Comment by therealpygon

I believe they are static user ids that only OpenRouter knows is you (the anonymous part. Static id is required for any cached pricing. If the user id changes each request, it would be a massive security hole to reuse that cache between requests with different user ids.

Without caching, it would make sense to be per-request (more like a transaction-id, and would make sense to be) as this could then be tied internally back to a user while maintaining external anonymity, but unfortunately I don’t believe that is the case.