diff --git a/langfuse/openai.py b/langfuse/openai.py
index ea9dcd68c..fa3c75682 100644
--- a/langfuse/openai.py
+++ b/langfuse/openai.py
@@ -727,8 +727,14 @@ def _parse_cost(usage: Optional[Any] = None) -> Any:
 
     # OpenRouter is returning total cost of the invocation
     # https://openrouter.ai/docs/use-cases/usage-accounting#cost-breakdown
-    if hasattr(usage, "cost") and isinstance(getattr(usage, "cost"), float):
-        return {"total": getattr(usage, "cost")}
+    cost = (
+        usage.get("cost") if isinstance(usage, dict) else getattr(usage, "cost", None)
+    )
+
+    # bool is a subclass of int, so it has to be excluded explicitly to avoid
+    # turning a stray `"cost": true` into a cost of 1.0
+    if isinstance(cost, (int, float)) and not isinstance(cost, bool):
+        return {"total": float(cost)}
 
     return None
 
diff --git a/tests/unit/test_parse_cost.py b/tests/unit/test_parse_cost.py
new file mode 100644
index 000000000..4527b2cb9
--- /dev/null
+++ b/tests/unit/test_parse_cost.py
@@ -0,0 +1,48 @@
+from langfuse.openai import _parse_cost
+
+
+class _Usage:
+    def __init__(self, **kwargs):
+        self.__dict__.update(kwargs)
+
+
+def test_none_usage():
+    assert _parse_cost(None) is None
+
+
+def test_object_with_float_cost():
+    assert _parse_cost(_Usage(cost=0.0021)) == {"total": 0.0021}
+
+
+def test_object_without_cost():
+    assert _parse_cost(_Usage(prompt_tokens=10)) is None
+
+
+def test_dict_with_float_cost():
+    """OpenRouter usage arrives as a dict on the streaming path.
+
+    _parse_usage already branches on isinstance(usage, dict) and both are called
+    with the same object, so cost has to handle a dict too.
+    """
+    assert _parse_cost({"cost": 0.0021, "prompt_tokens": 10}) == {"total": 0.0021}
+
+
+def test_dict_without_cost():
+    assert _parse_cost({"prompt_tokens": 10}) is None
+
+
+def test_integer_cost_is_not_dropped():
+    """A free or fully cached model reports "cost": 0, which is an int, not a float."""
+    assert _parse_cost({"cost": 0}) == {"total": 0.0}
+    assert _parse_cost(_Usage(cost=0)) == {"total": 0.0}
+    assert _parse_cost(_Usage(cost=2)) == {"total": 2.0}
+
+
+def test_bool_cost_is_rejected():
+    assert _parse_cost({"cost": True}) is None
+    assert _parse_cost(_Usage(cost=False)) is None
+
+
+def test_non_numeric_cost_is_rejected():
+    assert _parse_cost({"cost": "0.0021"}) is None
+    assert _parse_cost({"cost": None}) is None