diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b9b412efc..aac875584 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -8,7 +8,7 @@
 # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 name: Continuous integration checks
-run-name: CI checks for ${{github.event_name}} by ${{github.actor}}
+run-name: Continuous integration checks triggered by ${{github.event_name}}
 
 on:
   pull_request:
@@ -23,6 +23,11 @@ on:
   # Allow manual invocation.
   workflow_dispatch:
     inputs:
+      sha:
+        description: 'SHA of commit to run against:'
+        type: string
+        required: true
+
       python_ver:
         description: Normal version of Python to use
         type: string
@@ -38,7 +43,7 @@ concurrency:
 
 env:
   # Default Python version to use.
-  python_ver: "3.10.11"
+  python_ver: "3.12"
 
   # Oldest Python version to use, for max_compat tests.
   python_compat_ver: "3.10"
@@ -54,14 +59,61 @@ env:
     dev_tools/requirements/max_compat/pytest-max-compat.env.txt
 
 jobs:
-  Setup:
-    runs-on: ubuntu-latest
-    timeout-minutes: 10
+  Changes:
+    runs-on: ubuntu-24.04
+    timeout-minutes: 5
+    outputs:
+      python: ${{steps.filter.outputs.python}}
+      python_files: ${{steps.filter.outputs.python_files}}
     steps:
+      # When invoked manually, use the given SHA to figure out the change list.
+      - if: github.event_name == 'workflow_dispatch'
+        name: Use the user-provided SHA as the basis for comparison
+        env:
+          GH_TOKEN: ${{github.token}}
+        run: |
+          set -x +e
+          url="repos/${{github.repository}}/commits/${{inputs.sha}}"
+          full_sha="$(gh api $url -q '.sha')"
+          exit_code=$?
+          if [[ "$exit_code" == "0" ]]; then
+            echo "base=$full_sha" >> "$GITHUB_ENV"
+          else
+            {
+            echo "### :x: Workflow error"
+            echo "The SHA provided to _Run Workflow_ does not exist:"
+            echo "<code>${{inputs.sha}}</code>"
+            } >> "$GITHUB_STEP_SUMMARY"
+            exit 1
+          fi
+
+      - if: github.event_name != 'workflow_dispatch'
+        name: Use ref ${{github.ref_name}} as the basis for comparison
+        run: |
+          echo base=${{github.ref_name}} >> "$GITHUB_ENV"
+
       - name: Check out a copy of the OpenFermion git repository
         uses: actions/checkout@v4
+
+      - name: Determine files changed by this ${{github.event_name}} event
+        uses: dorny/paths-filter@v3
+        id: filter
         with:
-          fetch-depth: 0
+          base: ${{env.base}}
+          list-files: 'shell'
+          # The outputs will be variables named "foo_files" for a filter "foo".
+          filters: |
+            python:
+              - added|modified:
+                  - '**/*.py'
+
+  Setup:
+    if: needs.Changes.outputs.python == 'true'
+    needs: Changes
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out a copy of the git repository
+        uses: actions/checkout@v4
 
       - name: Set up Python with caching of pip dependencies
         uses: actions/setup-python@v5
@@ -82,17 +134,19 @@ jobs:
           pip list
           echo "::endgroup::"
 
-  format:
-    name: Format check
-    needs: Setup
+  Python-format:
+    if: needs.Changes.outputs.python == 'true'
+    name: Python format checks
+    needs: [Changes, Setup]
     runs-on: ubuntu-latest
-    timeout-minutes: 10
     steps:
-      - uses: actions/checkout@v4
+      - name: Check out a copy of the git repository
+        uses: actions/checkout@v4
         with:
           fetch-depth: 0
 
-      - uses: actions/setup-python@v5
+      - name: Set up Python and restore cache
+        uses: actions/setup-python@v5
         with:
           python-version: ${{inputs.python_ver || env.python_ver}}
           architecture: "x64"
@@ -105,15 +159,17 @@ jobs:
       - name: Format
         run: check/format-incremental
 
-  mypy:
-    name: Type check
-    needs: Setup
+  Python-mypy:
+    if: needs.Changes.outputs.python == 'true'
+    name: Python type checks
+    needs: [Changes, Setup]
     runs-on: ubuntu-latest
-    timeout-minutes: 10
     steps:
-      - uses: actions/checkout@v4
+      - name: Check out a copy of the git repository
+        uses: actions/checkout@v4
 
-      - uses: actions/setup-python@v5
+      - name: Set up Python and restore cache
+        uses: actions/setup-python@v5
         with:
           python-version: ${{inputs.python_ver || env.python_ver}}
           architecture: "x64"
@@ -126,15 +182,17 @@ jobs:
       - name: Type check
         run: check/mypy
 
-  pylint:
-    name: Lint check
-    needs: Setup
+  Python-lint:
+    if: needs.Changes.outputs.python == 'true'
+    name: Python lint checks
+    needs: [Changes, Setup]
     runs-on: ubuntu-latest
-    timeout-minutes: 10
     steps:
-      - uses: actions/checkout@v4
+      - name: Check out a copy of the git repository
+        uses: actions/checkout@v4
 
-      - uses: actions/setup-python@v5
+      - name: Set up Python and restore cache
+        uses: actions/setup-python@v5
         with:
           python-version: ${{inputs.python_ver || env.python_ver}}
           architecture: "x64"
@@ -147,41 +205,29 @@ jobs:
       - name: Lint
         run: check/pylint
 
-  pytest-max-compat:
-    name: Pytest max compat
-    runs-on: ubuntu-20.04
-    timeout-minutes: 15
-    steps:
-      - uses: actions/checkout@v4
-
-        # Note: deliberately not using our Python cache here b/c this runs
-        # a different version of Python.
-      - uses: actions/setup-python@v5
-        with:
-          python-version: ${{env.python_compat_ver}}
+  # The next set of matrix tests each consist of 2 job definitions. The job
+  # named "Thing-matrix" define a matrix of runs for different platforms. It's
+  # set with "fail-fast: false" so that a failure in one of matrix jobs doesn't
+  # cause this entire CI workflow to abort. Then, the job named "Thing" is the
+  # one that actually reports the results. It needs to be an independent job it
+  # has to test the results of all the matrix runs.
 
-      - name: Install requirements
-        run: |
-          pip install -r dev_tools/requirements/max_compat/pytest-max-compat.env.txt
-
-      - name: Pytest check
-        run: check/pytest
-        shell: bash
-
-  pytest-matrix:
+  Pytest-matrix:
+    if: needs.Changes.outputs.python == 'true'
     name: Pytest matrix
-    needs: Setup
+    needs: [Changes, Setup]
     runs-on: ${{ matrix.os }}
-    timeout-minutes: 15
     strategy:
       matrix:
         os: [ ubuntu-latest, macos-latest, windows-latest ]
         cirq-version: [ 1.4.1 ]
       fail-fast: false
     steps:
-      - uses: actions/checkout@v4
+      - name: Check out a copy of the git repository
+        uses: actions/checkout@v4
 
-      - uses: actions/setup-python@v5
+      - name: Set up Python and restore cache
+        uses: actions/setup-python@v5
         with:
           python-version: ${{inputs.python_ver || env.python_ver}}
           cache: pip
@@ -196,10 +242,10 @@ jobs:
         run: check/pytest
         shell: bash
 
-  pytest:
-    if: always()
+  Pytest:
+    if: needs.Changes.outputs.python == 'true' && (success() || failure())
     name: Pytest
-    needs: pytest-matrix
+    needs: [Changes, Pytest-matrix]
     runs-on: ubuntu-latest
     steps:
       - run: |
@@ -210,20 +256,22 @@ jobs:
             exit 1
           fi
 
-  pytest-extra-matrix:
+  Pytest-extra-matrix:
+    if: needs.Changes.outputs.python == 'true'
     name: Pytest extra matrix
-    needs: Setup
+    needs: [Changes, Setup]
     runs-on: ${{ matrix.os }}
-    timeout-minutes: 15
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest]
         cirq-version: [ 1.4.1 ]
       fail-fast: false
     steps:
-      - uses: actions/checkout@v4
+      - name: Check out a copy of the git repository
+        uses: actions/checkout@v4
 
-      - uses: actions/setup-python@v5
+      - name: Set up Python and restore cache
+        uses: actions/setup-python@v5
         with:
           python-version: ${{inputs.python_ver || env.python_ver}}
           cache: pip
@@ -238,10 +286,10 @@ jobs:
         run: check/pytest -m "not slow" src/openfermion/resource_estimates
         shell: bash
 
-  pytest-extra:
-    if: always()
+  Pytest-extra:
+    if: needs.Changes.outputs.python == 'true' && (success() || failure())
     name: Pytest extra
-    needs: pytest-extra-matrix
+    needs: [Changes, Pytest-extra-matrix]
     runs-on: ubuntu-latest
     steps:
       - run: |
@@ -252,16 +300,43 @@ jobs:
             exit 1
           fi
 
-  coverage:
-    name: Coverage check
-    needs: Setup
+  Pytest-max-compat:
+    if: needs.Changes.outputs.python == 'true'
+    name: Pytest max compatibility
+    needs: [Changes, Setup]
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Check out a copy of the git repository
+        uses: actions/checkout@v4
+
+        # Note: deliberately not using our Python cache here b/c this runs
+        # a different version of Python.
+      - name: Set up Python and restore cache
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{env.python_compat_ver}}
+
+      - name: Install requirements
+        run: |
+          pip install -r dev_tools/requirements/max_compat/pytest-max-compat.env.txt
+
+      - name: Pytest check
+        run: check/pytest
+        shell: bash
+
+  Coverage:
+    if: needs.Changes.outputs.python == 'true'
+    name: Code coverage checks
+    needs: [Changes, Setup]
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - name: Check out a copy of the git repository
+        uses: actions/checkout@v4
         with:
           fetch-depth: 0
 
-      - uses: actions/setup-python@v5
+      - name: Set up Python and restore cache
+        uses: actions/setup-python@v5
         with:
           python-version: ${{inputs.python_ver || env.python_ver}}
           cache: pip