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whats-new-fable-5-2 changedmodels/fable-5-2/whats-new-fable-5-2

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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:
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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).
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48Otherwise, test your code before you switch:
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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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