diff --git a/.github/actions/harden-pip/action.yml b/.github/actions/harden-pip/action.yml
index 868c30295f5..f4ccb7b0fb1 100644
--- a/.github/actions/harden-pip/action.yml
+++ b/.github/actions/harden-pip/action.yml
@@ -9,7 +9,10 @@ description: >-
   compromised eMudhra/emSign intermediate CAs. The sha256 fragment guarantees the
   bootstrap download's integrity even if the bootstrapping pip's trust store is
   compromised. See https://github.com/NVIDIA/cuda-python/issues/3195 and
-  https://discuss.python.org/t/109576.
+  https://discuss.python.org/t/109576. Before that, print sys.version and fail
+  if the interpreter is older than the first CPython release of its minor
+  series whose ssl module rejects the compromised intermediate keys (3.11.18,
+  3.12.16, 3.13.18, 3.14.10, 3.15.2).
 
 inputs:
   python:
@@ -19,6 +22,15 @@ inputs:
 runs:
   using: composite
   steps:
+    - name: Check Python interpreter is patched
+      shell: bash --noprofile --norc -xeuo pipefail {0}
+      env:
+        HARDEN_PIP_PYTHON: ${{ inputs.python }}
+      # The script lives next to this action so that it matches the action's
+      # commit even when the action is referenced from another checkout.
+      run: |
+        "${HARDEN_PIP_PYTHON}" "${GITHUB_ACTION_PATH}/check_python_patched.py"
+
     - name: Install hash-pinned pip 26.2.2
       shell: bash --noprofile --norc -xeuo pipefail {0}
       env:
diff --git a/.github/actions/harden-pip/check_python_patched.py b/.github/actions/harden-pip/check_python_patched.py
new file mode 100644
index 00000000000..51c16a6a022
--- /dev/null
+++ b/.github/actions/harden-pip/check_python_patched.py
@@ -0,0 +1,79 @@
+# SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+#
+# SPDX-License-Identifier: Apache-2.0
+
+"""Fail when the running CPython is older than the fix for the eMudhra/emSign CA compromise.
+
+Print ``sys.version`` and compare the interpreter with the first release of its
+minor series whose ``ssl`` module rejects the compromised eMudhra intermediate
+keys (CVE-2026-115557). Any release at or after that patch release passes.
+
+Release announcement:
+https://discuss.python.org/t/python-3-15-2-3-14-10-3-13-18-3-12-16-and-3-11-18-are-out/109678
+
+This file uses only the standard library because it runs on whichever
+interpreter the job is about to use, before any package is installed.
+"""
+
+from __future__ import annotations
+
+import sys
+
+# Minor series -> first patch release carrying the fix.
+FIXED_MICRO = {
+    (3, 11): 18,
+    (3, 12): 16,
+    (3, 13): 18,
+    (3, 14): 10,
+    (3, 15): 2,
+}
+
+
+def classify(version_info) -> tuple[str, str]:
+    """Return ``(status, detail)`` where status is ``"ok"``, ``"warn"`` or ``"fail"``."""
+    series = (version_info.major, version_info.minor)
+    found = f"{version_info.major}.{version_info.minor}.{version_info.micro}"
+
+    if series in FIXED_MICRO:
+        fixed = f"{series[0]}.{series[1]}.{FIXED_MICRO[series]}"
+        if version_info.releaselevel != "final":
+            return (
+                "fail",
+                f"Python {found} ({version_info.releaselevel}) is a pre-release; use a final release >= {fixed}.",
+            )
+        if version_info.micro >= FIXED_MICRO[series]:
+            return "ok", f"Python {found} includes the fix (first fixed release of this series: {fixed})."
+        return (
+            "fail",
+            f"Python {found} is older than {fixed}, the first release of this series that rejects the compromised eMudhra intermediate CA keys.",
+        )
+
+    if series > max(FIXED_MICRO):
+        # Newer than every series we know a fix for (for example 3.16 pre-releases).
+        return (
+            "warn",
+            f"Python {found}: no fixed release is known for the {series[0]}.{series[1]} series yet; update FIXED_MICRO once one exists.",
+        )
+
+    return (
+        "fail",
+        f"Python {series[0]}.{series[1]} has no release that rejects the compromised eMudhra intermediate CA keys.",
+    )
+
+
+def main() -> int:
+    print(f"sys.version: {sys.version}")
+    print(f"sys.executable: {sys.executable}")
+    status, detail = classify(sys.version_info)
+    if status == "ok":
+        print(detail)
+        return 0
+    if status == "warn":
+        print(f"::warning title=Python patch level unverified::{detail}")
+        return 0
+    print(f"::error title=Unpatched Python interpreter::{detail}")
+    return 1
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/ci/tools/tests/test_check_python_patched.py b/ci/tools/tests/test_check_python_patched.py
new file mode 100644
index 00000000000..4494804e626
--- /dev/null
+++ b/ci/tools/tests/test_check_python_patched.py
@@ -0,0 +1,83 @@
+# SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+
+"""Tests for the interpreter patch-level gate used by the harden-pip action."""
+
+from __future__ import annotations
+
+import importlib.util
+import subprocess
+import sys
+from pathlib import Path
+from types import SimpleNamespace
+
+import pytest
+
+ACTION_DIR = Path(__file__).resolve().parents[3] / ".github" / "actions" / "harden-pip"
+SCRIPT_PATH = ACTION_DIR / "check_python_patched.py"
+
+_spec = importlib.util.spec_from_file_location("check_python_patched", SCRIPT_PATH)
+assert _spec is not None and _spec.loader is not None
+check_python_patched = importlib.util.module_from_spec(_spec)
+_spec.loader.exec_module(check_python_patched)
+
+
+def _version(major: int, minor: int, micro: int, releaselevel: str = "final") -> SimpleNamespace:
+    return SimpleNamespace(major=major, minor=minor, micro=micro, releaselevel=releaselevel)
+
+
+@pytest.mark.agent_authored(model="claude-sonnet-5.5")
+@pytest.mark.parametrize(
+    ("version", "status"),
+    [
+        # First fixed release and anything later in the same series passes.
+        ((3, 11, 18), "ok"),
+        ((3, 11, 19), "ok"),
+        ((3, 12, 16), "ok"),
+        ((3, 13, 18), "ok"),
+        ((3, 14, 10), "ok"),
+        ((3, 14, 11), "ok"),
+        ((3, 15, 2), "ok"),
+        ((3, 15, 3), "ok"),
+        # The patch release before the fix fails.
+        ((3, 11, 17), "fail"),
+        ((3, 12, 15), "fail"),
+        ((3, 13, 17), "fail"),
+        ((3, 14, 9), "fail"),
+        ((3, 15, 1), "fail"),
+        ((3, 15, 0), "fail"),
+        # Series without any fixed release.
+        ((3, 10, 99), "fail"),
+        ((3, 9, 25), "fail"),
+    ],
+)
+def test_classify_final_releases(version: tuple[int, int, int], status: str) -> None:
+    assert check_python_patched.classify(_version(*version))[0] == status
+
+
+@pytest.mark.agent_authored(model="claude-sonnet-5.5")
+def test_classify_prerelease_of_fixed_series_fails() -> None:
+    assert check_python_patched.classify(_version(3, 15, 2, "candidate"))[0] == "fail"
+
+
+@pytest.mark.agent_authored(model="claude-sonnet-5.5")
+def test_classify_newer_series_warns() -> None:
+    assert check_python_patched.classify(_version(3, 16, 0, "alpha"))[0] == "warn"
+
+
+@pytest.mark.agent_authored(model="claude-sonnet-5.5")
+def test_script_prints_sys_version_and_matches_classification() -> None:
+    result = subprocess.run(  # noqa: S603
+        [sys.executable, str(SCRIPT_PATH)], capture_output=True, text=True, check=False
+    )
+    assert f"sys.version: {sys.version}" in result.stdout
+    expected_status, _ = check_python_patched.classify(sys.version_info)
+    assert (result.returncode == 1) == (expected_status == "fail")
+
+
+@pytest.mark.agent_authored(model="claude-sonnet-5.5")
+def test_harden_pip_action_runs_the_check_before_installing_pip() -> None:
+    text = (ACTION_DIR / "action.yml").read_text(encoding="utf-8")
+    check = text.index("check_python_patched.py")
+    install = text.index("pip install")
+    assert check < install
diff --git a/ruff.toml b/ruff.toml
index 5a2778cfb80..5b01f55bf09 100644
--- a/ruff.toml
+++ b/ruff.toml
@@ -158,6 +158,7 @@ inline-quotes = "double"
 "qa/**" = ["T201"]
 "toolshed/**" = ["T201"]
 "ci/**" = ["T201"]
+".github/actions/**" = ["T201"]
 "**/build_hooks.py" = ["T201"]
 "**/_build_shared.py" = ["T201"]
 "**/docs/**/conf.py" = ["T201", "ARG001"]