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4343Every thinking block records which model produced it, and it's preserved in one direction only: Claude Fable 5.1 reads earlier models' thinking blocks, and no earlier model reads Claude Fable 5.1's. A conversation that moves onto Claude Fable 5.1 (from Claude Opus 5, Claude Fable 5, or any earlier Claude model) keeps its reasoning. A conversation that moves from Claude Fable 5.1 to any of those models loses it for the turns that run there.
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45When a request carries a block the target model can't read (a router or fallback that switches models mid-conversation, for example), the API drops the block before the model sees it. Dropped blocks don't count toward `input_tokens` and aren't billed. With the `thinking-binding-controls-2026-08-01` beta header, the drop is reported in a top-level `input_transformations` array. Without it, the drop is silent. See [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-for-model).
45When a request carries a block the target model can't read (a router or fallback that switches models mid-conversation, for example), the API drops the block before the model sees it. Dropped blocks don't count toward `input_tokens` and aren't billed. With the `thinking-binding-controls-2026-08-01` beta header, the drop is reported in a top-level `input_transformations` array. Without it, the drop is silent. See [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models).
4646
4747### Editing earlier turns invalidates thinking blocks
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6060Where the check is enforced, a request that replays an invalidated block is rejected with a 400 whose message says `The block is bound to a different conversation`. To drop the block and continue instead, send the `thinking-binding-controls-2026-08-01` beta header with `thinking.block_binding.prefix_mismatch_behavior: "drop_block"`. The drop is reported in `input_transformations` with `reason: "prefix_binding_mismatch"`.
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62To keep thinking valid across a long session, treat the conversation as append-only. Add instructions with a [mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) ([turn-scoped](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#turn-scoped-system-messages-beta) if it should apply to one turn only) and change tools with [mid-conversation tool changes](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#mid-conversation-tool-changes) rather than editing `system` or `tools`. Trim context with server-side [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), which don't count as edits. These patterns also keep the [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) warm. To find out whether your integration edits history, run a session with `prefix_mismatch_behavior: "drop_block"` and log `input_transformations`: the [migration guide](https://platform.claude.com/docs/en/models/fable-5-1/migration-guide#fable-5-1-preserved-thinking) has the three-step check. See [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation) for the full rules.
62To keep thinking valid across a long session, treat the conversation as append-only. Add instructions with a [mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) ([turn-scoped](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#turn-scoped-system-messages-beta) if it should apply to one turn only) and change tools with [mid-conversation tool changes](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#mid-conversation-tool-changes) rather than editing `system` or `tools`. Trim context with server-side [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), which don't count as edits. These patterns also keep the [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) warm. To find out whether your integration edits history, run a session with `prefix_mismatch_behavior: "drop_block"` and log `input_transformations`: the [migration guide](https://platform.claude.com/docs/en/models/fable-5-1/migration-guide#fable-5-1-preserved-thinking) has the three-step check. See [Keeping the prefix unchanged](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check) for the full rules.
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6464## New features
6565