X-FB-Debug
The HTTP X-FB-Debug response header is an unofficial header containing a Base64-encoded debug token added by Meta's CDN infrastructure to responses from Facebook, Instagram, WhatsApp, and other Meta platform domains.
Note
The "X-" naming convention for HTTP headers, "X" referring to "experimental", has been deprecated and needs to be transitioned to the formal naming convention for HTTP headers.
Usage
The X-FB-Debug header appears on HTTP responses from Meta's family of services, including facebook.com, instagram.com, whatsapp.com, and messenger.com. The header carries an opaque Base64-encoded token generated by Meta's internal CDN and load-balancing infrastructure.
The token is intended for internal request tracing and debugging by Meta engineers. External consumers cannot decode the value into meaningful information. The header serves a similar purpose to the X-Request-Id header, providing a reference point for correlating a specific response with internal server-side logs and diagnostic data.
Values
Base64-encoded token
The value is a Base64-encoded string, typically 88 characters
long including trailing == padding. Each response receives a
unique token. The encoded content is opaque and intended for
Meta's internal debugging systems.
Example
A typical X-FB-Debug value from a facebook.com response. The Base64 string is an opaque token with no publicly documented structure. Each request produces a different token.
X-FB-Debug: nTcmDD1KT1GHzug7errmzqaim9G5NE39ON1++jFhDXTu
pG+rxpu3x20DS/QpmNpy3k9ngdJ6mM9E8vT66dJZkw
A response from instagram.com carries the same header format. The token structure is consistent across all Meta platform domains.
X-FB-Debug: rNlWaacU6C6BIBIIOXZPSM+qMa7TIfxIOpxk2CS6RG0B
DDutEAf5Lz0/f9yulsUueiwDy6JjU/loegS8OoaJZQ
Takeaway
The X-FB-Debug header carries an opaque Base64-encoded token attached to responses from Meta's CDN infrastructure. The token exists for internal debugging and request tracing across Meta's services.