From 030d4caa0e48b52ddf59ce675ce0020e8720aaee Mon Sep 17 00:00:00 2001
From: KoNit-K <124019182+KoNit-K@users.noreply.github.com>
Date: Tue, 15 Sep 2026 22:18:33 +0800
Subject: [PATCH] fix(mcp): bound nested connection error traversal

---
 tests/tools/test_mcp_tool_issue_948.py | 22 ++++++++++
 tools/mcp_tool_errors.py               | 59 +++++++++++++++++++-------
 2 files changed, 66 insertions(+), 15 deletions(-)

diff --git a/tests/tools/test_mcp_tool_issue_948.py b/tests/tools/test_mcp_tool_issue_948.py
index 230668b4a1..d31c02be7c 100644
--- a/tests/tools/test_mcp_tool_issue_948.py
+++ b/tests/tools/test_mcp_tool_issue_948.py
@@ -20,6 +20,28 @@ if not _MCP_AVAILABLE:
         _mcp_mod.ClientSession = MagicMock
 
 
+def test_format_connect_error_finds_missing_executable_in_deep_exception_chain():
+    """A deeply wrapped missing binary must not recurse while formatting it."""
+    missing = FileNotFoundError(2, "No such file or directory", "/opt/bin/removed-mcp")
+    current = missing
+    for _ in range(sys.getrecursionlimit() + 10):
+        wrapper = RuntimeError("stdio startup failed")
+        wrapper.__cause__ = current
+        current = wrapper
+
+    assert _format_connect_error(current) == "missing executable '/opt/bin/removed-mcp'"
+
+
+def test_format_connect_error_handles_cyclic_exception_chain():
+    """Malformed exception chains must fall back to a finite, sanitized message."""
+    first = RuntimeError("first failure")
+    second = RuntimeError("second failure")
+    first.__cause__ = second
+    second.__cause__ = first
+
+    assert _format_connect_error(first) == "first failure; second failure"
+
+
 def test_resolve_stdio_command_falls_back_to_hermes_node_bin(tmp_path):
     node_bin = tmp_path / "node" / "bin"
     node_bin.mkdir(parents=True)
diff --git a/tools/mcp_tool_errors.py b/tools/mcp_tool_errors.py
index 13167317e5..c0cb5b299f 100644
--- a/tools/mcp_tool_errors.py
+++ b/tools/mcp_tool_errors.py
@@ -307,23 +307,52 @@ def _exc_children(exc: BaseException) -> List[BaseException]:
 
 def _format_connect_error(exc: BaseException) -> str:
     """Render nested MCP connection errors into an actionable short message."""
-    def _find_missing(current: BaseException) -> Optional[str]:
-        if isinstance(current, FileNotFoundError):
-            if getattr(current, "filename", None):
-                return str(current.filename)
-            match = re.search(r"No such file or directory: '([^']+)'", str(current))
-            if match:
-                return match.group(1)
-        return next(filter(None, map(_find_missing, _exc_children(current))), None)
+    def _find_missing() -> Optional[str]:
+        """Find a missing executable without recursing through malformed chains."""
+        stack = [exc]
+        seen: set[int] = set()
+        budget = _EXC_TRAVERSAL_MAX_NODES
+        while stack and budget > 0:
+            current = stack.pop()
+            if id(current) in seen:
+                continue
+            seen.add(id(current))
+            budget -= 1
+            if isinstance(current, FileNotFoundError):
+                if getattr(current, "filename", None):
+                    return str(current.filename)
+                match = re.search(r"No such file or directory: '([^']+)'", str(current))
+                if match:
+                    return match.group(1)
+            # Reverse preserves the former left-to-right depth-first order.
+            stack.extend(reversed(_exc_children(current)))
+        return None
 
-    def _flatten_messages(current: BaseException) -> List[str]:
-        # A group's own str() is opaque — only its children speak.
-        text = "" if getattr(current, "exceptions", None) else str(current).strip()
-        messages = ([text] if text else []) + [m for child in _exc_children(current) for m in _flatten_messages(child)]
-        return messages or [current.__class__.__name__]
-    missing = _find_missing(exc)
+    def _flatten_messages() -> List[str]:
+        """Collect a short, cycle-safe rendering of an exception chain."""
+        stack = [exc]
+        seen: set[int] = set()
+        messages: List[str] = []
+        budget = _EXC_TRAVERSAL_MAX_NODES
+        while stack and budget > 0:
+            current = stack.pop()
+            if id(current) in seen:
+                continue
+            seen.add(id(current))
+            budget -= 1
+            children = _exc_children(current)
+            # A group's own str() is opaque — only its children speak.
+            text = "" if getattr(current, "exceptions", None) else str(current).strip()
+            if text:
+                messages.append(text)
+            elif not children:
+                messages.append(current.__class__.__name__)
+            stack.extend(reversed(children))
+        return messages or [exc.__class__.__name__]
+
+    missing = _find_missing()
     if not missing:
-        return _sanitize_error("; ".join(list(dict.fromkeys(_flatten_messages(exc)))[:3]))
+        return _sanitize_error("; ".join(list(dict.fromkeys(_flatten_messages()))[:3]))
     message = f"missing executable '{missing}'"
     if os.path.basename(missing) in {"npx", "npm", "node"}:
         message += (" (ensure Node.js is installed and PATH includes its bin directory, "