errors changedapi/errors
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477477### Thinking cannot be disabled
478478
479On Claude Fable 5.1, [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), Claude Fable 5, [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Opus 5.5, and [Claude Mythos Preview](https://anthropic.com/glasswing), thinking is always on. Sending `thinking: {"type": "disabled"}` to any of these models returns a 400 `invalid_request_error`. On all of these models except Claude Mythos Preview, the message reads:
479On Claude Fable 5.2, [Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview), Claude Fable 5.1, [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), Claude Fable 5, [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Opus 5.5, and [Claude Mythos Preview](https://anthropic.com/glasswing), thinking is always on. Sending `thinking: {"type": "disabled"}` to any of these models returns a 400 `invalid_request_error`. On all of these models except Claude Mythos Preview, the message reads:
480480
481481```text wrap
482482"thinking.type.disabled" is not supported for this model. Use "thinking.type.adaptive" and "output_config.effort" to control thinking behavior.
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522522
523523### Forced tool use not supported
524524
525Claude Opus 5.5, Claude Sonnet 5.5, Claude Fable 5.1, and [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview) don’t support forced tool use. Sending `tool_choice: {"type": "any"}` or `tool_choice: {"type": "tool", "name": "..."}` to any of these models, including on the [token counting endpoint](https://platform.claude.com/docs/en/build-with-claude/token-counting), returns a 400 `invalid_request_error`:
525Claude Fable 5.2, [Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview), Claude Opus 5.5, Claude Sonnet 5.5, Claude Fable 5.1, and [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview) don’t support forced tool use. Sending `tool_choice: {"type": "any"}` or `tool_choice: {"type": "tool", "name": "..."}` to any of these models, including on the [token counting endpoint](https://platform.claude.com/docs/en/build-with-claude/token-counting), returns a 400 `invalid_request_error`:
526526
527527```text wrap
528528tool_choice: type "tool" and "any" are not supported for this model.
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532532
533533### Computer use tool version not supported
534534
535Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5 return a 400 error for any [computer use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool) tool version they don’t accept. On the Claude API, that includes `computer_20251124` with its beta header. They don’t accept `computer_20250124` on any platform. The Claude API’s `invalid_request_error` message names the rejected type and, after `Did you mean one of`, lists the tool types the model accepts. For Claude Opus 5.5, the message begins:
535Claude Fable 5.2, Claude Mythos 5.2, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5 return a 400 error for any [computer use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool) tool version they don’t accept. On the Claude API, that includes `computer_20251124` with its beta header. They don’t accept `computer_20250124` on any platform. The Claude API’s `invalid_request_error` message names the rejected type and, after `Did you mean one of`, lists the tool types the model accepts. For Claude Opus 5.5, the message begins:
536536
537537```text wrap
538538'claude-opus-5-5' does not support tool types: computer_20251124.
539539```
540540
541The Claude API returns the same message for any Anthropic-defined tool type that the requested model doesn’t support. Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5 support computer use through the `computer_toolset_20260801` toolset. 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.
541The Claude API returns the same message for any Anthropic-defined tool type that the requested model doesn’t support. Claude Fable 5.2, Claude Mythos 5.2, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5 support computer use through the `computer_toolset_20260801` toolset. 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.
542542
543543On Amazon Bedrock, Claude Opus 5.5, Claude Sonnet 5.5, and Claude Haiku 5.5 also accept the `computer_20251124` tool with the `computer-use-2025-11-24` beta header. [Earlier tool versions](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#earlier-tool-versions) lists the models to use with each earlier version.
544544
545545### Thinking block bound to a different conversation
546546
547This 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 5.1 and later models 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:
547This 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 5.2, Claude Mythos 5.2, 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:
548548
549549```text wrap
550550messages.{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".
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