Migrating to Claude Fable 5.1 from Claude Opus 5

Claude Fable 5.1 uses the same Messages API and tool use patterns as Claude Opus 5. It keeps the 1M token context window by default, 128k max output tokens, the 512-token prompt caching minimum, and mid-conversation system message support. The prefill restriction, the sampling-parameter restriction, and the "omitted" default for thinking.display also carry over. Apply everything in Migrating to Claude Fable 5.1 from Claude Fable 5, plus the following.

Update your model name

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

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

What changed

1. Thinking can no longer be disabled: Claude Opus 5 accepts thinking: \{type: "disabled"\} at an effort level of high or lower. On claude-fable-5-1 and claude-mythos-5-1, adaptive thinking is always on, and thinking: \{type: "disabled"\} returns a 400 error at any effort level. Remove the field, control token spend with lower effort levels, and revisit max_tokens for workloads that ran with thinking disabled.

2. Forced tool choice is not supported: Claude Opus 5 accepts tool_choice any and tool. claude-fable-5-1 returns a 400 error. See Breaking changes.

3. Preserved thinking across models: Claude Fable 5.1 reads Claude Opus 5's thinking blocks: conversations moving from claude-opus-5 to claude-fable-5-1 keep their reasoning. Claude Opus 5 can't read Claude Fable 5.1's blocks. Claude Fable 5.1's blocks also stop being valid when earlier turns change: if your code edits earlier messages, rebuilds system or tools, or compacts on the client between requests, Claude Opus 5 didn't object, but claude-fable-5-1 rejects or drops every later thinking block. Run the three-step check in that section before switching traffic. See Breaking changes.

4. Text between tool calls is returned in thinking blocks: On Claude Opus 5, text the model writes between tool calls comes back as text blocks. On claude-fable-5-1, as on Claude Fable 5, that narration comes back as progress-update thinking blocks, one before each tool call. Under the default thinking.display of "omitted", they carry no readable text. If your interface renders that narration, set display: "updates" (beta) to receive progress updates as text while reasoning stays hidden, or "summarized" to receive both. Then render the non-empty thinking blocks between tool_use blocks. See Progress updates between tool calls.

5. Safety classifiers and fallback routing: Claude Fable 5.1 runs safety classifiers covering the same stop_details categories as Claude Fable 5, a broader set than Claude Opus 5's cybersecurity-only classifiers. Expect stop_details.category values beyond "cyber", such as "bio" and "reasoning_extraction"; see the refusal category table for the full set. For fallbacks configuration and permitted targets, see Use fallbacks: "default" for refusals.

6. Pricing: $10 USD per million input tokens and $50 USD per million output tokens, compared with $5 USD and $25 USD for Claude Opus 5. Prompt cache reads are $0.25 USD per million tokens, half the Claude Opus 5 rate. See Claude pricing.

7. Data retention: Claude Fable 5.1 and Claude Mythos 5.1 require 30-day data retention, aren't available under zero data retention (ZDR) arrangements unless expressly authorized by Anthropic, and are designated Covered Models. Claude Opus 5 is available under ZDR. See Model-specific data retention requirements.

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. See Model-specific data retention requirements.
• Update the model name from claude-opus-5 to claude-fable-5-1 (or claude-mythos-5-1).
• Remove any thinking: \{type: "disabled"\} configuration: it returns a 400 error on claude-fable-5-1. Control token spend with lower effort levels, and revisit max_tokens.
• 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.
• If your interface renders text between tool calls, set display: "updates" (beta) or "summarized" and render the non-empty thinking blocks.
• Apply the preserved-thinking, history-editing, behavior, effort, and fallback items from the Claude Fable 5 checklist.
• Re-baseline cost on your own workloads. The tokenizer is unchanged. Per-token pricing differs.