Bug: /model switch confirmed in UI but backend continues serving previous model for hours, silently draining weekly limit + usage credits

## Summary

`/model claude-opus-5` and later `/model claude-sonnet-5` both returned the expected CLI confirmation ("Set model to claude-opus-5" / "Set model to claude-sonnet-5") and the footer model selector updated correctly. Despite this, the backend continued serving requests with the previously-selected model (`claude-fable-5`) for roughly 14.5 hours, confirmed via the `message.model` field in the local session transcript (`~/.claude/projects//.jsonl`) — which is ground truth for what actually served each response, independent of what the UI displays.

This silently exhausted the Fable-5 weekly plan limit, then drained real usage-credit balance (pay-as-you-go overage), on model requests I never selected.

## Environment

- Plan: Claude Max 20x
- Product: Claude Code (desktop app + CLI)
- Session ID: `b112f44c-5dc5-4028-bf08-eccbc6b4f39c`

## Timeline (from session transcript `message.model` field)

- `2026-08-20 16:40:58 UTC` — switched to Opus via `/model claude-opus-5`; served correctly
- `2026-08-21 12:52 UTC` — still correctly on Opus
- `2026-08-21 13:49 UTC` — **with no user action, requests silently began being served by `claude-fable-5`**
- `2026-08-21 13:54 UTC` — briefly back to Opus
- `2026-08-21 14:04 UTC` — reverted to `claude-fable-5` and stayed there continuously until `2026-08-22 04:29 UTC` (~14.5 hours), despite the model selector displaying Opus throughout and Fable never being reselected
- `2026-08-22 04:33 UTC` — after explicitly running `/model claude-sonnet-5`, requests finally began being correctly served by `claude-sonnet-5`, and have been correct since

## Impact

- Fable-5 weekly limit exhausted on requests never intentionally routed to Fable-5
- Usage credits (real charges, pay-as-you-go) subsequently drained to fund the same misrouted requests
- The UI (model selector, `/model` confirmation output) gave no indication anything was wrong — the only way to detect this was manually grepping the local session transcript for the `message.model` field on each response

## How to verify

For any Claude Code session, the actual serving model per response is recorded in the local `.jsonl` transcript under `message.model` on assistant-type entries. This can diverge from both the UI's displayed selection and the CLI's own confirmation output for `/model`, as shown above.

## Request

- Investigation into why model selection stopped propagating to the serving backend for this session
- Ideally, a client-side or server-side consistency check that surfaces a warning when `message.model` diverges from the user's last explicit `/model` selection, rather than silently billing the mismatch

Happy to provide additional transcript excerpts if useful.

Start by examining the local session transcript at ~/.claude/projects//.jsonl and compare each assistant entry's message.model with the last explicit /model selection. Done means identifying why the displayed selection diverges from the serving model and defining a warning or other safeguard for mismatches, though the issue names no source files or tests.

索引モデルが issue の本文から書いたものです。