diff --git a/demo/App.vue b/demo/App.vue
index 3c0a2d1..e05893e 100644
--- a/demo/App.vue
+++ b/demo/App.vue
@@ -67,7 +67,8 @@
 
 <script setup lang="ts">
 import { ref, computed, onMounted } from "vue";
-import { plugin } from "../src/vue";
+import { plugin, executeMarkdown } from "../src/vue";
+import { isMarkdownArgs } from "./markdownArgs";
 import type { ToolResult, ToolSample, ToolPlugin } from "gui-chat-protocol/vue";
 
 // Plugin configuration - cast to base ToolPlugin for generic demo usage
@@ -96,18 +97,18 @@ const result = ref<ToolResult>({
 const loadSample = (sample: ToolSample) => {
   jsonInput.value = JSON.stringify(sample.args, null, 2);
   jsonError.value = "";
-  applyJson();
+  void applyJson();
 };
 
-const applyJson = () => {
+// Runs the plugin's own execute, as a host does, so View gets the result shape it reads (data.markdown).
+const applyJson = async () => {
   try {
-    const data = JSON.parse(jsonInput.value);
-    result.value = {
-      toolName: toolName.value,
-      message: "Data applied",
-      title: data.title || "",
-      jsonData: data,
-    };
+    const args: unknown = JSON.parse(jsonInput.value);
+    if (!isMarkdownArgs(args)) {
+      jsonError.value = "JSON must have string fields: title, markdown";
+      return;
+    }
+    result.value = { ...(await executeMarkdown(null, args)), toolName: toolName.value };
     jsonError.value = "";
     lastSentMessage.value = "";
   } catch (e) {
diff --git a/demo/markdownArgs.ts b/demo/markdownArgs.ts
new file mode 100644
index 0000000..06a89fa
--- /dev/null
+++ b/demo/markdownArgs.ts
@@ -0,0 +1,10 @@
+import type { MarkdownArgs } from "../src/core/types";
+
+const isRecord = (value: unknown): value is Record<string, unknown> =>
+  typeof value === "object" && value !== null && !Array.isArray(value);
+
+export const isMarkdownArgs = (value: unknown): value is MarkdownArgs =>
+  isRecord(value) &&
+  typeof value.title === "string" &&
+  typeof value.markdown === "string" &&
+  (value.filenameHint === undefined || typeof value.filenameHint === "string");
diff --git a/plans/fix-demo-view-markdown.md b/plans/fix-demo-view-markdown.md
new file mode 100644
index 0000000..f5aee7e
--- /dev/null
+++ b/plans/fix-demo-view-markdown.md
@@ -0,0 +1,10 @@
+# demo の View に markdown が出ない問題を直す
+
+## 原因
+- gui-chat-protocol では `data` が View に渡る描画用のデータ、`jsonData` は LLM に返す JSON
+- プラグインの `execute`（`presentDocument`）は `data: { markdown, filenameHint }` を返し、View は `data.markdown` を読む。ホストは `execute` の結果をそのまま View に渡す
+- demo だけが `execute` を通さずに結果を手で組み立て、入力した引数を `jsonData` に入れていた。`data` が無いので View は常に空
+
+## 修正
+- 直すのは demo 側（src は protocol どおり）
+- demo は入力 JSON を `isMarkdownArgs`（`demo/markdownArgs.ts`）で確かめてから `executeMarkdown(null, args)` を呼び、その結果を View / Preview に渡す