Hi Team,
In case there is any custom activity action present on the Zapier flow of the customer, we pass only 15 fields in the action of Zapier, irrespective of the number of fields present on that activity.
Ex: There are 20 fields in a custom activity, with schema name mx_Custom_1, mx_Custom_2……..mx_Custom_20. We only pass the data to Zapier until the mx_Custom_15 field.
As confirmed by the dev team, the limit of the activity fields in ZAP cannot be updated.
However, they have suggested a workaround for the same:
While creating the trigger, choose the Webhook option:

In Event, choose to Create a Hook:

Under the test trigger, you would get the webhook created:

Copy the Webhook URL.
Create one automation that triggers the addition of the activity which you are choosing. Use a PostWebhook card in the automation and add the URL:

Once done, choose the necessary action as per your use case and publish the ZAP.
Then, publish the automation.
In this, the automation would be sending all the activity field data to the ZAP action.
Hope this helps you.
Thanks and Regards,
Ayush Sharma
LeadSquared Support Team
