---
title: "Restricted Keywords for UIDs"
description: "Learn which keywords are restricted for UIDs in Contentstack content types and fields to avoid errors and ensure smooth functionality."
url: "https://www.contentstack.com/docs/headless-cms/restricted-keywords-for-uids"
product: "Contentstack"
doc_type: "guide"
audience:
  - developers
  - admins
version: "current"
last_updated: "2026-07-22"
---

# Restricted Keywords for UIDs

When providing user-defined unique IDs (UIDs) for [content type](/docs/headless-cms/about-content-types), [fields](/docs/headless-cms/about-fields/), and [taxonomies](/docs/headless-cms/about-taxonomy), avoid using certain reserved keywords.

**Warning**: Avoid using these keywords as their usage may lead to code failure.

## Restricted keywords for UIDs for Content Type

The following set of keywords cannot be used as the UID of a content type:

*   api\_key
*   built\_io\_application\_user
*   built\_io\_application\_user\_role
*   built\_io\_audit\_log
*   built\_io\_environment
*   built\_io\_extensions
*   built\_io\_installation\_data
*   built\_io\_label
*   built\_io\_language
*   built\_io\_publish\_queue
*   built\_io\_release
*   built\_io\_upload
*   cs\_branches
*   org\_uid
*   sys\_asset
*   sys\_metadata

**Note**: Avoid using hyphens (-) in UIDs for content types.

## Restricted keywords for UIDs for Fields

The following set of keywords cannot be used as the UID of a field:

*   \_exists
*   \_regex
*   \*\_ids - Fields cannot have UID ending with “\_ids.”
*   \_\_indexes
*   \_\_loc
*   \_\_meta
*   \_\_v
*   \_id
*   \_owner
*   \_publish\_locales
*   \_shouldFilter
*   \_shouldLean
*   \_version
*   ACL
*   api\_key
*   app\_user\_object\_uid
*   applikation\_id
*   built\_io\_upload
*   contentstackFilters
*   created\_at
*   created\_by
*   DEFAULT\_ACL
*   deleted\_at
*   dimension
*   domain
*   embedded\_items
*   hook
*   id
*   inbuilt\_class
*   isApplicationUser
*   isNew
*   isSystemUser
*   klass\_id
*   locale
*   options
*   org\_uid
*   publish\_details
*   save
*   shard\_account
*   shard\_app
*   shard\_random
*   SYS\_ACL
*   sys\_assets
*   sys\_metadata
*   tags
*   tags\_array
*   taxonomies
*   tenant\_id
*   toJSON
*   uid
*   update
*   updated\_at
*   updated\_by

## Restricted Keywords for UIDs for Taxonomies

When creating taxonomies, the following restrictions apply:

*   Do **not** use entries as the UID for a taxonomy.
*   Do **not** use a UID that starts with an underscore (\_).