From 8ee7455692049f6e9d88527440903771191c61cb Mon Sep 17 00:00:00 2001
From: Bortlesboat <bortstheboat@gmail.com>
Date: Thu, 19 Mar 2026 22:22:59 -0400
Subject: [PATCH] fix: widen errlog type hint to accept subprocess.DEVNULL

The errlog parameter in stdio_client and related functions was typed
as TextIO, which prevented passing subprocess.DEVNULL (an int) to
suppress stderr output from MCP server subprocesses. Both
subprocess.Popen and anyio.open_process already accept int values
for stderr, so widening the type to TextIO | int is safe.

Github-Issue: #1806
Reported-by: sesajad
---
 src/mcp/client/stdio.py       | 4 ++--
 src/mcp/os/win32/utilities.py | 6 +++---
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/mcp/client/stdio.py b/src/mcp/client/stdio.py
index 902dc8576..531ab480b 100644
--- a/src/mcp/client/stdio.py
+++ b/src/mcp/client/stdio.py
@@ -102,7 +102,7 @@ class StdioServerParameters(BaseModel):
 
 
 @asynccontextmanager
-async def stdio_client(server: StdioServerParameters, errlog: TextIO = sys.stderr):
+async def stdio_client(server: StdioServerParameters, errlog: TextIO | int = sys.stderr):
     """Client transport for stdio: this will connect to a server by spawning a
     process and communicating with it over stdin/stdout.
     """
@@ -230,7 +230,7 @@ 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,
 ):
     """Creates a subprocess in a platform-compatible way.
diff --git a/src/mcp/os/win32/utilities.py b/src/mcp/os/win32/utilities.py
index 6f68405f7..aeb5e8e50 100644
--- a/src/mcp/os/win32/utilities.py
+++ b/src/mcp/os/win32/utilities.py
@@ -138,7 +138,7 @@ 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 in a Windows-compatible way with Job Object support.
@@ -155,7 +155,7 @@ async def create_windows_process(
         command (str): The executable to run
         args (list[str]): List of command line arguments
         env (dict[str, str] | None): Environment variables
-        errlog (TextIO | None): Where to send stderr output (defaults to sys.stderr)
+        errlog (TextIO | int | None): Where to send stderr output (defaults to sys.stderr)
         cwd (Path | str | None): Working directory for the subprocess
 
     Returns:
@@ -196,7 +196,7 @@ 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:
     """Create a subprocess using subprocess.Popen as a fallback when anyio fails.