diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS
new file mode 100644
index 00000000..6810eeff
--- /dev/null
+++ b/.github/CODEOWNERS
@@ -0,0 +1,7 @@
+# Code owners are auto-requested as reviewers when a PR touches these paths.
+# This is an advisory flag for release/CI/build infrastructure, not a merge
+# gate (require_code_owner_reviews is intentionally off).
+
+/.github/       @igerber
+/pyproject.toml @igerber
+/rust/          @igerber
diff --git a/.github/workflows/ai_pr_review.yml b/.github/workflows/ai_pr_review.yml
index 991fddc4..2ac0ddf6 100644
--- a/.github/workflows/ai_pr_review.yml
+++ b/.github/workflows/ai_pr_review.yml
@@ -117,7 +117,7 @@ jobs:
       # ─────────────────────────────────────────────────────────────────
       - name: Resolve PR number + metadata
         id: pr
-        uses: actions/github-script@v9
+        uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9
         with:
           script: |
             const prNumber =
@@ -180,7 +180,7 @@ jobs:
       # durably even after the fork is deleted or branches removed.
       # The previous workflow used `refs/pull/<N>/merge`, which is
       # garbage-collected on closed PRs — this path replaces that.
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
         if: steps.pr.outputs.state != 'open' && steps.pr.outputs.is_fork == 'false'
         with:
           ref: refs/pull/${{ steps.pr.outputs.number }}/head
@@ -191,7 +191,7 @@ jobs:
       # has not yet mirrored a freshly API-created PR's head
       # (see .claude/commands/submit-pr.md:327-345). head_sha is
       # guaranteed to exist on the head repo for an open PR.
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
         if: steps.pr.outputs.state == 'open' && steps.pr.outputs.is_fork == 'false'
         with:
           repository: ${{ steps.pr.outputs.head_repo_full_name }}
@@ -211,7 +211,7 @@ jobs:
       - name: Fetch previous AI review (if any)
         id: prev_review
         if: steps.pr.outputs.is_fork == 'false'
-        uses: actions/github-script@v9
+        uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9
         with:
           script: |
             const { owner, repo } = context.repo;
@@ -509,7 +509,7 @@ jobs:
       - name: Run Codex
         id: run_codex
         if: steps.pr.outputs.is_fork == 'false'
-        uses: openai/codex-action@v1
+        uses: openai/codex-action@10cb888d2ed3b99867f7e7ccff174a861a75aeb6 # v1
         with:
           openai-api-key: ${{ secrets.OPENAI_API_KEY }}
           prompt-file: .github/codex/prompts/pr_review_compiled.md
@@ -521,7 +521,7 @@ jobs:
 
       - name: Post PR comment (new on every event except initial open)
         if: steps.pr.outputs.is_fork == 'false'
-        uses: actions/github-script@v9
+        uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9
         env:
           CODEX_FINAL_MESSAGE: ${{ steps.run_codex.outputs.final-message }}
           PR_NUMBER: ${{ steps.pr.outputs.number }}
diff --git a/.github/workflows/build-wheels.yml b/.github/workflows/build-wheels.yml
index 6cd90d6c..d4dae9b0 100644
--- a/.github/workflows/build-wheels.yml
+++ b/.github/workflows/build-wheels.yml
@@ -34,13 +34,15 @@ jobs:
             container: quay.io/pypa/manylinux_2_28_aarch64
             artifact: wheels-linux-aarch64
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
 
       - name: Install system dependencies
         run: dnf install -y openssl-devel perl-IPC-Cmd openblas-devel
 
       - name: Install Rust
-        uses: dtolnay/rust-toolchain@stable
+        uses: dtolnay/rust-toolchain@4be7066ada62dd38de10e7b70166bc74ed198c30 # stable branch
+        with:
+          toolchain: stable
 
       - name: Install maturin
         run: /opt/python/cp312-cp312/bin/pip install maturin
@@ -65,7 +67,7 @@ jobs:
           fi
 
       - name: Upload wheels
-        uses: actions/upload-artifact@v7
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7
         with:
           name: ${{ matrix.artifact }}
           path: dist/*.whl
@@ -80,15 +82,17 @@ jobs:
       matrix:
         python-version: ['3.9', '3.10', '3.11', '3.12', '3.13', '3.14']
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
 
       - name: Set up Python
-        uses: actions/setup-python@v6
+        uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6
         with:
           python-version: ${{ matrix.python-version }}
 
       - name: Install Rust
-        uses: dtolnay/rust-toolchain@stable
+        uses: dtolnay/rust-toolchain@4be7066ada62dd38de10e7b70166bc74ed198c30 # stable branch
+        with:
+          toolchain: stable
 
       - name: Install maturin
         run: pip install maturin
@@ -97,7 +101,7 @@ jobs:
         run: maturin build --release --out dist --features extension-module,accelerate
 
       - name: Upload wheels
-        uses: actions/upload-artifact@v7
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7
         with:
           name: wheels-macos-arm64-py${{ matrix.python-version }}
           path: dist/*.whl
@@ -111,15 +115,17 @@ jobs:
       matrix:
         python-version: ['3.9', '3.10', '3.11', '3.12', '3.13', '3.14']
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
 
       - name: Set up Python
-        uses: actions/setup-python@v6
+        uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6
         with:
           python-version: ${{ matrix.python-version }}
 
       - name: Install Rust
-        uses: dtolnay/rust-toolchain@stable
+        uses: dtolnay/rust-toolchain@4be7066ada62dd38de10e7b70166bc74ed198c30 # stable branch
+        with:
+          toolchain: stable
 
       - name: Install maturin
         run: pip install maturin
@@ -128,7 +134,7 @@ jobs:
         run: maturin build --release --out dist --features extension-module
 
       - name: Upload wheels
-        uses: actions/upload-artifact@v7
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7
         with:
           name: wheels-windows-py${{ matrix.python-version }}
           path: dist/*.whl
@@ -139,10 +145,10 @@ jobs:
     if: ${{ !inputs.linux_only }}
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
 
       - name: Set up Python
-        uses: actions/setup-python@v6
+        uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6
         with:
           python-version: '3.11'
 
@@ -153,7 +159,7 @@ jobs:
         run: maturin sdist --out dist
 
       - name: Upload sdist
-        uses: actions/upload-artifact@v7
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7
         with:
           name: sdist
           path: dist/*.tar.gz
diff --git a/.github/workflows/docs-tests.yml b/.github/workflows/docs-tests.yml
index 3fee2631..b4681d34 100644
--- a/.github/workflows/docs-tests.yml
+++ b/.github/workflows/docs-tests.yml
@@ -52,10 +52,10 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
 
       - name: Set up Python
-        uses: actions/setup-python@v6
+        uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6
         with:
           # 3.14 to mirror Pure Python Fallback (the only existing job
           # that actually ran these tests). notebooks.yml uses 3.11 for
@@ -90,14 +90,14 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
 
       - name: Install pandoc
         # nbsphinx invokes pandoc to render notebook markdown cells.
         run: sudo apt-get update && sudo apt-get install -y pandoc
 
       - name: Set up Python
-        uses: actions/setup-python@v6
+        uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6
         with:
           # Match the RTD build (.readthedocs.yaml:9) so CI catches drift
           # against what users actually see on diff-diff.readthedocs.io.
@@ -128,10 +128,10 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
 
       - name: Set up Python 3.9
-        uses: actions/setup-python@v6
+        uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6
         with:
           # 3.9 is the project's declared floor (pyproject.toml requires-python
           # >=3.9). The sphinx-build job above runs on 3.11; this job verifies
diff --git a/.github/workflows/notebooks.yml b/.github/workflows/notebooks.yml
index 1ca9bb62..c28fc9c9 100644
--- a/.github/workflows/notebooks.yml
+++ b/.github/workflows/notebooks.yml
@@ -35,10 +35,10 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
 
       - name: Set up Python
-        uses: actions/setup-python@v6
+        uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6
         with:
           python-version: '3.11'
 
@@ -66,7 +66,7 @@ jobs:
 
       - name: Upload failed notebook outputs
         if: failure()
-        uses: actions/upload-artifact@v7
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7
         with:
           name: failed-notebook-outputs
           path: docs/tutorials/*.ipynb
diff --git a/.github/workflows/publish.yml b/.github/workflows/publish.yml
index 303354b1..830d896a 100644
--- a/.github/workflows/publish.yml
+++ b/.github/workflows/publish.yml
@@ -23,10 +23,10 @@ jobs:
 
     steps:
       - name: Download all artifacts
-        uses: actions/download-artifact@v8
+        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8
         with:
           path: dist
           merge-multiple: true
 
       - name: Publish to PyPI
-        uses: pypa/gh-action-pypi-publish@release/v1
+        uses: pypa/gh-action-pypi-publish@cef221092ed1bacb1cc03d23a2d87d1d172e277b # release/v1
diff --git a/.github/workflows/rust-test.yml b/.github/workflows/rust-test.yml
index 3908f293..7c414b69 100644
--- a/.github/workflows/rust-test.yml
+++ b/.github/workflows/rust-test.yml
@@ -78,10 +78,12 @@ jobs:
         os: [ubuntu-latest, macos-latest, windows-latest, ubuntu-24.04-arm]
 
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
 
       - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@stable
+        uses: dtolnay/rust-toolchain@4be7066ada62dd38de10e7b70166bc74ed198c30 # stable branch
+        with:
+          toolchain: stable
 
       - name: Install OpenBLAS (Linux)
         if: runner.os == 'Linux'
@@ -116,15 +118,17 @@ jobs:
         python-version: ['3.11', '3.13', '3.14']
 
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
 
       - name: Set up Python
-        uses: actions/setup-python@v6
+        uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6
         with:
           python-version: ${{ matrix.python-version }}
 
       - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@stable
+        uses: dtolnay/rust-toolchain@4be7066ada62dd38de10e7b70166bc74ed198c30 # stable branch
+        with:
+          toolchain: stable
 
       - name: Install test dependencies
         # Keep in sync with pyproject.toml [project.dependencies] and [project.optional-dependencies.dev]
@@ -220,10 +224,10 @@ jobs:
       || github.event.label.name == 'ready-for-ci'))
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
 
       - name: Set up Python
-        uses: actions/setup-python@v6
+        uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6
         with:
           python-version: '3.14'
 
diff --git a/tests/test_openai_review.py b/tests/test_openai_review.py
index 3ae27dde..ba4b8f96 100644
--- a/tests/test_openai_review.py
+++ b/tests/test_openai_review.py
@@ -2451,7 +2451,8 @@ class TestWorkflowCodexActionContract:
     gates), or ``TestWorkflowDoesNotExecutePRHeadCode`` (``sandbox:
     read-only``):
 
-      - the action pin (``openai/codex-action@v1``) + its ``prompt-file`` input
+      - the action pin (``openai/codex-action`` at a full commit SHA with a
+        ``# v1`` version comment) + its ``prompt-file`` input
       - the compiled-prompt path agreeing between the build step (``PROMPT=``)
         and the action input (``prompt-file:``)
       - the ``final-message`` output flowing from the ``id:``-tagged Codex step
@@ -2520,11 +2521,14 @@ def _require_block(self, workflow_text, step_name):
     def test_run_codex_uses_pinned_action(self, workflow_text):
         block = self._require_block(workflow_text, self.RUN_CODEX_STEP)
         assert re.search(
-            r"^\s*uses:\s*openai/codex-action@v1\s*$", block, re.MULTILINE
+            r"^\s*uses:\s*openai/codex-action@[0-9a-f]{40}\s*#\s*v1(?:\.\d+)*\s*$",
+            block,
+            re.MULTILINE,
         ), (
-            "the Run Codex step must invoke the pinned openai/codex-action@v1; "
-            "a floating tag or a different action silently changes the review "
-            "contract."
+            "the Run Codex step must invoke openai/codex-action pinned to a "
+            "full commit SHA with a `# v1` version comment; a floating ref, a "
+            "different action, or a major-version bump silently changes the "
+            "review contract."
         )
 
     def test_run_codex_passes_prompt_file_input(self, workflow_text):
@@ -3359,7 +3363,7 @@ def _extract_open_pr_checkout_block(workflow_text):
         import re
 
         pattern = re.compile(
-            r"^      - uses: actions/checkout@\S+\s*\n"
+            r"^      - uses: actions/checkout@\S+(?:\s+#[^\n]*)?\n"
             r"        if: [^\n]*state == 'open'[^\n]*\n"
             r"((?:[ ]{8,}.*\n|[ ]*\n)*)",
             re.MULTILINE,