Anthropic model IDs: dateless IDs are pinned snapshots, not evergreen pointers

Pin your model ID on purpose, and know which kind you pinned

The two kinds

1. 4.6 generation and later: dateless IDs (claude-sonnet-5, claude-opus-4-6) are the canonical ID for that release and map to a single fixed snapshot. Anthropic does not update the weights or configuration of an existing ID. A common misconception is that these are evergreen pointers. They are not.
2. Before 4.6: dated snapshots (claude-sonnet-4-5-20250929) plus short aliases (claude-sonnet-4-5) that resolve to the most recent dated snapshot for that minor version. The alias can change under you when a new snapshot ships.

Every model ID, dated or dateless, has its own deprecation and retirement schedule.

What to do

1. For reproducible behavior, pin the full dated snapshot ID on pre-4.6 models, and use the dateless canonical ID on 4.6+ models.
2. If you use an alias (claude-sonnet-4-5 style), do it deliberately: you are opting into snapshot upgrades. Re-run your evals when the underlying snapshot changes.
3. Cross-platform IDs differ: Bedrock uses anthropic.claude-sonnet-5 style (Opus 4.6 was the last with a -v1 suffix), Google Cloud uses the same form as the Claude API for current models and an @ date separator for dated ones (claude-haiku-4-5@20251001). Do not paste a Claude API ID into Bedrock unchanged.

The trap

Assuming "dateless means latest." On 4.6+ models it means "this exact snapshot, forever." Teams that expected silent upgrades get stale models; teams that feared silent upgrades on aliases get surprise behavior changes. Both mistakes come from not knowing which kind of ID they hold.

Checklist

• Write down whether each model ID in your config is a pinned snapshot or a moving alias. If you cannot answer, you have an alias.
• Query the Models API for capabilities and token limits programmatically instead of hardcoding assumptions about what an ID supports.

Anthropic's model IDs page documents the dateless format from the 4.6 generation, that dateless IDs are pinned snapshots not evergreen pointers, that pre-4.6 aliases resolve to the newest dated snapshot, and the Bedrock/Google Cloud ID formats. Verified live at https://platform.claude.com/docs/en/about-claude/models/model-ids-and-versions.