docs: document Windows stdio subprocess stdin handling (#3079)

Author: AndreKalberer

docs/run/index.md

@@ -41,6 +41,30 @@ Nothing prints, and it doesn't return. It is waiting on stdin for a host to spea
 
 That also means stdout **is the wire**. A stray `print()` corrupts the stream; the `logging` module writes to stderr and is the right tool. That story is in **[Logging](../handlers/logging.md)**.
 
+On Windows, the same rule applies to child processes your tools start. A child
+that inherits the stdio server's stdin can block behind the server's protocol
+reader. If your tool starts a subprocess and you do not intend to feed it input,
+redirect the child's stdin:
+
+```python
+import asyncio
+import subprocess
+import sys
+
+
+async def run_script() -> tuple[bytes, bytes]:
+    process = await asyncio.create_subprocess_exec(
+        sys.executable,
+        "script.py",
+        stdin=subprocess.DEVNULL,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+    )
+    return await process.communicate()
+```
+
+The matching troubleshooting entry is **[My stdio tool hangs when it starts a subprocess on Windows](../troubleshooting.md#my-stdio-tool-hangs-when-it-starts-a-subprocess-on-windows)**.
+
 ### Try it
 
 ```console

docs/troubleshooting.md

@@ -141,6 +141,41 @@ There is no error string for this, which is exactly why it is hard to search. Th
 
 An "invalid" tool name is *not* on that list: a non-conforming name logs a warning but the tool is registered and listed anyway.
 
+## My stdio tool hangs when it starts a subprocess on Windows
+
+Your server is running over `stdio`, and a tool starts another process with
+`asyncio.create_subprocess_exec`, `asyncio.create_subprocess_shell`, or
+`subprocess.Popen`. The tool call never returns on Windows, while the same code
+works over an HTTP transport.
+
+The child inherited the server's stdin. In a stdio server, stdin is the protocol
+pipe and the server is already waiting on it for the next JSON-RPC message. A
+Python child process on Windows can block during startup when it inherits that
+same pipe.
+
+If you do not intend to send input to the child, redirect its stdin:
+
+```python
+import asyncio
+import subprocess
+import sys
+
+
+async def run_script() -> tuple[bytes, bytes]:
+    process = await asyncio.create_subprocess_exec(
+        sys.executable,
+        "script.py",
+        stdin=subprocess.DEVNULL,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+    )
+    return await process.communicate()
+```
+
+Use the same idea with `subprocess.Popen(..., stdin=subprocess.DEVNULL)`. Also
+capture or redirect the child's stdout. The stdio server's stdout is the MCP
+wire, so a child that writes there can corrupt the connection.
+
 ## `MCPError: Server returned an error response`
 
 The server refused the HTTP request outright, with a body that is not JSON-RPC, so the python `Client` has nothing better to show you than this stand-in.