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Use of the x- Prefix in HTTP Headers

Author: guiferviz

Created:

Last Modified:

The x- prefix in HTTP headers historically indicates a custom or non-standard header.

Background

Originally, x- was used to mark experimental or private headers that were not part of the official HTTP specification.

Over time, many x-* headers became widely adopted, which caused problems when trying to standardize them later (duplicate names, compatibility issues). For this reason, modern RFCs discourage using x- for headers intended to become public standards.

Practical Usage Today

Despite this, x- remains a common and accepted convention for:

  • private APIs
  • internal services
  • system-specific integrations
  • headers with no intention of standardization

In these contexts, x- clearly signals:

  • “this header is custom”
  • “this is not part of the HTTP standard”
  • “this header is specific to this system”

Conclusion

Using the x- prefix is discouraged only when designing headers meant to be standardized or publicly adopted. For private, internal, or integration-specific APIs, it remains a clear and practical choice.