Migrating to Claude Mythos 5.1 from Claude Mythos 5

Claude Mythos 5.1 is the access-gated counterpart to Claude Fable 5.1. Confirm your organization's access with your Anthropic account team before switching model IDs.

The API-level delta matches Migrating to Claude Fable 5.1 from Claude Fable 5: forced tool choice returns a 400 error, and thinking blocks are preserved only for the model that produced them or a newer one (Claude Mythos 5.1 reads Claude Mythos 5's blocks, not the reverse). Unlike Claude Fable 5.1, Claude Mythos 5.1 doesn't run the conversation check, so editing earlier turns doesn't invalidate thinking blocks, though it still restarts the prompt cache.

Update your model name

model = "claude-mythos-5"  # Before
model = "claude-mythos-5-1"  # After

Migration checklist

• Update the model name from claude-mythos-5 to claude-mythos-5-1.
• 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.
• Handle stop_reason: "refusal" and read stop_details.category before response content. See Refusals and fallback.
• Keep passing thinking blocks back unchanged on every turn, including empty ones.
• If your code builds the messages array itself, keep conversation history append-only to keep the prompt cache warm. Claude Mythos 5.1 doesn't run the conversation check, so edits don't invalidate its thinking blocks.
• Apply the behavior and recommended changes from the Claude Fable 5 section, except the history-editing items, which don't apply to Claude Mythos 5.1.
• Re-evaluate effort with a fresh sweep and re-baseline cost and latency. Prompt cache reads cost a quarter of the Claude Mythos 5 rate.