whats-new-fable-5-1 changedmodels/fable-5-1/whats-new-fable-5-1
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77Claude Fable 5.1 extends Claude Fable 5 at the same input and output prices, with cache reads at a quarter of the cost, and brings stronger long-running agentic coding, multistep research, and document, spreadsheet, and slide work. For most workloads, start with Claude Opus 5.5 (see [Choosing a model](https://platform.claude.com/docs/en/about-claude/models/choosing-a-model)). Use Claude Fable 5.1 for demanding reasoning and long-horizon agentic work, or when your evals on Claude Opus 5.5 at higher effort still don't meet your requirements. Claude Mythos 5.1 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).
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9If you already call Claude Fable 5, four changes are breaking: [forced tool use returns an error](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#forced-tool-use-is-not-supported), [only Claude Fable 5.1 and Claude Mythos 5.1 can read Claude Fable 5.1's thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#thinking-blocks-are-tied-to-the-model-that-produced-them), [editing earlier turns invalidates thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#editing-earlier-turns-invalidates-thinking-blocks), and [some advisor tool pairings are not supported](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#advisor-tool-pairings). Five are additive: [per-message effort](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#change-effort-mid-conversation-beta) (beta), [turn-scoped system messages](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#turn-scoped-system-messages-beta) (beta), [readable progress updates between tool calls](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#progress-updates-between-tool-calls-beta) (`display: "updates"`, beta), a [lower cache read price](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#pricing), and [content provenance](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#content-provenance).
9If you already call Claude Fable 5, four changes are breaking: [forced tool use returns an error](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#forced-tool-use-is-not-supported), only Claude Fable 5.1 and Claude Mythos 5.1 can [read Claude Fable 5.1's thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#thinking-blocks-are-tied-to-the-model-that-produced-them), [editing earlier turns invalidates thinking blocks](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#editing-earlier-turns-invalidates-thinking-blocks), and [some advisor tool pairings are not supported](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#advisor-tool-pairings). Five are additive: [mid-conversation effort changes](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#change-effort-mid-conversation-beta) (beta), [turn-scoped system messages](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#turn-scoped-system-messages-beta) (beta), [readable progress updates between tool calls](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#progress-updates-between-tool-calls-beta) (`display: "updates"`, beta), a [lower cache read price](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#pricing), and [content provenance](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#content-provenance).
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1111## Models
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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 [Keeping the prefix unchanged](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check) 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) (a [turn-scoped system message](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 has the three-step [history-editing check](https://platform.claude.com/docs/en/models/fable-5-1/migration-guide#fable-5-1-preserved-thinking). 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### Some advisor tool pairings are not supported
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343343### Turn-scoped system messages (beta)
344344
345A [mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) can be scoped to one turn. Set `clear_at: "next_user_message"` on a `role: "system"` message and its text carries system-prompt authority for the current turn, then stops rendering once a later `user` message exists. The message stays in `messages` and you keep sending it back verbatim, so nothing earlier in the conversation changes. The [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) keeps matching, later [thinking blocks stay valid](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation), and a cleared message costs no input tokens. Use it for per-turn reminders in a tool loop ("check your inbox before running more code," "the user can't see that tool output") instead of injecting text into the history and deleting it on the next request. Turn-scoped system messages are in beta: include the `mid-conversation-system-clear-at-2026-08-21` beta header. See [Turn-scoped system messages](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages).
345A [mid-conversation system message](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) can be scoped to one turn. Set `clear_at: "next_user_message"` on a `role: "system"` message and its text carries system-prompt authority for the current turn, then stops rendering once a later `user` message exists. The message stays in `messages` and you keep sending it back verbatim, so nothing earlier in the conversation changes. The [prompt cache](https://platform.claude.com/docs/en/build-with-claude/prompt-caching) keeps matching, later [thinking blocks stay valid](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-in-conversation), and a cleared message costs no input tokens. Use it for per-turn reminders in a tool loop ("check your inbox before running more code," "the user can't see that tool output") instead of injecting text into the history and deleting it on the next request. Turn-scoped system messages are in beta: include the `mid-conversation-system-clear-at-2026-08-21` beta header. See [how turn-scoped system messages work](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#turn-scoped-system-messages) in Mid-conversation system messages and tool changes.
346346
347347```json
348348{
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393393Claude Fable 5.1 improves on Claude Fable 5, and the gap is widest at higher [effort](https://platform.claude.com/docs/en/build-with-claude/effort) levels. The gains concentrate in six areas:
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395* **Agentic coding over long sessions**, including multifile features, large refactors and migrations, debugging, and code review across sessions that run for hours.
396* **Knowledge work with documents, spreadsheets, and slides**, taking an analysis from a first question to a finished document, live-formula spreadsheet, or slide deck built from a blank page.
397* **Research and search**, with higher accuracy on multistep web research and deep-research tasks that follow up on what they find.
398* **Vision**, reading dense charts, filings, and tables nested in PDFs, including with crop-and-zoom tools on charts.
399* **Long-context work**, reasoning over and connecting details across the full [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows).
400* **Computer use**, operating a browser and desktop applications more reliably and recovering from failed steps.
395* **Agentic coding over long sessions**, including multifile features, large refactors and migrations, debugging, and code review across sessions that run for hours
396* **Knowledge work with documents, spreadsheets, and slides**, taking an analysis from a first question to a finished document, live-formula spreadsheet, or slide deck built from a blank page
397* **Research and search**, with higher accuracy on multistep web research and deep-research tasks that follow up on what they find
398* **Vision**, reading dense charts, filings, and tables nested in PDFs, including with crop-and-zoom tools on charts
399* **Long-context work**, reasoning over and connecting details across the full [1M token context window](https://platform.claude.com/docs/en/build-with-claude/context-windows)
400* **Computer use**, operating a browser and desktop applications more reliably and recovering from failed steps
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402402Multilingual performance is comparable to Claude Fable 5.
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4814812. If you use the advisor tool (beta), choose an advisor at least as capable as Claude Fable 5.1. See [Some advisor tool pairings are not supported](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#advisor-tool-pairings).
4824823. Pass thinking blocks back unchanged and keep the history append-only. If your code builds the `messages` array itself, run the [history-editing check](https://platform.claude.com/docs/en/models/fable-5-1/migration-guide#fable-5-1-preserved-thinking): move per-turn reminders you currently inject and delete to [turn-scoped system messages](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#turn-scoped-system-messages-beta), move `system` and `tools` changes to mid-conversation system messages, trim context server-side or strip thinking blocks from turns you carry across a client-side summary, then pick a production [`prefix_mismatch_behavior`](https://platform.claude.com/docs/en/build-with-claude/thinking#preserved-thinking-controls) and monitor `input_transformations`.
4834834. Re-tune effort from the default (`high`), and consider [changing it mid-conversation](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#change-effort-mid-conversation-beta) instead of holding one level for the whole session.
4845. In agent loops, watch for one tool call per turn where Claude Fable 5 batched several, and add the per-turn note from [Prompting Claude Fable 5.1](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1).
4845. In agent loops, watch for one tool call in each turn where Claude Fable 5 batched several, and add the per-turn note from [Prompting Claude Fable 5.1](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1).
4854856. Re-run your evals. Refusal handling, fallback, fallback credit, and the tokenizer carry over unchanged. Cache reads cost less (see [cache read prices](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#pricing)), and default behavior differs in the ways listed under [Changed from Claude Fable 5](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#changed-from-claude-fable-5).
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487487See the [migration guide](https://platform.claude.com/docs/en/models/fable-5-1/migration-guide) for step-by-step instructions, including from Claude Opus 5 and earlier models.
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