diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index c10c5b4aa46ffb..dc2f4858be6e8c 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -22,25 +22,25 @@ env:
   FORCE_COLOR: 1
 
 jobs:
-  check_source:
+  build-context:
     name: Change detection
     # To use boolean outputs from this job, parse them as JSON.
     # Here's some examples:
     #
-    #   if: fromJSON(needs.check_source.outputs.run-docs)
+    #   if: fromJSON(needs.build-context.outputs.run-docs)
     #
     #   ${{
-    #        fromJSON(needs.check_source.outputs.run_tests)
+    #        fromJSON(needs.build-context.outputs.run-tests)
     #        && 'truthy-branch'
     #        || 'falsy-branch'
     #   }}
     #
-    uses: ./.github/workflows/reusable-change-detection.yml
+    uses: ./.github/workflows/reusable-context.yml
 
   check-docs:
     name: Docs
-    needs: check_source
-    if: fromJSON(needs.check_source.outputs.run-docs)
+    needs: build-context
+    if: fromJSON(needs.build-context.outputs.run-docs)
     uses: ./.github/workflows/reusable-docs.yml
 
   check_autoconf_regen:
@@ -51,8 +51,8 @@ jobs:
     container:
       image: ghcr.io/python/autoconf:2025.01.02.12581854023
     timeout-minutes: 60
-    needs: check_source
-    if: needs.check_source.outputs.run_tests == 'true'
+    needs: build-context
+    if: needs.build-context.outputs.run-tests == 'true'
     steps:
       - name: Install Git
         run: |
@@ -94,8 +94,8 @@ jobs:
     # reproducible: to get the same tools versions (autoconf, aclocal, ...)
     runs-on: ubuntu-24.04
     timeout-minutes: 60
-    needs: check_source
-    if: needs.check_source.outputs.run_tests == 'true'
+    needs: build-context
+    if: needs.build-context.outputs.run-tests == 'true'
     steps:
       - uses: actions/checkout@v4
         with:
@@ -110,7 +110,7 @@ jobs:
         with:
           path: config.cache
           # Include env.pythonLocation in key to avoid changes in environment when setup-python updates Python
-          key: ${{ github.job }}-${{ runner.os }}-${{ env.IMAGE_VERSION }}-${{ needs.check_source.outputs.config_hash }}-${{ env.pythonLocation }}
+          key: ${{ github.job }}-${{ runner.os }}-${{ env.IMAGE_VERSION }}-${{ needs.build-context.outputs.config-hash }}-${{ env.pythonLocation }}
       - name: Install Dependencies
         run: sudo ./.github/workflows/posix-deps-apt.sh
       - name: Add ccache to PATH
@@ -153,8 +153,8 @@ jobs:
     name: >-
       Windows
       ${{ fromJSON(matrix.free-threading) && '(free-threading)' || '' }}
-    needs: check_source
-    if: fromJSON(needs.check_source.outputs.run_tests)
+    needs: build-context
+    if: fromJSON(needs.build-context.outputs.run-tests)
     strategy:
       fail-fast: false
       matrix:
@@ -184,8 +184,8 @@ jobs:
   build_windows_msi:
     name: >-  # ${{ '' } is a hack to nest jobs under the same sidebar category
       Windows MSI${{ '' }}
-    needs: check_source
-    if: fromJSON(needs.check_source.outputs.run-win-msi)
+    needs: build-context
+    if: fromJSON(needs.build-context.outputs.run-windows-msi)
     strategy:
       matrix:
         arch:
@@ -200,8 +200,8 @@ jobs:
     name: >-
       macOS
       ${{ fromJSON(matrix.free-threading) && '(free-threading)' || '' }}
-    needs: check_source
-    if: needs.check_source.outputs.run_tests == 'true'
+    needs: build-context
+    if: needs.build-context.outputs.run-tests == 'true'
     strategy:
       fail-fast: false
       matrix:
@@ -226,7 +226,7 @@ jobs:
           free-threading: true
     uses: ./.github/workflows/reusable-macos.yml
     with:
-      config_hash: ${{ needs.check_source.outputs.config_hash }}
+      config_hash: ${{ needs.build-context.outputs.config-hash }}
       free-threading: ${{ matrix.free-threading }}
       os: ${{ matrix.os }}
 
@@ -235,8 +235,8 @@ jobs:
       Ubuntu
       ${{ fromJSON(matrix.free-threading) && '(free-threading)' || '' }}
       ${{ fromJSON(matrix.bolt) && '(bolt)' || '' }}
-    needs: check_source
-    if: needs.check_source.outputs.run_tests == 'true'
+    needs: build-context
+    if: needs.build-context.outputs.run-tests == 'true'
     strategy:
       matrix:
         bolt:
@@ -257,7 +257,7 @@ jobs:
           bolt: true
     uses: ./.github/workflows/reusable-ubuntu.yml
     with:
-      config_hash: ${{ needs.check_source.outputs.config_hash }}
+      config_hash: ${{ needs.build-context.outputs.config-hash }}
       bolt-optimizations: ${{ matrix.bolt }}
       free-threading: ${{ matrix.free-threading }}
       os: ${{ matrix.os }}
@@ -266,8 +266,8 @@ jobs:
     name: 'Ubuntu SSL tests with OpenSSL'
     runs-on: ${{ matrix.os }}
     timeout-minutes: 60
-    needs: check_source
-    if: needs.check_source.outputs.run_tests == 'true'
+    needs: build-context
+    if: needs.build-context.outputs.run-tests == 'true'
     strategy:
       fail-fast: false
       matrix:
@@ -289,7 +289,7 @@ jobs:
       uses: actions/cache@v4
       with:
         path: config.cache
-        key: ${{ github.job }}-${{ runner.os }}-${{ env.IMAGE_VERSION }}-${{ needs.check_source.outputs.config_hash }}
+        key: ${{ github.job }}-${{ runner.os }}-${{ env.IMAGE_VERSION }}-${{ needs.build-context.outputs.config-hash }}
     - name: Register gcc problem matcher
       run: echo "::add-matcher::.github/problem-matchers/gcc.json"
     - name: Install Dependencies
@@ -326,18 +326,18 @@ jobs:
 
   build_wasi:
     name: 'WASI'
-    needs: check_source
-    if: needs.check_source.outputs.run_tests == 'true'
+    needs: build-context
+    if: needs.build-context.outputs.run-tests == 'true'
     uses: ./.github/workflows/reusable-wasi.yml
     with:
-      config_hash: ${{ needs.check_source.outputs.config_hash }}
+      config_hash: ${{ needs.build-context.outputs.config-hash }}
 
   test_hypothesis:
     name: "Hypothesis tests on Ubuntu"
     runs-on: ubuntu-24.04
     timeout-minutes: 60
-    needs: check_source
-    if: needs.check_source.outputs.run_tests == 'true' && needs.check_source.outputs.run_hypothesis == 'true'
+    needs: build-context
+    if: needs.build-context.outputs.run-tests == 'true'
     env:
       OPENSSL_VER: 3.0.15
       PYTHONSTRICTEXTENSIONBUILD: 1
@@ -384,7 +384,7 @@ jobs:
       uses: actions/cache@v4
       with:
         path: ${{ env.CPYTHON_BUILDDIR }}/config.cache
-        key: ${{ github.job }}-${{ runner.os }}-${{ env.IMAGE_VERSION }}-${{ needs.check_source.outputs.config_hash }}
+        key: ${{ github.job }}-${{ runner.os }}-${{ env.IMAGE_VERSION }}-${{ needs.build-context.outputs.config-hash }}
     - name: Configure CPython out-of-tree
       working-directory: ${{ env.CPYTHON_BUILDDIR }}
       run: |
@@ -452,8 +452,8 @@ jobs:
     name: 'Address sanitizer'
     runs-on: ${{ matrix.os }}
     timeout-minutes: 60
-    needs: check_source
-    if: needs.check_source.outputs.run_tests == 'true'
+    needs: build-context
+    if: needs.build-context.outputs.run-tests == 'true'
     strategy:
       matrix:
         os: [ubuntu-24.04]
@@ -471,7 +471,7 @@ jobs:
       uses: actions/cache@v4
       with:
         path: config.cache
-        key: ${{ github.job }}-${{ runner.os }}-${{ env.IMAGE_VERSION }}-${{ needs.check_source.outputs.config_hash }}
+        key: ${{ github.job }}-${{ runner.os }}-${{ env.IMAGE_VERSION }}-${{ needs.build-context.outputs.config-hash }}
     - name: Register gcc problem matcher
       run: echo "::add-matcher::.github/problem-matchers/gcc.json"
     - name: Install Dependencies
@@ -515,8 +515,8 @@ jobs:
     name: >-
       Thread sanitizer
       ${{ fromJSON(matrix.free-threading) && '(free-threading)' || '' }}
-    needs: check_source
-    if: needs.check_source.outputs.run_tests == 'true'
+    needs: build-context
+    if: needs.build-context.outputs.run-tests == 'true'
     strategy:
       matrix:
         free-threading:
@@ -524,14 +524,14 @@ jobs:
         - true
     uses: ./.github/workflows/reusable-tsan.yml
     with:
-      config_hash: ${{ needs.check_source.outputs.config_hash }}
+      config_hash: ${{ needs.build-context.outputs.config-hash }}
       free-threading: ${{ matrix.free-threading }}
 
   cross-build-linux:
     name: Cross build Linux
     runs-on: ubuntu-latest
-    needs: check_source
-    if: needs.check_source.outputs.run_tests == 'true'
+    needs: build-context
+    if: needs.build-context.outputs.run-tests == 'true'
     steps:
       - uses: actions/checkout@v4
         with:
@@ -542,7 +542,7 @@ jobs:
         uses: actions/cache@v4
         with:
           path: config.cache
-          key: ${{ github.job }}-${{ runner.os }}-${{ env.IMAGE_VERSION }}-${{ needs.check_source.outputs.config_hash }}
+          key: ${{ github.job }}-${{ runner.os }}-${{ env.IMAGE_VERSION }}-${{ needs.build-context.outputs.config-hash }}
       - name: Register gcc problem matcher
         run: echo "::add-matcher::.github/problem-matchers/gcc.json"
       - name: Set build dir
@@ -571,8 +571,8 @@ jobs:
     name: CIFuzz
     runs-on: ubuntu-latest
     timeout-minutes: 60
-    needs: check_source
-    if: needs.check_source.outputs.run_cifuzz == 'true'
+    needs: build-context
+    if: needs.build-context.outputs.run-ci-fuzz == 'true'
     permissions:
       security-events: write
     strategy:
@@ -611,7 +611,7 @@ jobs:
     if: always()
 
     needs:
-    - check_source  # Transitive dependency, needed to access `run_tests` value
+    - build-context  # Transitive dependency, needed to access `run-tests` value
     - check-docs
     - check_autoconf_regen
     - check_generated_files
@@ -639,14 +639,14 @@ jobs:
           test_hypothesis,
         allowed-skips: >-
           ${{
-            !fromJSON(needs.check_source.outputs.run-docs)
+            !fromJSON(needs.build-context.outputs.run-docs)
             && '
             check-docs,
             '
             || ''
           }}
           ${{
-            needs.check_source.outputs.run_tests != 'true'
+            needs.build-context.outputs.run-tests != 'true'
             && '
             check_autoconf_regen,
             check_generated_files,
@@ -657,21 +657,15 @@ jobs:
             build_windows,
             build_asan,
             build_tsan,
+            test_hypothesis,
             '
             || ''
           }}
           ${{
-            !fromJSON(needs.check_source.outputs.run_cifuzz)
+            !fromJSON(needs.build-context.outputs.run-ci-fuzz)
             && '
             cifuzz,
             '
             || ''
           }}
-          ${{
-            !fromJSON(needs.check_source.outputs.run_hypothesis)
-            && '
-            test_hypothesis,
-            '
-            || ''
-          }}
         jobs: ${{ toJSON(needs) }}
diff --git a/.github/workflows/reusable-change-detection.yml b/.github/workflows/reusable-change-detection.yml
deleted file mode 100644
index c08c0cb8873f12..00000000000000
--- a/.github/workflows/reusable-change-detection.yml
+++ /dev/null
@@ -1,173 +0,0 @@
-name: Reusable change detection
-
-on:  # yamllint disable-line rule:truthy
-  workflow_call:
-    outputs:
-      # Some of the referenced steps set outputs conditionally and there may be
-      # cases when referencing them evaluates to empty strings. It is nice to
-      # work with proper booleans so they have to be evaluated through JSON
-      # conversion in the expressions. However, empty strings used like that
-      # may trigger all sorts of undefined and hard-to-debug behaviors in
-      # GitHub Actions CI/CD. To help with this, all of the outputs set here
-      # that are meant to be used as boolean flags (and not arbitrary strings),
-      # MUST have fallbacks with default values set. A common pattern would be
-      # to add ` || false` to all such expressions here, in the output
-      # definitions. They can then later be safely used through the following
-      # idiom in job conditionals and other expressions. Here's some examples:
-      #
-      #   if: fromJSON(needs.change-detection.outputs.run-docs)
-      #
-      #   ${{
-      #        fromJSON(needs.change-detection.outputs.run-tests)
-      #        && 'truthy-branch'
-      #        || 'falsy-branch'
-      #   }}
-      #
-      config_hash:
-        description: Config hash value for use in cache keys
-        value: ${{ jobs.compute-changes.outputs.config-hash }}  # str
-      run-docs:
-        description: Whether to build the docs
-        value: ${{ jobs.compute-changes.outputs.run-docs || false }}  # bool
-      run_tests:
-        description: Whether to run the regular tests
-        value: ${{ jobs.compute-changes.outputs.run-tests || false }}  # bool
-      run-win-msi:
-        description: Whether to run the MSI installer smoke tests
-        value: >-  # bool
-          ${{ jobs.compute-changes.outputs.run-win-msi || false }}
-      run_hypothesis:
-        description: Whether to run the Hypothesis tests
-        value: >-  # bool
-          ${{ jobs.compute-changes.outputs.run-hypothesis || false }}
-      run_cifuzz:
-        description: Whether to run the CIFuzz job
-        value: >-  # bool
-          ${{ jobs.compute-changes.outputs.run-cifuzz || false }}
-
-jobs:
-  compute-changes:
-    name: Compute changed files
-    runs-on: ubuntu-latest
-    timeout-minutes: 10
-    outputs:
-      config-hash: ${{ steps.config-hash.outputs.hash }}
-      run-cifuzz: ${{ steps.check.outputs.run-cifuzz }}
-      run-docs: ${{ steps.docs-changes.outputs.run-docs }}
-      run-hypothesis: ${{ steps.check.outputs.run-hypothesis }}
-      run-tests: ${{ steps.check.outputs.run-tests }}
-      run-win-msi: ${{ steps.win-msi-changes.outputs.run-win-msi }}
-    steps:
-    - run: >-
-        echo '${{ github.event_name }}'
-    - uses: actions/checkout@v4
-      with:
-        persist-credentials: false
-    - name: Check for source changes
-      id: check
-      run: |
-        if [ -z "$GITHUB_BASE_REF" ]; then
-          echo "run-tests=true" >> "$GITHUB_OUTPUT"
-        else
-          git fetch origin "$GITHUB_BASE_REF" --depth=1
-          # git diff "origin/$GITHUB_BASE_REF..." (3 dots) may be more
-          # reliable than git diff "origin/$GITHUB_BASE_REF.." (2 dots),
-          # but it requires to download more commits (this job uses
-          # "git fetch --depth=1").
-          #
-          # git diff "origin/$GITHUB_BASE_REF..." (3 dots) works with Git
-          # 2.26, but Git 2.28 is stricter and fails with "no merge base".
-          #
-          # git diff "origin/$GITHUB_BASE_REF.." (2 dots) should be enough on
-          # GitHub, since GitHub starts by merging origin/$GITHUB_BASE_REF
-          # into the PR branch anyway.
-          #
-          # https://github.com/python/core-workflow/issues/373
-          grep_ignore_args=(
-            # file extensions
-              -e '\.md$'
-              -e '\.rst$'
-            # top-level folders
-              -e '^Doc/'
-              -e '^Misc/'
-            # configuration files
-              -e '^\.github/CODEOWNERS$'
-              -e '^\.pre-commit-config\.yaml$'
-              -e '\.ruff\.toml$'
-              -e 'mypy\.ini$'
-          )
-          git diff --name-only "origin/$GITHUB_BASE_REF.."        \
-            | grep -qvE "${grep_ignore_args[@]}"                  \
-            && echo "run-tests=true" >> "$GITHUB_OUTPUT" || true
-        fi
-
-        # Check if we should run hypothesis tests
-        GIT_BRANCH=${GITHUB_BASE_REF:-${GITHUB_REF#refs/heads/}}
-        echo "$GIT_BRANCH"
-        if $(echo "$GIT_BRANCH" | grep -q -w '3\.\(8\|9\|10\|11\)'); then
-          echo "Branch too old for hypothesis tests"
-          echo "run-hypothesis=false" >> "$GITHUB_OUTPUT"
-        else
-          echo "Run hypothesis tests"
-          echo "run-hypothesis=true" >> "$GITHUB_OUTPUT"
-        fi
-
-        # oss-fuzz maintains a configuration for fuzzing the main branch of
-        # CPython, so CIFuzz should be run only for code that is likely to be
-        # merged into the main branch; compatibility with older branches may
-        # be broken.
-        FUZZ_RELEVANT_FILES='(\.c$|\.h$|\.cpp$|^configure$|^\.github/workflows/build\.yml$|^Modules/_xxtestfuzz)'
-        if [ "$GITHUB_BASE_REF" = "main" ] && [ "$(git diff --name-only "origin/$GITHUB_BASE_REF.." | grep -qE $FUZZ_RELEVANT_FILES; echo $?)" -eq 0 ]; then
-          # The tests are pretty slow so they are executed only for PRs
-          # changing relevant files.
-          echo "Run CIFuzz tests"
-          echo "run-cifuzz=true" >> "$GITHUB_OUTPUT"
-        else
-          echo "Branch too old for CIFuzz tests; or no C files were changed"
-          echo "run-cifuzz=false" >> "$GITHUB_OUTPUT"
-        fi
-    - name: Compute hash for config cache key
-      id: config-hash
-      run: |
-        echo "hash=${{ hashFiles('configure', 'configure.ac', '.github/workflows/build.yml') }}" >> "$GITHUB_OUTPUT"
-    - name: Get a list of the changed documentation-related files
-      if: github.event_name == 'pull_request'
-      id: changed-docs-files
-      uses: Ana06/get-changed-files@v2.3.0
-      with:
-        filter: |
-          Doc/**
-          Misc/**
-          .github/workflows/reusable-docs.yml
-        format: csv  # works for paths with spaces
-    - name: Check for docs changes
-      # We only want to run this on PRs when related files are changed,
-      # or when user triggers manual workflow run.
-      if: >-
-        (
-          github.event_name == 'pull_request'
-          && steps.changed-docs-files.outputs.added_modified_renamed != ''
-        ) || github.event_name == 'workflow_dispatch'
-      id: docs-changes
-      run: |
-        echo "run-docs=true" >> "${GITHUB_OUTPUT}"
-    - name: Get a list of the MSI installer-related files
-      if: github.event_name == 'pull_request'
-      id: changed-win-msi-files
-      uses: Ana06/get-changed-files@v2.3.0
-      with:
-        filter: |
-          Tools/msi/**
-          .github/workflows/reusable-windows-msi.yml
-        format: csv  # works for paths with spaces
-    - name: Check for changes in MSI installer-related files
-      # We only want to run this on PRs when related files are changed,
-      # or when user triggers manual workflow run.
-      if: >-
-        (
-          github.event_name == 'pull_request'
-          && steps.changed-win-msi-files.outputs.added_modified_renamed != ''
-        ) || github.event_name == 'workflow_dispatch'
-      id: win-msi-changes
-      run: |
-        echo "run-win-msi=true" >> "${GITHUB_OUTPUT}"
diff --git a/.github/workflows/reusable-context.yml b/.github/workflows/reusable-context.yml
new file mode 100644
index 00000000000000..fa4df6f29711db
--- /dev/null
+++ b/.github/workflows/reusable-context.yml
@@ -0,0 +1,100 @@
+name: Reusable build context
+
+on:  # yamllint disable-line rule:truthy
+  workflow_call:
+    outputs:
+      # Every referenced step MUST always set its output variable,
+      # either via ``Tools/build/compute-changes.py`` or in this workflow file.
+      # Boolean outputs (generally prefixed ``run-``) can then later be used
+      # safely through the following idiom in job conditionals and other
+      # expressions. Here's some examples:
+      #
+      #   if: fromJSON(needs.build-context.outputs.run-tests)
+      #
+      #   ${{
+      #        fromJSON(needs.build-context.outputs.run-tests)
+      #        && 'truthy-branch'
+      #        || 'falsy-branch'
+      #   }}
+      #
+      config-hash:
+        description: Config hash value for use in cache keys
+        value: ${{ jobs.compute-changes.outputs.config-hash }}  # str
+      run-docs:
+        description: Whether to build the docs
+        value: ${{ jobs.compute-changes.outputs.run-docs }}  # bool
+      run-tests:
+        description: Whether to run the regular tests
+        value: ${{ jobs.compute-changes.outputs.run-tests  }}  # bool
+      run-windows-msi:
+        description: Whether to run the MSI installer smoke tests
+        value: ${{ jobs.compute-changes.outputs.run-windows-msi }}  # bool
+      run-ci-fuzz:
+        description: Whether to run the CIFuzz job
+        value: ${{ jobs.compute-changes.outputs.run-ci-fuzz }}  # bool
+
+jobs:
+  compute-changes:
+    name: Create context from changed files
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    outputs:
+      config-hash: ${{ steps.config-hash.outputs.hash }}
+      run-ci-fuzz: ${{ steps.changes.outputs.run-ci-fuzz }}
+      run-docs: ${{ steps.changes.outputs.run-docs }}
+      run-tests: ${{ steps.changes.outputs.run-tests }}
+      run-windows-msi: ${{ steps.changes.outputs.run-windows-msi }}
+    steps:
+    - name: Set up Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: "3"
+
+    - run: >-
+        echo '${{ github.event_name }}'
+
+    - uses: actions/checkout@v4
+      with:
+        persist-credentials: false
+        ref: >-
+          ${{
+            github.event_name == 'pull_request'
+            && github.event.pull_request.head.sha
+            || ''
+          }}
+
+    # Adapted from https://github.com/actions/checkout/issues/520#issuecomment-1167205721
+    - name: Fetch commits to get branch diff
+      if: github.event_name == 'pull_request'
+      run: |
+        set -eux
+
+        # Fetch enough history to find a common ancestor commit (aka merge-base):
+        git fetch origin "${refspec_pr}" --depth=$(( commits + 1 )) \
+          --no-tags --prune --no-recurse-submodules
+
+        # This should get the oldest commit in the local fetched history (which may not be the commit the PR branched from):
+        COMMON_ANCESTOR=$( git rev-list --first-parent --max-parents=0 --max-count=1 "${branch_pr}" )
+        DATE=$( git log --date=iso8601 --format=%cd "${COMMON_ANCESTOR}" )
+
+        # Get all commits since that commit date from the base branch (eg: main):
+        git fetch origin "${refspec_base}" --shallow-since="${DATE}" \
+          --no-tags --prune --no-recurse-submodules
+      env:
+        branch_pr: 'origin/${{ github.event.pull_request.head.ref }}'
+        commits: ${{ github.event.pull_request.commits }}
+        refspec_base: '+${{ github.event.pull_request.base.sha }}:remotes/origin/${{ github.event.pull_request.base.ref }}'
+        refspec_pr: '+${{ github.event.pull_request.head.sha }}:remotes/origin/${{ github.event.pull_request.head.ref }}'
+
+    # We only want to run tests on PRs when related files are changed,
+    # or when someone triggers a manual workflow run.
+    - name: Compute changed files
+      id: changes
+      run: python Tools/build/compute-changes.py
+      env:
+        GITHUB_DEFAULT_BRANCH: ${{ github.event.repository.default_branch }}
+
+    - name: Compute hash for config cache key
+      id: config-hash
+      run: |
+        echo "hash=${{ hashFiles('configure', 'configure.ac', '.github/workflows/build.yml') }}" >> "$GITHUB_OUTPUT"
diff --git a/Tools/build/compute-changes.py b/Tools/build/compute-changes.py
new file mode 100644
index 00000000000000..105ba58cc9d941
--- /dev/null
+++ b/Tools/build/compute-changes.py
@@ -0,0 +1,183 @@
+"""Determine which GitHub Actions workflows to run.
+
+Called by ``.github/workflows/reusable-context.yml``.
+We only want to run tests on PRs when related files are changed,
+or when someone triggers a manual workflow run.
+This improves developer experience by not doing (slow)
+unnecessary work in GHA, and saves CI resources.
+"""
+
+from __future__ import annotations
+
+import os
+import subprocess
+from dataclasses import dataclass
+from pathlib import Path
+
+TYPE_CHECKING = False
+if TYPE_CHECKING:
+    from collections.abc import Set
+
+GITHUB_DEFAULT_BRANCH = os.environ["GITHUB_DEFAULT_BRANCH"]
+GITHUB_CODEOWNERS_PATH = Path(".github/CODEOWNERS")
+GITHUB_WORKFLOWS_PATH = Path(".github/workflows")
+CONFIGURATION_FILE_NAMES = frozenset({
+    ".pre-commit-config.yaml",
+    ".ruff.toml",
+    "mypy.ini",
+})
+SUFFIXES_C_OR_CPP = frozenset({".c", ".h", ".cpp"})
+SUFFIXES_DOCUMENTATION = frozenset({".rst", ".md"})
+
+
+@dataclass(kw_only=True, slots=True)
+class Outputs:
+    run_ci_fuzz: bool = False
+    run_docs: bool = False
+    run_tests: bool = False
+    run_windows_msi: bool = False
+
+
+def compute_changes() -> None:
+    target_branch, head_branch = git_branches()
+    if target_branch and head_branch:
+        # Getting changed files only makes sense on a pull request
+        files = get_changed_files(
+            f"origin/{target_branch}", f"origin/{head_branch}"
+        )
+        outputs = process_changed_files(files)
+    else:
+        # Otherwise, just run the tests
+        outputs = Outputs(run_tests=True)
+    outputs = process_target_branch(outputs, target_branch)
+
+    if outputs.run_tests:
+        print("Run tests")
+
+    if outputs.run_ci_fuzz:
+        print("Run CIFuzz tests")
+    else:
+        print("Branch too old for CIFuzz tests; or no C files were changed")
+
+    if outputs.run_docs:
+        print("Build documentation")
+
+    if outputs.run_windows_msi:
+        print("Build Windows MSI")
+
+    print(outputs)
+
+    write_github_output(outputs)
+
+
+def git_branches() -> tuple[str, str]:
+    target_branch = os.environ.get("GITHUB_BASE_REF", "")
+    target_branch = target_branch.removeprefix("refs/heads/")
+    print(f"target branch: {target_branch!r}")
+
+    head_branch = os.environ.get("GITHUB_HEAD_REF", "")
+    head_branch = head_branch.removeprefix("refs/heads/")
+    print(f"head branch: {head_branch!r}")
+    return target_branch, head_branch
+
+
+def get_changed_files(
+    ref_a: str = GITHUB_DEFAULT_BRANCH, ref_b: str = "HEAD"
+) -> Set[Path]:
+    """List the files changed between two Git refs, filtered by change type."""
+    args = ("git", "diff", "--name-only", f"{ref_a}...{ref_b}", "--")
+    print(*args)
+    changed_files_result = subprocess.run(
+        args, stdout=subprocess.PIPE, check=True, encoding="utf-8"
+    )
+    changed_files = changed_files_result.stdout.strip().splitlines()
+    return frozenset(map(Path, filter(None, map(str.strip, changed_files))))
+
+
+def process_changed_files(changed_files: Set[Path]) -> Outputs:
+    run_tests = False
+    run_ci_fuzz = False
+    run_docs = False
+    run_windows_msi = False
+
+    for file in changed_files:
+        # Documentation files
+        doc_or_misc = file.parts[0] in {"Doc", "Misc"}
+        doc_file = file.suffix in SUFFIXES_DOCUMENTATION or doc_or_misc
+
+        if file.parent == GITHUB_WORKFLOWS_PATH:
+            if file.name == "build.yml":
+                run_tests = run_ci_fuzz = True
+            if file.name == "reusable-docs.yml":
+                run_docs = True
+            if file.name == "reusable-windows-msi.yml":
+                run_windows_msi = True
+
+        if not (
+            doc_file
+            or file == GITHUB_CODEOWNERS_PATH
+            or file.name in CONFIGURATION_FILE_NAMES
+        ):
+            run_tests = True
+
+        # The fuzz tests are pretty slow so they are executed only for PRs
+        # changing relevant files.
+        if file.suffix in SUFFIXES_C_OR_CPP:
+            run_ci_fuzz = True
+        if file.parts[:2] in {
+            ("configure",),
+            ("Modules", "_xxtestfuzz"),
+        }:
+            run_ci_fuzz = True
+
+        # Check for changed documentation-related files
+        if doc_file:
+            run_docs = True
+
+        # Check for changed MSI installer-related files
+        if file.parts[:2] == ("Tools", "msi"):
+            run_windows_msi = True
+
+    return Outputs(
+        run_ci_fuzz=run_ci_fuzz,
+        run_docs=run_docs,
+        run_tests=run_tests,
+        run_windows_msi=run_windows_msi,
+    )
+
+
+def process_target_branch(outputs: Outputs, git_branch: str) -> Outputs:
+    if not git_branch:
+        outputs.run_tests = True
+
+    # CIFuzz / OSS-Fuzz compatibility with older branches may be broken.
+    if git_branch != GITHUB_DEFAULT_BRANCH:
+        outputs.run_ci_fuzz = False
+
+    if os.environ.get("GITHUB_EVENT_NAME", "").lower() == "workflow_dispatch":
+        outputs.run_docs = True
+        outputs.run_windows_msi = True
+
+    return outputs
+
+
+def write_github_output(outputs: Outputs) -> None:
+    # https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/store-information-in-variables#default-environment-variables
+    # https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/workflow-commands-for-github-actions#setting-an-output-parameter
+    if "GITHUB_OUTPUT" not in os.environ:
+        print("GITHUB_OUTPUT not defined!")
+        return
+
+    with open(os.environ["GITHUB_OUTPUT"], "a", encoding="utf-8") as f:
+        f.write(f"run-ci-fuzz={bool_lower(outputs.run_ci_fuzz)}\n")
+        f.write(f"run-docs={bool_lower(outputs.run_docs)}\n")
+        f.write(f"run-tests={bool_lower(outputs.run_tests)}\n")
+        f.write(f"run-windows-msi={bool_lower(outputs.run_windows_msi)}\n")
+
+
+def bool_lower(value: bool, /) -> str:
+    return "true" if value else "false"
+
+
+if __name__ == "__main__":
+    compute_changes()