diff --git a/internal/translator/codex/claude/codex_claude_request.go b/internal/translator/codex/claude/codex_claude_request.go
index 6373e693..4bc116b9 100644
--- a/internal/translator/codex/claude/codex_claude_request.go
+++ b/internal/translator/codex/claude/codex_claude_request.go
@@ -43,23 +43,32 @@ func ConvertClaudeRequestToCodex(modelName string, inputRawJSON []byte, _ bool)
 
 	// Process system messages and convert them to input content format.
 	systemsResult := rootResult.Get("system")
-	if systemsResult.IsArray() {
-		systemResults := systemsResult.Array()
+	if systemsResult.Exists() {
 		message := `{"type":"message","role":"developer","content":[]}`
 		contentIndex := 0
-		for i := 0; i < len(systemResults); i++ {
-			systemResult := systemResults[i]
-			systemTypeResult := systemResult.Get("type")
-			if systemTypeResult.String() == "text" {
-				text := systemResult.Get("text").String()
-				if strings.HasPrefix(text, "x-anthropic-billing-header: ") {
-					continue
+
+		appendSystemText := func(text string) {
+			if text == "" || strings.HasPrefix(text, "x-anthropic-billing-header: ") {
+				return
+			}
+
+			message, _ = sjson.Set(message, fmt.Sprintf("content.%d.type", contentIndex), "input_text")
+			message, _ = sjson.Set(message, fmt.Sprintf("content.%d.text", contentIndex), text)
+			contentIndex++
+		}
+
+		if systemsResult.Type == gjson.String {
+			appendSystemText(systemsResult.String())
+		} else if systemsResult.IsArray() {
+			systemResults := systemsResult.Array()
+			for i := 0; i < len(systemResults); i++ {
+				systemResult := systemResults[i]
+				if systemResult.Get("type").String() == "text" {
+					appendSystemText(systemResult.Get("text").String())
 				}
-				message, _ = sjson.Set(message, fmt.Sprintf("content.%d.type", contentIndex), "input_text")
-				message, _ = sjson.Set(message, fmt.Sprintf("content.%d.text", contentIndex), text)
-				contentIndex++
 			}
 		}
+
 		if contentIndex > 0 {
 			template, _ = sjson.SetRaw(template, "input.-1", message)
 		}
diff --git a/internal/translator/codex/claude/codex_claude_request_test.go b/internal/translator/codex/claude/codex_claude_request_test.go
new file mode 100644
index 00000000..bdd41639
--- /dev/null
+++ b/internal/translator/codex/claude/codex_claude_request_test.go
@@ -0,0 +1,89 @@
+package claude
+
+import (
+	"testing"
+
+	"github.com/tidwall/gjson"
+)
+
+func TestConvertClaudeRequestToCodex_SystemMessageScenarios(t *testing.T) {
+	tests := []struct {
+		name             string
+		inputJSON        string
+		wantHasDeveloper bool
+		wantTexts        []string
+	}{
+		{
+			name: "No system field",
+			inputJSON: `{
+				"model": "claude-3-opus",
+				"messages": [{"role": "user", "content": "hello"}]
+			}`,
+			wantHasDeveloper: false,
+		},
+		{
+			name: "Empty string system field",
+			inputJSON: `{
+				"model": "claude-3-opus",
+				"system": "",
+				"messages": [{"role": "user", "content": "hello"}]
+			}`,
+			wantHasDeveloper: false,
+		},
+		{
+			name: "String system field",
+			inputJSON: `{
+				"model": "claude-3-opus",
+				"system": "Be helpful",
+				"messages": [{"role": "user", "content": "hello"}]
+			}`,
+			wantHasDeveloper: true,
+			wantTexts:        []string{"Be helpful"},
+		},
+		{
+			name: "Array system field with filtered billing header",
+			inputJSON: `{
+				"model": "claude-3-opus",
+				"system": [
+					{"type": "text", "text": "x-anthropic-billing-header: tenant-123"},
+					{"type": "text", "text": "Block 1"},
+					{"type": "text", "text": "Block 2"}
+				],
+				"messages": [{"role": "user", "content": "hello"}]
+			}`,
+			wantHasDeveloper: true,
+			wantTexts:        []string{"Block 1", "Block 2"},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := ConvertClaudeRequestToCodex("test-model", []byte(tt.inputJSON), false)
+			resultJSON := gjson.ParseBytes(result)
+			inputs := resultJSON.Get("input").Array()
+
+			hasDeveloper := len(inputs) > 0 && inputs[0].Get("role").String() == "developer"
+			if hasDeveloper != tt.wantHasDeveloper {
+				t.Fatalf("got hasDeveloper = %v, want %v. Output: %s", hasDeveloper, tt.wantHasDeveloper, resultJSON.Get("input").Raw)
+			}
+
+			if !tt.wantHasDeveloper {
+				return
+			}
+
+			content := inputs[0].Get("content").Array()
+			if len(content) != len(tt.wantTexts) {
+				t.Fatalf("got %d system content items, want %d. Content: %s", len(content), len(tt.wantTexts), inputs[0].Get("content").Raw)
+			}
+
+			for i, wantText := range tt.wantTexts {
+				if gotType := content[i].Get("type").String(); gotType != "input_text" {
+					t.Fatalf("content[%d] type = %q, want %q", i, gotType, "input_text")
+				}
+				if gotText := content[i].Get("text").String(); gotText != wantText {
+					t.Fatalf("content[%d] text = %q, want %q", i, gotText, wantText)
+				}
+			}
+		})
+	}
+}