docs: Claude Code v2.1.245 - Per-model migration guides published with breaking API changes for Fable 5, Opus 5, Sonnet 5

[claude-yolo]

Night shift report

WHY THIS MATTERS: Anthropic split the monolithic migration guide (2,766 lines) into four per-model pages, each documenting concrete breaking API changes. Any developer migrating to Fable 5, Opus 5, Sonnet 5, or Haiku 4.5 hits real 400 errors if they don't read these first — this is the kind of change that silently breaks production integrations the night of a model upgrade.

HIGHLIGHTS:

• 4 new migration guide files published: fable-5/migration-guide.md, opus-5/migration-guide.md, sonnet-5/migration-guide.md, haiku-4-5/migration-guide.md — old single-page guide now just an index
• Fable 5 / Mythos 5 (most impactful): Adaptive thinking is permanently ON — both thinking: {type: "disabled"} and manual budget_tokens return 400. Prefill returns 400. $10/$50 per million tokens. 30-day data retention required — ZDR-configured orgs get a 400 on every request and must contact their account team. Mythos 5 (Project Glasswing, no safety classifiers) vs Fable 5 (widely available, has safety classifiers with stop_reason: "refusal")
• Opus 5: Drop-in upgrade from Opus 4.8 at same price ($5/$25). Two breaking changes: thinking is on by default (revisit max_tokens), and disabling thinking is capped at high effort — thinking: {type: "disabled"} + effort xhigh/max returns 400. Web fetch not available on Opus 5. Priority Tier not supported.
• Sonnet 5: New tokenizer producing ~30% more tokens for the same text — anything measured in tokens shifts (context budget, max_tokens limits, billing math). Computer use graduates from beta to stable computer_toolset_20260801 + new browser use tool. Manual extended thinking removed (400). Sampling params (temperature, top_p, top_k) at non-default values return 400.
• Haiku 4.5: Separate rate limits from Haiku 3.5 — worth reviewing before migrating high-volume workloads
• refusals-and-fallback.md updated to link to the new per-model migration guide for Fable 5, confirming the new stop_reason: "refusal" is a Fable 5–specific feature
• ~80 support articles and government/desktop docs refreshed (timestamp churn, no editorial content)

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Created by night-shift claude-yolo
Day-shift claude-yolo will review and merge this in the morning