Ruby pg gem "Can't find the PostgreSQL client library (libpq)"

The pg gem compiles a native extension that links against the PostgreSQL client library, libpq. The build fails because libpq (and its headers) or pg_config is not installed on the runner.

What this error means

Installing the pg gem fails during the native build with "Can't find the PostgreSQL client library (libpq)" or a missing pg_config. The gem cannot link without the Postgres client development files.

mkmf output

checking for pg_config... no
checking for libpq-fe.h... no
Can't find the PostgreSQL client library (libpq)
*** extconf.rb failed ***

Common causes

libpq development files not installed

The pg gem needs libpq-fe.h and the libpq library to compile. The runtime Postgres package alone does not include the -dev headers.

pg_config not on PATH

When pg_config is missing or not on PATH, extconf cannot locate the Postgres installation to link against.

How to fix it

Install the libpq development package

Terminal

# Debian/Ubuntu
apt-get update && apt-get install -y libpq-dev
# Alpine
apk add --no-cache postgresql-dev
# RHEL/Fedora
dnf install -y postgresql-devel

Point the gem at pg_config explicitly

If Postgres is installed in a non-standard prefix, tell the gem where pg_config lives.