scikit-build-core "CMake ... not found" Build Failure in CI

scikit-build-core drives a CMake build to compile a native Python extension. It needs cmake (and usually ninja) plus a C/C++ toolchain. A missing build tool, or a CMake error, aborts the wheel build.

What this error means

A pip install of a scikit-build-core project fails with Could not find CMake, ninja is required, or a CMake configure/compile error. There is no wheel for this platform, so pip ran the CMake build.

Build output

*** scikit-build-core 0.9 using CMake ...
CMake Error: CMake was not found on the PATH. Provide cmake or install it
(e.g. `pip install cmake ninja`).

Common causes

CMake or ninja not installed

The runner lacks cmake/ninja. scikit-build-core can pull them from PyPI as build requires, but only if declared - otherwise the build cannot configure.

Missing C/C++ compiler or library

Even with CMake present, the configure step fails if the compiler or a required system library/header is absent.

How to fix it

Provide CMake and ninja via build requires

Let pip supply the build tools from PyPI in the isolated build env.

pyproject.toml

[build-system]
requires = ["scikit-build-core", "cmake>=3.15", "ninja"]
build-backend = "scikit_build_core.build"

Install the toolchain on the runner

For system CMake plus a compiler and any needed dev libraries.

Terminal

# Debian/Ubuntu
apt-get update && apt-get install -y build-essential cmake ninja-build

Read the CMake configure error

Scroll to the first CMake Error: line for the real cause (missing compiler, library, or header).
Install the named dev package, then re-run.
Prefer a prebuilt wheel (--only-binary) when one exists.

How to prevent it

Declare cmake/ninja in [build-system] requires for portability.
Bake the compiler and dev libraries into images that build from source.
Prefer wheels in CI to skip the CMake build.