Crystal Ball checks covered

TEXT_COMPRESSION_NOT_DETECTED, CONTENT_ENCODING_INVALID

What this warning means

Missing compression means a text response was delivered without a recognized gzip, Brotli, or deflate encoding. Invalid compression is more serious: the server declared an encoding that the response body did not use correctly.

  • Invalid Content-Encoding can make a browser or scanner fail before it can parse the page.
  • Compression is normally useful for HTML, CSS, JavaScript, XML, JSON, and SVG, not already-compressed images, modern web fonts, video, or archives.

Common causes

The usual causes are duplicate compression at the origin and CDN, stale proxy metadata, a cache that mixes encoded and unencoded variants, or a server rule that labels a body without actually encoding it.

  • A CDN and origin may both try to transform the same response.
  • A cache key may ignore Accept-Encoding.
  • A plugin or proxy may decompress the body but leave the original Content-Encoding header.

How to fix it

Choose one dependable layer to negotiate compression and make caches vary by encoding.

  • Enable Brotli or gzip for compressible MIME types.
  • Send Vary: Accept-Encoding where shared caches can serve multiple variants.
  • Avoid compressing images, video, ZIP files, and other formats that are already compressed.
  • Purge CDN and proxy caches after changing compression rules.

How to verify

Request the same URL with and without Accept-Encoding and confirm that each response can be decoded and parsed.

  • Inspect Content-Encoding, Vary, Content-Type, and final status.
  • Test HTML and representative CSS, JavaScript, XML, and JSON assets.
  • Verify the response in a browser and rerun Crystal Ball after caches are cleared.

FAQ

Is Brotli always better than gzip?

Brotli often compresses text more efficiently, but correct negotiation, caching, and server cost matter more than choosing it blindly.

Why does the page fail only in one scanner?

Different clients send different Accept-Encoding values. A broken cache or proxy variant can therefore affect one client while another receives a valid body.

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