diff --git a/README.md b/README.md
index f051812..eae64ef 100644
--- a/README.md
+++ b/README.md
@@ -259,3 +259,28 @@ MIT – see `LICENSE`.
 ## Acknowledgements
 
 Inspired by PyO3, Tetragon and libsodium.
+
+## No-GIL readiness is a release axis
+
+PyIsolate distinguishes **parallel cells** from **scheduled compartments**. A
+host may claim parallel-cell semantics only when the interpreter is a
+`--disable-gil` build, the process GIL is not enabled, and loaded native
+extensions have explicit no-GIL safety declarations. Otherwise PyIsolate treats
+work as scheduled compartments: isolated and policy-controlled, but not a hard
+parallel execution guarantee.
+
+Use the doctor subcommands to make this visible in CI and fleet diagnostics:
+
+```bash
+pyisolate doctor gil
+pyisolate doctor gil --json
+pyisolate doctor extensions
+pyisolate doctor extensions --json
+```
+
+The legacy `pyisolate-doctor` command still prints the full provenance report,
+including the `no_gil.axis.mode` field. On free-threaded builds, PyIsolate emits
+a `RuntimeWarning` when native extensions are already imported but not declared
+safe through `PYISOLATE_NOGIL_SAFE_MODULES`. Only set that environment variable
+after auditing upstream support for subinterpreters and CPython no-GIL/free
+threading.
diff --git a/pyisolate/__init__.py b/pyisolate/__init__.py
index 9c314b1..5435838 100644
--- a/pyisolate/__init__.py
+++ b/pyisolate/__init__.py
@@ -77,6 +77,7 @@ def migrate(*args, **kwargs):  # type: ignore[no-redef]
 from .policy import refresh_remote  # noqa: F401
 from .sdk import Pipeline, sandbox  # noqa: F401
 from .subset import OwnershipError, RestrictedExec  # noqa: F401
+from .nogil import no_gil_readiness_report, warn_if_unsafe_native_extensions  # noqa: F401
 from .supervisor import (
     Sandbox,
     Supervisor,
@@ -131,5 +132,9 @@ def migrate(*args, **kwargs):  # type: ignore[no-redef]
     "migrate",
     "refresh_remote",
     "setup_structured_logging",
+    "no_gil_readiness_report",
+    "warn_if_unsafe_native_extensions",
     "bpf",
 ]
+
+warn_if_unsafe_native_extensions()
diff --git a/pyisolate/cli.py b/pyisolate/cli.py
new file mode 100644
index 0000000..2a0c403
--- /dev/null
+++ b/pyisolate/cli.py
@@ -0,0 +1,22 @@
+"""Top-level ``pyisolate`` command."""
+
+from __future__ import annotations
+
+import argparse
+
+from . import doctor
+
+
+def main(argv: list[str] | None = None) -> None:
+    parser = argparse.ArgumentParser(prog="pyisolate")
+    subparsers = parser.add_subparsers(dest="command")
+    subparsers.add_parser("doctor", help="Run installation and no-GIL diagnostics")
+    args, rest = parser.parse_known_args(argv)
+    if args.command == "doctor":
+        doctor.main(rest)
+        return
+    parser.print_help()
+
+
+if __name__ == "__main__":
+    main()
diff --git a/pyisolate/doctor.py b/pyisolate/doctor.py
index 310749d..b15a64f 100644
--- a/pyisolate/doctor.py
+++ b/pyisolate/doctor.py
@@ -3,16 +3,78 @@
 from __future__ import annotations
 
 import argparse
+import json
+from typing import Any
 
+from .nogil import imported_native_extensions, no_gil_readiness_report
 from .provenance import installation_report_json
 
 
+def _print_json(payload: object) -> None:
+    print(json.dumps(payload, indent=2, sort_keys=True))
+
+
+def _print_gil_human(report: dict[str, Any]) -> None:
+    axis = report["axis"]
+    build = report["build"]
+    runtime = report["runtime"]
+    extensions = report["extensions"]
+    print(f"mode: {axis['mode']}")
+    print(f"parallel_cells_ready: {axis['parallel_cells_ready']}")
+    print(f"reason: {axis['reason']}")
+    print(f"py_gil_disabled: {build['py_gil_disabled']}")
+    print(f"gil_enabled: {runtime['gil_enabled']}")
+    print(f"loaded_native_extensions: {extensions['loaded_native_count']}")
+    print(f"unknown_or_unmarked_extensions: {extensions['unknown_or_unmarked_count']}")
+
+
+def _print_extensions_human(extensions: list[dict[str, object]]) -> None:
+    if not extensions:
+        print("No imported native extension modules detected.")
+        return
+    for item in extensions:
+        marker = "OK" if item["no_gil_safe"] else "UNKNOWN"
+        print(f"{marker}\t{item['name']}\t{item['origin']}\t{item['reason']}")
+
+
 def main(argv: list[str] | None = None) -> None:
     parser = argparse.ArgumentParser(
         prog="pyisolate-doctor",
-        description="Print PyIsolate build provenance and kernel feature flags.",
+        description="Print PyIsolate build provenance and kernel/no-GIL feature flags.",
     )
-    parser.parse_args(argv)
+    subparsers = parser.add_subparsers(dest="command")
+
+    gil_parser = subparsers.add_parser(
+        "gil",
+        help="Report whether this host can run PyIsolate as parallel cells.",
+    )
+    gil_parser.add_argument(
+        "--json", action="store_true", help="Print machine-readable JSON"
+    )
+
+    ext_parser = subparsers.add_parser(
+        "extensions",
+        help="List loaded native extensions and their no-GIL audit status.",
+    )
+    ext_parser.add_argument(
+        "--json", action="store_true", help="Print machine-readable JSON"
+    )
+
+    args = parser.parse_args(argv)
+    if args.command == "gil":
+        report = no_gil_readiness_report()
+        if args.json:
+            _print_json(report)
+        else:
+            _print_gil_human(report)
+        return
+    if args.command == "extensions":
+        extensions = imported_native_extensions()
+        if args.json:
+            _print_json({"extensions": extensions})
+        else:
+            _print_extensions_human(extensions)
+        return
     print(installation_report_json())
 
 
diff --git a/pyisolate/nogil.py b/pyisolate/nogil.py
new file mode 100644
index 0000000..921218e
--- /dev/null
+++ b/pyisolate/nogil.py
@@ -0,0 +1,165 @@
+"""No-GIL readiness checks for PyIsolate runtime and extensions."""
+
+from __future__ import annotations
+
+import importlib.machinery
+import os
+import sys
+import sysconfig
+import warnings
+from pathlib import Path
+from types import ModuleType
+from typing import Any
+
+_NATIVE_SUFFIXES = tuple(importlib.machinery.EXTENSION_SUFFIXES)
+_SAFE_ENV = "PYISOLATE_NOGIL_SAFE_MODULES"
+_WARN_ENV = "PYISOLATE_WARN_UNSAFE_NOGIL_EXTENSIONS"
+
+
+def is_no_gil_build() -> bool:
+    """Return whether this interpreter was built with CPython's free-threaded ABI."""
+
+    return bool(sysconfig.get_config_var("Py_GIL_DISABLED"))
+
+
+def is_gil_enabled() -> bool | None:
+    """Return effective process GIL state when CPython exposes it.
+
+    Python 3.13+ free-threaded builds expose ``sys._is_gil_enabled``. Older
+    interpreters cannot report this distinction, so ``None`` means unknown.
+    """
+
+    checker = getattr(sys, "_is_gil_enabled", None)
+    if checker is None:
+        return None
+    return bool(checker())
+
+
+def _configured_safe_roots() -> set[str]:
+    raw = os.environ.get(_SAFE_ENV, "")
+    return {item.strip().split(".", 1)[0] for item in raw.split(",") if item.strip()}
+
+
+def _module_origin(module: ModuleType) -> str | None:
+    spec = getattr(module, "__spec__", None)
+    origin = getattr(spec, "origin", None)
+    if origin:
+        return origin
+    filename = getattr(module, "__file__", None)
+    return str(filename) if filename else None
+
+
+def _is_native_origin(origin: str | None) -> bool:
+    return bool(origin) and origin.endswith(_NATIVE_SUFFIXES)
+
+
+def imported_native_extensions() -> list[dict[str, Any]]:
+    """Return imported native extension modules with no-GIL audit status.
+
+    CPython does not expose a portable per-module flag that says whether an
+    already imported extension declared ``Py_MOD_GIL_NOT_USED``. PyIsolate
+    therefore treats native modules as unknown unless the deployment marks the
+    root module in ``PYISOLATE_NOGIL_SAFE_MODULES``.
+    """
+
+    safe_roots = _configured_safe_roots()
+    records: list[dict[str, Any]] = []
+    seen: set[tuple[str, str]] = set()
+    for name, module in sorted(sys.modules.items()):
+        if not isinstance(module, ModuleType):
+            continue
+        origin = _module_origin(module)
+        if not _is_native_origin(origin):
+            continue
+        assert origin is not None
+        root = name.split(".", 1)[0]
+        key = (name, origin)
+        if key in seen:
+            continue
+        seen.add(key)
+        marked_safe = root in safe_roots or name in safe_roots
+        records.append(
+            {
+                "name": name,
+                "root": root,
+                "origin": str(Path(origin)),
+                "suffix": next(
+                    (suffix for suffix in _NATIVE_SUFFIXES if origin.endswith(suffix)),
+                    Path(origin).suffix,
+                ),
+                "no_gil_safe": marked_safe,
+                "status": "declared-safe" if marked_safe else "unknown",
+                "reason": "declared in PYISOLATE_NOGIL_SAFE_MODULES"
+                if marked_safe
+                else "native extension has no PyIsolate no-GIL safety declaration",
+            }
+        )
+    return records
+
+
+def no_gil_readiness_report() -> dict[str, Any]:
+    """Return the hard no-GIL axis used to classify runtime behavior."""
+
+    no_gil_build = is_no_gil_build()
+    gil_enabled = is_gil_enabled()
+    extensions = imported_native_extensions()
+    unknown_extensions = [item for item in extensions if not item["no_gil_safe"]]
+
+    parallel_cells_ready = bool(no_gil_build) and gil_enabled is not True and not unknown_extensions
+    if parallel_cells_ready:
+        mode = "parallel_cells"
+        reason = "free-threaded runtime with no unknown native extensions loaded"
+    elif not no_gil_build:
+        mode = "scheduled_compartments"
+        reason = "Python was not built with --disable-gil"
+    elif gil_enabled is True:
+        mode = "scheduled_compartments"
+        reason = "the process GIL is currently enabled"
+    else:
+        mode = "scheduled_compartments"
+        reason = "native extension no-GIL safety is unknown"
+
+    return {
+        "build": {
+            "py_gil_disabled": no_gil_build,
+            "free_threaded_abi": no_gil_build,
+            "soabi": sysconfig.get_config_var("SOABI"),
+            "cache_tag": sys.implementation.cache_tag,
+        },
+        "runtime": {
+            "gil_enabled": gil_enabled,
+            "gil_state_known": gil_enabled is not None,
+        },
+        "extensions": {
+            "loaded_native_count": len(extensions),
+            "unknown_or_unmarked_count": len(unknown_extensions),
+            "safe_declaration_env": _SAFE_ENV,
+            "items": extensions,
+        },
+        "axis": {
+            "mode": mode,
+            "parallel_cells_ready": parallel_cells_ready,
+            "scheduled_compartments": not parallel_cells_ready,
+            "reason": reason,
+        },
+    }
+
+
+def warn_if_unsafe_native_extensions() -> None:
+    """Warn on free-threaded builds when native modules block parallel-cell claims."""
+
+    if os.environ.get(_WARN_ENV, "1").lower() in {"0", "false", "no"}:
+        return
+    if not is_no_gil_build():
+        return
+    report = no_gil_readiness_report()
+    unknown = report["extensions"]["unknown_or_unmarked_count"]
+    if unknown:
+        warnings.warn(
+            f"PyIsolate is running on a no-GIL build, but {unknown} imported native "
+            "extension module(s) are not declared no-GIL-safe; treating sandboxes as "
+            "scheduled compartments rather than parallel cells. Set "
+            f"{_SAFE_ENV}=module1,module2 only after auditing upstream no-GIL support.",
+            RuntimeWarning,
+            stacklevel=2,
+        )
diff --git a/pyisolate/provenance.py b/pyisolate/provenance.py
index 7475544..9fe709f 100644
--- a/pyisolate/provenance.py
+++ b/pyisolate/provenance.py
@@ -11,6 +11,8 @@
 from pathlib import Path
 from typing import Any
 
+from .nogil import no_gil_readiness_report
+
 
 def _safe_read_text(path: str) -> str | None:
     try:
@@ -117,6 +119,7 @@ def installation_report() -> dict[str, Any]:
             "features": kernel_feature_flags(),
         },
         "hardening": hardening_feature_flags(),
+        "no_gil": no_gil_readiness_report(),
     }
 
 
diff --git a/pyproject.toml b/pyproject.toml
index 4a044f8..68cfd47 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -14,6 +14,7 @@ build-backend = "setuptools.build_meta"
 include = ["pyisolate"]
 
 [project.scripts]
+pyisolate = "pyisolate.cli:main"
 pyisolate-doctor = "pyisolate.doctor:main"
 
 [project.optional-dependencies]
diff --git a/tests/test_provenance.py b/tests/test_provenance.py
index b57c937..17e2069 100644
--- a/tests/test_provenance.py
+++ b/tests/test_provenance.py
@@ -24,3 +24,34 @@ def test_doctor_cli_output(capsys):
     payload = json.loads(captured.out)
     assert "kernel" in payload
     assert "features" in payload["kernel"]
+
+
+def test_installation_report_exposes_no_gil_axis():
+    report = installation_report()
+    assert report["no_gil"]["axis"]["mode"] in {
+        "parallel_cells",
+        "scheduled_compartments",
+    }
+    assert "parallel_cells_ready" in report["no_gil"]["axis"]
+
+
+def test_doctor_gil_cli_output(capsys):
+    main(["gil"])
+    captured = capsys.readouterr()
+    assert "mode:" in captured.out
+    assert "parallel_cells_ready:" in captured.out
+
+
+def test_doctor_extensions_json_output(capsys):
+    main(["extensions", "--json"])
+    captured = capsys.readouterr()
+    payload = json.loads(captured.out)
+    assert "extensions" in payload
+
+
+def test_top_level_pyisolate_doctor_gil(capsys):
+    from pyisolate.cli import main as cli_main
+
+    cli_main(["doctor", "gil"])
+    captured = capsys.readouterr()
+    assert "parallel_cells_ready:" in captured.out