diff --git a/apps/ai-game-creator-shell/src-tauri/src/agent/direct_tool_bridge.rs b/apps/ai-game-creator-shell/src-tauri/src/agent/direct_tool_bridge.rs
index 4c791e640..00940673a 100644
--- a/apps/ai-game-creator-shell/src-tauri/src/agent/direct_tool_bridge.rs
+++ b/apps/ai-game-creator-shell/src-tauri/src/agent/direct_tool_bridge.rs
@@ -2900,12 +2900,16 @@ fn generate_image_grid_axis(
 fn bridge_image_generation_kind(
     arguments: &Value,
 ) -> Result<GameCreationAppAssetKind, GenerateImageError> {
-    let kind = arguments
-        .get("kind")
-        .and_then(Value::as_str)
-        .map(str::trim)
-        .filter(|value| !value.is_empty())
-        .unwrap_or("image");
+    // 缺省与显式 null 用 schema 声明的默认值；给了别的类型不许静默当 image。
+    let kind = match arguments.get("kind") {
+        None | Some(Value::Null) => "image",
+        Some(Value::String(value)) => value.trim(),
+        Some(value) => {
+            return Err(GenerateImageError::KindNotText {
+                got: value.to_string(),
+            })
+        }
+    };
     normalize_platform_art_asset_generation_kind(kind).ok_or_else(|| {
         GenerateImageError::KindUnsupported {
             got: kind.to_string(),
@@ -4860,6 +4864,40 @@ mod tests {
 
     #[test]
     fn non_text_field_values_report_a_type_error_instead_of_an_empty_value() {
+        let kind_not_text = match generate_image_input(&json!({
+            "prompt": "像素风主角",
+            "kind": true
+        })) {
+            Ok(_) => panic!("kind 不是字符串时必须被拒绝"),
+            Err(error) => error.to_user_msg(),
+        };
+        assert!(
+            kind_not_text.contains("kind 必须是字符串"),
+            "{kind_not_text}"
+        );
+
+        let kind_empty = match generate_image_input(&json!({
+            "prompt": "像素风主角",
+            "kind": ""
+        })) {
+            Ok(_) => panic!("空 kind 必须被拒绝，不能再静默当 image"),
+            Err(error) => error.to_user_msg(),
+        };
+        assert!(kind_empty.contains("不是受支持的素材类型"), "{kind_empty}");
+
+        assert_eq!(
+            generate_image_input(&json!({ "prompt": "像素风主角" }))
+                .map(|input| input.kind)
+                .expect("缺省 kind 仍默认 image"),
+            GameCreationAppAssetKind::Image
+        );
+        assert_eq!(
+            generate_image_input(&json!({ "prompt": "像素风主角", "kind": null }))
+                .map(|input| input.kind)
+                .expect("显式 null 仍按默认 image"),
+            GameCreationAppAssetKind::Image
+        );
+
         let trailing_newline = match resource_generation_input(&json!({
             "kind": "video",
             "mode": "create",
diff --git a/apps/ai-game-creator-shell/src-tauri/src/agent/tool/generate_image/error.rs b/apps/ai-game-creator-shell/src-tauri/src/agent/tool/generate_image/error.rs
index 847c35299..2bf337653 100644
--- a/apps/ai-game-creator-shell/src-tauri/src/agent/tool/generate_image/error.rs
+++ b/apps/ai-game-creator-shell/src-tauri/src/agent/tool/generate_image/error.rs
@@ -20,6 +20,7 @@ pub(crate) enum GenerateImageError {
     PromptTooLong { got_chars: usize, max_chars: usize },
     PromptHasControlCharacters,
     PromptNotText { got: String },
+    KindNotText { got: String },
     KindUnsupported { got: String },
     AspectRatioEmpty,
     AspectRatioTooLong { got_chars: usize, max_chars: usize },
@@ -83,6 +84,9 @@ impl ToolFailure for GenerateImageError {
             Self::PromptNotText { got } => format!(
                 "生成图片失败：prompt 必须是字符串，当前是「{got}」。"
             ),
+            Self::KindNotText { got } => {
+                format!("生成图片失败：kind 必须是字符串，当前是「{got}」。")
+            }
             Self::KindUnsupported { got } => format!(
                 "生成图片失败：kind「{got}」不是受支持的素材类型，只允许 {}。",
                 platform_art_asset_generation_kind_list()