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Claude Sonnet 4.5 Retires on November 30, 2026

2 min read AI-generated

The recommended replacement is two days old. And in the same table, claude-opus-4-5 carries 'not before November 24, 2026', just six days earlier.

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Anthropic announced the deprecation of Claude Sonnet 4.5 on September 30. The model name claude-sonnet-4-5-20250929 is now marked deprecated, and the retirement date on the Claude API is November 30, 2026. Recommended replacement: claude-sonnet-5-5.

What that means in practice

The numbers live in the deprecations table in the platform docs, and the order there never changes: announcement, then two months, then gone. Opus 4.1 was deprecated on June 5 and retired on August 5. Sonnet 4 and Opus 4 went on the list April 14 and left June 15. For Sonnet 4.5 it’s 61 days.

The date inside the model name tells you how long it served: 20250929. One year and two days.

If you pinned the dated model ID in your code — which is what Anthropic recommends for production systems, you get errors instead of answers from November 30. An alias like claude-sonnet-latest only moves the work forward: the model then changes without you asking, and Sonnet 5.5 just showed how much that can matter: five breaking changes against Sonnet 5, including tool_choice with types any and tool, which now answers with HTTP 400.

Who’s next in line

Worth reading the rest of that table before you migrate:

  • claude-opus-4-5-20251101 is active, retirement not before November 24, 2026. That’s the next date that can hurt.
  • claude-haiku-4-5-20251001 is active, not before October 15, 2026.
  • claude-sonnet-4-6 is active, not before February 17, 2027.
  • claude-sonnet-5 is active, not before June 30, 2027.
  • claude-sonnet-5-5 is active, not before September 28, 2027.

So the full year of runway is back on the new model.

A year is the cadence, not the exception

The pattern is clear enough to plan around now: a model runs roughly a year in full service, then gets two months of notice, then disappears. That the date sometimes arrives less predictably than that doesn’t improve the arithmetic.

My advice: treat November 30 as a testing deadline, not a migration deadline. Two months is plenty of time to work through Sonnet 5.5’s five breaking changes. But only if you don’t start on November 25.

Sources:

AnthropicClaude SonnetAPIDeprecationReleases