[FEATURE] Cowork: allow switching model mid-conversation (model is locked after the first message)

Environment

• Claude Desktop 1.10628.0 (Windows 11 x64, MSIX)
• Bundled Claude Code 2.1.160
• Cowork mode

Problem

In Cowork, the model can only be chosen when starting a new conversation. Once the first message is sent, the model picker is locked for the lifetime of that conversation.

Real-world pain points

1. Old conversations cannot be upgraded to new models. Opus 4.8 is out, but my long-lived Cowork conversations created earlier are permanently locked to Opus 4.7. The only way to use the new model is to abandon the conversation and all its accumulated context.
2. Cannot downgrade to save usage. Some conversations turn out to need far less reasoning than expected (e.g. they become routine file operations). I'd like to switch those to Sonnet mid-conversation to save quota, but I can't.

Expected behavior

Allow changing the model at any point in a conversation, taking effect from the next message — the same as claude.ai web chat and the Claude Code CLI (/model) already support. Cowork is currently the outlier among Anthropic's own surfaces.

Workaround today

None, other than starting a new conversation and manually re-feeding context, which defeats the purpose of a long-lived Cowork session.