diff --git a/backend/internal/pkg/apicompat/chatcompletions_responses_test.go b/backend/internal/pkg/apicompat/chatcompletions_responses_test.go
index 1731892735..1a27ef2374 100644
--- a/backend/internal/pkg/apicompat/chatcompletions_responses_test.go
+++ b/backend/internal/pkg/apicompat/chatcompletions_responses_test.go
@@ -92,6 +92,56 @@ func TestChatCompletionsToResponses_SystemMessage(t *testing.T) {
 	assert.Equal(t, "user", items[1].Role)
 }
 
+func TestChatCompletionsToResponses_MessageTypesWithReasoningAndTools(t *testing.T) {
+	req := &ChatCompletionsRequest{
+		Model: "step-5-preview",
+		Messages: []ChatMessage{
+			{Role: "system", Content: json.RawMessage(`"You are helpful."`)},
+			{Role: "user", Content: json.RawMessage(`"Check the directory"`)},
+			{
+				Role:             "assistant",
+				Content:          json.RawMessage(`"I will check."`),
+				ReasoningContent: "Need to inspect the directory.",
+				ToolCalls: []ChatToolCall{{
+					ID: "call_1", Type: "function",
+					Function: ChatFunctionCall{Name: "bash", Arguments: `{"cmd":"pwd"}`},
+				}},
+			},
+			{Role: "tool", ToolCallID: "call_1", Content: json.RawMessage(`"/tmp"`)},
+			{Role: "assistant", ReasoningContent: "The tool returned /tmp."},
+			{Role: "user", Content: json.RawMessage(`"Continue"`)},
+		},
+	}
+
+	resp, err := ChatCompletionsToResponses(req)
+	require.NoError(t, err)
+
+	var items []ResponsesInputItem
+	require.NoError(t, json.Unmarshal(resp.Input, &items))
+	require.Len(t, items, 7)
+	for i, want := range []struct{ typ, role string }{
+		{"message", "system"},
+		{"message", "user"},
+		{"message", "assistant"},
+		{"function_call", ""},
+		{"function_call_output", ""},
+		{"message", "assistant"},
+		{"message", "user"},
+	} {
+		assert.Equal(t, want.typ, items[i].Type, "input item %d type", i)
+		assert.Equal(t, want.role, items[i].Role, "input item %d role", i)
+	}
+	var assistantContent, reasoningOnlyContent []ResponsesContentPart
+	require.NoError(t, json.Unmarshal(items[2].Content, &assistantContent))
+	require.NoError(t, json.Unmarshal(items[5].Content, &reasoningOnlyContent))
+	require.Len(t, assistantContent, 1)
+	require.Len(t, reasoningOnlyContent, 1)
+	assert.Equal(t, "<thinking>Need to inspect the directory.</thinking>\nI will check.", assistantContent[0].Text)
+	assert.Equal(t, "<thinking>The tool returned /tmp.</thinking>", reasoningOnlyContent[0].Text)
+	assert.Equal(t, "call_1", items[3].CallID)
+	assert.Equal(t, "call_1", items[4].CallID)
+}
+
 func TestChatCompletionsToResponses_ToolCalls(t *testing.T) {
 	req := &ChatCompletionsRequest{
 		Model: "gpt-4o",
diff --git a/backend/internal/pkg/apicompat/chatcompletions_to_responses.go b/backend/internal/pkg/apicompat/chatcompletions_to_responses.go
index 773b12b69a..c6fdcb7352 100644
--- a/backend/internal/pkg/apicompat/chatcompletions_to_responses.go
+++ b/backend/internal/pkg/apicompat/chatcompletions_to_responses.go
@@ -143,7 +143,7 @@ func chatSystemToResponses(m ChatMessage) ([]ResponsesInputItem, error) {
 	if err != nil {
 		return nil, err
 	}
-	return []ResponsesInputItem{{Role: "system", Content: content}}, nil
+	return []ResponsesInputItem{{Type: "message", Role: "system", Content: content}}, nil
 }
 
 // chatUserToResponses converts a user message, handling both plain strings and
@@ -157,7 +157,7 @@ func chatUserToResponses(m ChatMessage) ([]ResponsesInputItem, error) {
 	if err != nil {
 		return nil, err
 	}
-	return []ResponsesInputItem{{Role: "user", Content: content}}, nil
+	return []ResponsesInputItem{{Type: "message", Role: "user", Content: content}}, nil
 }
 
 // chatAssistantToResponses converts an assistant message. If there is both
@@ -192,7 +192,7 @@ func chatAssistantToResponses(m ChatMessage) ([]ResponsesInputItem, error) {
 		if err != nil {
 			return nil, err
 		}
-		items = append(items, ResponsesInputItem{Role: "assistant", Content: partsJSON})
+		items = append(items, ResponsesInputItem{Type: "message", Role: "assistant", Content: partsJSON})
 	}
 
 	// Emit one function_call item per tool_call.
diff --git a/backend/internal/pkg/apicompat/types.go b/backend/internal/pkg/apicompat/types.go
index 44a5b4c79c..00b6e85c4d 100644
--- a/backend/internal/pkg/apicompat/types.go
+++ b/backend/internal/pkg/apicompat/types.go
@@ -263,7 +263,7 @@ type ResponsesText struct {
 // The Type field determines which other fields are populated.
 type ResponsesInputItem struct {
 	// Common
-	Type string `json:"type,omitempty"` // "" for role-based messages
+	Type string `json:"type,omitempty"` // "message" for role-based messages
 
 	// Role-based messages (developer/system/user/assistant)
 	Role    string          `json:"role,omitempty"`