From 55936172ee0ab9b09c3c5749d0099b1004499847 Mon Sep 17 00:00:00 2001
From: Lovish Arora <46993225+lavish0000@users.noreply.github.com>
Date: Thu, 5 Mar 2026 23:25:46 +0100
Subject: [PATCH 1/3] fix: avoid stdio cleanup BrokenResourceError race

---
 src/mcp/client/stdio.py    | 11 +++++++----
 tests/client/test_stdio.py | 29 +++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/src/mcp/client/stdio.py b/src/mcp/client/stdio.py
index 902dc8576..365836052 100644
--- a/src/mcp/client/stdio.py
+++ b/src/mcp/client/stdio.py
@@ -205,10 +205,13 @@ async def stdin_writer():
             except ProcessLookupError:  # pragma: no cover
                 # Process already exited, which is fine
                 pass
-            await read_stream.aclose()
-            await write_stream.aclose()
-            await read_stream_writer.aclose()
-            await write_stream_reader.aclose()
+            # Stop background stream tasks before closing the memory streams they use.
+            tg.cancel_scope.cancel()
+
+    await read_stream.aclose()
+    await write_stream.aclose()
+    await read_stream_writer.aclose()
+    await write_stream_reader.aclose()
 
 
 def _get_executable_command(command: str) -> str:
diff --git a/tests/client/test_stdio.py b/tests/client/test_stdio.py
index f70c24eee..68e4006c3 100644
--- a/tests/client/test_stdio.py
+++ b/tests/client/test_stdio.py
@@ -155,6 +155,35 @@ async def test_stdio_client_universal_cleanup():
         )
 
 
+@pytest.mark.anyio
+async def test_stdio_client_cleanup_cancels_backpressured_stdout_reader():
+    """Regression test for issue #1960.
+
+    Exiting the client without consuming the read stream leaves stdout_reader
+    blocked on a zero-buffer send. Cleanup must cancel the task before closing
+    its memory stream.
+    """
+    script_content = textwrap.dedent(
+        """
+        import sys
+        import time
+
+        sys.stdout.write('{"jsonrpc":"2.0","id":1,"result":{}}\\n')
+        sys.stdout.flush()
+        time.sleep(2.0)
+        """
+    )
+
+    server_params = StdioServerParameters(
+        command=sys.executable,
+        args=["-c", script_content],
+    )
+
+    with anyio.fail_after(5.0):
+        async with stdio_client(server_params) as (_, _):
+            await anyio.sleep(0.2)
+
+
 @pytest.mark.anyio
 @pytest.mark.skipif(sys.platform == "win32", reason="Windows signal handling is different")
 async def test_stdio_client_sigint_only_process():  # pragma: lax no cover

From 00920b55818850cbbca7857b8c2684ad3c5f026a Mon Sep 17 00:00:00 2001
From: Lovish Arora <46993225+lavish0000@users.noreply.github.com>
Date: Thu, 5 Mar 2026 23:34:44 +0100
Subject: [PATCH 2/3] fix: handle broken stdio streams during cleanup

---
 src/mcp/client/stdio.py | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/src/mcp/client/stdio.py b/src/mcp/client/stdio.py
index 365836052..e1557d029 100644
--- a/src/mcp/client/stdio.py
+++ b/src/mcp/client/stdio.py
@@ -158,7 +158,7 @@ async def stdout_reader():
 
                         session_message = SessionMessage(message)
                         await read_stream_writer.send(session_message)
-        except anyio.ClosedResourceError:  # pragma: lax no cover
+        except (anyio.BrokenResourceError, anyio.ClosedResourceError):  # pragma: lax no cover
             await anyio.lowlevel.checkpoint()
 
     async def stdin_writer():
@@ -174,7 +174,7 @@ async def stdin_writer():
                             errors=server.encoding_error_handler,
                         )
                     )
-        except anyio.ClosedResourceError:  # pragma: no cover
+        except (anyio.BrokenResourceError, anyio.ClosedResourceError):  # pragma: no cover
             await anyio.lowlevel.checkpoint()
 
     async with anyio.create_task_group() as tg, process:
@@ -205,13 +205,10 @@ async def stdin_writer():
             except ProcessLookupError:  # pragma: no cover
                 # Process already exited, which is fine
                 pass
-            # Stop background stream tasks before closing the memory streams they use.
-            tg.cancel_scope.cancel()
-
-    await read_stream.aclose()
-    await write_stream.aclose()
-    await read_stream_writer.aclose()
-    await write_stream_reader.aclose()
+            await read_stream.aclose()
+            await write_stream.aclose()
+            await read_stream_writer.aclose()
+            await write_stream_reader.aclose()
 
 
 def _get_executable_command(command: str) -> str:

From 54a97908cabb91033956545258eb83e1f2333d08 Mon Sep 17 00:00:00 2001
From: Lovish Arora <46993225+lavish0000@users.noreply.github.com>
Date: Fri, 6 Mar 2026 06:44:47 +0100
Subject: [PATCH 3/3] chore: retrigger CI after cancelled workflow