Merge pull request #2310 from shellus/fix/claude-openai-system-top-level

fix: preserve OpenAI system messages as Claude top-level system

internal/translator/claude/openai/chat-completions/claude_openai_request.go

@@ -165,29 +165,22 @@ func ConvertOpenAIRequestToClaude(modelName string, inputRawJSON []byte, stream
 	// Process messages and transform them to Claude Code format
 	if messages := root.Get("messages"); messages.Exists() && messages.IsArray() {
 		messageIndex := 0
-		systemMessageIndex := -1
 		messages.ForEach(func(_, message gjson.Result) bool {
 			role := message.Get("role").String()
 			contentResult := message.Get("content")
 
 			switch role {
 			case "system":
-				if systemMessageIndex == -1 {
-					systemMsg := []byte(`{"role":"user","content":[]}`)
-					out, _ = sjson.SetRawBytes(out, "messages.-1", systemMsg)
-					systemMessageIndex = messageIndex
-					messageIndex++
-				}
 				if contentResult.Exists() && contentResult.Type == gjson.String && contentResult.String() != "" {
 					textPart := []byte(`{"type":"text","text":""}`)
 					textPart, _ = sjson.SetBytes(textPart, "text", contentResult.String())
-					out, _ = sjson.SetRawBytes(out, fmt.Sprintf("messages.%d.content.-1", systemMessageIndex), textPart)
+					out, _ = sjson.SetRawBytes(out, "system.-1", textPart)
 				} else if contentResult.Exists() && contentResult.IsArray() {
 					contentResult.ForEach(func(_, part gjson.Result) bool {
 						if part.Get("type").String() == "text" {
 							textPart := []byte(`{"type":"text","text":""}`)
 							textPart, _ = sjson.SetBytes(textPart, "text", part.Get("text").String())
-							out, _ = sjson.SetRawBytes(out, fmt.Sprintf("messages.%d.content.-1", systemMessageIndex), textPart)
+							out, _ = sjson.SetRawBytes(out, "system.-1", textPart)
 						}
 						return true
 					})
@@ -269,6 +262,16 @@ func ConvertOpenAIRequestToClaude(modelName string, inputRawJSON []byte, stream
 			}
 			return true
 		})
+
+		// Preserve a minimal conversational turn for system-only inputs.
+		// Claude payloads with top-level system instructions but no messages are risky for downstream validation.
+		if messageIndex == 0 {
+			system := gjson.GetBytes(out, "system")
+			if system.Exists() && system.IsArray() && len(system.Array()) > 0 {
+				fallbackMsg := []byte(`{"role":"user","content":[{"type":"text","text":""}]}`)
+				out, _ = sjson.SetRawBytes(out, "messages.-1", fallbackMsg)
+			}
+		}
 	}
 
 	// Tools mapping: OpenAI tools -> Claude Code tools

internal/translator/claude/openai/chat-completions/claude_openai_request_test.go

@@ -135,3 +135,111 @@ func TestConvertOpenAIRequestToClaude_ToolResultURLImageOnly(t *testing.T) {
 		t.Fatalf("Unexpected image URL: %q", got)
 	}
 }
+
+func TestConvertOpenAIRequestToClaude_SystemRoleBecomesTopLevelSystem(t *testing.T) {
+	inputJSON := `{
+		"model": "gpt-4.1",
+		"messages": [
+			{"role": "system", "content": "You are a helpful assistant."},
+			{"role": "user", "content": "Hello"}
+		]
+	}`
+
+	result := ConvertOpenAIRequestToClaude("claude-sonnet-4-5", []byte(inputJSON), false)
+	resultJSON := gjson.ParseBytes(result)
+
+	system := resultJSON.Get("system")
+	if !system.IsArray() {
+		t.Fatalf("Expected top-level system array, got %s", system.Raw)
+	}
+	if len(system.Array()) != 1 {
+		t.Fatalf("Expected 1 system block, got %d. System: %s", len(system.Array()), system.Raw)
+	}
+	if got := system.Get("0.type").String(); got != "text" {
+		t.Fatalf("Expected system block type %q, got %q", "text", got)
+	}
+	if got := system.Get("0.text").String(); got != "You are a helpful assistant." {
+		t.Fatalf("Expected system text %q, got %q", "You are a helpful assistant.", got)
+	}
+
+	messages := resultJSON.Get("messages").Array()
+	if len(messages) != 1 {
+		t.Fatalf("Expected 1 non-system message, got %d. Messages: %s", len(messages), resultJSON.Get("messages").Raw)
+	}
+	if got := messages[0].Get("role").String(); got != "user" {
+		t.Fatalf("Expected remaining message role %q, got %q", "user", got)
+	}
+	if got := messages[0].Get("content.0.text").String(); got != "Hello" {
+		t.Fatalf("Expected user text %q, got %q", "Hello", got)
+	}
+}
+
+func TestConvertOpenAIRequestToClaude_MultipleSystemMessagesMergedIntoTopLevelSystem(t *testing.T) {
+	inputJSON := `{
+		"model": "gpt-4.1",
+		"messages": [
+			{"role": "system", "content": "Rule 1"},
+			{"role": "system", "content": [{"type": "text", "text": "Rule 2"}]},
+			{"role": "user", "content": "Hello"}
+		]
+	}`
+
+	result := ConvertOpenAIRequestToClaude("claude-sonnet-4-5", []byte(inputJSON), false)
+	resultJSON := gjson.ParseBytes(result)
+
+	system := resultJSON.Get("system").Array()
+	if len(system) != 2 {
+		t.Fatalf("Expected 2 system blocks, got %d. System: %s", len(system), resultJSON.Get("system").Raw)
+	}
+	if got := system[0].Get("text").String(); got != "Rule 1" {
+		t.Fatalf("Expected first system text %q, got %q", "Rule 1", got)
+	}
+	if got := system[1].Get("text").String(); got != "Rule 2" {
+		t.Fatalf("Expected second system text %q, got %q", "Rule 2", got)
+	}
+
+	messages := resultJSON.Get("messages").Array()
+	if len(messages) != 1 {
+		t.Fatalf("Expected 1 non-system message, got %d. Messages: %s", len(messages), resultJSON.Get("messages").Raw)
+	}
+	if got := messages[0].Get("role").String(); got != "user" {
+		t.Fatalf("Expected remaining message role %q, got %q", "user", got)
+	}
+	if got := messages[0].Get("content.0.text").String(); got != "Hello" {
+		t.Fatalf("Expected user text %q, got %q", "Hello", got)
+	}
+}
+
+func TestConvertOpenAIRequestToClaude_SystemOnlyInputKeepsFallbackUserMessage(t *testing.T) {
+	inputJSON := `{
+		"model": "gpt-4.1",
+		"messages": [
+			{"role": "system", "content": "You are a helpful assistant."}
+		]
+	}`
+
+	result := ConvertOpenAIRequestToClaude("claude-sonnet-4-5", []byte(inputJSON), false)
+	resultJSON := gjson.ParseBytes(result)
+
+	system := resultJSON.Get("system").Array()
+	if len(system) != 1 {
+		t.Fatalf("Expected 1 system block, got %d. System: %s", len(system), resultJSON.Get("system").Raw)
+	}
+	if got := system[0].Get("text").String(); got != "You are a helpful assistant." {
+		t.Fatalf("Expected system text %q, got %q", "You are a helpful assistant.", got)
+	}
+
+	messages := resultJSON.Get("messages").Array()
+	if len(messages) != 1 {
+		t.Fatalf("Expected 1 fallback message, got %d. Messages: %s", len(messages), resultJSON.Get("messages").Raw)
+	}
+	if got := messages[0].Get("role").String(); got != "user" {
+		t.Fatalf("Expected fallback message role %q, got %q", "user", got)
+	}
+	if got := messages[0].Get("content.0.type").String(); got != "text" {
+		t.Fatalf("Expected fallback content type %q, got %q", "text", got)
+	}
+	if got := messages[0].Get("content.0.text").String(); got != "" {
+		t.Fatalf("Expected fallback text %q, got %q", "", got)
+	}
+}