[BUG] Returning from agents/sessions side panel (→ key) silently downgrades main session from 1M to 200k context window

Summary

In Claude Code CLI v2.1.149, navigating into the agents/sessions side panel (via the → arrow key, the "Type a task below to dispatch a session alongside it" view) and returning to the main session silently drops the session's model from claude-opus-4-7[1m] (1M context) to bare claude-opus-4-7 (200K context). The /model selector and /context denominator both confirm the downgrade.

Environment

• OS: macOS (Darwin 25.5.0)
• Claude Code CLI: v2.1.149
• Model originally selected: Opus 4.7 (1M context) — set as default via /model
• 1M context entitlement: confirmed (fresh sessions correctly start on 1M)

Steps to reproduce

1. Start a fresh claude session. Confirm via /context that the denominator is /1m and the model line reads claude-opus-4-7[1m].
2. Press → (right arrow) at the prompt to enter the agents/sessions side panel (the view that says "Press → to return to your session anytime. Type a task below to dispatch a session alongside it.").
3. Press → again (or otherwise return) to the main session — no new task dispatched, just navigate in and back out.
4. Run /context again.

Expected

/context still shows claude-opus-4-7[1m] and /1m denominator.

Actual

/context now shows claude-opus-4-7 and /200k denominator. The 1M context window is silently lost. Re-running /model and re-selecting "Opus 4.7 (1M context)" restores it.

Workaround

Run /model and re-select "Opus 4.7 (1M context)" after every visit to the agents panel.

Related issues (likely shared root cause, different trigger path)

• #61068 — --resume overwrites settings.json model[1m] with API-returned bare model ID. The Bk_ function reverse-engineered there strips the [1m] suffix during session restore; the agents-panel return path may exercise the same code.
• #60913 — --resume sends literal claude-opus-4-7[1m] and falls back to 200K on 404.
• #49600 — Spawned agents themselves run on 200K (related but distinct; this report is about the main session being downgraded after merely visiting the panel).

This trigger path (agents-panel navigation, no resume/compact/--model flag involved) does not appear to be covered by existing issues.