fix(ENG-15479): stop mcp start quietly when the client closes stdout

CHANGELOG.md

@@ -82,6 +82,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - `cloudsmith credential-helper install` and `uninstall` no longer rewrite a config file when they make no change to its data. Previously, the CLI always rewrote a file in a different formatting, such as the tab-indented `config.json` that Docker writes. The command also made a `.bak` copy and reported changes that it did not make.
 - When the API host name or the proxy host name does not exist, for example because of a typo in `--api-host`, commands now fail immediately. Previously, they retried for approximately 62 seconds without output. The CLI still retries a DNS failure that the operating system reports as temporary.
 - When the CLI waits before it retries a failed connection or a 5xx response, it now writes `Request failed: retrying in N second(s) ...` to stderr. Previously, it waited without output.
+- `cloudsmith mcp start` now stops without an error when the MCP client closes the connection while the server writes a reply. Previously, the command stopped with `BrokenPipeError` and exit status 1.
 - The CLI now exits with status 1 when you interrupt it with Ctrl+C after the reader of its stderr has closed the pipe. Before, the CLI sent an error report and exited with status 120.
 - `cloudsmith check rates` no longer stops with `TypeError` when the API does not give a value for an interval, limit, remaining count, reset time or throttled status. The command now shows `None` for that value.
 

cloudsmith_cli/cli/tests/commands/test_mcp.py

@@ -1,6 +1,7 @@
 import json
 import os
 import stat
+import subprocess
 import sys
 from pathlib import Path
 from unittest.mock import patch
@@ -740,3 +741,40 @@ def test_configure_prints_on_cp1252_console(self, tmp_path):
         assert result.exit_code == 0, result.output
         assert "Configured Cursor" in result.stdout
         assert config_path.exists()
+
+
+def test_server_stops_quietly_when_the_client_closes_stdout():
+    """The stdio transport raises the closed pipe inside an ExceptionGroup."""
+    code = """
+import os
+from unittest.mock import patch
+
+import cloudsmith_api
+
+from cloudsmith_cli.core.mcp.server import DynamicMCPServer
+
+read_end, write_end = os.pipe()
+os.close(read_end)
+os.dup2(write_end, 1)
+with patch.object(DynamicMCPServer, "load_openapi_spec"):
+    DynamicMCPServer(api_config=cloudsmith_api.Configuration()).run()
+"""
+    initialize = {
+        "jsonrpc": "2.0",
+        "id": 1,
+        "method": "initialize",
+        "params": {
+            "protocolVersion": "2025-06-18",
+            "capabilities": {},
+            "clientInfo": {"name": "test", "version": "1"},
+        },
+    }
+    result = subprocess.run(
+        [sys.executable, "-c", code],
+        input=json.dumps(initialize) + "\n",
+        capture_output=True,
+        text=True,
+        check=False,
+    )
+
+    assert result.returncode == 0, result.stderr

cloudsmith_cli/core/mcp/server.py

@@ -883,7 +883,10 @@ def run(self):
         """Initialize and run the server"""
         asyncio.run(self.load_openapi_spec())
         try:
-            self.mcp.run(transport="stdio")
+            try:
+                self.mcp.run(transport="stdio")
+            except* BrokenPipeError:
+                pass  # The MCP client closed the connection.
         except asyncio.CancelledError:
             print("Server shutdown requested")