Manually selected model reverts to the agent's configured model after every turn

Description

A manually selected model reverts to the agent's configured model after every conversational turn, so it is impossible to stay on a non-default model.

My build agent is pinned to claude-opus-5 in opencode.json. If I pick a different model in the picker, the prompt runs on it, then the picker immediately shows the agent's model again. Next turn runs on the agent's model.

Happens in both the TUI and the desktop app, but for different reasons:

• TUI: Prompt restores the model from the last user message on session hydration, overwriting the pick.
• Desktop: the agent Select calls agents.select(...) even when the value is unchanged, and agent.set always rewrites the model as item.model ?? prev?.model. Since the agent has a configured model, that always wins.

Possibly related, but different triggers: #38770 (background subagent notifications), #34207 (answering a question). Mine needs no subagents, no questions, and no mid-stream switching.

Plugins

None

OpenCode version

Desktop app 1.18.7, CLI 1.18.4

Steps to reproduce

1. In opencode.json, set the build agent's model, e.g. "agent": { "build": { "model": "provider/model-a" } }
2. Start a session with build and confirm it shows model A
3. Pick a different model B in the picker
4. Send a prompt and wait for the reply
5. The picker shows model A again; the next prompt runs on A

Operating System

macOS 26.5.2

Terminal

Ghostty (also reproduced in the desktop app)