diff --git a/.github/actions/build-python/action.yml b/.github/actions/build-python/action.yml
new file mode 100644
index 00000000..fa310e81
--- /dev/null
+++ b/.github/actions/build-python/action.yml
@@ -0,0 +1,46 @@
+name: Build a Python version
+description: >
+  Install python-build's system dependencies for the current runner's OS, build
+  and check the requested version.
+inputs:
+  python-version:
+    description: A version as accepted by `pyenv install`.
+    required: true
+runs:
+  using: composite
+  steps:
+    # The two OSes differ only in this step.
+    - if: runner.os == 'macOS'
+      shell: bash
+      run: |
+        #prerequisites
+        brew install openssl readline sqlite3 xz tcl-tk@8 libb2 zstd
+    - if: runner.os == 'Linux'
+      shell: bash
+      run: |
+        #prerequisites
+        sudo apt-get update -q; sudo apt install -yq make build-essential libssl-dev zlib1g-dev \
+        libbz2-dev libreadline-dev libsqlite3-dev curl \
+        libncursesw5-dev xz-utils tk-dev libxml2-dev libxmlsec1-dev libffi-dev liblzma-dev
+    - shell: bash
+      run: |
+        #envvars
+        export PYENV_ROOT="$GITHUB_WORKSPACE"
+        echo "PYENV_ROOT=$PYENV_ROOT" >> $GITHUB_ENV
+        echo "$PYENV_ROOT/shims:$PYENV_ROOT/bin" >> $GITHUB_PATH
+    - shell: bash
+      run: |
+        #build
+        pyenv --debug install ${{ inputs.python-version }} && rc=$? || rc=$?
+        if [[ $rc -ne 0 ]]; then echo config.log:; cat "${TMPDIR:-/tmp}"/python-build*/*/config.log; false; fi
+        pyenv global ${{ inputs.python-version }}
+        pyenv rehash
+    - shell: bash
+      run: |
+        #print version
+        python --version
+        python -m pip --version
+    - shell: python  # Prove that actual Python == expected Python
+      env:
+        EXPECTED_PYTHON: ${{ inputs.python-version }}
+      run: import os, sys ; assert sys.version.startswith(os.getenv("EXPECTED_PYTHON"))
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
new file mode 100644
index 00000000..3fc2e674
--- /dev/null
+++ b/.github/workflows/build.yml
@@ -0,0 +1,62 @@
+name: build
+on:
+  push:
+    branches: [master]
+  pull_request: {}
+
+permissions:
+  contents: read
+
+jobs:
+  discover_build_logic_change:
+    if: github.event_name == 'pull_request'
+    runs-on: ubuntu-slim
+    env:
+      FULL_OS_MATRIX: '[
+        "macos-14",
+        "macos-15",
+        "macos-15-intel",
+        "macos-26",
+        "macos-26-intel",
+        "ubuntu-22.04",
+        "ubuntu-22.04-arm",
+        "ubuntu-24.04",
+        "ubuntu-24.04-arm"
+      ]'
+    outputs:
+      os: ${{ steps.detect.outputs.os }}
+    steps:
+      - uses: actions/checkout@v7
+      - run: git fetch origin "$GITHUB_BASE_REF"
+      - id: detect
+        shell: bash
+        run: |
+          if [[ -n $(git diff --name-only "origin/$GITHUB_BASE_REF" -- \
+                       plugins/python-build/bin/python-build \
+                       .github/actions/build-python/action.yml \
+                       .github/workflows/build.yml) ]]
+          then
+            printf "%s\n" "os<<!" "$FULL_OS_MATRIX" "!" >> "$GITHUB_OUTPUT"
+          fi
+
+  build:
+    needs: discover_build_logic_change
+    # Run when the detection job succeeded *or was skipped* -- but not failed
+    if: ${{ !cancelled() && !failure() }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: ${{ fromJson(needs.discover_build_logic_change.outputs.os || '["macos-latest","ubuntu-latest"]') }}
+        python-version:
+          - "3.10"
+          - "3.11"
+          - "3.12"
+          - "3.13"
+          - "3.14"
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v7
+      - uses: ./.github/actions/build-python
+        with:
+          python-version: ${{ matrix.python-version }}
+
diff --git a/.github/workflows/macos_build.yml b/.github/workflows/macos_build.yml
deleted file mode 100644
index b30511ac..00000000
--- a/.github/workflows/macos_build.yml
+++ /dev/null
@@ -1,43 +0,0 @@
-name: macos_build
-on:
-  push:
-    branches: [master]
-  pull_request: {}
-
-permissions:
-  contents: read  #  to fetch code (actions/checkout)
-
-jobs:
-  macos_build:
-    strategy:
-      fail-fast: false
-      matrix:
-        python-version:
-          - "3.10"
-          - "3.11"
-          - "3.12"
-          - "3.13"
-          - "3.14"
-    runs-on: macos-latest
-    steps:
-      - uses: actions/checkout@v7
-      - run: |
-          brew install openssl readline sqlite3 xz tcl-tk@8 libb2 zstd
-      - run: |
-          #envvars
-          export PYENV_ROOT="$GITHUB_WORKSPACE"
-          echo "PYENV_ROOT=$PYENV_ROOT" >> $GITHUB_ENV
-          echo "$PYENV_ROOT/shims:$PYENV_ROOT/bin" >> $GITHUB_PATH
-      - run: |
-          #build
-          pyenv --debug install ${{ matrix.python-version }} && rc=$? || rc=$?
-          if [[ $rc -ne 0 ]]; then echo config.log:; cat $TMPDIR/python-build*/*/config.log; false; fi
-          pyenv global ${{ matrix.python-version }}
-      - run: |
-          #print version
-          python --version
-          python -m pip --version
-      - shell: python  # Prove that actual Python == expected Python
-        env:
-          EXPECTED_PYTHON: ${{ matrix.python-version }}
-        run: import os, sys ; assert sys.version.startswith(os.getenv("EXPECTED_PYTHON"))
diff --git a/.github/workflows/ubuntu_build.yml b/.github/workflows/ubuntu_build.yml
deleted file mode 100644
index 697f4bd7..00000000
--- a/.github/workflows/ubuntu_build.yml
+++ /dev/null
@@ -1,43 +0,0 @@
-name: ubuntu_build
-on:
-  push:
-    branches: [master]
-  pull_request: {}
-
-permissions:
-  contents: read  #  to fetch code (actions/checkout)
-
-jobs:
-  ubuntu_build:
-    strategy:
-      fail-fast: false
-      matrix:
-        python-version:
-          - "3.10"
-          - "3.11"
-          - "3.12"
-          - "3.13"
-          - "3.14"
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v7
-      - run: |
-          sudo apt-get update -q; sudo apt install -yq make build-essential libssl-dev zlib1g-dev \
-          libbz2-dev libreadline-dev libsqlite3-dev curl \
-          libncursesw5-dev xz-utils tk-dev libxml2-dev libxmlsec1-dev libffi-dev liblzma-dev
-      # https://github.com/pyenv/pyenv#installation
-      - run: pwd
-      - env:
-          PYENV_ROOT: /home/runner/work/pyenv/pyenv
-        run: |
-          echo $PYENV_ROOT
-          echo "$PYENV_ROOT/shims:$PYENV_ROOT/bin" >> $GITHUB_PATH
-          bin/pyenv --debug install ${{ matrix.python-version }}
-          bin/pyenv global ${{ matrix.python-version }}
-          bin/pyenv rehash
-      - run: python --version
-      - run: python -m pip --version
-      - shell: python  # Prove that actual Python == expected Python
-        env:
-          EXPECTED_PYTHON: ${{ matrix.python-version }}
-        run: import os, sys ; assert sys.version.startswith(os.getenv("EXPECTED_PYTHON"))