Model routing mismatch: session set to Fable 5 (xHigh), served model self-identifies as Haiku 4.5

Summary

A Claude Code session configured for Fable 5 (effort xHigh) is being served by a model that self-identifies as Haiku 4.5 (claude-haiku-4-5-20251001). Every UI surface says Fable; the model says Haiku.

Evidence

• /model dialog: Fable ✓ selected ("Fable 5 · Most capable for your hardest and longest-running tasks"), effort xHigh.
• Status line (custom statusline script reading the session's model): Fable 5.
• The model's own system prompt in this session states: "You are powered by the model named Haiku 4.5. The exact model ID is claude-haiku-4-5-20251001." When asked directly which model it is, it answered Haiku 4.5.
• Session ID: 6433e68f-c6de-4c46-b3b0-0ae2badbc9fd
• Date: 2026-08-27 (session ran across 2026-08-26 → 27, UTC+0 machine clock)

Environment

• Claude Code CLI (terminal), auto mode on
• macOS Darwin 24.6.0, Intel (x86_64)
• Project: a Vite/React repo; nothing project-specific involved

Impact

The whole session's output quality matched a small model, not the one selected: repeated instruction-following failures (an explicit "no Playwright" ignored four times, a dev server started on an arbitrary port that collided with the user's own server, layout work that ignored the app's rail geometry). The user only discovered the cause by asking the model which model it was — nothing in the UI indicated a substitution.

Expected

Either the selected model is served, or the UI (status line, /model, and ideally a visible notice) reflects the model actually serving the session. A silent downgrade from the most expensive tier to the cheapest is the worst case for a paying user.

Steps to reproduce (as observed)

1. Start a session with Fable 5 selected as default (/model → Fable, Enter).
2. Work normally; check /model — shows Fable ✓.
3. Ask the model: "which model are you?" — it reports Haiku 4.5.

Happy to provide the session transcript on request.