[BUG] Outdated CLI (2.1.211) session hits spurious 'session limit' on Fable 5 and silently falls back to Opus 4.8 — no guidance to update/restart (works fine on 2.1.215)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet (closest matches: #74379, #74279 — related symptoms but a different failure signature; this one is tied to CLI version skew)
• [x] This is a single bug report
• [x] The bug occurred on 2.1.211; a fresh session on the latest version (2.1.215) does not reproduce it — that version dependency is the point of this report

What's Wrong?

Sessions that had been running on Claude Code 2.1.211 with Fable 5 suddenly started returning "you've hit your session limit"-style errors today (2026-07-20). In the affected session, /status also showed the model as claude-opus-4-8[1m] rather than Fable 5.

After killing that session and starting a new one — which picked up CLI 2.1.215 — Fable 5 works completely normally under the same account, with no limit errors. So the "session limit" message on the old version appears to be spurious (or at least not an account-level limit), and seems related to the older CLI build rather than actual usage.

/status from the affected 2.1.211 session (email/org redacted):

Version:         2.1.211
Session ID:      0cb22d12-faba-47fb-88ff-7d9254d2c3c2
cwd:             /Users/hypark5540/hwahae-agents
Login method:    Claude Max account
Model:           claude-opus-4-8[1m]
MCP servers:     13 connected, 2 disabled

Expected Behavior

If an outdated CLI version can no longer serve the selected model (or needs a restart/update to do so), Claude Code should say so explicitly — e.g. "This version of Claude Code can't use this model. Please update to the latest version and start a new session."

Instead, the user gets a misleading "session limit reached" error and a silent model fallback (claude-opus-4-8[1m] in /status), with no hint that simply updating/restarting fixes it. That's a confusing and unfriendly failure mode — it looks like an account/billing problem when it's actually a client-version problem.

Steps to Reproduce

1. Run a long-lived session on an older CLI build (2.1.211) with Fable 5 selected.
2. Keep the session alive across days while newer CLI versions ship.
3. Session starts emitting session-limit errors; /status shows claude-opus-4-8[1m].
4. Kill the session, launch a new one (auto-updated to 2.1.215) → Fable 5 works fine, no limit errors.

Environment

• Platform: macOS (darwin 25.5.0)
• Affected version: 2.1.211
• Working version: 2.1.215
• Login method: Claude Max account
• Model: Fable 5 (claude-fable-5), fell back to claude-opus-4-8[1m]

Related Issues

• #74379 — model silently switches to Fable 5 / limit block not cleared by /model (similar "limit error doesn't match selected model" symptom)
• #74279 — Fable 5 limit burned unexpectedly
• This report's distinct signature: the limit error + silent fallback only occurs on the outdated CLI build, and a restart on the latest version resolves it. Likely a duplicate family — feel free to dedupe, but please consider the UX ask above (explicit "please update" messaging) either way.