From c541bb8603b966ebf92f593b0a267140198a2c26 Mon Sep 17 00:00:00 2001
From: Arpan Mondal <arpanmondal@Arpans-MacBook-Pro.local>
Date: Tue, 15 Sep 2026 20:50:25 +0530
Subject: [PATCH] [v1.x] Fall back to "unknown" when package version metadata
 is None

`Server.create_initialization_options` fills `server_version` from
`pkg_version("mcp")` when no explicit version is set. That helper caught
exceptions from `importlib.metadata.version` but not the case where it returns
None instead of a string, which happens on some environments (an embedded
Python whose package metadata is present but unreadable). Because
`InitializationOptions.server_version` is typed `str`, the None propagated into
the model and raised a ValidationError, killing the server before the stdio
handshake and surfacing to the client as a bare "Connection closed".

Fall back to "unknown" for a None (or empty) version, matching the helper's
existing behaviour on failure. Add a regression test.

Github-Issue: #3487
Reported-by: j-brumback
---
 src/mcp/server/lowlevel/server.py             |  6 +++++-
 .../server/test_lowlevel_input_validation.py  | 19 +++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/src/mcp/server/lowlevel/server.py b/src/mcp/server/lowlevel/server.py
index 25a8fde37c..0c25ac9b05 100644
--- a/src/mcp/server/lowlevel/server.py
+++ b/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
 
diff --git a/tests/server/test_lowlevel_input_validation.py b/tests/server/test_lowlevel_input_validation.py
index 0614ad7c46..f317925935 100644
--- a/tests/server/test_lowlevel_input_validation.py
+++ b/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"