# ACTIVATION_TIMESTAMP_INVALID

- Surface: API
- HTTP status: 400
- Details: `{ activationTs: string, firstExposureTs: string, message: "activation must occur after first exposure" }`

## Cause

An activation timestamp was placed at or before the first Exposure it would filter, which would silently drop the Entities the Run already measured.

## Fix

Move the activation timestamp after `details.firstExposureTs`. An activation point earlier than the first Exposure cannot be honored without discarding measured data, so the platform refuses it rather than quietly truncating the analysis window.

## Related

- [VALIDATION_ERROR](https://splitch.dev/docs/error/VALIDATION_ERROR)

Source: https://splitch.dev/docs/error/ACTIVATION_TIMESTAMP_INVALID