migration-guide changedmodels/fable-5-2/migration-guide
Nearest release: v2.1.321, published an hour before this site recorded the change. Shown because the two are within 24 hours of each other. Nothing here says the release caused the edit.
Recorded here
Lines+4added
Lines−4removed
From line
14
where the diff opens
First seen
9 Nov 2026
this site's first read of the page
Recorded edits4to this page, all time
The whole hunk
from line 14, old and new numbered
/
from line 14
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).
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.
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
from line 64
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. It 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. 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.
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
from line 99
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. 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`.
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. On the Claude API, Claude Mythos 5.1 already returns the error for every account (see [When `prefix_mismatch_behavior` is unset](https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#enforcement)). 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 changes `system`, `tools`, or earlier messages between requests, 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
from line 143
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 edits earlier turns, 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 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.
149149
No line in this hunk matches that.