CMake Error: Compatibility with CMake < 3.5 has been removed from CMake.

CMake 4.0 removed compatibility with versions older than 3.5, so cmake_minimum_required(VERSION X) with X below 3.5 is now a fatal error: Compatibility with CMake < 3.5 has been removed from CMake. Raise the version in cmake_minimum_required (3.10 or later avoids the follow-up deprecation warning). If the line is in a dependency you cannot change, configure with -DCMAKE_POLICY_VERSION_MINIMUM=3.5, as the message itself suggests.

Reproduce it

The block runs CMake 4.4 from the cmake PyPI package with the Ninja generator; project(demo NONE) needs no compiler:

cat > CMakeLists.txt <<'EOF'
cmake_minimum_required(VERSION 3.0)
project(demo NONE)
message(STATUS "configured with CMake ${CMAKE_VERSION}")
EOF
uv run -q --no-project --python 3.13 --with cmake==4.4.4 --with ninja==1.13.0 cmake -S . -B build -G Ninja

CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.

-- Configuring incomplete, errors occurred!

Fix your own project

cat > CMakeLists.txt <<'EOF'
cmake_minimum_required(VERSION 3.5...3.31)
project(demo NONE)
message(STATUS "configured with CMake ${CMAKE_VERSION}")
EOF
uv run -q --no-project --python 3.13 --with cmake==4.4.4 --with ninja==1.13.0 cmake -S . -B build -G Ninja | grep -vE 'Configuring done|Generating done|Build files'

-- configured with CMake 4.4.4

VERSION 3.5...3.31 means "requires at least 3.5, and behaves like 3.31 when available": older CMake 3 releases still accept the file, while CMake 4 sets every policy up to 3.31 to NEW. A plain VERSION 3.5 would also configure, but prints a deprecation warning because compatibility with versions below 3.10 is scheduled for removal next.

Third-party code: CMAKE_POLICY_VERSION_MINIMUM

cat > CMakeLists.txt <<'EOF'
cmake_minimum_required(VERSION 3.0)
project(demo NONE)
message(STATUS "configured with CMake ${CMAKE_VERSION}")
EOF
uv run -q --no-project --python 3.13 --with cmake==4.4.4 --with ninja==1.13.0 cmake -S . -B build -G Ninja -DCMAKE_POLICY_VERSION_MINIMUM=3.5 2>&1 | grep -vE 'Configuring done|Generating done|Build files'

CMake Warning (deprecated) at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.
This warning is for project developers.  Use -Wno-author or -Wno-deprecated
to suppress it.

-- configured with CMake 4.4.4

CMAKE_POLICY_VERSION_MINIMUM (CMake 4.0 and later) raises the effective minimum of every cmake_minimum_required call, including those in subprojects. It can also be set as an environment variable, which helps when CMake is called by another tool (pip, cargo, vcpkg, conan):

export CMAKE_POLICY_VERSION_MINIMUM=3.5
pip install some-package-with-cmake-build

Find out where the old version comes from

The first line of the error names the file and line: CMake Error at CMakeLists.txt:1 (cmake_minimum_required) is your own top-level file, while a path such as build/_deps/somelib-src/CMakeLists.txt or third_party/somelib/CMakeLists.txt points to a dependency. To list every declaration in a source tree at once:

grep -rn --include=CMakeLists.txt --include='*.cmake' -iE 'cmake_(minimum_required|policy)\s*\(\s*VERSION' .

Any version below 3.5 in that list fails on CMake 4; versions from 3.5 up to 3.9 configure but print the deprecation warning shown above. Run cmake --version to confirm which CMake is actually running: a Python virtual environment, a CI image or a package manager can put a newer cmake first on PATH than the one you installed.

Which fix to choose

After changing the minimum, delete the build directory or configure with --fresh, so no cached state from the failed run is reused. See CMake 3 to 4 for the other changes in CMake 4.0.

Prevention

Declare a range (min...max) rather than a single version, and raise max when you test a new CMake release. Pin the CMake version in CI (for example cmake==4.4.4 from PyPI, as in the blocks above) and upgrade it deliberately, so a new major release cannot break the build without a code change. When you vendor or fetch a library, check its cmake_minimum_required line as part of the review.

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