Claude 3 Sonnet

Claude 3 Sonnet, API id claude-3-sonnet-20240229, was a mid-tier model in Anthropic's Claude 3 family. It is no longer available: on January 21, 2025 Anthropic notified developers that Claude Sonnet 3 would be retired, and the model deprecations page lists the retirement date as July 21, 2025. Requests to a retired model fail, and the recommended replacement on that page is Claude Sonnet 5.5 (claude-sonnet-5-5).

Status at a glance

Anthropic's page defines "Retired" as no longer available for use, with requests failing. It also says the dates apply to Anthropic-operated platforms, while Amazon Bedrock and Google-managed Claude on Google Cloud set their own schedules, so confirm the status on the platform you use.

Key Features of the Replacement

• Claude Sonnet 5.5: according to Anthropic's models overview, it is priced at $2 per million input tokens and $10 per million output tokens, has a 1M-token context window, 128K max output, and adaptive thinking (checked 2026-10-29).
• Same upgrade path for older Sonnets: the deprecations page points Claude 3.5 Sonnet (retired October 28, 2025) and Claude Sonnet 3.7 (retired February 19, 2026) to the same replacement.
• Higher tiers: the overview describes Claude Opus 5.5 as the starting point for most workloads and for long-running agentic coding and knowledge work.

Use Cases for This Page

• You find claude-3-sonnet-20240229 in old code, notebooks or tutorials and need to know why it errors.
• You are comparing historical Claude generations in a research or audit context.
• You maintain a prompt set written for Claude 3 Sonnet and need a migration target.

A limitation to be aware of: this entry does not list per-token prices for Claude 3 Sonnet, because the current Anthropic pages no longer publish them. Past benchmark comparisons for it are not a guide to current models.

Migration Steps

1. Search your code and config for claude-3-sonnet-20240229.
2. Switch the model value to claude-sonnet-5-5, then read Anthropic's migration guide for parameter changes.
3. Re-run your own evals, since prompts tuned for an older model can behave differently.
4. Set a reminder to review model status again: the same deprecations page shows Claude Sonnet 4.5 as deprecated on September 30, 2026 with retirement on November 30, 2026.

FAQ

Is Claude 3 Sonnet still available?

No. Anthropic lists it as retired on July 21, 2025 after a January 21, 2025 notice.

What replaced Claude 3 Sonnet?

Anthropic's recommended replacement is Claude Sonnet 5.5, API id claude-sonnet-5-5.

Why does claude-3-sonnet-20240229 return an error?

Requests to retired models fail on Anthropic-operated platforms. Update the model id.

Is Claude 3.5 Sonnet still available?

No. The same page lists both Claude 3.5 Sonnet snapshots as retired on October 28, 2025. See Claude 3.5 Sonnet.

Alternatives

• Claude Sonnet 5.5: the official replacement and current mid-tier model.
• Claude Opus 5.5: the stronger tier for complex work.
• Claude Haiku 5.5: for high-volume, latency-sensitive tasks.
• Claude Sonnet 4.5: an intermediate Sonnet that is itself scheduled for retirement.

Conclusion

Claude 3 Sonnet is a retired model, so treat any reference to it as a migration task rather than a choice. Use claude-sonnet-5-5 unless your evals say otherwise, and keep the deprecations page bookmarked. More Claude entries are in the models category.