Allow subprocess constants (e.g. DEVNULL) for stdio errlog parameter

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.

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.

tests/client/test_stdio.py

@@ -1,5 +1,6 @@
 import errno
 import shutil
+import subprocess
 import sys
 import textwrap
 import time
@@ -70,6 +71,45 @@ async def test_stdio_client():
         assert read_messages[1] == JSONRPCResponse(jsonrpc="2.0", id=2, result={})
 
 
+@pytest.mark.anyio
+async def test_stdio_client_devnull_errlog():
+    """Test that stdio_client accepts subprocess.DEVNULL for errlog,
+    allowing callers to suppress stderr output from the child process.
+
+    Regression test for https://github.com/modelcontextprotocol/python-sdk/issues/1806
+    """
+    # A script that writes to stderr then echoes stdin to stdout
+    script_content = textwrap.dedent(
+        """
+        import sys
+        sys.stderr.write("this goes to devnull\\n")
+        sys.stderr.flush()
+        for line in sys.stdin:
+            sys.stdout.write(line)
+            sys.stdout.flush()
+        """
+    )
+
+    server_params = StdioServerParameters(
+        command=sys.executable,
+        args=["-c", script_content],
+    )
+
+    async with stdio_client(server_params, errlog=subprocess.DEVNULL) as (read_stream, write_stream):
+        message = JSONRPCRequest(jsonrpc="2.0", id=1, method="ping")
+        session_message = SessionMessage(message)
+
+        async with write_stream:
+            await write_stream.send(session_message)
+
+        async with read_stream:
+            async for received in read_stream:
+                if isinstance(received, Exception):  # pragma: no cover
+                    raise received
+                assert received.message == message
+                break
+
+
 @pytest.mark.anyio
 async def test_stdio_client_bad_path():
     """Check that the connection doesn't hang if process errors."""