Harden runtime compatibility policy checks

backend/src/backend/main.py

@@ -15,12 +15,22 @@ REQUIRED_PACKAGE_PINS: dict[str, str] = {
 }
 
 
+def _parse_required_python(required_python: str) -> tuple[int, int] | None:
+    """Parse required Python version from strict major.minor format."""
+    value = required_python.strip()
+    pieces = value.split(".")
+    if len(pieces) != 2 or not all(piece.isdigit() for piece in pieces):
+        return None
+    return int(pieces[0]), int(pieces[1])
+
+
 def _runtime_meets_required_python(required_python: str) -> bool:
-    """Return True when the interpreter version satisfies required major.minor."""
-    major_str, minor_str = required_python.split(".", maxsplit=1)
-    required = (int(major_str), int(minor_str))
+    """Return True when the interpreter major.minor exactly matches required policy."""
+    required = _parse_required_python(required_python)
+    if required is None:
+        return False
     current = (sys.version_info.major, sys.version_info.minor)
-    return current >= required
+    return current == required
 
 
 def _installed_version(package_name: str) -> str:
@@ -32,22 +42,32 @@ def compatibility_status() -> dict[str, object]:
     """Compute runtime and pinned-dependency compatibility status."""
     required_python = os.getenv("BACKEND_REQUIRED_PYTHON", "3.14")
     current_python = f"{sys.version_info.major}.{sys.version_info.minor}"
+    required_python_parsed = _parse_required_python(required_python)
 
     package_checks: dict[str, bool] = {}
+    package_errors: dict[str, str] = {}
     for package_name, required_version in REQUIRED_PACKAGE_PINS.items():
-        package_checks[package_name] = (
-            _installed_version(package_name) == required_version
-        )
+        try:
+            package_checks[package_name] = (
+                _installed_version(package_name) == required_version
+            )
+        except metadata.PackageNotFoundError as exc:
+            package_checks[package_name] = False
+            package_errors[package_name] = str(exc)
 
-    runtime_ok = _runtime_meets_required_python(required_python)
+    runtime_ok = required_python_parsed is not None and _runtime_meets_required_python(
+        required_python
+    )
     packages_ok = all(package_checks.values())
 
     return {
         "ok": runtime_ok and packages_ok,
         "required_python": required_python,
         "current_python": current_python,
+        "python_policy_valid": required_python_parsed is not None,
         "required_packages": REQUIRED_PACKAGE_PINS,
         "package_checks": package_checks,
+        "package_errors": package_errors,
     }
 
 

backend/tests/integration/test_api.py

@@ -1,9 +1,11 @@
 """Integration tests for API endpoints."""
 
+from importlib import metadata
+
 import pytest
 from fastapi.testclient import TestClient
 
-from backend.main import app
+from backend.main import app, compatibility_status
 
 client = TestClient(app)
 
@@ -49,8 +51,42 @@ class TestAPIIntegration:
         assert payload["ok"] is True
         assert payload["required_python"] == "3.14"
         assert "current_python" in payload
+        assert payload["python_policy_valid"] is True
         assert "required_packages" in payload
         assert payload["required_packages"]["fastapi"] == "0.120.2"
         assert payload["required_packages"]["uvicorn"] == "0.38.0"
+        assert payload["package_errors"] == {}
 
         monkeypatch.delenv("BACKEND_REQUIRED_PYTHON", raising=False)
+
+    def test_startup_rejects_malformed_required_python(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """Startup should fail fast when Python policy is not strict major.minor."""
+        monkeypatch.setenv("BACKEND_REQUIRED_PYTHON", "3.14.1")
+
+        with pytest.raises(RuntimeError), TestClient(app):
+            pass
+
+        monkeypatch.delenv("BACKEND_REQUIRED_PYTHON", raising=False)
+
+    def test_compatibility_status_handles_missing_package_metadata(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        """Compatibility checks should handle missing package metadata gracefully."""
+
+        def raise_not_found(_: str) -> str:
+            raise metadata.PackageNotFoundError("fake")
+
+        monkeypatch.setattr("backend.main._installed_version", raise_not_found)
+        status = compatibility_status()
+        package_checks = status["package_checks"]
+        package_errors = status["package_errors"]
+
+        assert status["ok"] is False
+        assert isinstance(package_checks, dict)
+        assert isinstance(package_errors, dict)
+        assert package_checks["fastapi"] is False
+        assert package_checks["uvicorn"] is False
+        assert "fastapi" in package_errors
+        assert "uvicorn" in package_errors