[BUG] OpenCode Go DeepSeek V4.1 Flash intermittently returns reasoning-only responses after model ID migratio

Description

After OpenCode Go changed the DeepSeek model ID from opencode-go/deepseek-flash to opencode-go/deepseek-v4.1-flash, I started seeing intermittent empty responses in existing sessions.

I reproduced this independently on two Linux servers.

When the problem occurs, OpenCode persists a reasoning part but no text part. The reasoning sometimes contains both the internal planning and what is clearly the complete final user-facing answer, concatenated together.

The turn then ends normally with finish: stop.

For example, the persisted structure is:

step-start
reasoning
step-finish

instead of the normal:

step-start
reasoning
text
step-finish

Restarting OpenCode or the server does not reliably fix it. However, after several more messages in the same session using deepseek-v4.1-flash, responses eventually return to normal without any configuration change.

This happened on both servers specifically after switching existing sessions from deepseek-flash to deepseek-v4.1-flash.

I have not captured the raw OpenCode Go SSE response, so I cannot tell whether the final content is already missing upstream or whether it is being classified as reasoning inside the OpenCode/OpenCode Go pipeline.

Plugins

None relevant to the issue.

OpenCode version

1.18.30

Steps to reproduce

1. Use an existing OpenCode session with opencode-go/deepseek-flash.
2. Switch the session/agent model to opencode-go/deepseek-v4.1-flash.
3. Continue chatting in the same session.
4. In some of the first several turns, the response is empty.
5. Inspect the persisted session data in ~/.local/share/opencode/opencode.db.
6. The affected assistant message contains a reasoning part but no text part and finishes with stop.
7. Continue sending messages without restarting or changing configuration.
8. After several turns, normal reasoning -> text responses eventually resume.

Observed independently on two Linux servers(Ubuntu 24.04).

Screenshot and/or share link

No public share link because the affected sessions contain private conversation history.

I can provide sanitized SQLite session records if needed.

Operating System

Ubuntu 24.04

Terminal

bash