Migrating to Claude Fable 5.1 from Claude Opus 4.8 or earlier

First apply Migrating to Claude Mythos 5 and Claude Fable 5 from Claude Opus 4.8 for the API-level changes from Claude Opus 4.8. It covers adaptive thinking, thinking output, refusals, effort, the caching minimum, pricing, and data retention. Then apply the remaining delta in Migrating to Claude Fable 5.1 from Claude Fable 5. On Claude Opus 4.7 or earlier, start with the matching Migrating to Claude Opus 5 section.

Update your model name

model = "claude-opus-4-8"  # Before
model = "claude-fable-5-1"  # After

# Or, for the Project Glasswing model with the same capabilities:
model = "claude-mythos-5-1"  # After

Migration checklist

• If your organization has a zero data retention (ZDR) arrangement, confirm eligibility first: these models aren't available under ZDR unless expressly authorized by Anthropic. Claude Opus 4.8 is available under ZDR.
• Update the model name from claude-opus-4-8 to claude-fable-5-1 (or claude-mythos-5-1).
• Remove any thinking: \{type: "disabled"\} configuration and revisit max_tokens. Requests without a thinking field run with adaptive thinking.
• Replace forced tool_choice (any or tool) with auto plus an explicit instruction (user turn or mid-conversation system message) and strict: true tools, or with JSON outputs.
• Pass thinking blocks back unchanged and treat their text as display-only. Claude Fable 5.1 reads Claude Opus 4.8's thinking blocks: a conversation that moves onto claude-fable-5-1 keeps its earlier reasoning. Claude Opus 4.8 can't read Claude Fable 5.1's blocks.
• If your code builds the messages array itself, check whether it edits earlier turns. Integrations written for Claude Opus 4.8 and earlier often truncate old turns, strip or rebuild earlier messages, or refresh the system prompt each request, and Claude Opus 4.8 never objected. On claude-fable-5-1 each of those invalidates later thinking blocks.
• Handle stop_reason: "refusal", read stop_details.category, and consider fallbacks: "default" (beta).
• Apply the preserved-thinking, history-editing, behavior, per-message effort, and progress-update items from the Claude Fable 5 checklist.
• Re-evaluate effort (start at high), review prompts near the 512-token caching minimum, and re-baseline cost and latency. Per-token pricing differs.