Surface the Copilot usage model in response metadata

provider/copilotprovider/copilot.go

@@ -847,6 +847,15 @@ func (p *provider) assistantUsageUpdate(event copilot.SessionEvent, data *copilo
 	case data.FinishReason != nil:
 		update.FinishReason = *data.FinishReason
 	}
+	// Surface which model actually served the request, matching the Python
+	// client which carries it as additional_properties["model"]. Initialize the
+	// map lazily so existing/future properties are not overwritten.
+	if data.Model != "" {
+		if update.AdditionalProperties == nil {
+			update.AdditionalProperties = make(map[string]any)
+		}
+		update.AdditionalProperties["model"] = data.Model
+	}
 	return update
 }
 

provider/copilotprovider/copilot_test.go

@@ -488,6 +488,28 @@ func TestConvertToAgentResponseUpdate_UsageEvent_SurfacesReasoningTokens(t *test
 	}
 }
 
+// The usage event reports which model actually served the request; it must be
+// surfaced on AdditionalProperties, matching the Python client.
+func TestConvertToAgentResponseUpdate_UsageEvent_SurfacesModel(t *testing.T) {
+	runtime := newFakeRuntime(t,
+		sessionEvent("assistant.usage", map[string]any{
+			"model":        "gpt-5",
+			"inputTokens":  10,
+			"outputTokens": 20,
+		}),
+		idleEvent(),
+	)
+	agent := copilotprovider.NewAgent(runtime.client(), copilotprovider.AgentConfig{})
+
+	response, err := runText(t, agent, "hello")
+	if err != nil {
+		t.Fatalf("RunText: %v", err)
+	}
+	if got, _ := response.AdditionalProperties["model"].(string); got != "gpt-5" {
+		t.Fatalf("AdditionalProperties[model] = %q, want gpt-5", got)
+	}
+}
+
 // A content-filter-triggered usage event must report the canonical
 // "content_filter" finish reason, taking precedence over the raw finishReason,
 // matching the Python client.