From 7ae772a21aa59779f08054cbd4c07c569411aa60 Mon Sep 17 00:00:00 2001
From: Rudrendu <RudrenduPaul@users.noreply.github.com>
Date: Thu, 9 Apr 2026 01:14:27 -0700
Subject: [PATCH] fix: allow integer file descriptors for errlog in
 stdio_client

Github-Issue: #1806
---
 src/mcp/client/stdio.py       | 10 ++++++++--
 src/mcp/os/win32/utilities.py |  7 +++++--
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/src/mcp/client/stdio.py b/src/mcp/client/stdio.py
index 3e03eef9ef..19aae0c462 100644
--- a/src/mcp/client/stdio.py
+++ b/src/mcp/client/stdio.py
@@ -112,10 +112,16 @@ class StdioServerParameters(BaseModel):
 
 @asynccontextmanager
 async def stdio_client(
-    server: StdioServerParameters, errlog: TextIO = sys.stderr
+    server: StdioServerParameters, errlog: TextIO | int = sys.stderr
 ) -> AsyncGenerator[TransportStreams, None]:
     """Spawns an MCP server subprocess and connects to it over stdin/stdout.
 
+    Args:
+        server: Parameters for the server process to spawn.
+        errlog: Where to send the server's stderr. Accepts a text stream
+            (e.g. ``sys.stderr``) or an integer file descriptor, including
+            ``subprocess.DEVNULL`` to discard stderr entirely.
+
     Raises:
         OSError: If the server process cannot be spawned.
         ValueError: If the spawn parameters are invalid (embedded NUL bytes).
@@ -329,7 +335,7 @@ async def _create_platform_compatible_process(
     command: str,
     args: list[str],
     env: dict[str, str] | None = None,
-    errlog: TextIO = sys.stderr,
+    errlog: TextIO | int = sys.stderr,
     cwd: Path | str | None = None,
 ) -> ServerProcess:
     """Spawns the server in its own kill scope.
diff --git a/src/mcp/os/win32/utilities.py b/src/mcp/os/win32/utilities.py
index 321fda8a66..88a06263bc 100644
--- a/src/mcp/os/win32/utilities.py
+++ b/src/mcp/os/win32/utilities.py
@@ -137,7 +137,7 @@ async def create_windows_process(
     command: str,
     args: list[str],
     env: dict[str, str] | None = None,
-    errlog: TextIO | None = sys.stderr,
+    errlog: TextIO | int | None = sys.stderr,
     cwd: Path | str | None = None,
 ) -> Process | FallbackProcess:
     """Creates a subprocess with Job Object support for tree termination.
@@ -149,6 +149,9 @@ async def create_windows_process(
     with it; children spawned before the assignment completes are not captured
     (see the inline note below).
 
+    errlog accepts a text stream (e.g. ``sys.stderr``), an integer file
+    descriptor (e.g. ``subprocess.DEVNULL``), or ``None``.
+
     Returns:
         Process | FallbackProcess: The spawned process with async stdin/stdout streams.
     """
@@ -177,7 +180,7 @@ async def _create_windows_fallback_process(
     command: str,
     args: list[str],
     env: dict[str, str] | None = None,
-    errlog: TextIO | None = sys.stderr,
+    errlog: TextIO | int | None = sys.stderr,
     cwd: Path | str | None = None,
 ) -> FallbackProcess:
     """Spawns via subprocess.Popen and wraps it in FallbackProcess."""