diff --git a/.github/workflows/reusable-standard.yml b/.github/workflows/reusable-standard.yml
index 6081f801eb..0e79aacb42 100644
--- a/.github/workflows/reusable-standard.yml
+++ b/.github/workflows/reusable-standard.yml
@@ -101,3 +101,7 @@ jobs:
         run: |
           uv pip install --python=python --system setuptools
           pytest tests/extra_setuptools
+
+      # This tests ABI compatibility
+      - name: ABI test
+        run: pytest tests/extra_abi
diff --git a/.gitignore b/.gitignore
index 798e05a7e6..3fc1c67972 100644
--- a/.gitignore
+++ b/.gitignore
@@ -45,6 +45,7 @@ pybind11Targets.cmake
 /docs/_build/*
 .ipynb_checkpoints/
 tests/main.cpp
+tests/*/temp*
 CMakeUserPresents.json
 
 /Python
diff --git a/noxfile.py b/noxfile.py
index 778025229c..d0843bad66 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -61,6 +61,16 @@ def tests_packaging(session: nox.Session) -> None:
     session.run("pytest", "tests/extra_python_package", *session.posargs)
 
 
+@nox.session
+def tests_abi(session: nox.Session) -> None:
+    """
+    Run the cross-version ABI check.
+    """
+
+    session.install("pytest")
+    session.run("pytest", "tests/extra_abi", *session.posargs)
+
+
 @nox.session(reuse_venv=True, default=False)
 def docs(session: nox.Session) -> None:
     """
diff --git a/tests/conftest.py b/tests/conftest.py
index 9d9815b88c..5e77f55d13 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -24,6 +24,8 @@
 # Early diagnostic for failed imports
 try:
     import pybind11_tests
+except ModuleNotFoundError:
+    pybind11_tests = None
 except Exception:
     # pytest does not show the traceback without this.
     traceback.print_exc()
@@ -260,6 +262,8 @@ def pytest_configure():
 
 
 def pytest_report_header():
+    if pybind11_tests is None:
+        return None
     assert pybind11_tests.compiler_info is not None, (
         "Please update pybind11_tests.cpp if this assert fails."
     )
diff --git a/tests/extra_abi/check_installed.py b/tests/extra_abi/check_installed.py
new file mode 100755
index 0000000000..1650d464dd
--- /dev/null
+++ b/tests/extra_abi/check_installed.py
@@ -0,0 +1,33 @@
+from __future__ import annotations
+
+import sys
+
+expect_incompatible = "--expect-incompatible" in sys.argv
+
+# dog alone must fail: it needs pet to register the Pet base class first.
+# Otherwise this check would not be exercising cross-extension ABI at all.
+try:
+    import dog
+except Exception:
+    pass
+else:
+    raise SystemExit("Broken: dog imported without pet; the check is not cross-module")
+
+import pet  # noqa: E402
+
+try:
+    import dog
+except Exception:
+    if expect_incompatible:
+        print("OK: incompatible internals are isolated, dog did not load")
+        sys.exit(0)
+    raise
+
+if expect_incompatible:
+    raise SystemExit("Broken: dog loaded against pet with mismatched internals")
+
+d = dog.Dog("Bluey")
+assert d.bark() == "woof!"
+assert d.name == "Bluey"
+assert isinstance(d, pet.Pet)
+print("OK: cross-version ABI is compatible")
diff --git a/tests/extra_abi/dog.cpp b/tests/extra_abi/dog.cpp
new file mode 100644
index 0000000000..da40d5885e
--- /dev/null
+++ b/tests/extra_abi/dog.cpp
@@ -0,0 +1,11 @@
+#include <pybind11/pybind11.h>
+
+#include "dog.hpp"
+
+#include <string>
+
+namespace py = pybind11;
+
+PYBIND11_MODULE(dog, m) {
+    py::class_<Dog, Pet>(m, "Dog").def(py::init<const std::string &>()).def("bark", &Dog::bark);
+}
diff --git a/tests/extra_abi/dog.hpp b/tests/extra_abi/dog.hpp
new file mode 100644
index 0000000000..f604716a9a
--- /dev/null
+++ b/tests/extra_abi/dog.hpp
@@ -0,0 +1,10 @@
+#include <pybind11/pybind11.h>
+
+#include "pet.hpp"
+
+#include <string>
+
+struct PYBIND11_EXPORT Dog : Pet {
+    explicit Dog(const std::string &name) : Pet(name) {}
+    std::string bark() const { return "woof!"; }
+};
diff --git a/tests/extra_abi/pet.cpp b/tests/extra_abi/pet.cpp
new file mode 100644
index 0000000000..7830b90e24
--- /dev/null
+++ b/tests/extra_abi/pet.cpp
@@ -0,0 +1,12 @@
+#include <pybind11/pybind11.h>
+
+#include "pet.hpp"
+
+#include <string>
+
+namespace py = pybind11;
+
+PYBIND11_MODULE(pet, m) {
+    py::class_<Pet> pet(m, "Pet");
+    pet.def(py::init<const std::string &>()).def_readwrite("name", &Pet::name);
+}
diff --git a/tests/extra_abi/pet.hpp b/tests/extra_abi/pet.hpp
new file mode 100644
index 0000000000..37fa90b186
--- /dev/null
+++ b/tests/extra_abi/pet.hpp
@@ -0,0 +1,6 @@
+#include <string>
+
+struct Pet {
+    explicit Pet(const std::string &name) : name(name) {}
+    std::string name;
+};
diff --git a/tests/extra_abi/setup.py b/tests/extra_abi/setup.py
new file mode 100644
index 0000000000..caf17c5169
--- /dev/null
+++ b/tests/extra_abi/setup.py
@@ -0,0 +1,14 @@
+from __future__ import annotations
+
+import os
+
+from setuptools import setup
+
+from pybind11.setup_helpers import Pybind11Extension
+
+name = os.environ["EXAMPLE_NAME"]
+assert name in {"pet", "dog"}
+
+ext = Pybind11Extension(name, [f"{name}.cpp"], include_dirs=["."], cxx_std=17)
+
+setup(name=name, version="0.0.0", ext_modules=[ext])
diff --git a/tests/extra_abi/test_cross_version_abi.py b/tests/extra_abi/test_cross_version_abi.py
new file mode 100644
index 0000000000..08a773bf5a
--- /dev/null
+++ b/tests/extra_abi/test_cross_version_abi.py
@@ -0,0 +1,64 @@
+from __future__ import annotations
+
+import os
+import re
+import subprocess
+import sys
+import venv
+from pathlib import Path
+
+DIR = Path(__file__).parent.resolve()
+MAIN_DIR = DIR.parent.parent
+
+# The newest release used as the cross-version ABI baseline, and the
+# PYBIND11_INTERNALS_VERSION it ships with.
+BASELINE_VERSION = "3.0.4"
+BASELINE_INTERNALS_VERSION = 11
+
+# The internals version of this checkout. If test_internals_version_pinned
+# fails, the ABI was bumped: make sure that was intentional, then update this
+# number (and the baseline above once a compatible release exists).
+EXPECTED_INTERNALS_VERSION = 12
+
+
+def read_internals_version() -> int:
+    header = MAIN_DIR / "include/pybind11/detail/internals.h"
+    match = re.search(
+        r"^#\s*define\s+PYBIND11_INTERNALS_VERSION\s+(\d+)",
+        header.read_text(encoding="utf-8"),
+        flags=re.MULTILINE,
+    )
+    assert match, "PYBIND11_INTERNALS_VERSION not found in internals.h"
+    return int(match.group(1))
+
+
+def test_internals_version_pinned():
+    """Any bump of PYBIND11_INTERNALS_VERSION must be a conscious decision."""
+    assert read_internals_version() == EXPECTED_INTERNALS_VERSION
+
+
+def test_cross_version_abi(tmp_path: Path) -> None:
+    venv_dir = tmp_path / "venv"
+    venv.create(venv_dir, with_pip=True)
+    bin_dir = "Scripts" if sys.platform.startswith("win") else "bin"
+    python = venv_dir / bin_dir / "python"
+
+    def run(*args: str, name: str | None = None) -> None:
+        env = os.environ.copy()
+        if name is not None:
+            env["EXAMPLE_NAME"] = name
+        subprocess.run([os.fspath(python), *args], check=True, env=env)
+
+    # Build pet against the baseline release; no build isolation, so that the
+    # pinned pybind11 (not the latest release) provides the headers.
+    run("-m", "pip", "install", f"pybind11=={BASELINE_VERSION}", "setuptools>=70.1")
+    run("-m", "pip", "install", os.fspath(DIR), "--no-build-isolation", name="pet")
+
+    # Build dog against this checkout.
+    run("-m", "pip", "install", os.fspath(MAIN_DIR))
+    run("-m", "pip", "install", os.fspath(DIR), "--no-build-isolation", name="dog")
+
+    check_args = []
+    if BASELINE_INTERNALS_VERSION != EXPECTED_INTERNALS_VERSION:
+        check_args.append("--expect-incompatible")
+    run(os.fspath(DIR / "check_installed.py"), *check_args)