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Delivering work changedbuild-with-claude/prompt-engineering/prompting-claude-fable-5-1

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751751Append each assistant turn to the history exactly as the API returned it, thinking blocks included, and don’t edit earlier turns between requests. A thinking block is valid [only while everything before it is unchanged](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation): the system prompt, the tool list, and every earlier message. To choose what happens to a block that fails the prefix check, set `thinking.block_binding.prefix_mismatch_behavior`, which requires the `thinking-binding-controls-2026-08-01` beta header. With `"error"`, the API rejects the request with a 400 error; with `"drop_block"`, it drops the invalid blocks. [When `prefix_mismatch_behavior` is unset](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement) covers requests that don’t set it. On Claude Fable 5.2 and Claude Mythos 5.2, a request that leaves it unset and sends back a block that fails the check gets the 400 error on every account (see [Editing earlier turns invalidates thinking blocks on every account](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#editing-earlier-turns-invalidates-thinking-blocks) in What’s new in Claude Fable 5.2).
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753Most history edits that trip the prefix check also restart the [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching): injecting and removing per-turn reminders, summarizing older turns in place, or changing the system prompt mid-session. Send per-turn reminders as [turn-scoped system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages), change instructions or tools with a [mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) instead of rewriting `system` or `tools`, and let server-side [compaction](https://platform.claude.com/docs/en/build-with-claude/compaction) or [context editing](https://platform.claude.com/docs/en/build-with-claude/context-editing) do any trimming. If you compact on the client, the simplest shape is to replace the whole history with one summary message plus the new user turn and replay nothing else. No thinking blocks carry over, so nothing fails, and the model thinks afresh on the compacted conversation (see [Compact on the client](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#custom-compaction-on-the-client)). Cache reads on Claude Fable 5.1 cost $0.25 USD per million tokens, 2.5 percent of the input price (see [Prompt caching pricing](https://platform.claude.com/docs/en/about-claude/pricing#prompt-caching)). Compacting early to save cost can be the wrong trade-off between cost and intelligence, so experiment with later compaction points.
753Most history edits that trip the prefix check also restart the [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching): injecting and removing per-turn reminders, summarizing older turns in place, or changing the system prompt mid-session. Send per-turn reminders as [turn-scoped system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages). To change instructions or tools, send a [mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) instead of rewriting `system` or `tools`. Let server-side [compaction](https://platform.claude.com/docs/en/build-with-claude/compaction) or [context editing](https://platform.claude.com/docs/en/build-with-claude/context-editing) do any trimming. If you compact on the client, the simplest approach is to replace the whole history with one summary message plus the new user turn, and nothing else. No thinking blocks carry over, so nothing fails, and the model thinks afresh on the compacted conversation (see [Compact on the client](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#custom-compaction-on-the-client)). Cache reads on Claude Fable 5.1 cost $0.25 USD per million tokens, 2.5 percent of the input price (see [Prompt caching pricing](https://platform.claude.com/docs/en/about-claude/pricing#prompt-caching)). Compacting early to save cost can be the wrong trade-off between cost and intelligence, so experiment with later compaction points.
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755755To find edits your harness makes, run a session with `prefix_mismatch_behavior: "drop_block"` and log `input_transformations`, as described 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). You can also capture the exact requests it sends over a few normal turns and confirm that consecutive requests are byte-identical up to the appended turns.
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