[BUG] REGRESSION: Cannot switch between Opus and Sonnet mid-thread in Cowork after April 15 desktop redesign

Description

After the April 15, 2026 desktop app redesign, mid-thread model switching is broken in Cowork. Previously, users could freely switch between Opus and Sonnet within the same Cowork thread — including within Projects. After relaunching the app post-update, this capability is gone.

This may be related to the regression in #40095 (automatic model switching from Opus to Sonnet), but the behavior here is different: the option to manually switch models mid-thread appears to be entirely removed, not just silently failing.

Environment

• Product: Claude Desktop (Cowork mode)
• Platform: macOS
• Subscription: Max plan
• App version: Post-April 15, 2026 update (desktop redesign)

Steps to Reproduce

1. Open a Cowork session
2. Start a conversation on one model (e.g., Opus)
3. Attempt to switch to a different model (e.g., Sonnet) mid-thread
4. Model switching is no longer available mid-thread

Expected Behavior

Ability to switch between Opus and Sonnet mid-thread in Cowork, as was possible before the April 15 update.

Actual Behavior

Model cannot be changed mid-thread in Cowork after the update.

Notes

• This worked reliably in previous versions of the desktop app
• Mid-thread switching was never available in Chat mode (expected), but was available in Cowork
• Likely a regression introduced by the April 14-15 desktop redesign
• Related: #40095