Summary
Anthropic has retired the Claude Opus 4.1 model (claude-opus-4-1-20250805) on the Claude API. Requests to the model now return an error, and Anthropic recommends upgrading to Claude Opus 5, with continued access for researchers through its External Researcher Access Program.
What changed
On August 5, 2026 Anthropic retired claude-opus-4-1-20250805. All requests to that model now return an error, with Claude Opus 5 as the recommended migration target and an External Researcher Access Program path for ongoing research access.
Why it matters
Model retirements force real migration work for teams that pinned a specific Opus version in production. The hard cutover to errors, rather than silent redirection, signals Anthropic's steady consolidation onto the Claude 5 line and is a reminder for builders to track model lifecycle and avoid hardcoding deprecated IDs.
Evidence excerpt
We've retired the Claude Opus 4.1 model (claude-opus-4-1-20250805). All requests to this model will now return an error. We recommend upgrading to Claude Opus 5.