fix(ai): capture pre-calculated cost from OpenAI Agents responses

.sampo/changesets/doughty-iceseeker-tursas.md

@@ -0,0 +1,5 @@
+---
+pypi/posthog: patch
+---
+
+Capture pre-calculated total cost from OpenAI Agents Responses API usage.

posthog/ai/openai_agents/processor.py

@@ -682,6 +682,7 @@ def _handle_response_span(
 
         # Try to extract usage from response
         usage = getattr(response, "usage", None) if response else None
+        total_cost_usd = getattr(usage, "cost", None) if usage else None
         input_tokens = 0
         output_tokens = 0
         if usage:
@@ -703,6 +704,9 @@ def _handle_response_span(
             "$ai_total_tokens": input_tokens + output_tokens,
         }
 
+        if total_cost_usd is not None:
+            properties["$ai_total_cost_usd"] = total_cost_usd
+
         # Extract output content from response
         if response:
             output_items = getattr(response, "output", None)

posthog/test/ai/openai_agents/test_processor.py

@@ -536,6 +536,7 @@ def test_response_span_with_output_and_total_tokens(
         mock_response.usage = MagicMock()
         mock_response.usage.input_tokens = 25
         mock_response.usage.output_tokens = 10
+        mock_response.usage.cost = None
 
         span_data = ResponseSpanData(
             response=mock_response,
@@ -554,6 +555,29 @@ def test_response_span_with_output_and_total_tokens(
             {"type": "message", "content": "Hello!"}
         ]
         assert call_kwargs["properties"]["$ai_response_id"] == "resp_123"
+        assert "$ai_total_cost_usd" not in call_kwargs["properties"]
+
+    @pytest.mark.parametrize("total_cost_usd", [0.01234, 0.0])
+    def test_response_span_includes_total_cost(
+        self, processor, mock_client, mock_span, total_cost_usd
+    ):
+        """Test ResponseSpanData includes a pre-calculated total cost."""
+        mock_response = MagicMock()
+        mock_response.id = "resp_123"
+        mock_response.model = "gpt-4o"
+        mock_response.output = []
+        mock_response.usage = MagicMock()
+        mock_response.usage.input_tokens = 25
+        mock_response.usage.output_tokens = 10
+        mock_response.usage.cost = total_cost_usd
+
+        mock_span.span_data = ResponseSpanData(response=mock_response)
+
+        processor.on_span_start(mock_span)
+        processor.on_span_end(mock_span)
+
+        call_kwargs = mock_client.capture.call_args[1]
+        assert call_kwargs["properties"]["$ai_total_cost_usd"] == total_cost_usd
 
     def test_speech_span_with_pass_through_properties(
         self, processor, mock_client, mock_span