> ## Documentation Index
> Fetch the complete documentation index at: https://docs.open.cx/llms.txt
> Use this file to discover all available pages before exploring further.

# Create a contact segment

> Create a contact segment through the public API with segment write permission. Learn the request fields, responses, and membership behavior.

Requires `segments:write`.

Supply `name` and `definition`. Omit `description` to use `null`. Membership is calculated in the background. A `null` definition matches all contacts.

See [contact segment guidance](/api-reference/contacts/segments/introduction) for filter examples and membership timing.