[v1.x] Fall back to "unknown" when package version metadata is None

src/mcp/server/lowlevel/server.py

@@ -172,7 +172,11 @@ def pkg_version(package: str) -> str:
             try:
                 from importlib.metadata import version
 
-                return version(package)
+                # `version()` can return None (not just raise) on some
+                # environments, e.g. an embedded Python whose package metadata
+                # is present but unreadable; `server_version` is typed `str`,
+                # so fall back to "unknown" rather than let None crash init.
+                return version(package) or "unknown"
             except Exception:  # pragma: no cover
                 pass
 

tests/server/test_lowlevel_input_validation.py

@@ -309,3 +309,22 @@ async def test_callback(client_session: ClientSession) -> CallToolResult:
     assert any(
         "Tool 'unknown_tool' not listed, no validation will be performed" in record.message for record in caplog.records
     )
+
+
+def test_create_initialization_options_falls_back_when_package_version_is_none(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """A None from importlib.metadata.version does not crash create_initialization_options.
+
+    On some environments (e.g. an embedded Python whose package metadata is present but
+    unreadable) importlib.metadata.version returns None instead of raising. server_version is
+    typed str, so without a guard that None reaches InitializationOptions and raises a
+    ValidationError, killing the server before the stdio handshake. It must fall back to
+    "unknown" instead.
+    """
+    monkeypatch.setattr("importlib.metadata.version", lambda package: None)
+
+    # No explicit version, so create_initialization_options falls back to pkg_version("mcp").
+    options = Server("test-server").create_initialization_options()
+
+    assert options.server_version == "unknown"