whats-new-fable-5-1 changedmodels/fable-5-1/whats-new-fable-5-1
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44description: Overview of new features, breaking changes, and capability improvements in Claude Fable 5.1 and Claude Mythos 5.1.
55---
66
7Claude 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).
7Claude 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).
88
99If you already call Claude Fable 5, four changes are breaking:
1010
1111* [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).
12* 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).
12* 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).
1313* [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).
1414* [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).
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2626| Model | Claude API ID | Description | Availability |
2727| ----------------- | ----------------- | ------------------------------------------------------------------------------------------ | ----------------------------------------------------------------------------------------------------------------------------------------------------------- |
2828| Claude Fable 5.1 | claude-fable-5-1 | Successor to Claude Fable 5, for long-running agentic coding, knowledge work, and research | All customers, on the Claude API and partner platforms |
29| Claude Mythos 5.1 | claude-mythos-5-1 | Same capabilities as Claude Fable 5.1. Successor to Claude Mythos 5. | Organizations verified through Anthropic's verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842) |
29| Claude Mythos 5.1 | claude-mythos-5-1 | Same capabilities as Claude Fable 5.1. Successor to Claude Mythos 5. | Organizations verified through Anthropic’s verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842) |
3030
3131Claude Fable 5.1 and Claude Mythos 5.1 share specs and pricing:
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4141
4242### Forced tool use is not supported
4343
44Claude Fable 5.1 and Claude Mythos 5.1 don't support forced tool use. `tool_choice` set to `{"type": "any"}` or `{"type": "tool", "name": "..."}` returns a 400 `invalid_request_error`:
44Claude Fable 5.1 and Claude Mythos 5.1 don’t support forced tool use. `tool_choice` set to `{"type": "any"}` or `{"type": "tool", "name": "..."}` returns a 400 `invalid_request_error`:
4545
4646```text wrap
4747tool_choice: type "tool" and "any" are not supported for this model.
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4949
5050`tool_choice: {"type": "auto"}` (the default) and `{"type": "none"}` are unchanged. The same validation applies to the [token counting](https://platform.claude.com/docs/en/build-with-claude/token-counting) endpoint.
5151
52Thinking is always on for these models, and a forced tool call would skip it. The model would write its working-out into the tool arguments instead, which lowers argument quality. For schema-valid JSON, keep `tool_choice: {"type": "auto"}` and set `strict: true` with [strict tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use), or move the schema to [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs). To make the model call a tool rather than reply in text, state in the prompt when the tool applies (for example, "Use the `get_weather` tool to answer"). Claude Fable 5.1 follows explicit tool instructions reliably.
52Thinking is always on for these models, and a forced tool call would skip it. The model would write its working-out into the tool arguments instead, which lowers argument quality. For schema-valid JSON, keep `tool_choice: {"type": "auto"}` and set `strict: true` with [strict tool use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/strict-tool-use), or move the schema to [structured outputs](https://platform.claude.com/docs/en/build-with-claude/structured-outputs). To make the model call a tool rather than reply in text, state in the prompt when the tool applies (for example, “Use the `get_weather` tool to answer”). Claude Fable 5.1 follows explicit tool instructions reliably.
5353
5454### Only Claude Fable 5.1 and Claude Mythos 5.1 read Claude Fable 5.1 thinking blocks
5555
56Every thinking block records which model produced it. Claude Fable 5.1 reads earlier models' thinking blocks, but only Claude Fable 5.1 and Claude Mythos 5.1 read 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 other model loses it for the turns that run there.
56Every thinking block records which model produced it. Claude Fable 5.1 reads earlier models’ thinking blocks, but only Claude Fable 5.1 and Claude Mythos 5.1 read 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 other model loses it for the turns that run there.
5757
58When 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 receives 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).
58When 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 receives 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).
5959
6060### Editing earlier turns invalidates thinking blocks
6161
62Modifying anything before a Claude Fable 5.1 thinking block (the `system` prompt, the `tools`, or an earlier message) results in an error on the next request, or in the block being dropped if you opt into that. Claude Mythos 5.1 doesn't run this check. Claude Code, claude.ai, [Claude Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview), and the [Claude Agent SDK](https://code.claude.com/docs/en/agent-sdk/overview) keep that prefix intact for you. If your code builds the `messages` array itself, check it before you migrate: [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) walks through the check and each fix. The check is enforced for new accounts created on or after August 31, 2026. For accounts created earlier, the API records the mismatch but acts on it only when the request sets `thinking.block_binding.prefix_mismatch_behavior`.
62Modifying anything before a Claude Fable 5.1 thinking block (the `system` prompt, the `tools`, or an earlier message) results in an error on the next request, or in the block being dropped if you opt into that. Claude Mythos 5.1 doesn’t run this check. Claude Code, claude.ai, [Claude Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview), and the [Claude Agent SDK](https://code.claude.com/docs/en/agent-sdk/overview) keep that prefix intact for you. If your code builds the `messages` array itself, check it before you migrate: [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) walks through the check and each fix. The check is enforced for new accounts created on or after August 31, 2026. For accounts created earlier, the API records the mismatch but acts on it only when the request sets `thinking.block_binding.prefix_mismatch_behavior`.
6363
6464These patterns invalidate every later thinking block:
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7373Where 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"`.
7474
75To 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.
75To 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.
7676
7777### Some advisor tool pairings are not supported
7878
79With the [advisor tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool) (beta), the advisor must be at least as capable as the executor. Claude Mythos 5, Claude Fable 5, Claude Opus 5.5, and Claude Opus 5 advisors work with a Claude Fable 5 or Claude Mythos 5 executor. With a Claude Fable 5.1 or Claude Mythos 5.1 executor, they return a 400 `invalid_request_error`. See the advisor tool's [Model compatibility](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool#model-compatibility).
79With the [advisor tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool) (beta), the advisor must be at least as capable as the executor. Claude Mythos 5, Claude Fable 5, Claude Opus 5.5, and Claude Opus 5 advisors work with a Claude Fable 5 or Claude Mythos 5 executor. With a Claude Fable 5.1 or Claude Mythos 5.1 executor, they return a 400 `invalid_request_error`. See the advisor tool’s [Model compatibility](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool#model-compatibility).
8080
8181## New features
8282
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354354
355355### Turn-scoped system messages (beta)
356356
357A [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.
357A [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.
358358
359359```json
360360{
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366366
367367### Progress updates between tool calls (beta)
368368
369Like Claude Fable 5, Claude Fable 5.1 writes short progress updates between tool calls on what it found and what it will do next, though fewer of them (see [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)). Each update arrives as its own `thinking` block immediately before the tool call. Under the default `thinking.display` of `"omitted"` those blocks come back empty, like reasoning, so a long agentic turn can look silent to your users. What's new is the `display: "updates"` option: set it with the `thinking-display-updates-2026-08-18` beta header to receive the progress updates as text while reasoning stays hidden. Any `thinking` block with non-empty text is then a status line you can show the user. `"summarized"` returns them too, mixed with summarized reasoning. See [Progress updates between tool calls](https://platform.claude.com/docs/en/build-with-claude/thinking#progress-updates).
369Like Claude Fable 5, Claude Fable 5.1 writes short progress updates between tool calls on what it found and what it will do next, though fewer of them (see [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)). Each update arrives as its own `thinking` block immediately before the tool call. Under the default `thinking.display` of `"omitted"` those blocks come back empty, like reasoning, so a long agentic turn can look silent to your users. What’s new is the `display: "updates"` option: set it with the `thinking-display-updates-2026-08-18` beta header to receive the progress updates as text while reasoning stays hidden. Any `thinking` block with non-empty text is then a status line you can show the user. `"summarized"` returns them too, mixed with summarized reasoning. See [Progress updates between tool calls](https://platform.claude.com/docs/en/build-with-claude/thinking#progress-updates).
370370
371371### Content provenance
372372
373Text generated by Claude Fable 5.1 and Claude Mythos 5.1 carries Anthropic's statistical text watermark on every platform where the model is available. Supported image, video, and audio files Claude produces (through the [code execution tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/code-execution-tool), for example) carry signed [C2PA](https://c2pa.org/) Content Credentials when you retrieve them through the [Files API](https://platform.claude.com/docs/en/build-with-claude/files) on the Claude API.
373Text generated by Claude Fable 5.1 and Claude Mythos 5.1 carries Anthropic’s statistical text watermark on every platform where the model is available. Supported image, video, and audio files Claude produces (through the [code execution tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/code-execution-tool), for example) carry signed [C2PA](https://c2pa.org/) Content Credentials when you retrieve them through the [Files API](https://platform.claude.com/docs/en/build-with-claude/files) on the Claude API.
374374
375The watermark doesn't change the meaning, quality, or readability of the output. It adds no tokens or hidden characters, carries no information about you or your organization, and needs no changes to your requests or responses. For background, see [How Claude marks AI-generated content](https://support.claude.com/en/articles/16266773-how-claude-marks-ai-generated-content) and [How Claude's text watermark works](https://www.anthropic.com/news/claude-text-watermark).
375The watermark doesn’t change the meaning, quality, or readability of the output. It adds no tokens or hidden characters, carries no information about you or your organization, and needs no changes to your requests or responses. For background, see [How Claude marks AI-generated content](https://support.claude.com/en/articles/16266773-how-claude-marks-ai-generated-content) and [How Claude’s text watermark works](https://www.anthropic.com/news/claude-text-watermark).
376376
377377## Behavior differences
378378
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380380
381381Claude Fable 5.1 differs from Claude Fable 5 in several ways that show up without any code change. Each has a prompting fix in [Prompting Claude Fable 5.1](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1):
382382
383* **Parallel tool calling is more variable.** Claude Fable 5.1 might issue one tool call in each turn where Claude Fable 5 batched several. This shows up in long agent loops where the next independent reads are only implied: custom coding agents, bash-and-editor harnesses, computer use. The extra turns cost tokens, round trips, and wall-clock time but don't reduce answer quality. Requests naming several things to fetch still run in parallel. Add the one-line batching instruction from [Batch independent tool calls in agent loops](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#batch-independent-tool-calls-in-agent-loops).
383* **Parallel tool calling is more variable.** Claude Fable 5.1 might issue one tool call in each turn where Claude Fable 5 batched several. This shows up in long agent loops where the next independent reads are only implied: custom coding agents, bash-and-editor harnesses, computer use. The extra turns cost tokens, round trips, and wall-clock time but don’t reduce answer quality. Requests naming several things to fetch still run in parallel. Add the one-line batching instruction from [Batch independent tool calls in agent loops](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#batch-independent-tool-calls-in-agent-loops).
384384* **Fewer progress updates during long tool runs.** The model writes less user-facing text between tool calls, especially at higher effort. Set `thinking.display` to `"updates"` (beta) to receive the [progress updates](https://platform.claude.com/docs/en/build-with-claude/thinking#progress-updates) it does write, and remove any prompt line that tells it to hold findings for the final response. If your UI depends on narration, ask explicitly for an opening line, periodic updates, and a closing recap. See [Ask for user-facing progress updates](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#ask-for-user-facing-progress-updates).
385385* **Answers from memory more often at `low` effort.** At the lowest effort level the model calls a search or retrieval tool less often. Raise effort for turns that need fresh information, including [mid-conversation](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1#change-effort-mid-conversation-beta), or add the verification nudge from [Search triggering at low effort](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#search-triggering-at-low-effort).
386* **Denser prose in places.** In some cases its prose is denser than Claude Fable 5's, with longer sentences and fewer paragraph breaks. See [Writing density](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#writing-density).
386* **Denser prose in places.** In some cases its prose is denser than Claude Fable 5’s, with longer sentences and fewer paragraph breaks. See [Writing density](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#writing-density).
387387* **Less formatting in chat.** The model uses bold, headers, and lists less than earlier Claude models, so anti-formatting rules written for those models can suppress structure the content needs. See [Formatting in chat](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#formatting-in-chat).
388388* **Unmarked quotations in summaries.** When summarizing documents, the model is more likely to reproduce passages of the source without marking them as quotations. See [Quoting retrieved sources](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#quoting-retrieved-sources).
389389* **Whole-file rewrites for small changes.** When editing text files, the model is more likely to rewrite the entire file than make a targeted edit. The result is usually the same, but the rewrite costs more output tokens and time. See [Prefer targeted edits over whole-file rewrites](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1#prefer-targeted-edits-over-whole-file-rewrites).
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411411* **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)
412412* **Computer use**, operating a browser and desktop applications more reliably and recovering from failed steps
413413
414Multilingual performance is at the same level as Claude Fable 5's.
414Multilingual performance is at the same level as Claude Fable 5’s.
415415
416416## Refusals, fallback, and billing
417417
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419419
420420* **Refusals:** a declined request returns HTTP 200 with `stop_reason: "refusal"` and a [`stop_details`](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#refusal-response) object naming the policy area that fired.
421421* **Fallback:** retry a refused request on another model with [server-side fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#server-side-fallback), the [SDK middleware](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#client-side-fallback), or your own retry. `fallbacks: "default"` (beta) retries a declined request on the model Anthropic recommends for that category. The permitted fallback targets for Claude Fable 5.1 are Claude Opus 4.8 and Claude Opus 5.
422* **Billing:** a refusal that arrives before any output is billed when it's in a category with low volumes of false positives, to disrupt attempts to circumvent Anthropic's safeguards at scale ([How refusals are billed](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#how-refusals-are-billed)). Before September 24, 2026, these refusals weren't billed. A mid-stream refusal bills the input tokens and the output already streamed at normal rates. For Claude Fable 5.1, [fallback credit](https://platform.claude.com/docs/en/build-with-claude/fallback-credit) refunds the prompt-cache cost of switching models.
422* **Billing:** a refusal that arrives before any output is billed when it’s in a category with low volumes of false positives, to disrupt attempts to circumvent Anthropic’s safeguards at scale ([How refusals are billed](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#how-refusals-are-billed)). Before September 24, 2026, these refusals weren’t billed. A mid-stream refusal bills the input tokens and the output already streamed at normal rates. For Claude Fable 5.1, [fallback credit](https://platform.claude.com/docs/en/build-with-claude/fallback-credit) refunds the prompt-cache cost of switching models.
423423
424424## Pricing
425425
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442442* **Google Cloud:** [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed), as `claude-fable-5-1`, and [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed), as `claude-fable-5-1`
443443* **Microsoft Foundry:** [Claude in Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry), on Anthropic infrastructure
444444
445Claude Mythos 5.1 is available only to organizations verified through Anthropic's verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842). To request access, apply to the program that covers your use case, or contact your Anthropic, AWS, or Google Cloud account team.
445Claude Mythos 5.1 is available only to organizations verified through Anthropic’s verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842). To request access, apply to the program that covers your use case, or contact your Anthropic, AWS, or Google Cloud account team.
446446
447Claude Fable 5.1 and Claude Mythos 5.1 carry 30-day data retention and aren't available under zero data retention unless expressly authorized by Anthropic. Both are [Covered Models](https://support.claude.com/en/articles/15425695), like Claude Fable 5 and Claude Mythos 5. See [Model-specific data retention requirements](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements).
447Claude Fable 5.1 and Claude Mythos 5.1 carry 30-day data retention and aren’t available under zero data retention unless expressly authorized by Anthropic. Both are [Covered Models](https://support.claude.com/en/articles/15425695), like Claude Fable 5 and Claude Mythos 5. See [Model-specific data retention requirements](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements).
448448
449449## Migrate from Claude Fable 5
450450
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