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4646
4747On most models, thinking is on by default or one parameter away. Which configuration each model accepts, and what it defaults to, is listed in the [per-model configuration table](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#supported-models) on the Troubleshooting page.
4848
49On Claude Opus 5, Claude Sonnet 5, Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, and Claude Mythos Preview, thinking is already on and needs no configuration. `display` defaults to `"omitted"` on these models, so the thinking text is hidden until you opt in. Opt in with `thinking: {"type": "adaptive", "display": "summarized"}`, which is exactly the following request with the [model string](https://platform.claude.com/docs/en/models/overview) swapped.
49On Claude Opus 5.5, Claude Opus 5, Claude Sonnet 5, Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, and Claude Mythos Preview, thinking is already on and needs no configuration. `display` defaults to `"omitted"` on these models, so the thinking text is hidden until you opt in. Opt in with `thinking: {"type": "adaptive", "display": "summarized"}`, which is exactly the following request with the [model string](https://platform.claude.com/docs/en/models/overview) swapped.
5050
5151On Claude Opus 4.8, Claude Opus 4.7, Claude Opus 4.6, and Claude Sonnet 4.6, thinking is off until you set `thinking: {type: "adaptive"}`, which lets Claude decide when and how deeply to think based on the request. The following examples do that, set `display: "summarized"` so the thinking text is visible, and use a roomy `max_tokens`:
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438438 ```
439439</CodeGroup>
440440
441Claude Opus 5 also has thinking on by default and accepts `thinking: {type: "disabled"}` at [effort](https://platform.claude.com/docs/en/build-with-claude/effort) `high` or below. At `xhigh` or `max` effort, thinking cannot be turned off: requests that combine `thinking: {type: "disabled"}` with those effort levels return a 400 error. This restriction applies to Claude Opus 5 and later models and is enforced on each request. With thinking disabled, Claude Opus 5 can occasionally emit tool calls as plain text or include internal XML tags in its visible output. See [Running with thinking disabled](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5#running-with-thinking-disabled) for prompting mitigations.
441Claude Opus 5 also has thinking on by default and accepts `thinking: {type: "disabled"}` at [effort](https://platform.claude.com/docs/en/build-with-claude/effort) `high` or below. At `xhigh` or `max` effort, thinking cannot be turned off: requests that combine `thinking: {type: "disabled"}` with those effort levels return a 400 error. This restriction is enforced on each request. With thinking disabled, Claude Opus 5 can occasionally emit tool calls as plain text or include internal XML tags in its visible output. See [Running with thinking disabled](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5#running-with-thinking-disabled) for prompting mitigations.
442442
443Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, and Claude Mythos Preview reject `thinking: {type: "disabled"}`. Thinking can't be turned off on these models.
443Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, Claude Opus 5.5, and Claude Mythos Preview reject `thinking: {type: "disabled"}`. Thinking can't be turned off on these models.
444444
445445If your model supports only extended thinking (see the [per-model configuration table](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#supported-models)), configure it with `type: "enabled"` and a `budget_tokens` value instead. The [Extended thinking](https://platform.claude.com/docs/en/build-with-claude/extended-thinking) page covers that configuration. And if any thinking configuration comes back with a 400 error, [Troubleshooting thinking](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting) matches each error message to its fix.
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451451The `display` field on the thinking configuration controls how thinking content is returned in API responses. `display` works in both modes: set it alongside `type: "adaptive"` or `type: "enabled"`. It accepts these values:
452452
453453* `"summarized"`: thinking blocks contain [summarized thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#summarized-thinking) text, a readable summary of Claude's reasoning. This is the default on Claude Opus 4.6, Claude Sonnet 4.6, and earlier models.
454* `"omitted"`: thinking blocks are returned with an empty `thinking` field. The `signature` field still carries the encrypted full thinking for multi-turn continuity (see [Thinking encryption](https://platform.claude.com/docs/en/build-with-claude/thinking#thinking-encryption)). This is the default on Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, Claude Opus 5, Claude Sonnet 5, Claude Opus 4.8, Claude Opus 4.7, and [Claude Mythos Preview](https://anthropic.com/glasswing).
454* `"omitted"`: thinking blocks are returned with an empty `thinking` field. The `signature` field still carries the encrypted full thinking for multi-turn continuity (see [Thinking encryption](https://platform.claude.com/docs/en/build-with-claude/thinking#thinking-encryption)). This is the default on Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, Claude Opus 5.5, Claude Opus 5, Claude Sonnet 5, Claude Opus 4.8, Claude Opus 4.7, and [Claude Mythos Preview](https://anthropic.com/glasswing).
455455* `"updates"` (beta): reasoning blocks are returned with an empty `thinking` field, as with `"omitted"`, and the short [progress updates](https://platform.claude.com/docs/en/build-with-claude/thinking#progress-updates) some models write between tool calls come back as readable text. Requires the beta header `thinking-display-updates-2026-08-18`.
456456
457457Set `display: "omitted"` when your application doesn't surface thinking content to users. The primary benefit is faster time-to-first-text-token when streaming: the server skips streaming thinking tokens entirely and delivers only the signature, so the final text response begins streaming sooner.
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504504 In rare cases where you need access to full thinking output, [contact Anthropic sales](mailto:[email protected]).
505505</Note>
506506
507To see the model's reasoning, read the `thinking` blocks rather than prompting for reasoning in the response text. On Claude Fable 5.1 and Claude Fable 5, a request that attempts to elicit the model's internal reasoning as part of the response text can be refused with `stop_details.category: "reasoning_extraction"`. See [Refusal categories](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#refusal-response) for the field reference and handling guidance.
507To see the model's reasoning, read the `thinking` blocks rather than prompting for reasoning in the response text. On Claude Fable 5.1, Claude Opus 5.5, and Claude Fable 5, a request that attempts to elicit the model's internal reasoning as part of the response text can be refused with `stop_details.category: "reasoning_extraction"`. See [Refusal categories](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#refusal-response) for the field reference and handling guidance.
508508
509509### Streaming thinking
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877877
878878Thinking works alongside [tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview), letting Claude reason through tool selection and process tool results. Two constraints apply:
879879
8801. **Tool choice limitation (manual mode):** tool use with manual extended thinking (`thinking: {type: "enabled"}`) only supports `tool_choice: {"type": "auto"}` (the default) or `tool_choice: {"type": "none"}`. Using `tool_choice: {"type": "any"}` or `tool_choice: {"type": "tool", "name": "..."}` results in an error because these options force tool use, which is incompatible with manual extended thinking. Adaptive thinking, including on models where thinking is on by default, supports forced tool use, except on Claude Fable 5.1 and Claude Mythos 5.1 (see [Response prefill and forced tool use](https://platform.claude.com/docs/en/build-with-claude/thinking#limits-and-feature-compatibility)).
8801. **Tool choice limitation (manual mode):** tool use with manual extended thinking (`thinking: {type: "enabled"}`) only supports `tool_choice: {"type": "auto"}` (the default) or `tool_choice: {"type": "none"}`. Using `tool_choice: {"type": "any"}` or `tool_choice: {"type": "tool", "name": "..."}` results in an error because these options force tool use, which is incompatible with manual extended thinking. Adaptive thinking, including on models where thinking is on by default, supports forced tool use, except on Claude Opus 5.5, Claude Fable 5.1, and Claude Mythos 5.1 (see [Response prefill and forced tool use](https://platform.claude.com/docs/en/build-with-claude/thinking#limits-and-feature-compatibility)).
8818812. **Preserving thinking blocks:** when you return tool results, you must pass the thinking blocks from the assistant message back to the API, complete and unmodified. See [Preserving thinking blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#preserving-thinking-blocks).
882882
883883**A tool-use loop is one assistant turn.** From the model's perspective, an assistant turn doesn't complete until Claude finishes its full response, which may include multiple tool calls and results. This whole sequence is a single assistant turn:
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941941 Consecutive tool calls do not require interleaved thinking. Claude can chain tool calls with or without interleaved thinking. Interleaving changes where thinking blocks appear between tool calls, not whether tool calls can chain.
942942</Note>
943943
944With adaptive thinking, interleaved thinking is automatic on every model that supports adaptive thinking. No beta header is needed. On Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, Claude Mythos Preview, Claude Opus 5, Claude Opus 4.8, and Claude Opus 4.7, reasoning between tool calls always appears in thinking blocks. Claude Haiku 4.5 does not support interleaved thinking. On models using manual extended thinking, interleaving requires a beta header and changes how the thinking budget is counted. [Interleaved thinking in manual mode](https://platform.claude.com/docs/en/build-with-claude/extended-thinking#interleaved-thinking) covers the per-model rules and platform-specific header behavior.
944With adaptive thinking, interleaved thinking is automatic on every model that supports adaptive thinking. No beta header is needed. On Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, Claude Mythos Preview, Claude Opus 5.5, Claude Opus 5, Claude Opus 4.8, and Claude Opus 4.7, reasoning between tool calls always appears in thinking blocks. Claude Haiku 4.5 does not support interleaved thinking. On models using manual extended thinking, interleaving requires a beta header and changes how the thinking budget is counted. [Interleaved thinking in manual mode](https://platform.claude.com/docs/en/build-with-claude/extended-thinking#interleaved-thinking) covers the per-model rules and platform-specific header behavior.
945945
946946With interleaved thinking, the thinking allocation can span the entire assistant turn rather than a single response. Interleaved thinking is only supported for [tools used through the Messages API](https://platform.claude.com/docs/en/agents-and-tools/tool-use/overview).
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949949
950950### Progress updates between tool calls
951951
952On Claude Fable 5.1, Claude Mythos 5.1, and Claude Fable 5, the model can write a progress update between tool calls. A progress update is a sentence or two on what the model just found and what it's about to do next, written for the person watching the agent rather than as reasoning. Each one comes back as its own `thinking` block with its own `signature`, separate from any reasoning block at the same point. It sits immediately before the `tool_use` or `server_tool_use` block it introduces. At most one progress update precedes each tool call, and the model can skip any of them. Progress updates aren't [interleaved thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#interleaved-thinking): they appear whether or not reasoning blocks appear between tool calls, and a response can contain both.
952On Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, and Claude Fable 5, the model can write a progress update between tool calls. A progress update is a sentence or two on what the model just found and what it's about to do next, written for the person watching the agent rather than as reasoning. Each one comes back as its own `thinking` block with its own `signature`, separate from any reasoning block at the same point. It sits immediately before the `tool_use` or `server_tool_use` block it introduces. At most one progress update precedes each tool call, and the model can skip any of them. Progress updates aren't [interleaved thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#interleaved-thinking): they appear whether or not reasoning blocks appear between tool calls, and a response can contain both.
953953
954954What a progress-update block contains depends on [`display`](https://platform.claude.com/docs/en/build-with-claude/thinking#controlling-thinking-display):
955955
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10211021* A progress-update block can come back with an empty `thinking` field under any `display` value. Render nothing for an empty block. Under `"updates"` it looks the same as an empty reasoning block and needs no separate handling.
10221022* When a response stops on `max_tokens`, `model_context_window_exceeded`, or `stop_sequence` soon after a tool call or tool result, its last block can be a progress-update block standing in for the work the model hadn't finished. Under `"updates"` and `"summarized"` its text is exactly `This part of the response was interrupted before it finished.` and you can show it like any other update. Under `"omitted"` it's empty. To continue, pass the assistant turn back unchanged and append a new `user` message (with a `tool_result` for each `tool_use` block in that turn).
10231023* When [streaming](https://platform.claude.com/docs/en/build-with-claude/thinking#streaming-thinking), expect a pause of several seconds before a progress-update block opens. See the `"updates"` trace in [Streaming thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#streaming-thinking).
1024* These models write fewer progress updates at higher [effort](https://platform.claude.com/docs/en/build-with-claude/effort) and in long tool chains. If your interface depends on them, see [Ask for user-facing progress updates](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#ask-for-user-facing-progress-updates).
1024* These models write fewer progress updates at higher [effort](https://platform.claude.com/docs/en/build-with-claude/effort) and in long tool chains. If your interface depends on them, see [Ask for user-facing progress updates](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#ask-for-user-facing-progress-updates) or, for Claude Opus 5.5, [User-facing progress updates](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#user-facing-progress-updates).
10251025
10261026### Thinking block preservation by model
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10371037
10381038The tradeoff is context usage: long conversations consume more context space on keep-all models, because retained thinking blocks count as input like any other conversation history (see [Thinking and the context window](https://platform.claude.com/docs/en/build-with-claude/thinking#thinking-and-the-context-window)). The behavior is automatic in both regimes. No code changes or beta headers are required, and you should keep passing complete, unmodified thinking blocks back as described in [Preserving thinking blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#preserving-thinking-blocks). To override the default in either direction, use [thinking block clearing](https://platform.claude.com/docs/en/build-with-claude/context-editing#thinking-block-clearing).
10391039
1040**Switching models mid-conversation.** Keep passing thinking blocks back unchanged when you switch models, for example after a [classifier refusal fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback). A thinking block is readable only by the model that produced it or a newer one, and the API ignores or drops the blocks the target model can't read. On Claude Fable 5.1 and Claude Mythos 5.1 the direction matters: they read every earlier model's thinking blocks and no earlier model reads theirs, so switching up to them keeps the conversation's reasoning and switching down drops it (see [how dropped blocks are billed and reported](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models)). Strip prior `thinking` and `redacted_thinking` blocks yourself only to save input tokens on models that ignore rather than drop them, and never when redeeming a [fallback credit](https://platform.claude.com/docs/en/build-with-claude/fallback-credit), which requires the body unchanged.
1040**Switching models mid-conversation.** Keep passing thinking blocks back unchanged when you switch models, for example after a [classifier refusal fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback). A thinking block is readable only by the model that produced it and certain other models, and the API ignores or drops the blocks the target model can't read. On Claude Fable 5.1 and Claude Mythos 5.1 the direction matters: they read every earlier model's thinking blocks and no earlier model reads theirs, so switching up to them keeps the conversation's reasoning and switching down drops it (see [how dropped blocks are billed and reported](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models)). Claude Opus 5.5 reads Claude Opus 5's thinking blocks and those of earlier Opus, Sonnet, and Haiku models, but not those of the Claude Fable and Claude Mythos models; on the Claude API, Claude Fable 5.1 and Claude Mythos 5.1 read Claude Opus 5.5's blocks. A switch from Claude Opus 5.5 up to Claude Fable 5.1 on the Claude API keeps the earlier turns' reasoning; a switch from Claude Fable 5.1 to Claude Opus 5.5 drops it. Strip prior `thinking` and `redacted_thinking` blocks yourself only to save input tokens on models that ignore rather than drop them, and never when redeeming a [fallback credit](https://platform.claude.com/docs/en/build-with-claude/fallback-credit), which requires the body unchanged.
10411041
10421042## Preserved thinking
10431043
10441044[Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) decides whether the model can use a thinking block that you send back from an earlier turn. Starting with Claude Fable 5.1, the API checks the `signature` of every `thinking` or `redacted_thinking` block in a request for two things:
10451045
1046* **The model that produced it.** A model reads its own thinking blocks and those of earlier models, never those of a newer model. Claude Fable 5.1 reads blocks from Claude Opus 5, but Claude Opus 5 can't read blocks from Claude Fable 5.1. The API drops a block the current model can't read, without an error and without billing it. See [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models).
1046* **The model that produced it.** Each model reads its own thinking blocks and those of a fixed set of other models. Claude Fable 5.1 reads blocks from Claude Opus 5 and, on the Claude API, from Claude Opus 5.5; neither Claude Opus 5 nor Claude Opus 5.5 reads blocks from Claude Fable 5.1. The API drops a block the current model can't read, without an error and without billing it. See [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models).
10471047* **Everything sent before it.** A block stays valid only while the top-level `system` prompt, the `tools`, and the messages before it are unchanged. If any of them changes, that block and every later thinking block are invalid, and the API rejects the request with a 400 error or drops the invalid blocks, whichever you choose. See [Keeping the prefix unchanged](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check).
10481048
1049The model check applies to every account. The API enforces the prefix check by default for accounts created on or after August 31, 2026, 00:00 UTC. On older accounts it enforces the check only on requests that set `thinking.block_binding.prefix_mismatch_behavior`. Later models will enforce it for all accounts, so make your integration append-only now.
1049The model check applies to every account. The API enforces the prefix check by default for accounts created on or after August 31, 2026, 00:00 UTC. On older accounts it enforces the check only on requests that set `thinking.block_binding.prefix_mismatch_behavior`. Make your integration append-only regardless of your account's age, so the same code works on every account, including newer accounts enforced by default.
10501050
10511051To keep thinking valid, send every assistant turn back exactly as you received it and add new messages only at the end of `messages`. If your code builds the `messages` array itself, the Preserved thinking page covers:
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11531153
11541154### Sampling parameters
11551155
1156On Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, Claude Mythos Preview, Claude Opus 5, Claude Opus 4.8, Claude Opus 4.7, and Claude Sonnet 5, non-default `temperature`, `top_p`, or `top_k` values return a 400 error on every request, regardless of whether thinking is used. On older models, the restriction applies only while thinking is on: `temperature` and `top_k` are incompatible with thinking, and `top_p` is allowed at values between 0.95 and 1.
1156On Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, Claude Mythos Preview, Claude Opus 5.5, Claude Opus 5, Claude Opus 4.8, Claude Opus 4.7, and Claude Sonnet 5, non-default `temperature`, `top_p`, or `top_k` values return a 400 error on every request, regardless of whether thinking is used. On older models, the restriction applies only while thinking is on: `temperature` and `top_k` are incompatible with thinking, and `top_p` is allowed at values between 0.95 and 1.
11571157
11581158### Response prefill and forced tool use
11591159
1160You can't prefill the assistant response while thinking is on. Forced tool use (`tool_choice: {"type": "any"}` or `{"type": "tool", ...}`) is incompatible with manual extended thinking but works with adaptive thinking. The exceptions are Claude Fable 5.1 and Claude Mythos 5.1, which reject forced tool use on every request with a 400 error. On those models, use `tool_choice: {"type": "auto"}` with [strict tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use) or [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs) instead. See [Thinking with tool use](https://platform.claude.com/docs/en/build-with-claude/thinking#thinking-with-tool-use).
1160You can't prefill the assistant response while thinking is on. Forced tool use (`tool_choice: {"type": "any"}` or `{"type": "tool", ...}`) is incompatible with manual extended thinking but works with adaptive thinking. The exceptions are Claude Opus 5.5, Claude Fable 5.1, and Claude Mythos 5.1, which reject forced tool use on every request with a 400 error. On those models, use `tool_choice: {"type": "auto"}` with [strict tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use) or [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs) instead. See [Thinking with tool use](https://platform.claude.com/docs/en/build-with-claude/thinking#thinking-with-tool-use).
11611161
11621162### Output limits
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11701170| Claude Fable 5 | 128K | — |
11711171| Claude Mythos 5 | 128K | — |
11721172| Claude Mythos Preview | 128K | Not available |
1173| Claude Opus 5.5 | 128K | 300K |
11731174| Claude Opus 5 | 128K | 300K |
11741175| Claude Opus 4.8 | 128K | 300K |
11751176| Claude Opus 4.7 | 128K | 300K |