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agents-and-tools/tool-use/browser-use-sdk Changed · +4 / -5 lines

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13891389 
13901390* Availability: Available
13911391 
1392| Status | Cloud platforms | Models |
1393| ------ | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
1394| Beta | Claude API, [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed) | [Claude Fable 5.2](https://platform.claude.com/docs/en/models/fable-5-2/overview), [Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview), [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview), [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Mythos Preview, [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Sonnet 5](https://platform.claude.com/docs/en/models/sonnet-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview), [Claude Opus 4.8](https://platform.claude.com/docs/en/models/opus-4-8/overview) |
1395| Beta | [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), [Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry) | [Claude Fable 5.2](https://platform.claude.com/docs/en/models/fable-5-2/overview), [Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview), [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview), [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Sonnet 5](https://platform.claude.com/docs/en/models/sonnet-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview), [Claude Opus 4.8](https://platform.claude.com/docs/en/models/opus-4-8/overview) |
1396| Beta | [Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock) | [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview), [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Sonnet 5](https://platform.claude.com/docs/en/models/sonnet-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview), [Claude Opus 4.8](https://platform.claude.com/docs/en/models/opus-4-8/overview) |
1392| Status | Cloud platforms | Models |
1393| ------ | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
1394| Beta | Claude API, [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed) | [Claude Fable 5.2](https://platform.claude.com/docs/en/models/fable-5-2/overview), [Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview), [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview), [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Mythos Preview, [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Sonnet 5](https://platform.claude.com/docs/en/models/sonnet-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview), [Claude Opus 4.8](https://platform.claude.com/docs/en/models/opus-4-8/overview) |
1395| Beta | [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), [Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), [Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry) | [Claude Fable 5.2](https://platform.claude.com/docs/en/models/fable-5-2/overview), [Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview), [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview), [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Sonnet 5](https://platform.claude.com/docs/en/models/sonnet-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview), [Claude Opus 4.8](https://platform.claude.com/docs/en/models/opus-4-8/overview) |
13971396 
13981397#### [Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview)
13991398 

agents-and-tools/tool-use/browser-use-tool Changed · +4 / -5 lines

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16231623 
16241624* Availability: Available
16251625 
1626| Status | Cloud platforms | Models |
1627| --------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
1628| Available | Claude API, [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed) | [Claude Fable 5.2](https://platform.claude.com/docs/en/models/fable-5-2/overview), [Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview), [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview), [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Mythos Preview, [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Sonnet 5](https://platform.claude.com/docs/en/models/sonnet-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview), [Claude Opus 4.8](https://platform.claude.com/docs/en/models/opus-4-8/overview) |
1629| Available | [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), [Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry) | [Claude Fable 5.2](https://platform.claude.com/docs/en/models/fable-5-2/overview), [Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview), [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview), [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Sonnet 5](https://platform.claude.com/docs/en/models/sonnet-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview), [Claude Opus 4.8](https://platform.claude.com/docs/en/models/opus-4-8/overview) |
1630| Available | [Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock) | [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview), [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Sonnet 5](https://platform.claude.com/docs/en/models/sonnet-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview), [Claude Opus 4.8](https://platform.claude.com/docs/en/models/opus-4-8/overview) |
1626| Status | Cloud platforms | Models |
1627| --------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
1628| Available | Claude API, [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed) | [Claude Fable 5.2](https://platform.claude.com/docs/en/models/fable-5-2/overview), [Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview), [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview), [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), Claude Mythos Preview, [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Sonnet 5](https://platform.claude.com/docs/en/models/sonnet-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview), [Claude Opus 4.8](https://platform.claude.com/docs/en/models/opus-4-8/overview) |
1629| Available | [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), [Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), [Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry) | [Claude Fable 5.2](https://platform.claude.com/docs/en/models/fable-5-2/overview), [Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview), [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview), [Claude Mythos 5.1](https://platform.claude.com/docs/en/models/mythos-5-1/overview), [Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5/overview), [Claude Mythos 5](https://platform.claude.com/docs/en/models/mythos-5/overview), [Claude Opus 5.5](https://platform.claude.com/docs/en/models/opus-5-5/overview), [Claude Opus 5](https://platform.claude.com/docs/en/models/opus-5/overview), [Claude Sonnet 5.5](https://platform.claude.com/docs/en/models/sonnet-5-5/overview), [Claude Sonnet 5](https://platform.claude.com/docs/en/models/sonnet-5/overview), [Claude Haiku 5.5](https://platform.claude.com/docs/en/models/haiku-5-5/overview), [Claude Opus 4.8](https://platform.claude.com/docs/en/models/opus-4-8/overview) |
16311630 
16321631#### [Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview)
16331632 

models/fable-5-2/migration-guide Changed · +4 / -4 lines

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1414/claude-api migrate this project to claude-fable-5-2
1515```
1616 
17If your Claude Managed Agents agent has an advisor, the advisor’s `model` is a second model name to update: set it to `claude-fable-5-2` or `claude-mythos-5-2` when you change the agent’s model. See [A Claude Fable 5.2 executor needs a Claude Fable 5.2 or Claude Mythos 5.2 advisor](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-advisor-pairs).
17If your Claude Managed Agents agent has an advisor, the advisor’s `model` is a second model name to update: set it to `claude-fable-5-2` or `claude-mythos-5-2` when you change the agent’s model. See [A Claude Fable 5.2 executor needs a Claude Fable 5.2 or Claude Mythos 5.2 advisor](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-advisor-pairs). If the agent’s `model` object sets `speed: "fast"`, remove it too.
1818 
1919The following sections cover moving to Claude Fable 5.2, or to Claude Mythos 5.2, from Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, or an earlier Claude Fable, Claude Mythos, or Claude Opus model. For the changes themselves, see [What’s new in Claude Fable 5.2](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2). For behavioral differences and model-specific prompting patterns, see [Prompting Claude Fable 5.2](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2).
2020 
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6464 messages.5.content.0: 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". That setting requires the `thinking-binding-controls-2026-08-01` value in the `anthropic-beta` header.
6565 ```
6666 
67 Retrying the same request body doesn’t clear the error. To find these edits before you switch production traffic, run the check in [Check whether your code edits the prefix](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#how-to-tell-whether-your-integration-is-impacted) against the model you use now, Claude Fable 5.1 or Claude Opus 5.5. It works from any account, because setting `prefix_mismatch_behavior` opts the request into the check. [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) gives the fix for each pattern.
67 Retrying the same request body doesn’t clear the error. To find these edits before you switch production traffic, run the check in [Check whether your code edits the prefix](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#how-to-tell-whether-your-integration-is-impacted) against the model you use now, Claude Fable 5.1 or Claude Opus 5.5. If you use the [MCP connector](https://platform.claude.com/docs/en/agents-and-tools/mcp-connector) without [pinning a server’s tool list](https://platform.claude.com/docs/en/agents-and-tools/mcp-connector#pin-mcp-tool-list), also run the check against `claude-fable-5-2`: a tool list that changes between requests is an edit for Claude Fable 5.2 thinking blocks, but not for Claude Fable 5.1 or Claude Opus 5.5 blocks. The check works from any account, because setting `prefix_mismatch_behavior` opts the request into the check. [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) gives the fix for each pattern.
6868 
6969 In production, leave `prefix_mismatch_behavior` at its default, `"error"`, if a mismatch can only mean a bug in your code. To drop the affected blocks and continue without that reasoning while you fix the edit, set it to `"drop_block"`. Treat that as a stopgap, not a long-term setting: [dropping blocks can raise output tokens](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#drop-block-cost).
7070 
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9999 
100100[Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview) is the counterpart to Claude Fable 5.2 for organizations verified through Anthropic’s verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842). Confirm that your organization has access to Claude Mythos 5.2 before you switch model IDs.
101101 
102Claude Mythos 5.2 enforces the [prefix check](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check) for every account, as Claude Fable 5.2 does. [When `prefix_mismatch_behavior` is unset](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement) shows where Claude Mythos 5.1 already returns the error. On Amazon Bedrock, Claude Mythos 5.2 returns the 400 error for edited earlier turns, where Claude Mythos 5.1 accepts such a request. If your code edits earlier turns, see [Editing earlier turns invalidates thinking blocks](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-edited-history) for what to change and how to test, and run the test against `claude-mythos-5-2`.
102Claude Mythos 5.2 enforces the [prefix check](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check) for every account, as Claude Fable 5.2 does. On the Claude API, Claude Mythos 5.1 already returns the error for every account (see [When `prefix_mismatch_behavior` is unset](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement)). On Amazon Bedrock, Claude Mythos 5.2 returns the 400 error for edited earlier turns, where Claude Mythos 5.1 accepts such a request. If your code changes `system`, `tools`, or earlier messages between requests, see [Editing earlier turns invalidates thinking blocks](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-edited-history) for what to change and how to test, and run the test against `claude-mythos-5-2`.
103103 
104104On the Claude API and Claude Models on Google Cloud (Google managed), Claude Mythos 5.2 reads Claude Mythos 5.1’s thinking blocks, so a conversation that moves from `claude-mythos-5-1` onto `claude-mythos-5-2` keeps its earlier reasoning. In the other direction, Claude Mythos 5.1 can’t read Claude Mythos 5.2’s blocks: the API drops them, and the request succeeds. See [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models).
105105 
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143143 
1441441. **Data retention is required:** Claude Fable 5.2 requires 30-day data retention and isn’t available under a zero data retention (ZDR) arrangement unless expressly authorized by Anthropic. If your organization or workspace runs Claude Opus 5.5 under ZDR, enable 30-day retention for a workspace or contact your Anthropic account team before you switch. Otherwise, on the Claude API, every request to Claude Fable 5.2 returns a 400 `invalid_request_error`. See [Model-specific data retention requirements](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements).
145145 
1462. **The prefix check applies to every account:** Claude Opus 5.5 enforces the prefix check by default for accounts created on or after August 31, 2026. Claude Fable 5.2 enforces it for every account, whatever its age. If your account is older and your code edits earlier turns, requests that work on Claude Opus 5.5 fail on Claude Fable 5.2. To test before you switch, see [The prefix check applies to every account](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-preserved-thinking).
1462. **The prefix check applies to every account:** Claude Opus 5.5 enforces the prefix check by default for accounts created on or after August 31, 2026. Claude Fable 5.2 enforces it for every account, whatever its age. If your account is older and your code changes `system`, `tools`, or earlier messages between requests, requests that work on Claude Opus 5.5 fail on Claude Fable 5.2. To test before you switch, see [The prefix check applies to every account](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-preserved-thinking).
147147 
1481483. **Claude Opus 5.5 can’t read Claude Fable 5.2’s thinking blocks:** A conversation that moves from Claude Fable 5.2 to Claude Opus 5.5 keeps its text and tool calls, but the turns after the switch run without Claude Fable 5.2’s reasoning. The API drops those blocks before the model receives them, the request succeeds, and the dropped blocks aren’t billed. Expect higher cost and latency on the first turn after the switch. In the other direction, on the Claude API and Claude Models on Google Cloud (Google managed), Claude Fable 5.2 reads Claude Opus 5.5’s thinking blocks, so a conversation that moves from `claude-opus-5-5` onto `claude-fable-5-2` keeps its earlier reasoning.
149149 

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3232 
3333### Editing earlier turns invalidates thinking blocks on every account
3434 
35The API’s prefix check (see [Keeping the prefix unchanged](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check)) tests whether anything before a Claude Fable 5.2 or Claude Mythos 5.2 thinking block has changed since the model produced it: the `system` prompt, the `tools`, or an earlier message. On Claude Fable 5.2 and Claude Mythos 5.2, a request that sends back an invalid thinking block and leaves `prefix_mismatch_behavior` unset gets a 400 error, whatever the account’s age (see [When `prefix_mismatch_behavior` is unset](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement)). On Claude Fable 5.1, such a request gets the error on accounts created on or after August 31, 2026. Claude Mythos 5.1 already returns it on every account, on the platforms listed in [When `prefix_mismatch_behavior` is unset](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement). So code on Claude Fable 5.1 that edits its history can run without errors on an older account before the switch and fail after it.
35The API’s prefix check (see [Keeping the prefix unchanged](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check)) tests whether anything before a Claude Fable 5.2 or Claude Mythos 5.2 thinking block has changed since the model produced it: the `system` prompt, the `tools`, or an earlier message. On Claude Fable 5.2 and Claude Mythos 5.2, a request that sends back an invalid thinking block and leaves `prefix_mismatch_behavior` unset gets a 400 error, whatever the account’s age (see [When `prefix_mismatch_behavior` is unset](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement)). On Claude Fable 5.1, such a request gets the error on accounts created on or after August 31, 2026. On the Claude API, Claude Mythos 5.1 already returns it on every account. So code on Claude Fable 5.1 that edits its history can run without errors on an older account before the switch and fail after it.
3636 
3737When the check fails, the request returns a 400 error whose message says `The block is bound to a different conversation`. To drop the affected blocks and continue instead, send the `thinking-binding-controls-2026-08-01` beta header and set `thinking.block_binding.prefix_mismatch_behavior` to `"drop_block"`. The response then lists each dropped block in a top-level `input_transformations` array. Treat `"drop_block"` as a stopgap while you fix the edit, because [dropping blocks can raise output tokens](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#drop-block-cost).
3838 
39Nothing changes for you if Claude Code, claude.ai, [Claude Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview), or the [Claude Agent SDK](https://code.claude.com/docs/en/agent-sdk/overview) builds your requests. The same holds if your code keeps `system` and `tools` fixed for a session and only appends to `messages`. Otherwise, run the test in the migration guide before you switch: the [prefix check section](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-preserved-thinking) for code on Claude Fable 5.1, or [Migrating to Claude Mythos 5.2 from Claude Mythos 5.1](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#migrating-from-claude-mythos-5-1-to-claude-mythos-5-2) for code on Claude Mythos 5.1. [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) lists what counts as an edit and how to make each change without one.
39Nothing changes for you if Claude Code, claude.ai, [Claude Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview), or the [Claude Agent SDK](https://code.claude.com/docs/en/agent-sdk/overview) builds your requests.
40 
41If your own code builds your requests, nothing changes for you either, as long as all of these are true:
42 
43* Your code keeps `system` and `tools` the same for the whole conversation.
44* Your code only adds to `messages`. It never changes, reorders, or removes an earlier message.
45* Your code [sends each assistant turn back exactly as the API returned it](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#append-assistant-turns-exactly-as-returned): every block, in the same order.
46* Your code does none of the things listed in [What to check in your integration](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#what-to-check).
47 
48Otherwise, test your code before you switch:
49 
50* **Code on Claude Fable 5.1:** Follow the [prefix check section](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-preserved-thinking) of the migration guide.
51* **Code on Claude Mythos 5.1:** Follow [Migrating to Claude Mythos 5.2 from Claude Mythos 5.1](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#migrating-from-claude-mythos-5-1-to-claude-mythos-5-2).
52 
53[Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) explains what counts as an edit. It also shows how to make each change without an edit.
4054 
4155### Only Claude Fable 5.2 and Claude Mythos 5.2 read Claude Fable 5.2 thinking blocks
4256 

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5555 
5656**Claude Opus 5.5** (`claude-opus-5-5`) is built for long-running agentic coding and knowledge work, with [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) always on. If your integration forces tool use (`tool_choice` of type `any` or `tool`), turns thinking off, or uses the `computer_20251124` computer use tool, see [Breaking changes](https://platform.claude.com/docs/en/models/opus-5-5/whats-new-opus-5-5#breaking-changes) before switching.
5757 
58**Claude Fable 5.2** (`claude-fable-5-2`) is Anthropic’s most capable model open to all customers. It’s built for demanding reasoning and long-horizon agentic work. **Claude Mythos 5.2** (`claude-mythos-5-2`) offers the same capabilities only to organizations verified through Anthropic’s verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842). Both models use always-on [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking). If your code edits earlier turns of a conversation, moves a conversation between models, uses the `computer_20251124` computer use tool, or uses the advisor tool, see [Claude Fable 5.2 breaking changes](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#breaking-changes) before switching.
58**Claude Fable 5.2** (`claude-fable-5-2`) is Anthropic’s most capable model open to all customers. It’s built for demanding reasoning and long-horizon agentic work. **Claude Mythos 5.2** (`claude-mythos-5-2`) offers the same capabilities only to organizations verified through Anthropic’s verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842). Both models use always-on [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking). If your code changes `system`, `tools`, or earlier messages in a conversation, moves a conversation between models, uses the `computer_20251124` computer use tool, or uses the advisor tool, see [Claude Fable 5.2 breaking changes](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#breaking-changes) before switching.
5959 
6060For context windows, output limits, and prices, see the [model comparison table](https://platform.claude.com/docs/en/models/overview#latest-models-comparison).
6161 

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410410 
411411### Per-message effort (beta)
412412 
413Per-message effort is in beta. On the Claude API and [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed), it’s available on Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, Claude Opus 5, Claude Sonnet 5.5, and Claude Haiku 5.5. On the Claude API, it’s also available on Claude Fable 5.2 and Claude Mythos 5.2. On Claude Models on Google Cloud (Google managed), it’s also available on Claude Fable 5.2 and Claude Mythos 5.2. On [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed) and [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), it’s available on Claude Fable 5.2, Claude Fable 5.1, Claude Opus 5.5, Claude Opus 5, Claude Sonnet 5.5, and Claude Haiku 5.5. On [Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), it’s available on Claude Fable 5.2, Claude Mythos 5.2, Claude Fable 5.1, Claude Mythos 5.1, and Claude Opus 5.5. It requires the [beta header](https://platform.claude.com/docs/en/api/beta-headers) `mid-conversation-output-config-2026-07-01`. With the Amazon Bedrock [InvokeModel API](https://platform.claude.com/docs/en/build-with-claude/claude-on-amazon-bedrock-legacy), it’s available on Claude Fable 5.2, Claude Mythos 5.2, Claude Fable 5.1, and Claude Opus 5.5, and you send that value in the `anthropic_beta` array of the request body instead.
413Per-message effort is in beta. On the Claude API and [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed), it’s available on Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, Claude Opus 5, Claude Sonnet 5.5, and Claude Haiku 5.5. On the Claude API, it’s also available on Claude Fable 5.2 and Claude Mythos 5.2. On Claude Models on Google Cloud (Google managed), it’s also available on Claude Fable 5.2 and Claude Mythos 5.2. On [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed), it’s available on Claude Fable 5.2, Claude Mythos 5.2, Claude Fable 5.1, Claude Opus 5.5, Claude Opus 5, Claude Sonnet 5.5, and Claude Haiku 5.5. On [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), it’s available on Claude Fable 5.2, Claude Fable 5.1, Claude Opus 5.5, Claude Opus 5, Claude Sonnet 5.5, and Claude Haiku 5.5. On [Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), it’s available on Claude Fable 5.2, Claude Mythos 5.2, Claude Fable 5.1, Claude Mythos 5.1, and Claude Opus 5.5. It requires the [beta header](https://platform.claude.com/docs/en/api/beta-headers) `mid-conversation-output-config-2026-07-01`. With the Amazon Bedrock [InvokeModel API](https://platform.claude.com/docs/en/build-with-claude/claude-on-amazon-bedrock-legacy), it’s available on Claude Fable 5.2, Claude Mythos 5.2, Claude Fable 5.1, and Claude Opus 5.5, and you send that value in the `anthropic_beta` array of the request body instead.
414414 
415415Without the beta value, a per-message `output_config` returns a 400 error: `messages.N.output_config: Extra inputs are not permitted`, where `N` is the index of the `system` message in `messages`. With the beta value, models without per-message effort, including Claude Fable 5, return a 400 error: `output_config.effort requires a model that supports per-turn effort; this model does not`. On Amazon Bedrock, those models and Claude Opus 5 return the `Extra inputs are not permitted` error instead. On Claude Sonnet 5.5 with `thinking: {"type": "between_tools"}` and on Claude Haiku 5.5 with `thinking: {"type": "disabled"}`, effort can’t change mid-conversation: a per-message `output_config.effort` that differs from the level in effect returns a 400 error. To vary effort for each turn, use adaptive thinking.
416416 

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475475 
476476With the `inline-tools-2026-09-15` beta header, a `tool_addition` block can define a tool by value, carrying its full definition, instead of naming it by reference. This lets you introduce a tool that is unknown at the start of the conversation, or whose schema changes later, by appending a `role: "system"` message. The `tools` array and every earlier message stay exactly as sent, so the prompt cache still hits and only the appended message is processed as new input. The one exception, a `tools` array with no non-deferred tool, is covered in the rules that follow. The header also covers adding and removing tools by reference, so you don’t need to send `mid-conversation-tool-changes-2026-07-01` as well.
477477 
478Wrap the definition in a `tool` object of type `tool_definition`. The `definition` is a `tools` entry, such as a custom tool or an Anthropic-defined client or server tool, with its usual configuration, including `cache_control` and `defer_loading`. During the beta, some tool types (the computer use tool among them) can’t be defined in a message yet and return a 400 error that says so; declare those in `tools` and add them by reference. For example, to define a custom tool mid-conversation:
478Wrap the definition in a `tool` object of type `tool_definition`. The `definition` is a `tools` entry, such as a custom tool or an Anthropic-defined client or server tool, with its usual configuration, including `cache_control` and `defer_loading`. During the beta, the computer use and browser use tools can’t be defined in a message and return a 400 error that says so; declare those in `tools` and add them by reference. For example, to define a custom tool mid-conversation:
479479 
480480```json
481481{

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9494 
9595### What the API does with an invalid block
9696 
97The prefix check always runs on these models. Set `thinking.block_binding.prefix_mismatch_behavior` to choose what happens to a block that fails it. The field and the `input_transformations` array require the `thinking-binding-controls-2026-08-01` [beta header](https://platform.claude.com/docs/en/api/beta-headers). [Set the mismatch behavior and read `input_transformations`](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#preserved-thinking-controls) shows the request in each SDK.
97On the Claude API, the prefix check always runs on these models. Set `thinking.block_binding.prefix_mismatch_behavior` to choose what happens to a block that fails it. The field and the `input_transformations` array require the `thinking-binding-controls-2026-08-01` [beta header](https://platform.claude.com/docs/en/api/beta-headers). [Set the mismatch behavior and read `input_transformations`](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#preserved-thinking-controls) shows the request in each SDK.
9898 
9999* **`"error"`:** The API rejects the request with a 400 `invalid_request_error` that names the first failing block.
100100* **`"drop_block"`:** The API drops each failing block and every thinking block after it, and the request succeeds. Dropped blocks aren’t billed. The model answers that turn without using reasoning from dropped blocks, and the prompt cache restarts at the edit. The response lists each dropped block in `input_transformations` (on the `message_start` event when streaming) with `reason: "prefix_binding_mismatch"`. It’s a stopgap with an [output-token cost](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#drop-block-cost).

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543543 
544544Claude’s latest models offer thinking capabilities that can be especially helpful for tasks involving reflection after tool use or complex multistep reasoning. You can guide its initial or interleaved thinking for better results.
545545 
546Claude 4.6 and later models and Claude Mythos Preview use [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) (`thinking: {type: "adaptive"}`), where Claude dynamically determines when and how much to think. On Claude Fable 5.2, Claude Mythos 5.2, Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, and Claude Opus 5.5, thinking is always on and adaptive thinking is the only mode. Claude calibrates its thinking based on two factors: the `effort` parameter and query complexity. Higher effort elicits more thinking, and more complex queries do the same. On easier queries that don’t require thinking, the model responds directly. In internal evaluations, adaptive thinking reliably drives better performance than extended thinking. Consider moving to adaptive thinking.
546Claude 4.6 and later models and Claude Mythos Preview use [adaptive thinking](https://platform.claude.com/docs/en/build-with-claude/thinking) (`thinking: {type: "adaptive"}`), where Claude dynamically determines when and how much to think. On Claude Fable 5.2, Claude Mythos 5.2, Claude Fable 5.1, Claude Mythos 5.1, Claude Fable 5, Claude Mythos 5, and Claude Opus 5.5, thinking is always on and adaptive thinking is the only mode. Claude calibrates its thinking based on two factors: the `effort` parameter and query complexity. Higher effort elicits more thinking, and more complex queries do the same. On easier queries that don’t require thinking, the model responds directly. Claude Fable 5.2 and Claude Mythos 5.2 are exceptions: at `high` effort and above, they think before almost every reply. In internal evaluations, adaptive thinking reliably drives better performance than extended thinking. Consider moving to adaptive thinking.
547547 
548548Use adaptive thinking for workloads that require agentic behavior such as multistep tool use, complex coding tasks, and long-horizon agent loops. Older models use manual [extended thinking](https://platform.claude.com/docs/en/build-with-claude/extended-thinking) with `budget_tokens`; see the [per-model configuration table](https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#supported-models) for which configuration each model accepts.
549549 

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163163 
164164* **Standing instructions to delegate:** Lines that push work to subagents can sit in system prompts, skills, and agent files. Read those files before you migrate. If, in your tests, the model starts more subagents than your tasks need, remove the lines you no longer need. See [Set expectations for subagents](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2#set-expectations-for-subagents).
165165* **A token budget that your harness writes into the prompt:** Some harnesses tell the model in the prompt how many tokens the task may use and how many are left. The size of that budget changes how much Claude Fable 5.2 spends. In Anthropic’s testing on coding tasks at `high` and `xhigh` effort, removing a large budget lowered cost, with equivalent performance. Test your tasks without the budget. This finding is about text in the prompt, not the API’s [task budgets](https://platform.claude.com/docs/en/build-with-claude/task-budgets), a beta feature that the testing didn’t cover.
166* **Harness code that edits earlier turns:** If your harness changes the system prompt, the tools, or an earlier message between requests, the thinking blocks that follow the changed point in the history become invalid. By default, the Messages API then rejects the request, and the Message Batches API drops those blocks. To have the Messages API drop them too, set `thinking.block_binding.prefix_mismatch_behavior` to `"drop_block"` (beta, `thinking-binding-controls-2026-08-01` header). In Anthropic’s testing, when earlier thinking was dropped on every turn, the model thought again and used more output tokens. That is the worst case, and the increase depends on how often your harness edits earlier turns. Keep the history append-only, so that no thinking is dropped, and measure output tokens on your own tasks before you rely on `"drop_block"`. See [What the API does with an invalid block](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#mismatch-behavior) and [Make changes without editing the prefix](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#replace-prefix-edits).
166* **Harness code that edits earlier turns:** Claude Fable 5.2 uses [preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking). If your harness changes the system prompt, the tools, or an earlier message between requests, the thinking blocks that follow the changed point in the history become invalid. By default, the Messages API then rejects the request, and the Message Batches API drops those blocks. To shorten the history without this error, use [context editing](https://platform.claude.com/docs/en/build-with-claude/context-editing), or [compaction](https://platform.claude.com/docs/en/build-with-claude/compaction) that keeps no earlier turns after the summary. To have the Messages API drop the blocks too, set `thinking.block_binding.prefix_mismatch_behavior` to `"drop_block"` (beta, `thinking-binding-controls-2026-08-01` header). In Anthropic’s testing, when earlier thinking was dropped on every turn, the model thought again and used more output tokens. That is the worst case, and the increase depends on how often your harness edits earlier turns. Apart from compaction and context editing, keep the history append-only. That keeps the cache hit rate high, and no thinking is dropped. Measure output tokens on your own tasks before you rely on `"drop_block"`. See [What the API does with an invalid block](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#mismatch-behavior) and [Make changes without editing the prefix](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#replace-prefix-edits).
167167* **The “treat that answer as done” lines:** [Prompting Claude Opus 5.5](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5-5#thinking-instructions-in-chat-system-prompts) offers two sentences for chat system prompts. They tell the model to treat an earlier answer as done and to focus its thinking on the new message. In Anthropic’s testing at `high` effort, these sentences reduced Claude Fable 5.2’s thinking on follow-up questions, but the model also became far less likely to point out mistakes in its earlier answers. Leave these lines out where you want the model to keep re-examining its earlier answers. Elsewhere, test whether the model still points out its own mistakes before you adopt the lines.
168168* **Instructions to think less:** Claude Fable 5.1 usually follows a line such as “answer directly without deliberating.” In Anthropic’s testing, such a line didn’t stop Claude Fable 5.2 from thinking at `medium` and above. Remove these lines and lower the effort level, as described in [Calibrate effort](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2#calibrate-effort).
169169* **Style rules for writing:** Claude Fable 5.2 follows style rules closely, so a hard limit such as “one instruction per sentence, in 20 words or fewer” can make its prose choppy. Remove the rules you no longer need before you add the text in [Make reports easy to follow](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2#make-reports-easy-to-follow).

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11061106 
11071107## Preserved thinking
11081108 
1109[Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) determines 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:
1109[Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) determines whether the model can use a thinking block that you send back from an earlier turn. The API checks the `signature` of every `thinking` or `redacted_thinking` block in a request for two things:
11101110 
11111111* **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).
11121112* **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 fail the prefix check. See [Keeping the prefix unchanged](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check).

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173173* A rebuilt `system` prompt or `tools` array
174174* Client-side compaction that kept recent turns and their thinking verbatim
175175 
176Server-side [compaction](https://platform.claude.com/docs/en/build-with-claude/compaction) and [context editing](https://platform.claude.com/docs/en/build-with-claude/context-editing) never trigger this error. For requests that don’t set `thinking.block_binding.prefix_mismatch_behavior`, see [When `prefix_mismatch_behavior` is unset](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement).
176Server-side [compaction at a token threshold](https://platform.claude.com/docs/en/build-with-claude/compaction-threshold) and [context editing](https://platform.claude.com/docs/en/build-with-claude/context-editing) never trigger this error. With [on-demand compaction](https://platform.claude.com/docs/en/build-with-claude/compaction-on-demand), thinking in the turns you keep after the summary triggers it when one of the [conditions for kept thinking](https://platform.claude.com/docs/en/build-with-claude/compaction-thinking-blocks#conditions-for-kept-thinking-to-stay-valid) doesn’t hold. For requests that don’t set `thinking.block_binding.prefix_mismatch_behavior`, see [When `prefix_mismatch_behavior` is unset](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement).
177177 
178178Retrying the same request body doesn’t clear the error. Neither does fixing your code, because the stored history holds the edit. To continue the conversation, do one of the following:
179179 
Feedback