user_properties from ExperimentUser are not forwarded to exposure/assignment events

[FabricioFFC]

Expected Behavior

When calling evaluateV2() with tracksExposure: true, the custom user_properties passed in ExperimentUser should be included in the [Experiment] Exposure event's user_properties.$set, so they are associated with the user profile in Amplitude.

const user: ExperimentUser = {
  user_id: 'user-123',
  device_id: 'device-456',
  user_properties: { anonymous: true, plan: 'premium' },
};

const variants = client.evaluateV2(user, ['my-flag'], { tracksExposure: true });

// Expected: Exposure event should include user_properties in $set
// {
//   event_type: '[Experiment] Exposure',
//   user_id: 'user-123',
//   device_id: 'device-456',
//   user_properties: {
//     $set: {
//       anonymous: true,           // <-- from ExperimentUser.user_properties
//       plan: 'premium',           // <-- from ExperimentUser.user_properties
//       '[Experiment] my-flag': 'treatment'
//     },
//     $unset: {}
//   }
// }

Possible Solution

Merge exposure.user.user_properties into the $set object when building the event:

user_properties: {
  $set: { ...exposure.user.user_properties, ...set },
  $unset: unset,
},

Current Behavior

The user_properties from ExperimentUser are only used for local evaluation targeting and are not forwarded to exposure events.

The same issue exists in assignment-service.ts.

Steps to Reproduce

1. Initialize local evaluation client with exposureConfig
2. Create an ExperimentUser with custom user_properties
3. Call evaluateV2() with tracksExposure: true
4. Check the exposure event in Amplitude - custom user_properties are missing

import { Experiment } from '@amplitude/experiment-node-server';

const client = Experiment.initializeLocal('DEPLOYMENT_KEY', {
  exposureConfig: { apiKey: 'API_KEY' },
});
await client.start();

const user = {
  user_id: 'user-123',
  device_id: 'device-456',
  user_properties: { anonymous: true },
};

// user_properties is used for targeting but NOT sent to Amplitude
const variants = client.evaluateV2(user, ['my-flag'], { tracksExposure: true });

Environment
JS Server SDK Version: 1.13.2
Installation Method: yarn
Runtime: Node.js

[bgiori]

Hi @FabricioFFC We dont currently track the user properties passed into evaluation on purpose. Can you elaborate more on the use case? We generally expect the application to explicitly track user properties using on of our analytics SDKs.

I can see the value for this, just want to understand better! Thanks again for submitting such a detailed ticket.

[FabricioFFC]

Hi @bgiori, for our use case, we need the user properties passed in the exposure event to be available so we can filter and analyze exposure events in the Amplitude dashboard using those same properties.