diff --git a/bubus/models.py b/bubus/models.py
index 4079e49..dd2280f 100644
--- a/bubus/models.py
+++ b/bubus/models.py
@@ -295,6 +295,33 @@ async def wait_for_handlers_to_complete_then_return_event():
                 max_iterations = 1000  # Prevent infinite loops
                 iterations = 0
 
+                # Snapshot the buses once for the whole drain: the ancestry
+                # walk and the per-candidate descendant walk both look up
+                # events across this set, and rebuilding it per hop would make
+                # the drain O(candidates × bus_count × chain_depth) per
+                # iteration (review feedback on #5509).
+                buses_snapshot = list(EventBus.all_instances)
+
+                def _find_event_by_id(event_id: str) -> BaseEvent[Any] | None:
+                    for bus_ in buses_snapshot:
+                        if bus_ and event_id in bus_.event_history:
+                            return bus_.event_history[event_id]
+                    return None
+
+                # Compute this event's ancestry chain once, so the drain loop
+                # below only processes events that belong to this waiting
+                # chain. Draining unrelated events from other buses (cross-loop
+                # contamination) consumes them with the wrong handler table and
+                # can stall or drop the owning bus's work — see issue #5509.
+                ancestor_ids: set[str] = {self.event_id}
+                cursor: BaseEvent[Any] | None = self
+                while cursor is not None and cursor.event_parent_id:
+                    parent_event = _find_event_by_id(cursor.event_parent_id)
+                    if parent_event is None or parent_event.event_id in ancestor_ids:
+                        break
+                    ancestor_ids.add(parent_event.event_id)
+                    cursor = parent_event
+
                 try:
                     while not self.event_completed_signal.is_set() and iterations < max_iterations:
                         iterations += 1
@@ -309,10 +336,51 @@ async def wait_for_handlers_to_complete_then_return_event():
                             # Process one event from this bus if available
                             try:
                                 if bus.event_queue.qsize() > 0:
-                                    event = bus.event_queue.get_nowait()
-                                    await bus.process_event(event)
-                                    bus.event_queue.task_done()
-                                    processed_any = True
+                                    # Only process events that belong to this
+                                    # waiting chain. Locate the first such event
+                                    # in the queue and process just that one;
+                                    # unrelated events (from other buses or
+                                    # independent tasks) keep their FIFO position
+                                    # untouched and are left for their own bus's
+                                    # run loop — draining them here is cross-loop
+                                    # contamination that steals the owning bus's
+                                    # scheduling (issue #5509).
+                                    # The underlying deque is indexed directly
+                                    # (same approach as the memory-usage check);
+                                    # task_done() pairs with the put() that
+                                    # enqueued the event.
+                                    for idx, candidate in enumerate(bus.event_queue._queue):
+                                        # The candidate belongs to this waiting
+                                        # chain iff it is this event or a
+                                        # descendant of it (its ancestor chain
+                                        # contains self.event_id). Matching on
+                                        # parent-id alone would also drain
+                                        # siblings that merely share a parent
+                                        # with the awaited event, stealing
+                                        # their normal scheduling.
+                                        if candidate.event_id in ancestor_ids:
+                                            matches = True
+                                        else:
+                                            matches = False
+                                            cursor_candidate = candidate
+                                            seen_ids: set[str] = set()
+                                            while cursor_candidate.event_parent_id and cursor_candidate.event_parent_id not in seen_ids:
+                                                seen_ids.add(cursor_candidate.event_parent_id)
+                                                if cursor_candidate.event_parent_id == self.event_id:
+                                                    matches = True
+                                                    break
+                                                parent_candidate = _find_event_by_id(cursor_candidate.event_parent_id)
+                                                if parent_candidate is None:
+                                                    break
+                                                cursor_candidate = parent_candidate
+                                        if matches:
+                                            del bus.event_queue._queue[idx]
+                                            try:
+                                                await bus.process_event(candidate)
+                                            finally:
+                                                bus.event_queue.task_done()
+                                            processed_any = True
+                                            break
                                     # Check if the event we're waiting for is now complete
                                     if self.event_completed_signal.is_set():
                                         break
diff --git a/tests/test_issue5509_cross_loop.py b/tests/test_issue5509_cross_loop.py
new file mode 100644
index 0000000..339b15d
--- /dev/null
+++ b/tests/test_issue5509_cross_loop.py
@@ -0,0 +1,144 @@
+"""
+Reproduction for issue #5509: cross-loop contamination in the EventBus drain loop.
+
+Scenario: two EventBus instances running in parallel (like two concurrent
+agent sessions in browser-use). A handler chain on bus A (parent -> child ->
+grandchild) enters the inner `__await__` drain loop, which iterates over
+`EventBus.all_instances` and processes queued events from ALL buses.
+
+Before the fix, a bus B event queued during that window is processed by the
+drain loop — i.e. INSIDE bus A's handler context (while bus A holds the global
+lock and bus B's own run loop is starved). This cross-loop execution steals
+bus B's scheduling, and with many parallel sessions it is what drives the
+EventBus capacity errors reported in the issue.
+
+After the fix, the drain loop only processes events belonging to its own
+waiting chain; bus B's independent event stays queued and is handled by bus
+B's own run loop once the lock is released.
+"""
+
+import asyncio
+
+import pytest
+
+from bubus import BaseEvent, EventBus
+
+
+class ParentAEvent(BaseEvent[str]):
+    message: str
+
+
+class ChildAEvent(BaseEvent[str]):
+    data: str
+
+
+class GrandchildAEvent(BaseEvent[str]):
+    value: int
+
+
+class BusBEvent(BaseEvent[str]):
+    """Independent event that should only be handled on bus B."""
+
+    payload: str
+
+
+class StartEvent(BaseEvent[str]):
+    """Handler-less event used only to auto-start a bus's run loop."""
+
+    data: str
+
+
+@pytest.fixture
+async def buses():
+    """Two isolated buses simulating parallel sessions."""
+    # Create bus_b first so it appears before bus_a in the
+    # `EventBus.all_instances` iteration order (WeakSet keeps insertion
+    # order); the drain loop scans buses in that order, so bus B's queued
+    # event is hit before the drain completes its own chain and breaks.
+    bus_b = EventBus(name="bus_b")
+    bus_a = EventBus(name="bus_a")
+    # First dispatch auto-starts each bus's run loop (and creates event_queue).
+    bus_a.dispatch(StartEvent(data="__start__"))
+    bus_b.dispatch(StartEvent(data="__start__"))
+    await asyncio.gather(
+        bus_a.wait_until_idle(timeout=5),
+        bus_b.wait_until_idle(timeout=5),
+    )
+    yield bus_a, bus_b
+    await bus_a.stop(clear=True)
+    await bus_b.stop(clear=True)
+
+
+@pytest.mark.asyncio
+async def test_bus_b_event_not_processed_inside_bus_a_drain(buses):
+    bus_a, bus_b = buses
+
+    ready = asyncio.Event()
+    proceed = asyncio.Event()
+    order: list[str] = []
+
+    async def grandchild_handler(event: GrandchildAEvent) -> int:
+        await asyncio.sleep(0.5)
+        return event.value
+
+    async def child_handler(event: ChildAEvent) -> str:
+        # Tell the test the chain is armed; wait until bus B's independent
+        # event is queued BEFORE forwarding the grandchild, so bus B's queue
+        # holds [independent event, grandchild] when the inner drain loop
+        # scans it — the drain must skip the unrelated event and still
+        # process the grandchild.
+        ready.set()
+        await proceed.wait()
+        order.append("a_drain_enter")
+        grandchild = GrandchildAEvent(value=1)
+        bus_b.dispatch(grandchild)
+        await grandchild
+        order.append("a_drain_exit")
+        return "child_handled"
+
+    async def handler_a(event: ParentAEvent) -> str:
+        child = ChildAEvent(data=f"child_of_{event.message}")
+        bus_a.dispatch(child)
+        await child
+        return "a_handled"
+
+    async def handler_b(event: BusBEvent) -> str:
+        order.append("b_handled")
+        return "b_handled"
+
+    bus_a.on("GrandchildAEvent", grandchild_handler)
+    bus_a.on("ChildAEvent", child_handler)
+    bus_a.on("ParentAEvent", handler_a)
+    bus_b.on("BusBEvent", handler_b)
+
+    # Start bus A's event chain; wait until its handler holds the global lock
+    # (child_handler is parked on `proceed` inside bus A's run loop).
+    bus_a.dispatch(ParentAEvent(message="trigger"))
+    await asyncio.wait_for(ready.wait(), timeout=5)
+
+    # Dispatch an event on bus B: its run loop `get()`s it (outside the lock)
+    # and then blocks acquiring the global lock held by bus A — so bus B's
+    # consumer is parked and will not `get()` again until the lock is free.
+    bus_b.dispatch(BusBEvent(payload="first"))
+    await asyncio.sleep(0.05)
+
+    # Queue a SECOND independent event on bus B (directly into its queue):
+    # with bus B's run loop parked on the lock, this event is only reachable
+    # by bus A's drain loop — the cross-loop window (issue #5509).
+    bus_b.event_queue.put_nowait(BusBEvent(payload="second"))
+    proceed.set()
+
+    await asyncio.gather(
+        bus_a.wait_until_idle(timeout=5),
+        bus_b.wait_until_idle(timeout=5),
+    )
+
+    # Bus B's events must NOT be processed while bus A's drain loop is running
+    # (inside bus A's handler context, holding the global lock): that is the
+    # cross-loop contamination. Both events must be handled afterwards by bus
+    # B's own run loop, once the lock is released.
+    assert order[:2] == ["a_drain_enter", "a_drain_exit"], f"drain markers missing: {order!r}"
+    assert order[2:] == ["b_handled", "b_handled"], (
+        f"cross-loop contamination: bus B events were processed inside bus A's "
+        f"drain loop (order was {order!r}, expected both bus B events after the drain)"
+    )