Fable 5.1 and Mythos 5.1 change tool choice and thinking-history handling
Fable 5.1 and Mythos 5.1 reject forced tool choice. Thinking replay depends on the target model. Fable’s prefix-history check also depends on account age, request settings and API surface; Mythos does not run that check.
Required action
Before routing a workflow to claude-fable-5-1 or claude-mythos-5-1, audit forced-tool requests, history edits and fallback routes. Keep replayed thinking unchanged and history append-only. Validate access, retention and schema-feature availability for the serving account and platform.
Replace forced tool choice
Both exact 5.1 models reject tool_choice types any and tool with a 400 invalid_request_error. auto and none remain supported. The migration guide applies this validation to Messages, Message Batches and token counting. Use auto with explicit tool instructions; it does not guarantee a call. Strict tools or structured outputs can enforce schema validity where available. In CMEK organizations, the guide excludes structured outputs, including strict tools, on Claude Fable models.
Check model routes explicitly
A block’s producing model and the receiving model determine whether its thinking can be read. Opus 5.5 and Sonnet 5.5 cannot read Fable/Mythos thinking. On the Claude API, Fable 5.1 and Mythos 5.1 can read Opus 5.5 thinking. Do not extend that reverse route to other platforms or assume that a newer release reads every older model’s blocks. The API drops unreadable blocks; that model mismatch alone does not reject the whole request.
Keep Fable history stable
Fable 5.1 checks the system prompt, tools and messages preceding replayed thinking. Edits can invalidate later blocks. Mythos 5.1 does not run this prefix check. Enforcement defaults on for accounts created on or after August 31, 2026 at 00:00 UTC; older accounts opt in by setting prefix_mismatch_behavior. Keep history append-only and pass complete thinking blocks back unchanged.
Handle prefix mismatches by surface
The thinking-binding-controls-2026-08-01 beta header enables mismatch settings and transformation reporting. Where Fable’s prefix check is enforced, ordinary requests default to error unless drop_block is chosen. In Message Batches, leaving the setting unset drops failing blocks instead; explicitly setting error makes the item fail. Dropping blocks loses reasoning and does not repair the history edit. Monitor input_transformations for these losses.
Verify access and retention
Mythos 5.1 is restricted to approved Project Glasswing customers. Both models require 30-day retention unless Anthropic expressly authorizes an exception. The migration guide documents a Claude API validation error when required retention is absent. Account entitlement, retention configuration and platform feature support need their own checks.
No shutdown deadline
September 1, 2026 is the documented launch and API-change notice. This is an opt-in migration record. No mandatory migration or retirement deadline is established.
Evidence limits
Historical first-detection date is unverified. Publication and source-check dates do not establish when Kingy first detected this change. Kingy has not reproduced an API error or tested inference, SDK behavior, migration, account access, retention or output equivalence.
The Sonnet 5.5 migration report covers a separate target model. Its account, effort and platform rules do not become Fable/Mythos rules by analogy.