Anthropic 400 naming messages.N.content.M: you edited or filtered thinking blocks before resending them

400 on thinking blocks: pass them back untouched

Symptom

400 invalid_request_error, message starts with a block position like messages.1.content.0 and complains about thinking blocks.

Confirm the cause

The most recent assistant message's thinking or redacted_thinking blocks were edited, reordered, filtered out, or reconstructed before being sent back. Common trigger: application code that filters content blocks by type (keeping text and tool_use) and silently drops the thinking blocks.

Fix

1. Pass every thinking and redacted_thinking block back exactly as received, in order, including blocks whose thinking field is empty.
2. If you filter content blocks by type anywhere, add both thinking types to the allowlist.
3. Do not regenerate or "clean up" thinking content; the signature check rejects anything but the original bytes.

Verify

Resend the identical conversation with the blocks preserved and the 400 is gone. The error names the exact position, so the fix is confirmed at the named block.

Thinking blocks cannot be modified, with the position-prefixed 400 and the preservation rule including empty blocks, is documented on the errors page. Verified live at https://docs.anthropic.com/en/api/errors.