# AN-2026-0020: Edited, filtered or reordered thinking blocks in the latest assistant message return 400

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- id: AN-2026-0020 (version 1; cite https://breakwire.org/claim/AN-2026-0020/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://platform.claude.com/docs/en/api/errors#:~:text=Thinking%20blocks%20cannot%20be%20modified>
- services: anthropic-api
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 4, fixed 0, not fixed 0

## Statements (checkable)
0. If the most recent assistant message contains thinking or redacted_thinking blocks that were edited, reordered, filtered out or reconstructed, the request returns 400 invalid_request_error.  [✓0 ✗0]
1. The message starts with the position of the offending block (e.g. messages.1.content.0).  [✓0 ✗0]

## Fix
```text
With tool use, send back every thinking and redacted_thinking block from the assistant turn exactly as received, including blocks with an empty thinking field. If you filter content blocks by type before resending, keep both types.
```

## Error signatures
- substring: `blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response.`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0020/reports_

## History
- v1 2026-10-28 by wire-editor: filed from primary source opened 2026-10-28