fix(stdio): handle bufferless in-memory text streams in fallback

steps-re · claude · steps-re · commit 9a57007148a2 · 2026-07-13T17:46:41.000-04:00
The no-fd fallback reached for std.buffer, which raises AttributeError on bufferless text streams such as io.StringIO. Guard with hasattr and wrap the text stream directly in that case (nothing to tear down). Adds a regression test. Addresses the cubic review on #3090. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
diff --git a/src/mcp/server/stdio.py b/src/mcp/server/stdio.py
@@ -62,6 +62,13 @@ def wrap_std(std: TextIO, mode: str, errors: str | None) -> anyio.AsyncFile[str]
             wrapper = TextIOWrapper(binary, encoding="utf-8", errors=errors)
             to_close.append(wrapper)
         except (AttributeError, OSError, ValueError, io.UnsupportedOperation):
+            # No real fd. A bufferless in-memory text stream (e.g. io.StringIO)
+            # has no .buffer to re-wrap, so use it directly -- it is already text,
+            # and we did not create it, so there is nothing to tear down. Otherwise
+            # re-wrap .buffer and detach() on exit so the finalizer cannot close the
+            # real handle.
+            if not hasattr(std, "buffer"):
+                return anyio.wrap_file(std)
             wrapper = TextIOWrapper(std.buffer, encoding="utf-8", errors=errors)
             to_detach.append(wrapper)
         return anyio.wrap_file(wrapper)
diff --git a/tests/server/test_stdio.py b/tests/server/test_stdio.py
@@ -118,6 +118,24 @@ async def test_stdio_server_does_not_close_real_std_handles(monkeypatch: pytest.
         stdout_file.flush()
 
 
+@pytest.mark.anyio
+async def test_stdio_server_bufferless_text_streams(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Regression: default startup must not crash on bufferless in-memory text streams.
+
+    In-memory text streams like `io.StringIO` have neither a real `fileno()` nor a
+    `.buffer`, so both the fd-dup path and the `.buffer` re-wrap path fail. In that
+    case `stdio_server()` must use the stream directly. Before the fix, the fallback
+    reached for `std.buffer` and raised `AttributeError` during startup.
+    """
+    monkeypatch.setattr(sys, "stdin", io.StringIO())  # empty -> immediate EOF
+    monkeypatch.setattr(sys, "stdout", io.StringIO())
+
+    with anyio.fail_after(5):
+        async with stdio_server() as (read_stream, write_stream):
+            async with read_stream, write_stream:
+                pass
+
+
 @pytest.mark.anyio
 async def test_stdio_server_invalid_utf8(monkeypatch: pytest.MonkeyPatch) -> None:
     """Non-UTF-8 stdin bytes surface as an in-stream exception without killing the stream.
