Why it was retired
Anthropic pins each Claude release to a dated id and retires older snapshots as newer Sonnet-class models ship. This one reached its retirement date above. A request naming the retired id no longer succeeds — Anthropic does not silently redirect it to a newer Claude, so the call fails outright.
Anything with this id baked into a config file — an editor plugin, a CI job, a saved agent — keeps sending the dead string until someone updates it, which is why these breaks often outlive the person who first configured them.
What to move to
The replacement shown above is the current Sonnet-class model, the direct successor for the same balance of capability, latency, and cost that made 3.5 Sonnet popular. For most workloads it is a straightforward upgrade.
Its live status is tracked on the Model Deprecation Radar so you can see if the successor itself is approaching a deprecation of its own before you standardize on it.
Migration notes
The Messages API shape is unchanged across these Claude versions, so the swap is usually just the model string. The behavior differences are the part to test: newer Claude models handle system prompts, tool use, and long context differently enough that a prompt tuned tightly to 3.5 Sonnet can produce noticeably different output.
Look for the id in more than your application code — Anthropic model strings commonly live in .cursor/editor settings, agent YAML, and shared team config, and a single stale copy is enough to break one workflow while the rest keep working.