Hidden Fields

Sometimes we surface hidden fields that are too long to be added as hidden fields in Typeform. These can be accessed in the text of a question/statement using the following syntax:

{{hidden:e_payment_http_result_message_success}}

Where the “too long” hidden field is e_payment_http_result_message_success which would be populated, for example, if you had a http payment result that looked like this: {"message": {"success": "foo"}} and you wanted to show “foo”.

Fly supports mustache-style interpolation in question titles and descriptions. You can reference:

• Hidden fields: {{hidden:field_name}} — metadata from the user, referral params, or payment results
• Previous answers: {{field:question_ref}} — the raw answer to a previous question

You can apply transforms to interpolated values using pipe syntax:

{{field:phone|e164}}

This looks up the answer to the phone question, then applies the e164 transform to normalize it. Transforms are applied left-to-right, so chaining is possible:

{{field:phone|e164|lower}}

When a user answers a phone number question, their response may include trailing text — for example +254712345678 use this instead of just +254712345678. The raw answer is preserved as-is in the survey data, which is important for the audit trail.

However, payment providers require a clean E.164 phone number. Without the |e164 transform, the messy raw value would be sent directly to the provider, which can cause:

1. Payment failures — the provider rejects the malformed number
2. Duplicate payments — two submissions of the same number with different trailing text produce different custom_identifier values, bypassing duplicate detection

To avoid these issues, always use |e164 when referencing phone fields in payment details:

"details": {
    "mobile": "{{field:phone|e164}}",
    "custom_identifier": "survey_x_{{field:phone|e164}}_1"
}

Without the transform, {{field:phone}} resolves to the raw stored value (e.g. +254712345678 use this). With |e164, it resolves to the normalized E.164 number (+254712345678).