Make errlog optional in stdio_client and update default to None

examples/snippets/clients/logging_client.py

@@ -0,0 +1,46 @@
+"""
+cd to the `examples/snippets/clients` directory and run:
+    uv run logging-client
+"""
+
+import asyncio
+import os
+
+from mcp import ClientSession, StdioServerParameters
+from mcp.client.stdio import stdio_client
+from mcp.types import LoggingMessageNotificationParams
+
+# Create server parameters for stdio connection
+server_params = StdioServerParameters(
+    command="uv",  # Using uv to run the server
+    args=["run", "server", "logging", "stdio"],  # We're already in snippets dir
+    env={"UV_INDEX": os.environ.get("UV_INDEX", "")},
+)
+
+
+async def logging_callback(params: LoggingMessageNotificationParams):
+    print(f"Log Level: {params.level}, Message: {params.data}")
+
+
+async def run():
+    fds = os.pipe()
+    reader = os.fdopen(fds[0], "r")
+    writer = os.fdopen(fds[1], "w")
+    async with stdio_client(server_params, errlog=writer) as (read, write):
+        async with ClientSession(read, write, logging_callback=logging_callback) as session:
+            await session.initialize()
+
+            await session.list_tools()
+            await session.call_tool("log", arguments={})
+    writer.close()
+    print("Captured stderr logs:")
+    print(reader.read())
+
+
+def main():
+    """Entry point for the client script."""
+    asyncio.run(run())
+
+
+if __name__ == "__main__":
+    main()

examples/snippets/pyproject.toml

@@ -22,3 +22,4 @@ direct-execution-server = "servers.direct_execution:main"
 display-utilities-client = "clients.display_utilities:main"
 oauth-client = "clients.oauth_client:run"
 elicitation-client = "clients.url_elicitation_client:run"
+logging-client = "clients.logging_client:main"

examples/snippets/servers/logging.py

@@ -0,0 +1,24 @@
+"""
+FastMCP Echo Server that sends log messages and prints to stderr
+"""
+
+import sys
+
+from mcp.server.fastmcp import Context, FastMCP
+from mcp.server.session import ServerSession
+
+mcp = FastMCP("Logging and stdio test")
+
+
+@mcp.tool()
+async def log(ctx: Context[ServerSession, None]) -> str:
+    await ctx.debug("Debug message")
+    await ctx.info("Info message")
+    print("Stderr message", file=sys.stderr)
+    await ctx.warning("Warning message")
+    await ctx.error("Error message")
+    return "done"
+
+
+if __name__ == "__main__":
+    mcp.run(transport="stdio")

src/mcp/client/stdio/__init__.py

@@ -103,7 +103,7 @@ class StdioServerParameters(BaseModel):
 
 
 @asynccontextmanager
-async def stdio_client(server: StdioServerParameters, errlog: TextIO = sys.stderr):
+async def stdio_client(server: StdioServerParameters, errlog: TextIO | None = None):
     """
     Client transport for stdio: this will connect to a server by spawning a
     process and communicating with it over stdin/stdout.
@@ -236,7 +236,7 @@ async def _create_platform_compatible_process(
     command: str,
     args: list[str],
     env: dict[str, str] | None = None,
-    errlog: TextIO = sys.stderr,
+    errlog: TextIO | None = None,
     cwd: Path | str | None = None,
 ):
     """