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44description: Behavioral differences and prompting patterns for Claude Fable 5.1 and Claude Mythos 5.1, covering effort, progress updates, tool-call batching, conversation history, writing style, formatting, task completion, compaction summaries, scope and test coverage, search triggering, safeguard false positives, file edits, long outputs, subagents, and vision.
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7For the model’s capabilities, API changes, pricing, and availability, see [What’s new in Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1). For techniques that apply across Claude models, see [Prompting best practices](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/claude-prompting-best-practices).
7For the model’s specifications, pricing, and availability, see [Claude Fable 5.1](https://platform.claude.com/docs/en/models/fable-5-1/overview). For techniques that apply across Claude models, see [Prompting best practices](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/claude-prompting-best-practices).
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99Your existing Claude Fable 5 prompts should perform well on Claude Fable 5.1 without changes, but a handful of behavioral differences are worth knowing about. Start with the section that matches what you observe:
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2626* Answers about charts and dense images miss detail: [Give vision work tools to crop and zoom](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#give-vision-work-tools-to-crop-and-zoom)
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2828<Note>
29 Claude Fable 5.1 runs safety classifiers and can return `stop_reason: "refusal"`. See [Refusals, fallback, and billing](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#refusals-fallback-and-billing) and [Reduce safeguard false positives](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#reduce-safeguard-false-positives).
29 Claude Fable 5.1 runs safety classifiers and can return `stop_reason: "refusal"`. See [Refusals and fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback) and [Reduce safeguard false positives](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#reduce-safeguard-false-positives).
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3131 Prompts, skills, and tool descriptions that ask the model to write its thinking or reasoning might be declined with the `reasoning_extraction` category. Ask for a short explanation or a summary of the actions taken instead, or read the reasoning from [summarized thinking](https://platform.claude.com/docs/en/build-with-claude/thinking#summarized-thinking) blocks (`display: "summarized"`). See [Keep reasoning in thinking blocks](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#keep-reasoning-in-thinking-blocks).
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749749## Keep the conversation history append-only
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751Append 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.
751Append 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 [Pricing](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#pricing)). 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), 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.
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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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852852## Reduce safeguard false positives
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854Claude Fable 5.1’s safety classifiers produce fewer false positives than Claude Fable 5’s did at launch, and finding vulnerabilities in source code is permitted. False positives still occur, and a blocked request returns `stop_reason: "refusal"` (see [Refusals, fallback, and billing](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#refusals-fallback-and-billing)). Three situations make them more likely:
854Claude Fable 5.1’s safety classifiers produce fewer false positives than Claude Fable 5’s did at launch, and finding vulnerabilities in source code is permitted. False positives still occur, and a blocked request returns `stop_reason: "refusal"` (see [Refusals and fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback)). Three situations make them more likely:
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856856* **Compile-check phrasing:** Instead of “Does this program compile without errors?”, ask “Are there any bugs in this program?”
857857* **Lesser-known programming languages:** Give the model context about what the language is and how it works, for example by giving it access to the language’s documentation.
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