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### Thinking block bound to a different conversation ### Thinking block no longer matches the conversation

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538538 
539539The API returns the same message for any Anthropic-defined tool type that the requested model doesn’t support. Declare `{"type": "computer_toolset_20260801"}` without the beta header and update your agent loop as described in [Migrate from `computer_20251124`](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#migrate-from-computer-20251124). To move from `computer_20250124`, see [Move computer use to the toolset](https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide#computer-use-toolset) in the Claude Haiku 5.5 migration guide. Earlier models that support the toolset keep accepting `computer_20251124`, as do Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5 on Amazon Bedrock.
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541### Thinking block no longer matches the conversation
541### Thinking block bound to a different conversation
542542 
543On Claude Fable 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5, the API accepts a replayed thinking block only while the `system` prompt, `tools`, and messages that preceded it are unchanged. For new accounts created on or after August 31, 2026, and for any request that sets `thinking.block_binding.prefix_mismatch_behavior` to `"error"`, a replayed block whose earlier history changed is rejected with a 400 `invalid_request_error` (with `"drop_block"`, the API drops the block and the request succeeds). The message starts with the position of the first failing block:
543This 400 `invalid_request_error` usually means a thinking block you sent back failed the prefix check: something before the block changed after the block was produced, such as the `system` prompt, `tools`, or an earlier message. Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5 run this check. For requests that don’t set `prefix_mismatch_behavior`, [When `prefix_mismatch_behavior` is unset](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement) shows which ones get this error. The message starts with the position of the first failing block:
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545545```text wrap
546546messages.{i}.content.{j}: Invalid `signature` in `thinking` block. The block is bound to a different conversation. Remove the block, or set `thinking.block_binding.prefix_mismatch_behavior` to "drop_block".
547547```
548548 
549Without the `thinking-binding-controls-2026-08-01` beta header, the message also names that header. Keep the conversation history append-only, or send the beta header with `prefix_mismatch_behavior: "drop_block"` to drop the block and continue. On Claude Sonnet 5.5 and Claude Haiku 5.5, `block_binding` works only with `thinking: {"type": "adaptive"}`. With `between_tools` on Claude Sonnet 5.5, or `thinking: {"type": "disabled"}` on Claude Haiku 5.5, keep the history append-only, or strip the thinking blocks from the edited turn on. A block from a model the target model can’t read is dropped rather than rejected. See [Keeping the prefix unchanged](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check) and the [fix for an invalid thinking block signature](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#error-thinking-block-signature) in Troubleshooting thinking.
549Without the `thinking-binding-controls-2026-08-01` beta header, the message also names that header. Resending the same history fails the same way, even after you fix the code that made the edit. To continue the conversation, send the beta header with `prefix_mismatch_behavior: "drop_block"` on the retry and on every later request. Alternatively, strip the block the error names and every later `thinking` and `redacted_thinking` block, then retry once. On Claude Sonnet 5.5 and Claude Haiku 5.5, `block_binding` works only with `thinking: {"type": "adaptive"}`, so with any other thinking setting, strip the blocks. To stop new conversations from failing, keep the history append-only. A block from a model the target model can’t read is dropped rather than rejected. See [Keeping the prefix unchanged](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check) and the [fix for an invalid thinking block signature](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#error-thinking-block-signature) in Troubleshooting thinking.
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551551Sending `thinking.block_binding` without the `thinking-binding-controls-2026-08-01` [beta header](https://platform.claude.com/docs/en/api/beta-headers) returns a 400 `invalid_request_error` whose message ends in:
552552 
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