CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1.

Suspected cause

The default is the 1M-context lane (claude-fable-5[1m]). When that lane is briefly unavailable, the client appears to fall back to Opus 4.8 rather than staying on a Fable-5 tier. The fallback is also not surfaced in the UI.

Environment

• Claude Code: 2.1.212
• macOS: 26.3.1 (25D2128)
• Node: v20.19.5
• Model picker offers: Fable 5, Opus 5, Sonnet 5, Haiku 4.5
• Pinned: Fable 5 (claude-fable-5[1m])

Impact

Unpredictable model behavior and cost/quality: I pin the top model for hard work and silently get an older Opus tier mid-turn, with no notification.