diff --git a/pkg/providers/http_provider.go b/pkg/providers/http_provider.go
index 5028fa1..72d5fce 100644
--- a/pkg/providers/http_provider.go
+++ b/pkg/providers/http_provider.go
@@ -141,11 +141,13 @@ func (p *HTTPProvider) callResponses(ctx context.Context, messages []Message, to
 func toResponsesInputItems(msg Message) []map[string]interface{} {
 	role := strings.ToLower(strings.TrimSpace(msg.Role))
 	switch role {
-	case "system", "developer", "assistant", "user":
-		return []map[string]interface{}{responsesMessageItem(role, msg.Content)}
+	case "system", "developer", "user":
+		return []map[string]interface{}{responsesMessageItem(role, msg.Content, "input_text")}
+	case "assistant":
+		return []map[string]interface{}{responsesMessageItem(role, msg.Content, "output_text")}
 	case "tool":
 		if strings.TrimSpace(msg.ToolCallID) == "" {
-			return []map[string]interface{}{responsesMessageItem("user", msg.Content)}
+			return []map[string]interface{}{responsesMessageItem("user", msg.Content, "input_text")}
 		}
 		return []map[string]interface{}{map[string]interface{}{
 			"type":    "function_call_output",
@@ -153,17 +155,21 @@ func toResponsesInputItems(msg Message) []map[string]interface{} {
 			"output":  msg.Content,
 		}}
 	default:
-		return []map[string]interface{}{responsesMessageItem("user", msg.Content)}
+		return []map[string]interface{}{responsesMessageItem("user", msg.Content, "input_text")}
 	}
 }
 
-func responsesMessageItem(role, text string) map[string]interface{} {
+func responsesMessageItem(role, text, contentType string) map[string]interface{} {
+	ct := strings.TrimSpace(contentType)
+	if ct == "" {
+		ct = "input_text"
+	}
 	return map[string]interface{}{
 		"type": "message",
 		"role": role,
 		"content": []map[string]interface{}{
 			{
-				"type": "input_text",
+				"type": ct,
 				"text": text,
 			},
 		},
@@ -523,7 +529,7 @@ func (p *HTTPProvider) BuildSummaryViaResponsesCompact(ctx context.Context, mode
 	}
 	input := make([]map[string]interface{}, 0, len(messages)+1)
 	if strings.TrimSpace(existingSummary) != "" {
-		input = append(input, responsesMessageItem("system", "Existing summary:\n"+strings.TrimSpace(existingSummary)))
+		input = append(input, responsesMessageItem("system", "Existing summary:\n"+strings.TrimSpace(existingSummary), "input_text"))
 	}
 	for _, msg := range messages {
 		input = append(input, toResponsesInputItems(msg)...)
diff --git a/pkg/providers/provider_test.go b/pkg/providers/provider_test.go
index bc9a678..70bc61b 100644
--- a/pkg/providers/provider_test.go
+++ b/pkg/providers/provider_test.go
@@ -117,6 +117,24 @@ func TestEndpointForResponsesCompact(t *testing.T) {
 	}
 }
 
+func TestToResponsesInputItems_AssistantUsesOutputText(t *testing.T) {
+	items := toResponsesInputItems(Message{
+		Role:    "assistant",
+		Content: "hello",
+	})
+	if len(items) != 1 {
+		t.Fatalf("expected 1 item, got %d", len(items))
+	}
+	content, ok := items[0]["content"].([]map[string]interface{})
+	if !ok || len(content) == 0 {
+		t.Fatalf("unexpected content shape: %#v", items[0]["content"])
+	}
+	gotType, _ := content[0]["type"].(string)
+	if gotType != "output_text" {
+		t.Fatalf("assistant content type = %q, want output_text", gotType)
+	}
+}
+
 func containsFunctionCallMarkup(s string) bool {
 	return len(s) > 0 && (strings.Contains(s, "<function_call>") || strings.Contains(s, "</function_call>"))
 }