errors changedapi/errors
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5858The API always returns errors as JSON, with a top-level `error` object that always includes a `type` and `message` value. The response also includes a `request_id` field for easier tracking and debugging. For example:
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60```text JSON wrap
60```json JSON
6161{
6262 "type": "error",
6363 "error": {
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431431Claude 4.6 and later models and [Claude Mythos Preview](https://anthropic.com/glasswing) do not support prefilling assistant messages. Sending a request with a prefilled last assistant message to any of these models returns a 400 `invalid_request_error`:
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433```text
433```json
434434{
435435 "type": "error",
436436 "error": {
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450450`thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response.
451451```
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453With tool use, every `thinking` and `redacted_thinking` block from the assistant turn must be passed back exactly as received, including blocks whose `thinking` field is empty. Pass thinking blocks back unchanged, and if your application filters content blocks by type before resending, include both `thinking` and `redacted_thinking`. See the [fix for modified thinking blocks](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#error-thinking-blocks-modified) in Troubleshooting thinking, and also [Preserving thinking blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#preserving-thinking-blocks) and [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-thinking).
453With tool use, pass back every `thinking` and `redacted_thinking` block from the assistant turn exactly as received, including blocks whose `thinking` field is empty. If your application filters content blocks by type before resending, include both `thinking` and `redacted_thinking`. See the [fix for modified thinking blocks](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#error-thinking-blocks-modified) in Troubleshooting thinking, and also [Preserving thinking blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#preserving-thinking-blocks) and [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-thinking).
454454
455455### Extended thinking not supported
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