User ID is not merging – new user is created instead of merging with existing one

[tequila1410]

Context / Problem:
We're trying to set a custom user_id for our users, but instead of merging with the existing anonymous user, a new user is created with that ID. This breaks our logic.

Expected behavior:
If we assign a known user_id to an anonymous user, we expect Rybbit to merge the existing anonymous profile with the new identified profile. This is necessary because we will later be sending paid event data via API to that user_id.

Actual behavior:
When we set user_id, Rybbit creates a completely new user. The old anonymous user remains in the system, which leads to duplicate users (anonymous + new identified), even though they are clearly the same person.

Why this matters:
This duplication causes problems for analytics and payment attribution. For example, if the anonymous user performed some actions (e.g. installed the app, trial started), but the payment comes to the identified user, we lose the link between events. We need to have one merged user to properly track full user journey.

Steps to reproduce:
A new user visits the app anonymously (anonymous ID is generated).
User logs in or registers.
We assign a user_id to that session.
Instead of merging with the anonymous user, Rybbit creates a new one.

Expected result:
One user, with both anonymous and identified activity merged.

Actual result:
Two users — one anonymous with pre-login actions, and one identified with post-login actions.
Please advise if this is expected behavior or a bug, and how we can force merging correctly. Thanks.

[goldflag]

Unfortunately this is unintended but expected behavior. But it is very clear what the issue is, so I am thinking about ways to fix it right now!

[bkonia]

Any progress on this issue? I'm considering switching to Rybbit from PostHog, but this limitation is a deal breaker.

[goldflag]

No, not yet. I know this is important though.

[Enjer]

Maybe a Distinct ID like Umami can be a solution. Any plan on this?