Understanding Mixpanel's Legacy ID Management and Distinct IDs

Thang D. · 2025-06-04T04:22:17.697Z

In my Mixpanel project, the Merge ID setting shows: "This project does not have ID Merge enabled." This means my project is using the Legacy ID Management system. However, in the data, I see that some distinct_id values have a $device: prefix. Can you explain what this means under the Legacy ID Management? Also, I thought that in Legacy ID Management, the distinct_id does not change until the user logs out. But in my data, I see a single distinct_id associated with more than two different user_ids. Can you help explain why this happens?

Certainly! Here’s what the knowledge sources say about your situation:

Under Legacy ID Management, Mixpanel did not natively use the $device: prefix for distinct_id values. The $device: prefix is a convention introduced with the Simplified ID Merge system, where anonymous users are tracked with a $device_id, and Mixpanel sets the distinct_id as $device:<device_id> for anonymous events. In Legacy, the distinct_id was typically just a randomly generated value (often a UUID) for anonymous users, and then switched to a user identifier upon login or identification.If you are seeing distinct_id values with a $device: prefix in a project that claims to be on Legacy ID Management, this is unexpected based on the official documentation. It may indicate that: - Some events were ingested using a newer SDK or integration that applies the newer convention, even though the project is set to Legacy. - There may have been a migration or partial update in your implementation, or data was imported from another project using a different ID management system.The documentation does not explicitly describe $device: prefixes appearing in Legacy projects, so this is not standard behavior for Legacy ID Management Understanding Simplified ID Merge.

In Legacy ID Management, the distinct_id is supposed to represent a single user until logout or reset. When a user logs in, the distinct_id is typically set to their user ID, and when they log out, it is reset to a new anonymous ID. The documentation states that aliasing (linking anonymous and identified states) can only be done once, and subsequent attempts to alias the same user ID to a different anonymous ID will fail Deciding to Migrate.If you are seeing a single distinct_id associated with more than two different user_ids, this is not expected in Legacy ID Management. Possible explanations (not directly covered in the documentation) could include: - Custom or non-standard implementation that reuses or incorrectly assigns distinct_id values. - Data imported or merged from other projects or systems with different ID management logic. - Use of SDKs or integrations that do not strictly follow the Legacy conventions.

- The $device: prefix in distinct_id is not standard for Legacy ID Management and is a convention from newer ID Merge systems. - In Legacy, a distinct_id should not be associated with multiple user_ids; if you see this, it likely results from custom implementation, data import, or SDK behavior outside the documented Legacy flow.If you need to ensure correct identity management, consider reviewing your SDK versions, event ingestion methods, and any data import processes for consistency with your project's ID management setting.If you need more precise troubleshooting, you may need to audit your implementation or contact Mixpanel support, as the documentation does not provide further details on this scenario.

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Understanding Mixpanel's Legacy ID Management and Distinct IDs

17 comments

1. `$device:` Prefix in distinct_id under Legacy ID Management

2. distinct_id Associated with Multiple user_ids in Legacy

Summary

1. distinct_id is the same as user_id

2. distinct_id is a device_id

3. distinct_id has a $device: prefix

4. distinct_id is neither a user_id nor a device_id

Summary

17 comments

1. $device: Prefix in distinct_id under Legacy ID Management

2. distinct_id Associated with Multiple user_ids in Legacy

Summary

1. distinct_id is the same as user_id

2. distinct_id is a device_id

3. distinct_id has a $device: prefix

4. distinct_id is neither a user_id nor a device_id

Summary

1. `$device:` Prefix in distinct_id under Legacy ID Management