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### Forced tool use not supported ### Thinking block no longer matches the conversation
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451451`thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response.
452452```
453453
454With 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 [Troubleshooting thinking](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#error-thinking-blocks-modified), [Preserving thinking blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#preserving-thinking-blocks), and [Thinking output on Claude Fable 5 and Claude Mythos 5](https://platform.claude.com/docs/en/build-with-claude/thinking#thinking-output-on-claude-fable-5-and-claude-mythos-5).
454With 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 [Troubleshooting thinking](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#error-thinking-blocks-modified), [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).
455455
456456### Extended thinking not supported
457457
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475475
476476### Thinking cannot be disabled
477477
478On Claude Fable 5, [Claude Mythos 5](https://anthropic.com/glasswing), 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`:
478On Claude Fable 5.1, [Claude Mythos 5.1](https://anthropic.com/glasswing), Claude Fable 5, [Claude Mythos 5](https://anthropic.com/glasswing), 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`:
479479
480480```text wrap
481481"thinking.type.disabled" is not supported for this model. Thinking defaults to adaptive mode when not specified; use "thinking.type.enabled" with "budget_tokens" for extended thinking.
482482```
483483
484On Claude Fable 5 and Claude Mythos 5, the error message's own suggestion of `"thinking.type.enabled"` is also rejected. Omit the `thinking` parameter and the request runs with adaptive thinking. To keep thinking content out of responses without turning thinking off, set `display: "omitted"` on the thinking configuration. See [Troubleshooting thinking](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#error-thinking-type-disabled).
484On Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, and Claude Mythos 5, the error message's own suggestion of `"thinking.type.enabled"` is also rejected. Omit the `thinking` parameter and the request runs with adaptive thinking. To keep thinking content out of responses without turning thinking off, set `display: "omitted"` on the thinking configuration. See [Troubleshooting thinking](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#error-thinking-type-disabled).
485485
486### Forced tool use not supported
487
488Claude Fable 5.1 and [Claude Mythos 5.1](https://anthropic.com/glasswing) don't support forced tool use. Sending `tool_choice: {"type": "any"}` or `tool_choice: {"type": "tool", "name": "..."}` to either model, including on the [token counting endpoint](https://platform.claude.com/docs/en/build-with-claude/token-counting), returns a 400 `invalid_request_error`:
489
490```text wrap
491tool_choice: type "tool" and "any" are not supported for this model.
492```
493
494`tool_choice: {"type": "auto"}` (the default) and `{"type": "none"}` are accepted. Use `auto` with [strict tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use) to keep tool inputs schema-valid, or [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs) when you need the response itself in a fixed JSON shape. See [Forcing tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/define-tools#forcing-tool-use).
495
496### Thinking block no longer matches the conversation
497
498On Claude Fable 5.1, 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:
499
500```text wrap
501messages.{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".
502```
503
504Without 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. A block from a model the target model can't read is dropped rather than rejected. See [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation) and [Troubleshooting thinking](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#error-thinking-block-signature).
505
506Sending `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:
507
508```text wrap
509block_binding: Extra inputs are not permitted
510```
511
512Add the header, or remove the field.
513
486514### Outbound web identity federation disabled (Claude Platform on AWS)
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488516If every request to [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws) returns `"Outbound web identity federation is disabled for your account"`, run `aws iam enable-outbound-web-identity-federation` once per AWS account. See [Enable outbound web identity federation](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws#enable-outbound-web-identity-federation) for details.
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490518## Next steps
491519
492520<CardGroup cols={3}>
493 <Card title="Trigger a routine through the API" icon="play" href="https://platform.claude.com/docs/en/api/claude-code/routines-fire">
494 Start a Claude Code routine session on demand by sending an authenticated POST request.
521 <Card title="Troubleshooting thinking" icon="wrench" href="https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting">
522 Symptom-first fixes for thinking configuration 400 errors, empty thinking blocks, and `max_tokens` stops.
495523 </Card>
496524
497525 <Card title="Rate limits" icon="gauge" href="https://platform.claude.com/docs/en/api/rate-limits">
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