fix: handle non-file stdio in tests by catching UnsupportedOperation

When sys.stdin/sys.stdout are monkeypatched with StringIO/BytesIO
(e.g. in unit tests), fileno() raises io.UnsupportedOperation.
Fall back to the original non-duping behavior in that case.

Fixes CI test failures on all Python versions.

Author: Hermes PR Bot

src/mcp/server/stdio.py

@@ -41,14 +41,24 @@ async def stdio_server(stdin: anyio.AsyncFile[str] | None = None, stdout: anyio.
     # re-wrap the underlying binary stream to ensure UTF-8.
     # We duplicate the file descriptors to avoid closing the original stdio
     # when the transport is closed (fixes issue #1933).
+    # When stdin/stdout are not real OS files (e.g. StringIO/BytesIO in tests),
+    # fileno() raises io.UnsupportedOperation; fall back to the original path.
     if not stdin:
-        stdin_fd = os.dup(sys.stdin.fileno())
-        stdin_bin = os.fdopen(stdin_fd, "rb", closefd=True)
-        stdin = anyio.wrap_file(TextIOWrapper(stdin_bin, encoding="utf-8", errors="replace"))
+        try:
+            stdin_fd = os.dup(sys.stdin.fileno())
+        except io.UnsupportedOperation:
+            stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8", errors="replace"))
+        else:
+            stdin_bin = os.fdopen(stdin_fd, "rb", closefd=True)
+            stdin = anyio.wrap_file(TextIOWrapper(stdin_bin, encoding="utf-8", errors="replace"))
     if not stdout:
-        stdout_fd = os.dup(sys.stdout.fileno())
-        stdout_bin = os.fdopen(stdout_fd, "wb", closefd=True)
-        stdout = anyio.wrap_file(TextIOWrapper(stdout_bin, encoding="utf-8"))
+        try:
+            stdout_fd = os.dup(sys.stdout.fileno())
+        except io.UnsupportedOperation:
+            stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))
+        else:
+            stdout_bin = os.fdopen(stdout_fd, "wb", closefd=True)
+            stdout = anyio.wrap_file(TextIOWrapper(stdout_bin, encoding="utf-8"))
 
     read_stream_writer, read_stream = create_context_streams[SessionMessage | Exception](0)
     write_stream, write_stream_reader = create_context_streams[SessionMessage](0)