# 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.