migration-guide changedmodels/fable-5-2/migration-guide
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1919The following sections cover moving to Claude Fable 5.2, or to Claude Mythos 5.2, from Claude Fable 5.1, Claude Mythos 5.1, Claude Opus 5.5, or an earlier Claude Fable, Claude Mythos, or Claude Opus model. For the changes themselves, see [What’s new in Claude Fable 5.2](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2). For behavioral differences and model-specific prompting patterns, see [Prompting Claude Fable 5.2](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2).
2020
21Claude Fable 5.2 is available on the Claude API, [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), and [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed). It’s also available on [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed). [Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview) has the same capabilities and is available only to organizations verified through Anthropic’s verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842).
21Claude Fable 5.2 is available on the Claude API, [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), and [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed). It’s also available on [Amazon Bedrock](https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock). It’s also available on [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed). It’s also available on [Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry). [Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview) has the same capabilities and is available only to organizations verified through Anthropic’s verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842).
2222
2323The baseline settings shared by `claude-fable-5-2` and `claude-mythos-5-2`:
2424
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4343
4444## Migrating to Claude Fable 5.2 from Claude Fable 5.1
4545
46Migration is mostly drop-in. The API surface, always-on adaptive thinking, the forced tool choice restriction, refusal handling, `stop_details` categories, and the data retention requirement all match Claude Fable 5.1. Claude Fable 5.2 costs less: $8 USD per million input tokens and $40 USD per million output tokens, compared with $10 USD and $50 USD on Claude Fable 5.1. Four changes can break your code. The [prefix check](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check), which catches edits to earlier turns, applies to every account. Claude Fable 5.1 can’t read Claude Fable 5.2’s thinking blocks. The earlier computer use tool, `computer_20251124`, returns a 400 error. With the advisor tool, a Claude Fable 5.2 executor accepts only Claude Fable 5.2 or Claude Mythos 5.2 as its advisor.
46Migration is mostly drop-in. The API surface, always-on adaptive thinking, the forced tool choice restriction, refusal handling, `stop_details` categories, and the data retention requirement all match Claude Fable 5.1. Claude Fable 5.2 costs less: $8 USD per million input tokens and $40 USD per million output tokens, compared with $10 USD and $50 USD on Claude Fable 5.1. Four changes can break your code. The [prefix check](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check), which catches edits to earlier turns, applies to every account. Claude Fable 5.1 can’t read Claude Fable 5.2’s thinking blocks. The earlier computer use tool, `computer_20251124`, returns a 400 error on every platform except Amazon Bedrock. With the advisor tool, a Claude Fable 5.2 executor accepts only Claude Fable 5.2 or Claude Mythos 5.2 as its advisor.
4747
4848### Update your model name
4949
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5252model = "claude-fable-5-2" # After
5353```
5454
55The model ID is `claude-fable-5-2` on every platform. For each platform’s ID, see the [Claude Fable 5.2](https://platform.claude.com/docs/en/models/fable-5-2/overview) model page.
55The model ID is `claude-fable-5-2` on every platform except Amazon Bedrock, where it’s `anthropic.claude-fable-5-2`. For each platform’s ID, see the [Claude Fable 5.2](https://platform.claude.com/docs/en/models/fable-5-2/overview) model page.
5656
5757### Breaking changes
5858
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7070
71712. **Claude Fable 5.1 can’t read Claude Fable 5.2’s thinking blocks:** On the Claude API and Claude Models on Google Cloud (Google managed), Claude Fable 5.2 reads Claude Fable 5.1’s blocks, so a conversation that moves from `claude-fable-5-1` onto `claude-fable-5-2` keeps its earlier reasoning. In the other direction, Claude Fable 5.1 can’t read Claude Fable 5.2’s blocks. If a router switch, a client-side retry, or a [classifier refusal fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback) moves a conversation from Claude Fable 5.2 to another model, Claude Fable 5.1 included, the API drops the blocks that model can’t read. The request succeeds, and you aren’t billed for the dropped input tokens. The model then works without that reasoning, which can raise cost and latency on the first turn after the switch. With the `thinking-binding-controls-2026-08-01` beta header, `input_transformations` lists each dropped block with `reason: "model_binding_mismatch"`. See [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models).
7272
733. **Use the computer use toolset instead of `computer_20251124`:** A `tools` entry of type `computer_20251124` returns a 400 error. Declare the `computer_toolset_20260801` toolset instead: drop the beta header and send the entry with no `name` or display dimensions. Then update your agent loop for the toolset, as [Migrate from `computer_20251124`](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#migrate-from-computer-20251124) describes.
733. **Use the computer use toolset instead of `computer_20251124`:** A `tools` entry of type `computer_20251124` returns a 400 error on every platform except Amazon Bedrock. Declare the `computer_toolset_20260801` toolset instead: drop the beta header and send the entry with no `name` or display dimensions. Then update your agent loop for the toolset, as [Migrate from `computer_20251124`](https://platform.claude.com/docs/en/agents-and-tools/tool-use/computer-use-tool#migrate-from-computer-20251124) describes. On Amazon Bedrock, the earlier tool works as it does on Claude Fable 5.1.
7474
75754. **A Claude Fable 5.2 executor needs a Claude Fable 5.2 or Claude Mythos 5.2 advisor:** With the [advisor tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool), which is in beta, the advisor must be at least as capable as the executor. A Claude Fable 5.2 or Claude Mythos 5.2 executor accepts only Claude Fable 5.2 or Claude Mythos 5.2 as its advisor, and any other advisor, including Claude Fable 5.1 and Claude Mythos 5.1, returns a 400 `invalid_request_error`. When you change the executor’s model ID, set the `model` in the advisor tool definition to `claude-fable-5-2` or `claude-mythos-5-2`. Changing the advisor’s `model` is an edit to `tools`, so make the change in new conversations. A request that sends earlier thinking blocks back after the change fails the prefix check (see [The prefix check applies to every account](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-preserved-thinking)) and returns a 400 error, unless you set `prefix_mismatch_behavior` to `"drop_block"`. To keep your current executor and change only the advisor, set the advisor’s `model` to `claude-fable-5-2`: every executor model accepts it. A Claude Fable 5.2 advisor returns its advice encrypted, so if your code reads the advice text, see [Result variants](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool#result-variants). The same pairs apply to the advisor of a [Claude Managed Agents](https://platform.claude.com/docs/en/managed-agents/overview) agent, so set that advisor’s `model` to one of these two models when you change the agent’s. See [Model compatibility](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool#model-compatibility) and [Give the session an advisor](https://platform.claude.com/docs/en/managed-agents/multiagent-orchestration#give-the-session-an-advisor).
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8686
8787* Update the model name from `claude-fable-5-1` to `claude-fable-5-2`.
8888* Check your [rate limits](https://platform.claude.com/docs/en/api/rate-limits#rate-limits) before you move production traffic. Claude Fable 5.2 has its own rate limit, separate from the one Claude Fable 5.1 and Claude Fable 5 share.
89* If you use computer use through `computer_20251124`, Claude Fable 5.2 returns a 400 error for that tool. Declare `computer_toolset_20260801` instead and update your agent loop for the toolset.
89* If you use computer use through `computer_20251124` anywhere except Amazon Bedrock, Claude Fable 5.2 returns a 400 error for that tool. Declare `computer_toolset_20260801` instead and update your agent loop for the toolset.
9090* 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`.
9191* If your code builds the `messages` array itself, run a session with the `thinking-binding-controls-2026-08-01` beta header and `prefix_mismatch_behavior: "drop_block"`, log `input_transformations`, and fix every `prefix_binding_mismatch` entry. Do this even if your account never hit the check on Claude Fable 5.1.
9292* Pick a production `prefix_mismatch_behavior`: `"error"` (the default), or `"drop_block"` as a stopgap while you fix the edits, because [dropping blocks can raise output tokens](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#drop-block-cost). Monitor the 400 errors or the `input_transformations` entries.
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9999
100100[Claude Mythos 5.2](https://platform.claude.com/docs/en/models/mythos-5-2/overview) is the counterpart to Claude Fable 5.2 for organizations verified through Anthropic’s verification programs, such as the [Cyber Verification Program](https://support.claude.com/en/articles/14604842). Confirm that your organization has access to Claude Mythos 5.2 before you switch model IDs.
101101
102Claude Mythos 5.2 enforces the [prefix check](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check) for every account, as Claude Fable 5.2 does. [When `prefix_mismatch_behavior` is unset](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement) shows where Claude Mythos 5.1 already returns the error. If your code edits earlier turns, see [Editing earlier turns invalidates thinking blocks](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-edited-history) for what to change and how to test, and run the test against `claude-mythos-5-2`.
102Claude Mythos 5.2 enforces the [prefix check](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#prefix-check) for every account, as Claude Fable 5.2 does. [When `prefix_mismatch_behavior` is unset](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement) shows where Claude Mythos 5.1 already returns the error. On Amazon Bedrock, Claude Mythos 5.2 returns the 400 error for edited earlier turns, where Claude Mythos 5.1 accepts such a request. If your code edits earlier turns, see [Editing earlier turns invalidates thinking blocks](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-edited-history) for what to change and how to test, and run the test against `claude-mythos-5-2`.
103103
104104On the Claude API and Claude Models on Google Cloud (Google managed), Claude Mythos 5.2 reads Claude Mythos 5.1’s thinking blocks, so a conversation that moves from `claude-mythos-5-1` onto `claude-mythos-5-2` keeps its earlier reasoning. In the other direction, Claude Mythos 5.1 can’t read Claude Mythos 5.2’s blocks: the API drops them, and the request succeeds. See [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models).
105105
106106With the [advisor tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool), which is in beta, a Claude Mythos 5.2 executor accepts only Claude Mythos 5.2 or Claude Fable 5.2 as its advisor. A Claude Mythos 5.1 or Claude Fable 5.1 advisor returns a 400 error. 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/migration-guide#fable-5-2-advisor-pairs).
107107
108Claude Mythos 5.2 supports computer use through the `computer_toolset_20260801` toolset. A `tools` entry of type `computer_20251124` returns a 400 error. See [Use the computer use toolset instead of `computer_20251124`](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-computer-use).
108Claude Mythos 5.2 supports computer use through the `computer_toolset_20260801` toolset. A `tools` entry of type `computer_20251124` returns a 400 error on every platform except Amazon Bedrock, where the earlier tool works. See [Use the computer use toolset instead of `computer_20251124`](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-computer-use).
109109
110110Claude Mythos 5.2 costs $8 USD per million input tokens and $40 USD per million output tokens, compared with $10 USD and $50 USD on Claude Mythos 5.1. Prompt cache reads are $0.20 USD per million tokens, compared with $0.25 USD.
111111
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119119### Migration checklist
120120
121121* Update the model name from `claude-mythos-5-1` to `claude-mythos-5-2`.
122* If you use computer use through `computer_20251124`, Claude Mythos 5.2 returns a 400 error for that tool. Declare `computer_toolset_20260801` instead and update your agent loop for the toolset.
122* If you use computer use through `computer_20251124` anywhere except Amazon Bedrock, Claude Mythos 5.2 returns a 400 error for that tool. Declare `computer_toolset_20260801` instead and update your agent loop for the toolset.
123123* If you use the advisor tool with Claude Mythos 5.2 as the executor, set the advisor’s `model` to `claude-mythos-5-2` or `claude-fable-5-2`.
124124* Keep passing `thinking` blocks back unchanged on every turn, including empty ones.
125125* If you replay stored conversations through a different account, replay each one through the account that produced it, or an account linked to it. The API drops Claude Mythos 5.2 thinking blocks that another account produced. See [Thinking blocks stay with the account that produced them](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#account-bound-thinking).
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157157
158158 Claude Fable 5.2 also supports fast mode on [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed), with the same `speed: "fast"` setting. Google Cloud sets its prices there. See [Cloud platform pricing](https://platform.claude.com/docs/en/about-claude/pricing#cloud-platform-pricing).
159159
160 Claude Fable 5.2 also supports fast mode on [Claude in Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry) for deployments hosted on Anthropic. There, fast mode is a separate model: send requests to a `claude-fable-5-2-fast` deployment and leave out `speed`.
161
160162 Claude Mythos 5.2 also supports fast mode in Messages API requests on the Claude API, at Claude Fable 5.2’s fast mode prices. [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.
161163
162164 Remove `speed: "fast"` in Claude Managed Agents. Also remove it on any platform or hosting option where [Availability of fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode#feature-availability) doesn’t list the model you move to.
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176178* Handle every `stop_details.category`. If you set an explicit `fallbacks` list, name only Claude Fable 5.2’s permitted targets.
177179* If a router or fallback moves conversations from Claude Fable 5.2 to Claude Opus 5.5, expect the turns after the switch to run without Claude Fable 5.2’s thinking blocks.
178180* If you replay stored conversations through a different account, replay each one through the account that produced it, or an account linked to it. The API drops Claude Fable 5.2 thinking blocks that another account produced. See [Thinking blocks stay with the account that produced them](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#account-bound-thinking).
179* If you use fast mode in Messages API requests, keep `speed: "fast"` only on a platform or hosting option where [Availability of fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode#feature-availability) lists the model you move to. On those platforms and hosting options, re-baseline cost against the [fast mode prices](https://platform.claude.com/docs/en/about-claude/pricing#fast-mode-pricing). On every other platform or hosting option, and in Claude Managed Agents, remove `speed: "fast"`. You don’t need the `fast-mode-2026-02-01` beta header. On the Claude API, requests that send it keep working.
181* If you use fast mode in Messages API requests, keep `speed: "fast"` only on a platform or hosting option where [Availability of fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode#feature-availability) lists the model you move to. On those platforms and hosting options, re-baseline cost against the [fast mode prices](https://platform.claude.com/docs/en/about-claude/pricing#fast-mode-pricing). On every other platform or hosting option, and in Claude Managed Agents, remove `speed: "fast"`. You don’t need the `fast-mode-2026-02-01` beta header. On the Claude API, requests that send it keep working. Claude in Microsoft Foundry is the exception: there, remove `speed: "fast"` and send requests to a `claude-fable-5-2-fast` deployment.
180182* 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`.
181183* In an organization with [customer-managed encryption keys](https://platform.claude.com/docs/en/manage-claude/cmek), remove `strict: true` and `output_config.format`, and validate the output in your code.
182184* Set `effort` explicitly: Claude Fable 5.2’s default is `high`, where Claude Opus 5.5’s is `medium`, so a request that omits it runs one level higher. Then re-run your [effort sweep](https://platform.claude.com/docs/en/build-with-claude/effort#recommended-effort-levels-for-claude-fable-5-2) on your own evals, and re-baseline cost and latency.
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209211
2102123. **Claude Fable 5 can’t read Claude Fable 5.2’s thinking blocks:** On the Claude API and Claude Models on Google Cloud (Google managed), Claude Fable 5.2 reads Claude Fable 5’s blocks, so a conversation that moves from `claude-fable-5` onto `claude-fable-5-2` keeps its earlier reasoning. In the other direction, Claude Fable 5 can’t read Claude Fable 5.2’s blocks. If a router switch, a client-side retry, or a [classifier refusal fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback) moves a conversation from Claude Fable 5.2 to another model, Claude Fable 5 included, the API drops the blocks that model can’t read. The request succeeds, and you aren’t billed for the dropped input tokens. With the `thinking-binding-controls-2026-08-01` beta header, `input_transformations` lists each dropped block with `reason: "model_binding_mismatch"`. See [Switching models mid-conversation](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#switching-models).
211213
2124. **Use the computer use toolset instead of `computer_20251124`:** On Claude Fable 5.2, a `tools` entry of type `computer_20251124` returns a 400 error. Claude Fable 5 accepts it with its beta header. Declare the `computer_toolset_20260801` toolset instead, as [Use the computer use toolset instead of `computer_20251124`](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-computer-use) describes.
2144. **Use the computer use toolset instead of `computer_20251124`:** On Claude Fable 5.2, a `tools` entry of type `computer_20251124` returns a 400 error on every platform except Amazon Bedrock. Claude Fable 5 accepts it with its beta header. Declare the `computer_toolset_20260801` toolset instead, as [Use the computer use toolset instead of `computer_20251124`](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-computer-use) describes.
213215
2142165. **A Claude Fable 5.2 executor needs a Claude Fable 5.2 or Claude Mythos 5.2 advisor:** With the [advisor tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool), which is in beta, a Claude Fable 5.2 executor accepts only Claude Fable 5.2 or Claude Mythos 5.2 as its advisor. The other advisors a Claude Fable 5 executor accepts, such as Claude Fable 5 and Claude Opus 5.5, return a 400 error. Set the advisor’s `model` to `claude-fable-5-2` or `claude-mythos-5-2` when you change the executor’s. 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/migration-guide#fable-5-2-advisor-pairs).
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229231* Pick a production `prefix_mismatch_behavior`: `"error"` (the default), or `"drop_block"` as a stopgap while you fix the edits, because [dropping blocks can raise output tokens](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#drop-block-cost). Monitor the 400 errors or the `input_transformations` entries.
230232* Keep passing `thinking` blocks back unchanged on every turn, including empty ones. If a router or fallback can move a conversation from Claude Fable 5.2 to Claude Fable 5, expect Claude Fable 5 to run without Claude Fable 5.2’s blocks.
231233* If you replay stored conversations through a different account, replay each one through the account that produced it, or an account linked to it. The API drops Claude Fable 5.2 thinking blocks that another account produced. See [Thinking blocks stay with the account that produced them](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#account-bound-thinking).
232* If you use computer use through `computer_20251124`, Claude Fable 5.2 returns a 400 error for that tool. Declare `computer_toolset_20260801` instead and update your agent loop for the toolset.
234* If you use computer use through `computer_20251124` anywhere except Amazon Bedrock, Claude Fable 5.2 returns a 400 error for that tool. Declare `computer_toolset_20260801` instead and update your agent loop for the toolset.
233235* 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`.
234236* If you change effort between requests, move the change to a per-message effort `role: "system"` message, which is in beta, where your platform supports it.
235237* If your organization has a Priority Tier commitment, plan capacity separately.
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261263
262264* Update the model name from `claude-mythos-5` to `claude-mythos-5-2`.
263265* Replace forced `tool_choice` (`any` or `tool`) with `auto` plus an explicit instruction and `strict: true` tools, or with JSON outputs. In an organization with [customer-managed encryption keys](https://platform.claude.com/docs/en/manage-claude/cmek), strict tool use and JSON outputs aren’t available for Claude Mythos 5.2. There, keep `auto` and the instruction, and validate the tool input in your code.
264* If you use computer use through `computer_20251124`, Claude Mythos 5.2 returns a 400 error for that tool. Declare `computer_toolset_20260801` instead and update your agent loop for the toolset.
266* If you use computer use through `computer_20251124` anywhere except Amazon Bedrock, Claude Mythos 5.2 returns a 400 error for that tool. Declare `computer_toolset_20260801` instead and update your agent loop for the toolset.
265267* If you use the advisor tool with Claude Mythos 5.2 as the executor, set the advisor’s `model` to `claude-mythos-5-2` or `claude-fable-5-2`.
266268* Keep passing `thinking` blocks back unchanged on every turn, including empty ones.
267269* If you replay stored conversations through a different account, replay each one through the account that produced it, or an account linked to it. The API drops Claude Mythos 5.2 thinking blocks that another account produced. See [Thinking blocks stay with the account that produced them](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#account-bound-thinking).
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293295
2942965. **Text between tool calls is returned in thinking blocks:** On Claude Opus 5, text the model writes between tool calls comes back as `text` blocks. On Claude Fable 5.2, that narration comes back as progress-update `thinking` blocks, at most one before each tool call. If your interface renders that narration, set `thinking.display` to `"updates"`, which is in beta, to receive the progress updates as text while reasoning stays hidden, or to `"summarized"` to receive both. Then render the non-empty `thinking` blocks between `tool_use` blocks. See [Progress updates between tool calls](https://platform.claude.com/docs/en/build-with-claude/thinking#progress-updates).
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2966. **Use the computer use toolset instead of `computer_20251124`:** Claude Opus 5 accepts the earlier `computer_20251124` tool with its beta header. Claude Fable 5.2 returns a 400 error for it. See [Use the computer use toolset instead of `computer_20251124`](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-computer-use).
2986. **Use the computer use toolset instead of `computer_20251124`:** Claude Opus 5 accepts the earlier `computer_20251124` tool with its beta header. Claude Fable 5.2 returns a 400 error for it on every platform except Amazon Bedrock. See [Use the computer use toolset instead of `computer_20251124`](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-computer-use).
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2983007. **Safety classifiers and fallback:** Claude Fable 5.2’s safety classifiers cover a broader set of `stop_details` categories than Claude Opus 5’s classifiers. Expect additional `stop_details.category` values, such as `"bio"`. Handle every category in the [refusal category table](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#refusal-response). To re-run declined requests on another model, set `fallbacks: "default"` or build your own retry. That parameter is in beta on the Claude API and Claude Models on Google Cloud (Anthropic managed), and needs the `server-side-fallback-2026-07-01` beta header. [Refusals, fallback, and billing](https://platform.claude.com/docs/en/models/fable-5-2/whats-new-fable-5-2#refusals-fallback-and-billing) lists the permitted fallback targets.
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308310 Claude Fable 5.2 also supports fast mode on [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed), with the same `speed: "fast"` setting. Google Cloud sets its prices there. See [Cloud platform pricing](https://platform.claude.com/docs/en/about-claude/pricing#cloud-platform-pricing).
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312 Claude Fable 5.2 also supports fast mode on [Claude in Microsoft Foundry](https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry) for deployments hosted on Anthropic. There, fast mode is a separate model: send requests to a `claude-fable-5-2-fast` deployment and leave out `speed`.
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310314 Claude Mythos 5.2 also supports fast mode in Messages API requests on the Claude API, without the header and at Claude Fable 5.2’s fast mode prices. [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.
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312316 Remove `speed: "fast"` in Claude Managed Agents. Also remove it if [Availability of fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode#feature-availability) doesn’t list the model you move to.
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324328* Pass `thinking` blocks back unchanged on every turn, including empty ones. In a tool-use loop, send them back with the tool results. See [Preserving thinking blocks](https://platform.claude.com/docs/en/build-with-claude/thinking#preserving-thinking-blocks).
325329* Replace forced `tool_choice` (`any` or `tool`) with `auto` plus an explicit instruction and `strict: true` tools, or with JSON outputs. In an organization with [customer-managed encryption keys](https://platform.claude.com/docs/en/manage-claude/cmek), strict tool use and JSON outputs aren’t available for Claude Fable 5.2. There, keep `auto` and the instruction, and validate the tool input in your code.
326330* If your code builds the `messages` array itself, run the check in [Editing earlier turns invalidates thinking blocks](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#fable-5-2-edited-history) against `claude-fable-5-2`, fix every `prefix_binding_mismatch` entry, and keep the history append-only.
327* If you use computer use through `computer_20251124`, Claude Fable 5.2 returns a 400 error for that tool. Declare `computer_toolset_20260801` instead and update your agent loop for the toolset.
331* If you use computer use through `computer_20251124` anywhere except Amazon Bedrock, Claude Fable 5.2 returns a 400 error for that tool. Declare `computer_toolset_20260801` instead and update your agent loop for the toolset.
328332* 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`.
329333* If a router or fallback moves conversations from Claude Fable 5.2 to Claude Opus 5, expect the turns after the switch to run without Claude Fable 5.2’s thinking blocks.
330334* If you replay stored conversations through a different account, replay each one through the account that produced it, or an account linked to it. The API drops Claude Fable 5.2 thinking blocks that another account produced. See [Thinking blocks stay with the account that produced them](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#account-bound-thinking).
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