[BUG]: Error switching from thinking model to non-thinking model mid-session

[okossa]

Description

When switching from a model with extended thinking (e.g., Claude Opus with thinking enabled) to a model without thinking support (e.g., GPT 5.2, Claude Sonnet without thinking) mid-session, an error occurs:

messages.1.content.0.thinking: each thinking block must contain thinking

Root Cause

The reasoning parts from the previous model's responses are stored in the message history. When switching to a model that doesn't support interleaved reasoning (capabilities.interleaved === false), these parts are still sent to the API, causing validation errors.

Steps to Reproduce

1. Start a conversation with Claude Opus (or any model with extended thinking)
2. Get a response that includes thinking/reasoning blocks
3. Switch to GPT 5.2 or Claude Sonnet (without thinking enabled) mid-session using Ctrl+M
4. Send a new message

Expected Behavior

Switching models mid-session should work seamlessly. OpenCode should strip reasoning parts from message history when the target model doesn't support them.

Actual Behavior

API error about invalid thinking blocks.

Environment

• OpenCode version: 0.0.0-dev
• OS: macOS
• Provider: Anthropic / OpenAI

Related Issues

• Invalid signature in thinking block when switching Claude models (Copilot provider) #11546 (similar but for Claude-to-Claude switching with signature issues)

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Invalid signature in thinking block when switching Claude models (Copilot provider) #11546: Invalid signature in thinking block when switching Claude models (Copilot provider) - very similar issue with Claude model switching
• DeepSeek V3.2 : reasoning_content not cleared from message history on new turns, causing excess token usage and violating API spec #5577: DeepSeek V3.2: reasoning_content not cleared from message history on new turns - related to reasoning blocks persisting in message history
• Does OpenCode send back all previous reasoning blocks to Claude (or any provider)? #8185: Does OpenCode send back all previous reasoning blocks to Claude - discusses if reasoning blocks are properly handled in message history
• Error: Invalid signature in thinking block #6418: Error: Invalid signature in thinking block - general thinking block validation error

Feel free to ignore if none of these address your specific case.