Ruby gem build "checking for libxml-2.0 ... no" in CI

During a native gem install, mkmf probes for a system library and prints "checking for libxml-2.0 ... no" when it cannot find it. Nokogiri and similar gems then fail because the dependency they compile against is not available where they look.

What this error means

Installing nokogiri (or a libxml2/libxslt-backed gem) prints "checking for libxml-2.0 ... no" and aborts with guidance to install libxml2 or use the precompiled gem. The runtime library or its -dev package is missing or unfindable.

checking for libxml-2.0... no
ERROR: Failed to build gem native extension.

Please install libxml2 and libxslt development packages, or run:
  gem install nokogiri -- --use-system-libraries

Common causes

libxml2/libxslt development packages are missing

The gem can use system libxml2 but the -dev packages providing headers and pkg-config metadata are not installed, so the check fails.

pkg-config cannot locate the library

The library is installed in a non-standard prefix that pkg-config does not search, so mkmf still reports "no".

How to fix it

Install the libxml2/libxslt dev packages

Provide the headers and pkg-config files mkmf is probing for.

# Debian/Ubuntu
sudo apt-get update
sudo apt-get install -y libxml2-dev libxslt1-dev pkg-config
bundle install --jobs 4

Use the precompiled nokogiri instead

Let Bundler install the platform-native nokogiri that bundles libxml2, avoiding the system probe entirely.

# ensure the platform variant is in the lockfile
bundle lock --add-platform x86_64-linux
bundle install

How to prevent it

• Prefer precompiled platform gems for nokogiri so no system libxml2 is needed.
• If building from source, install libxml2-dev, libxslt1-dev, and pkg-config in CI.
• Add the right platform to the lockfile so the precompiled gem is selected on CI.

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