---
title: "Profiles that don't join across devices: confirm the problem"
description: "Confirm by hand whether the same signed-in account shows up as one Amplitude user across web and mobile."
product: data
token_estimate: 1052
---
# Profiles that don't join across devices: confirm the problem

> For AI agents: a documentation index is available at [/docs/llms.txt](/docs/llms.txt). Append `.md` to any page URL for markdown, or send `Accept: text/markdown`.

Amplitude should represent a person who uses your product on the web and on their phone as one user. When each platform sends a different User ID for the same account, Amplitude creates separate profiles. Cross-device journeys break, retention appears lower than the actual rate, and active-user counts increase.

Data Assistant doesn't compare identities across platforms. Use a two-device test to confirm an identity mismatch. Fix the mismatch in your instrumentation by sending one canonical User ID from every platform.

Run this audit when you suspect that Amplitude has split one account into separate web and mobile profiles or when you add a platform. The audit requires a dedicated test account with a known canonical account ID and access to two devices or platforms.

![Diagram showing web and mobile activity joining into one user profile when both send the same User ID, versus splitting into two profiles when identity differs by platform](https://amplitude.com/docs/images/diagrams/cross-device-join.svg) ![Diagram showing web and mobile activity joining into one user profile when both send the same User ID, versus splitting into two profiles when identity differs by platform](https://amplitude.com/docs/images/diagrams/cross-device-join.dark.svg)

## Run a cross-device test

1. Choose a test account with a known canonical account ID.
2. Sign in on the first platform, such as web, and trigger several identifiable actions.
3. Sign in with the same account on the second platform, such as mobile, and trigger different identifiable actions.
4. In Amplitude, go to _Users > User Profiles_ and search for the canonical User ID.
5. Open the profile and confirm its activity timeline holds events from both platforms.
6. Open one event from each platform, select **Raw**, and compare the `user_id` values.
7. Search for any platform-specific ID you suspect and check whether it creates a separate profile.

## Interpret cross-device test results

- If the same profile holds both journeys and both platforms send the identical `user_id`, the sample passes.
- If each platform creates a different profile, record both User IDs, their casing, source libraries, and event times.
- If one platform never sends a User ID, treat the issue as a missing-User-ID problem and use the [missing User IDs](https://amplitude.com/docs/data/data-assistant-user-solutions/missing-user-ids-audit) guide.

This audit doesn't scan every account. To audit all accounts, ask your data team to compare platform identifiers against your canonical account table in a privacy-approved environment.

## Limitations of cross-device identity in Amplitude

Amplitude joins a person across devices when every platform sends the same User ID. Amplitude can't merge two profiles that already formed from mismatched identity in a standard project. Align the platform instrumentation based on which platform sends the mismatched User ID in the cross-device test.

## Report the mismatch to the tracking owner

Send the mismatch to the engineers who own tracking. Copy and complete this template:

```text
In Amplitude, the same account shows up as separate profiles per platform. On
[PLATFORM A] the user_id is [VALUE A]; on [PLATFORM B] it's [VALUE B] (or
missing). Same person, two profiles, so cross-device retention and journeys are
broken. Send the same immutable, case-consistent user_id from every
platform for a signed-in person.
```

## Get help without engineering resources

You can't align the platforms inside Amplitude. Contact the following resources in order:

- Your Amplitude account team or Customer Success Manager.
- A partner or agency that manages your website or app analytics.
- Amplitude Support at [gethelp.amplitude.com](https://gethelp.amplitude.com) for product questions.

When you're ready to align identity across platforms, continue with the [engineering solution](https://amplitude.com/docs/data/data-assistant-user-solutions/cross-device-identity-engineering).