migration-guide changedmodels/fable-5-2/migration-guide
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1414/claude-api migrate this project to claude-fable-5-2
1515```
1616
17If your Claude Managed Agents agent has an advisor, the advisor’s `model` is a second model name to update: set it to `claude-fable-5-2` or `claude-mythos-5-2` when you change the agent’s model. 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). If the agent’s `model` object sets `speed: "fast"`, remove it too.
17If your Claude Managed Agents agent has an advisor, the advisor’s `model` is a second model name to update: set it to `claude-fable-5-2` or `claude-mythos-5-2` when you change the agent’s model. 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). If the agent’s `model` object sets `speed: "fast"`, remove that setting too.
1818
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 [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).
21Claude Fable 5.2 is available on the Claude API, [Amazon Bedrock (`bedrock-runtime`)](https://platform.claude.com/docs/en/build-with-claude/claude-on-amazon-bedrock-legacy), [Claude Platform on AWS](https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws), [Claude Models on Google Cloud (Anthropic managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/anthropic-managed), [Claude Models on Google Cloud (Google managed)](https://platform.claude.com/docs/en/build-with-claude/claude-models-on-google-cloud/google-managed), and [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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5252model = "claude-fable-5-2" # After
5353```
5454
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.
55The model ID is `claude-fable-5-2` on every platform except Amazon Bedrock. There, it’s `global.anthropic.claude-fable-5-2` or `us.anthropic.claude-fable-5-2`, and only the `bedrock-runtime` endpoint serves the model.
5656
5757### Breaking changes
5858
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6464 messages.5.content.0: Invalid `signature` in `thinking` block. The block is bound to a different conversation. Remove the block, or set `thinking.block_binding.prefix_mismatch_behavior` to "drop_block". That setting requires the `thinking-binding-controls-2026-08-01` value in the `anthropic-beta` header.
6565 ```
6666
67 Retrying the same request body doesn’t clear the error. To find these edits before you switch production traffic, run the check 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) against the model you use now, Claude Fable 5.1 or Claude Opus 5.5. If you use the [MCP connector](https://platform.claude.com/docs/en/agents-and-tools/mcp-connector) without [pinning a server’s tool list](https://platform.claude.com/docs/en/agents-and-tools/mcp-connector#pin-mcp-tool-list), also run the check against `claude-fable-5-2`: a tool list that changes between requests is an edit for Claude Fable 5.2 thinking blocks, but not for Claude Fable 5.1 or Claude Opus 5.5 blocks. The check works from any account, because setting `prefix_mismatch_behavior` opts the request into the check. [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) gives the fix for each pattern.
67 Retrying the same request body doesn’t clear the error. To find these edits before you switch production traffic, run the check 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) against the model you use now, Claude Fable 5.1 or Claude Opus 5.5. If you use the [MCP connector](https://platform.claude.com/docs/en/agents-and-tools/mcp-connector) without [pinning a server’s tool list](https://platform.claude.com/docs/en/agents-and-tools/mcp-connector#pin-mcp-tool-list), also run the check against `claude-fable-5-2`: a tool list that changes between requests is an edit for Claude Fable 5.2 thinking blocks, but not for Claude Fable 5.1 or Claude Opus 5.5 blocks. You can run this check from any account. [Preserved thinking](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking) gives the fix for each pattern.
6868
6969 In production, leave `prefix_mismatch_behavior` at its default, `"error"`, if a mismatch can only mean a bug in your code. To drop the affected blocks and continue without that reasoning while you fix the edit, set it to `"drop_block"`. Treat that as a stopgap, not a long-term setting: [dropping blocks can raise output tokens](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#drop-block-cost).
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 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.
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, Claude Fable 5.2 and Claude Mythos 5.2 accept the earlier tool: with the InvokeModel API, send `computer-use-2025-11-24` in the `anthropic_beta` array of the request body.
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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9393* 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 5.1, expect Claude Fable 5.1 to run without Claude Fable 5.2’s blocks.
9494* 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).
9595* 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.
96* Check the system prompts, skills, and other instructions you carry over against [Review instructions written for earlier models](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2#review-instructions-written-for-earlier-models).
96* Check the system prompts, skills, and other instructions you carry over against [Review prompts and harnesses designed for earlier models](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2#review-instructions-written-for-earlier-models).
9797
9898## Migrating to Claude Mythos 5.2 from Claude Mythos 5.1
9999
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126126* 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-mythos-5-2`, fix every `prefix_binding_mismatch` entry, and keep conversation history append-only (see [What counts as an edit](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#what-counts-as-an-edit)). An append-only history also keeps the prompt cache warm.
127127* Handle `stop_reason: "refusal"` and read `stop_details.category` before response content. See [Refusals and fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback).
128128* Re-run your effort sweep, and re-baseline cost and latency.
129* Check the system prompts, skills, and other instructions you carry over against [Review instructions written for earlier models](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2#review-instructions-written-for-earlier-models).
129* Check the system prompts, skills, and other instructions you carry over against [Review prompts and harnesses designed for earlier models](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2#review-instructions-written-for-earlier-models).
130130
131131## Migrating to Claude Fable 5.2 from Claude Opus 5.5
132132
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143143
1441441. **Data retention is required:** Claude Fable 5.2 requires 30-day data retention and isn’t available under a zero data retention (ZDR) arrangement unless expressly authorized by Anthropic. If your organization or workspace runs Claude Opus 5.5 under ZDR, enable 30-day retention for a workspace or contact your Anthropic account team before you switch. Otherwise, on the Claude API, every request to Claude Fable 5.2 returns a 400 `invalid_request_error`. See [Model-specific data retention requirements](https://platform.claude.com/docs/en/manage-claude/api-and-data-retention#model-specific-data-retention-requirements).
145145
1462. **The prefix check applies to every account:** Claude Opus 5.5 enforces the prefix check by default for accounts created on or after August 31, 2026. Claude Fable 5.2 enforces it for every account, whatever its age. If your account is older and your code changes `system`, `tools`, or earlier messages between requests, requests that work on Claude Opus 5.5 fail on Claude Fable 5.2. To test before you switch, 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).
1462. **The prefix check applies to every account:** Claude Opus 5.5 enforces the prefix check by default for accounts created on or after August 31, 2026. Claude Fable 5.2 enforces it for every account, whatever its age. If your account is older and your code changes `system`, `tools`, or earlier messages between requests, requests that work on Claude Opus 5.5 can fail on Claude Fable 5.2. To test before you switch, 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).
147147
1481483. **Claude Opus 5.5 can’t read Claude Fable 5.2’s thinking blocks:** A conversation that moves from Claude Fable 5.2 to Claude Opus 5.5 keeps its text and tool calls, but the turns after the switch run without Claude Fable 5.2’s reasoning. The API drops those blocks before the model receives them, the request succeeds, and the dropped blocks aren’t billed. Expect higher cost and latency on the first turn after the switch. In the other direction, on the Claude API and Claude Models on Google Cloud (Google managed), Claude Fable 5.2 reads Claude Opus 5.5’s thinking blocks, so a conversation that moves from `claude-opus-5-5` onto `claude-fable-5-2` keeps its earlier reasoning.
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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`.
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 has its own model ID: send requests to a `claude-fable-5-2-fast` deployment and leave out `speed`.
161161
162162 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.
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182182* 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`.
183183* 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.
184184* 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.
185* Check the system prompts, skills, and other instructions you carry over against [Review instructions written for earlier models](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2#review-instructions-written-for-earlier-models).
185* Check the system prompts, skills, and other instructions you carry over against [Review prompts and harnesses designed for earlier models](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2#review-instructions-written-for-earlier-models).
186186
187187## Migrating to Claude Fable 5.2 from Claude Fable 5
188188
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219219
220220* **Pricing:** $8 USD per million input tokens and $40 USD per million output tokens, compared with $10 USD and $50 USD on Claude Fable 5. Prompt cache reads are $0.20 USD per million tokens, compared with $1 USD. See [Pricing](https://platform.claude.com/docs/en/about-claude/pricing).
221221* **Priority Tier:** Claude Fable 5.2 isn’t supported on [Priority Tier](https://platform.claude.com/docs/en/api/service-tiers#supported-models). Claude Fable 5 is. If your organization has a Priority Tier commitment, plan capacity separately.
222* **Per-message effort (beta):** To change effort partway through a conversation without losing prompt cache hits, send a [per-message effort](https://platform.claude.com/docs/en/build-with-claude/effort#change-effort-mid-conversation-beta) `role: "system"` message instead of changing `output_config.effort` between requests. That section lists the platforms where it’s available.
222* **Per-message effort (beta):** To change effort partway through a conversation without losing prompt cache hits, send a [per-message effort](https://platform.claude.com/docs/en/build-with-claude/effort-per-message) `role: "system"` message instead of changing `output_config.effort` between requests. The Per-message effort page lists the platforms where it’s available.
223223
224224### Migration checklist
225225
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237237* If your organization has a Priority Tier commitment, plan capacity separately.
238238* Handle `stop_reason: "refusal"` and read `stop_details.category`. Consider `fallbacks: "default"`, in beta on the Claude API and Claude Models on Google Cloud (Anthropic managed).
239239* 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.
240* Check the system prompts, skills, and other instructions you carry over against [Review instructions written for earlier models](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2#review-instructions-written-for-earlier-models).
240* Check the system prompts, skills, and other instructions you carry over against [Review prompts and harnesses designed for earlier models](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2#review-instructions-written-for-earlier-models).
241241
242242## Migrating to Claude Mythos 5.2 from Claude Mythos 5
243243
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270270* 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-mythos-5-2`, fix every `prefix_binding_mismatch` entry, and keep conversation history append-only (see [What counts as an edit](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#what-counts-as-an-edit)). An append-only history also keeps the prompt cache warm.
271271* Handle `stop_reason: "refusal"` and read `stop_details.category` before response content. See [Refusals and fallback](https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback).
272272* Re-run your effort sweep, and re-baseline cost and latency.
273* Check the system prompts, skills, and other instructions you carry over against [Review instructions written for earlier models](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2#review-instructions-written-for-earlier-models).
273* Check the system prompts, skills, and other instructions you carry over against [Review prompts and harnesses designed for earlier models](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2#review-instructions-written-for-earlier-models).
274274
275275## Migrating to Claude Fable 5.2 from Claude Opus 5
276276
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309309
310310 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).
311311
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`.
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 has its own model ID: send requests to a `claude-fable-5-2-fast` deployment and leave out `speed`.
313313
314314 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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337337* If you use fast mode in Messages API requests on the Claude API, keep `speed: "fast"` only if [Availability of fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode#feature-availability) lists the model you move to. If you keep it, re-baseline cost against the [fast mode prices](https://platform.claude.com/docs/en/about-claude/pricing#fast-mode-pricing). Remove `speed: "fast"` in Claude Managed Agents. You don’t need the `fast-mode-2026-02-01` beta header, and requests that send it keep working.
338338* 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.
339339* 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.
340* Check the system prompts, skills, and other instructions you carry over against [Review instructions written for earlier models](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2#review-instructions-written-for-earlier-models).
340* Check the system prompts, skills, and other instructions you carry over against [Review prompts and harnesses designed for earlier models](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-2#review-instructions-written-for-earlier-models).
341341
342342## Migrating to Claude Fable 5.2 from Claude Opus 4.8 or earlier
343343
344344First work through [Migrating to Claude Fable 5.2 from Claude Opus 5](https://platform.claude.com/docs/en/models/fable-5-2/migration-guide#migrating-from-claude-opus-5-to-claude-fable-5-2), with `claude-opus-4-8` as the model ID you replace. That section applies to code on Claude Opus 4.8 as written, because Claude Opus 4.8, like Claude Opus 5:
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346* Accepts `thinking: {type: "disabled"}`, forced tool choice, the `computer_20251124` tool, and fast mode.
346* Accepts `thinking: {type: "disabled"}`, forced tool choice, the `computer_20251124` tool, and fast mode, which is [deprecated on Claude Opus 4.8](https://platform.claude.com/docs/en/build-with-claude/fast-mode).
347347* Returns the text between tool calls as `text` blocks.
348348* Is available under zero data retention (ZDR) arrangements.
349349* Costs $5 USD per million input tokens and $25 USD per million output tokens, with prompt cache reads at $0.50 USD.
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3613614. **Advice from the advisor tool comes back encrypted:** With the [advisor tool](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool), which is in beta, a Claude Opus 4.8 executor can pair with an advisor that returns its advice as plaintext, such as Claude Opus 4.8. The only advisors a Claude Fable 5.2 executor accepts, Claude Fable 5.2 and Claude Mythos 5.2, return the advice encrypted, as an `advisor_redacted_result` block, so your client can’t read its text. If your code reads the advice text, remove that dependency. See [Result variants](https://platform.claude.com/docs/en/agents-and-tools/tool-use/advisor-tool#result-variants).
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363If your code is on Claude Opus 4.7 or earlier, also apply part of the Claude Opus 5.5 guide’s migration checklist. Start at the group [Claude Opus 4.7 or earlier](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#migration-checklist-from-claude-opus-47) and go down to the group that names your model. These groups cover request changes made before Claude Opus 4.8, such as removed sampling parameters and beta headers. Apply that checklist’s fast mode item only if [Availability of fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode#feature-availability) lists the model you move to. In an organization with [customer-managed encryption keys](https://platform.claude.com/docs/en/manage-claude/cmek), structured outputs aren’t available for Claude Fable 5.2, so don’t apply the `output_config.format` item there. Remove `output_format` from your requests instead, and validate the JSON in your code.
363If your code is on Claude Opus 4.7 or earlier, also apply part of the Claude Opus 5.5 guide’s migration checklist. Start at the group [Claude Opus 4.7 or earlier](https://platform.claude.com/docs/en/models/opus-5-5/migration-guide#migration-checklist-from-claude-opus-47) and apply each group after it in turn, up to and including the one that names your model. These groups cover request changes made before Claude Opus 4.8, such as removed sampling parameters and beta headers. Apply that checklist’s fast mode item only if [Availability of fast mode](https://platform.claude.com/docs/en/build-with-claude/fast-mode#feature-availability) lists the model you move to. In an organization with [customer-managed encryption keys](https://platform.claude.com/docs/en/manage-claude/cmek), structured outputs aren’t available for Claude Fable 5.2, so don’t apply the `output_config.format` item there. Remove `output_format` from your requests instead, and validate the JSON in your code.
364364
365365### Migration checklist
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