claude-code - 💡(How to fix) Fix [BUG] API Error 400: thinking block missing in multi-turn conversations with extended thinking enabled [1 participants]

When using Claude Code CLI in a multi-turn conversation with extended thinking enabled, after several exchanges the following error appears and the session becomes unusable: API Error: 400 messages.1.content.0.type: Expected thinking or redacted_thinking, but found text. When thinking is enabled, a final assistant message must start with a thinking block (preceding the lastmost set of tool_use and tool_result blocks). We recommend you include thinking blocks from previous turns. To avoid this requirement, disable thinking.

Error Messages/Logs

1. After several exchanges, the error appears and repeats on every message

TL;DR

Preserve thinking blocks in the conversation history to fix the API Error 400 when using Claude Code CLI with extended thinking enabled.

Guidance

• Verify that the issue is indeed caused by the missing thinking blocks in the conversation history by checking the API request payload.
• Modify the Claude Code CLI to include thinking blocks from previous turns in the conversation history.
• Test the fix by having a multi-turn conversation with several tool uses and checking if the error appears.
• If the issue persists, try disabling extended thinking to confirm that it's the root cause.

Example

No code snippet is provided as the issue does not contain sufficient information about the code.

Notes

The issue seems to be a regression, and the previous version that worked is not specified. The fix may require changes to the Claude Code CLI to preserve the thinking blocks in the conversation history.

Recommendation

Apply workaround: Modify the Claude Code CLI to preserve thinking blocks in the conversation history, as disabling extended thinking degrades response quality and is not a suitable long-term solution.