whats-new-fable-5-2 changedmodels/fable-5-2/whats-new-fable-5-2
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1111* [The prefix check applies to every account](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#editing-earlier-turns-invalidates-thinking-blocks).
1212* [Only Claude Fable 5.2 and Claude Mythos 5.2 read Claude Fable 5.2 thinking blocks](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#thinking-blocks-are-tied-to-the-model-that-produced-them).
13* The earlier [`computer_20251124` computer use tool returns an error](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#computer-20251124-is-not-supported).
13* The earlier [`computer_20251124` computer use tool returns an error](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#computer-20251124-is-not-supported) on every platform except Amazon Bedrock.
1414* With the advisor tool, a Claude Fable 5.2 executor [needs a Claude Fable 5.2 or Claude Mythos 5.2 advisor](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#advisor-tool-pairs).
1515
1616## Models
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4949
5050### Use the computer use toolset instead of `computer_20251124`
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52Claude Fable 5.1 accepts [computer use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool) both as the `computer_toolset_20260801` toolset and, with the `computer-use-2025-11-24` beta header, as the earlier `computer_20251124` tool. Claude Fable 5.2 and Claude Mythos 5.2 support computer use through the toolset. A request to either model that declares a `computer_20251124` tool returns a 400 `invalid_request_error`.
52Claude Fable 5.1 accepts [computer use](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool) both as the `computer_toolset_20260801` toolset and, with the `computer-use-2025-11-24` beta header, as the earlier `computer_20251124` tool. Claude Fable 5.2 and Claude Mythos 5.2 support computer use through the toolset. A request to either model that declares a `computer_20251124` tool returns a 400 `invalid_request_error` on every platform except Amazon Bedrock, where the earlier tool works as it does on Claude Fable 5.1.
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5454To move an existing integration, follow [Migrate from `computer_20251124`](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#migrate-from-computer-20251124).
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7373* **Effort:** all five [effort levels](https://platform.claude.com/docs/en/build-with-claude/effort#effort-levels): `low`, `medium`, `high`, `xhigh`, and `max`. The API default is `high`. Start there, because most of the gain in capability is already present at `high`. Step up to `xhigh` for your hardest long-running coding and agentic work, where you’ve measured a gain. Reserve `max` for measuring the model’s ceiling and for the rare task where time and tokens don’t matter. See [Recommended effort levels for Claude Fable 5.2](https://platform.claude.com/docs/en/build-with-claude/effort#recommended-effort-levels-for-claude-fable-5-2).
7474* **Compaction on demand (beta):** with the `compact-2026-09-04` beta header, you choose when to replace older turns with a summary. See [Compaction on demand](https://platform.claude.com/docs/en/build-with-claude/compaction-on-demand).
7575* **Mid-conversation tool changes (beta):** you add or remove tools in a `role: "system"` message instead of editing `tools`. On the Claude API, Claude Platform on AWS, and Claude Models on Google Cloud (Anthropic managed), send the `inline-tools-2026-09-15` beta header. The `mid-conversation-tool-changes-2026-07-01` header works for changes that name a tool by reference, on the platforms that [Mid-conversation system messages and tool changes](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages) lists. See [Mid-conversation tool changes](https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages#mid-conversation-tool-changes).
76* **Computer use:** through the `computer_toolset_20260801` toolset. The earlier `computer_20251124` tool returns a 400 error (see the [computer use breaking change](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#computer-20251124-is-not-supported)). See [Computer use tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool).
76* **Computer use:** through the `computer_toolset_20260801` toolset. The earlier `computer_20251124` tool returns a 400 error on every platform except Amazon Bedrock, where both models also accept the earlier tool (see the [computer use breaking change](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#computer-20251124-is-not-supported)). See [Computer use tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool).
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7878Compaction on demand and mid-conversation tool changes help you keep the history append-only, which the [prefix check](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#editing-earlier-turns-invalidates-thinking-blocks) requires. See each feature’s page for model and platform availability.
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8484Fast mode is also available for Claude Fable 5.2 on [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed).
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86Fast mode is also available for Claude Fable 5.2 on [Claude in Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry) for deployments hosted on Anthropic, as a separate model, `claude-fable-5-2-fast`.
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8688Fast mode is also available for Claude Mythos 5.2 in Messages API requests on the Claude API. [Availability of fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode#feature-availability) lists every platform where Claude Mythos 5.2 has fast mode.
8789
8890### Thinking blocks stay with the account that produced them
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123125Claude Fable 5.2 is available on:
124126
125127* **Claude API:** all customers, as `claude-fable-5-2`
126* **AWS:** [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), as `claude-fable-5-2`
128* **AWS:** [Claude in Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), as `anthropic.claude-fable-5-2`, and [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), as `claude-fable-5-2`
127129* **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-5-2`, 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-5-2`
130* **Microsoft Foundry:** [Claude in Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry), as `claude-fable-5-2`, on Anthropic infrastructure (see [Hosting options](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry#hosting-options))
128131
129132Claude Mythos 5.2 is available only to organizations verified through Anthropic’s verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842). It’s available on:
130133
131134* **Claude API:** as `claude-mythos-5-2`
135* **AWS:** [Claude in Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock), as `anthropic.claude-mythos-5-2`
132136* **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-mythos-5-2`, 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-mythos-5-2`
137* **Microsoft Foundry:** [Claude in Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry), as `claude-mythos-5-2`, on Anthropic infrastructure
133138
134139To request access, apply to the program that covers your use case, or contact your account team.
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188193
1891941. **Check whether your code edits history.** If your code builds the `messages` array itself, run the test in the migration guide’s [prefix check section](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-preserved-thinking), even if Claude Fable 5.1 has never returned the error for your account.
1901952. **Check model switches.** If a router or fallback moves conversations between models, expect every model except Claude Mythos 5.2 to drop Claude Fable 5.2’s thinking blocks, Claude Fable 5.1 and Claude Opus 5.5 included. See [Only Claude Fable 5.2 and Claude Mythos 5.2 read Claude Fable 5.2 thinking blocks](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#thinking-blocks-are-tied-to-the-model-that-produced-them).
1913. **Move computer use to the toolset.** If you use the `computer_20251124` tool, Claude Fable 5.2 returns a 400 error for that tool. Declare `computer_toolset_20260801` instead. For the change itself, see [Use the computer use toolset instead of `computer_20251124`](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#computer-20251124-is-not-supported).
1963. **Move computer use to the toolset.** If you use the `computer_20251124` tool anywhere except Amazon Bedrock, Claude Fable 5.2 returns a 400 error for that tool. Declare `computer_toolset_20260801` instead. For the change itself, see [Use the computer use toolset instead of `computer_20251124`](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#computer-20251124-is-not-supported).
1921974. **Update the advisor.** If you use the advisor tool with Claude Fable 5.2 as the executor, set the advisor’s `model` to `claude-fable-5-2` or `claude-mythos-5-2`. See [A Claude Fable 5.2 executor needs a Claude Fable 5.2 or Claude Mythos 5.2 advisor](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#advisor-tool-pairs).
1931985. **Re-run your evals.** Include an effort sweep that starts at the default, `high`. See [Recommended effort levels for Claude Fable 5.2](https://platform.claude.com/docs/en/build-with-claude/effort#recommended-effort-levels-for-claude-fable-5-2).
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