fix: skip deeply nested upstream events in the stats observer

routstr/upstream/terminal_outcome_tracking.py

@@ -160,8 +160,9 @@ def observe_terminal_sse_bytes(
             continue
         try:
             parsed = json.loads(payload)
-        except ValueError:
-            # Bytes cut inside a character raise UnicodeDecodeError, not JSONDecodeError.
+        except (ValueError, RecursionError):
+            # Bytes cut inside a character raise UnicodeDecodeError, not
+            # JSONDecodeError; an upstream's deep nesting raises RecursionError.
             continue
         if isinstance(parsed, dict):
             state.observe(parsed)

tests/unit/test_streaming_billing_finalization.py

@@ -1388,6 +1388,14 @@ def test_stream_cut_inside_a_character_does_not_raise() -> None:
     assert observe_terminal_sse_bytes(state, b"", tail, final=True) == b""
 
 
+def test_deeply_nested_event_does_not_raise() -> None:
+    state = TerminalOutcomeState()
+    event = b"data: " + b"[" * 200_000 + b"]" * 200_000 + b"\n\n"
+
+    assert observe_terminal_sse_bytes(state, b"", event, final=True) == b""
+    assert state.usage is None
+
+
 def test_routstr_upstream_cost_event_is_not_provider_usage() -> None:
     state = TerminalOutcomeState()
     stream = (