diff --git a/internal/translator/claude/openai/chat-completions/claude_openai_request.go b/internal/translator/claude/openai/chat-completions/claude_openai_request.go
index f6489894..e9d8d35b 100644
--- a/internal/translator/claude/openai/chat-completions/claude_openai_request.go
+++ b/internal/translator/claude/openai/chat-completions/claude_openai_request.go
@@ -262,6 +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
diff --git a/internal/translator/claude/openai/chat-completions/claude_openai_request_test.go b/internal/translator/claude/openai/chat-completions/claude_openai_request_test.go
index ef8f0036..ead08d72 100644
--- a/internal/translator/claude/openai/chat-completions/claude_openai_request_test.go
+++ b/internal/translator/claude/openai/chat-completions/claude_openai_request_test.go
@@ -209,3 +209,37 @@ func TestConvertOpenAIRequestToClaude_MultipleSystemMessagesMergedIntoTopLevelSy
 		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)
+	}
+}