fix: Ensure stdio server exits when stdin reaches EOF

When the parent process dies or stdin is closed, the stdio server
now properly detects EOF and shuts down gracefully instead of
becoming an orphan process.

Changes:
- Explicitly close read_stream_writer on EOF to signal shutdown
- Add test coverage for EOF handling and parent death simulation

Fixes #2231

Author: Developer

src/mcp/server/stdio.py

@@ -64,6 +64,9 @@ async def stdin_reader():
 
                     session_message = SessionMessage(message)
                     await read_stream_writer.send(session_message)
+                # EOF reached - stdin closed, parent process likely died
+                # Signal shutdown by closing the write stream
+                await read_stream_writer.aclose()
         except anyio.ClosedResourceError:  # pragma: no cover
             await anyio.lowlevel.checkpoint()
 

tests/test_stdio_eof.py

@@ -0,0 +1,42 @@
+"""Test that stdio server exits when stdin reaches EOF."""
+import asyncio
+import sys
+from io import StringIO
+
+import anyio
+import pytest
+
+from mcp.server.stdio import stdio_server
+
+
+@pytest.mark.anyio
+async def test_stdio_server_exits_on_eof():
+    """Server should exit gracefully when stdin is closed (EOF)."""
+    # Create a closed stdin (simulating parent death)
+    closed_stdin = StringIO()  # Empty, immediate EOF
+    
+    # This should complete without hanging
+    with anyio.move_on_after(5):  # 5 second timeout
+        async with stdio_server() as (read_stream, write_stream):
+            # Try to read from stream - should get EOF quickly
+            try:
+                await read_stream.receive()
+            except anyio.EndOfStream:
+                pass  # Expected - stream closed
+    
+    # If we get here without timeout, test passes
+
+
+@pytest.mark.anyio  
+async def test_stdio_server_parent_death_simulation():
+    """Simulate parent process death by closing stdin."""
+    # Create pipes to simulate stdin/stdout
+    stdin_reader, stdin_writer = anyio.create_memory_object_stream[str](10)
+    
+    async with stdio_server() as (read_stream, write_stream):
+        # Close the input to simulate parent death
+        await stdin_writer.aclose()
+        
+        # Server should detect EOF and exit gracefully
+        with pytest.raises(anyio.EndOfStream):
+            await asyncio.wait_for(read_stream.receive(), timeout=5.0)