diff --git a/agentscope-core/src/main/java/io/agentscope/core/tool/ToolValidator.java b/agentscope-core/src/main/java/io/agentscope/core/tool/ToolValidator.java
index 8f0d0fe81..c997becaa 100644
--- a/agentscope-core/src/main/java/io/agentscope/core/tool/ToolValidator.java
+++ b/agentscope-core/src/main/java/io/agentscope/core/tool/ToolValidator.java
@@ -79,6 +79,13 @@ public static String validateInput(String input, Map<String, Object> schema) {
             return null; // No schema, validation passes
         }
 
+        // Defensive: treat null/blank input as empty JSON object so that
+        // networknt-schema does not throw NPE ("argument 'content' is null")
+        // when a ToolUseBlock carries parsed arguments but no raw content string.
+        if (input == null || input.isBlank()) {
+            input = "{}";
+        }
+
         try {
             // Convert schema to JSON string
             String schemaJson = JsonUtils.getJsonCodec().toJson(schema);