From 1ae1dd8b2c63fdd3b950553c56d9f5e56f3d23ab Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?B=C3=B6rje?= <borje@dqsverige.se>
Date: Sat, 18 Jul 2026 00:07:11 +0200
Subject: [PATCH] fix(mcp): harden nested interruption detection

---
 tests/tools/test_mcp_tool_session_expired.py | 33 ++++++++++++++++++++
 tools/mcp_tool.py                            | 18 +++++++----
 2 files changed, 45 insertions(+), 6 deletions(-)

diff --git a/tests/tools/test_mcp_tool_session_expired.py b/tests/tools/test_mcp_tool_session_expired.py
index c794236e04f2e..f6aa9ee590bc6 100644
--- a/tests/tools/test_mcp_tool_session_expired.py
+++ b/tests/tools/test_mcp_tool_session_expired.py
@@ -121,6 +121,39 @@ def test_is_session_expired_rejects_mixed_group_with_user_interruption():
     assert _is_session_expired_error(exc) is False
 
 
+def test_exception_tree_finds_interruption_beyond_recursion_limit():
+    """Arbitrarily deep wrapper trees must not overflow Python's call stack."""
+    import sys
+
+    from tools.mcp_tool import _exception_tree_contains_interruption
+
+    class NestedException(Exception):
+        exceptions: tuple[BaseException, ...]
+
+    exc = InterruptedError("cancel")
+    for _ in range(sys.getrecursionlimit() + 100):
+        wrapper = NestedException("wrapped")
+        wrapper.exceptions = (exc,)
+        exc = wrapper
+
+    assert _exception_tree_contains_interruption(exc) is True
+
+
+def test_exception_tree_handles_cyclic_exceptions_graph():
+    """Malformed exception graphs may contain cycles and must terminate safely."""
+    from tools.mcp_tool import _exception_tree_contains_interruption
+
+    class CyclicException(Exception):
+        exceptions: tuple[BaseException, ...]
+
+    first = CyclicException("first")
+    second = CyclicException("second")
+    first.exceptions = (second,)
+    second.exceptions = (first,)
+
+    assert _exception_tree_contains_interruption(first) is False
+
+
 def test_is_session_expired_rejects_empty_message():
     """Bare exceptions with no message shouldn't match."""
     from tools.mcp_tool import _is_session_expired_error
diff --git a/tools/mcp_tool.py b/tools/mcp_tool.py
index aca2651597d48..c2b9503a84e2c 100644
--- a/tools/mcp_tool.py
+++ b/tools/mcp_tool.py
@@ -3902,12 +3902,18 @@ _SESSION_EXPIRED_MARKERS: tuple = (
 
 def _exception_tree_contains_interruption(exc: BaseException) -> bool:
     """Return whether ``exc`` or any nested exception is user cancellation."""
-    if isinstance(exc, InterruptedError):
-        return True
-    return any(
-        _exception_tree_contains_interruption(child)
-        for child in getattr(exc, "exceptions", ())
-    )
+    stack = [exc]
+    seen: set[int] = set()
+    while stack:
+        current = stack.pop()
+        identity = id(current)
+        if identity in seen:
+            continue
+        seen.add(identity)
+        if isinstance(current, InterruptedError):
+            return True
+        stack.extend(getattr(current, "exceptions", ()))
+    return False
 
 
 def _is_session_expired_error(exc: BaseException) -> bool: