> ## 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.

# Update a contact segment

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

Requires `segments:write`.

Supply at least one of `name`, `description`, or `definition`. Omitted fields remain unchanged. A supplied definition replaces all filter rules; `null` matches all contacts. Filter changes recalculate membership in the background.

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