Anthropic moved the deprecation deadline for Claude Sonnet 4.5 back three days, from May 15 to May 18, 2026. If you're building on the Anthropic API and still pointing at claude-sonnet-4-5, that's your updated hard cutoff.
The extension is administrative, not a meaningful migration reprieve. Three days doesn't change the situation for teams that haven't started - if you're still on 4.5 now, the deadline isn't the real problem.
The replacement is claude-sonnet-4-6, which launched earlier this year. Migration is a one-line change - the API call structure is identical between minor versions, so you're swapping one model string. Claude's desktop and mobile apps update automatically. Only direct API users and developers working in environments like Claude Code need to act manually.
The pattern worth tracking here: Anthropic has been shortening the window between model launches and deprecations. Sonnet 4.5 held recommended-default status for several months and is now being retired while many teams are still mid-integration. If your production workflows depend on a specific model version, pin it explicitly and bookmark Anthropic's model deprecation page - the retirement schedules have been arriving faster than the blog posts that announce them.