Fix mimeType, CSP settings and QR code example (#111)

- Fix RESOURCE_MIME_TYPE to match spec: "text/html;profile=mcp-app"
- Add CSP field to McpUiSandboxResourceReadyNotification type and schema
- Update basic-host to extract and pass CSP metadata from resources
- Update sandbox.ts to dynamically inject CSP meta tag into inner iframe
- Remove static CSP from sandbox.html (now dynamic based on resource metadata)
- Update QR server to use custom read_resource handler for _meta injection
- Update requirements.txt to require mcp[cli]>=1.23.3

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

examples/basic-host/sandbox.html

@@ -2,22 +2,8 @@
 <html>
   <head>
     <meta charset="utf-8" />
-    <!-- Permissive CSP so nested content is not constrained by host CSP -->
+    <!-- CSP is set by serve.ts HTTP header - no meta tag needed here
-    <meta http-equiv="Content-Security-Policy" content="
+         The inner iframe's CSP is dynamically injected based on resource metadata -->
-      default-src 'self';
-      img-src * data: blob: 'unsafe-inline';
-      media-src * blob: data:;
-      font-src * blob: data:;
-      script-src 'self'
-                 'wasm-unsafe-eval'
-                 'unsafe-inline'
-                 'unsafe-eval'
-                 blob: data: http://localhost:* https://localhost:*;
-      style-src * blob: data: 'unsafe-inline';
-      connect-src *;
-      frame-src * blob: data: http://localhost:* https://localhost:*;
-      base-uri 'self';
-    " />
     <title>MCP-UI Proxy</title>
     <style>
       html,

examples/basic-host/src/implementation.ts

@@ -40,17 +40,25 @@ export async function connectToServer(serverUrl: URL): Promise<ServerInfo> {
 }
 
 
+interface UiResourceData {
+  html: string;
+  csp?: {
+    connectDomains?: string[];
+    resourceDomains?: string[];
+  };
+}
+
 export interface ToolCallInfo {
   serverInfo: ServerInfo;
   tool: Tool;
   input: Record<string, unknown>;
   resultPromise: Promise<CallToolResult>;
-  appHtmlPromise?: Promise<string>;
+  appResourcePromise?: Promise<UiResourceData>;
 }
 
 
 export function hasAppHtml(toolCallInfo: ToolCallInfo): toolCallInfo is Required<ToolCallInfo> {
-  return !!toolCallInfo.appHtmlPromise;
+  return !!toolCallInfo.appResourcePromise;
 }
 
 
@@ -71,7 +79,7 @@ export function callTool(
 
   const uiResourceUri = getUiResourceUri(tool);
   if (uiResourceUri) {
-    toolCallInfo.appHtmlPromise = getUiResourceHtml(serverInfo, uiResourceUri);
+    toolCallInfo.appResourcePromise = getUiResource(serverInfo, uiResourceUri);
   }
 
   return toolCallInfo;
@@ -88,13 +96,7 @@ function getUiResourceUri(tool: Tool): string | undefined {
 }
 
 
-async function getUiResourceHtml(serverInfo: ServerInfo, uri: string): Promise<string> {
+async function getUiResource(serverInfo: ServerInfo, uri: string): Promise<UiResourceData> {
-  let html = serverInfo.appHtmlCache.get(uri);
-  if (html) {
-    log.info("Read UI resource from cache:", uri);
-    return html;
-  }
-
   log.info("Reading UI resource:", uri);
   const resource = await serverInfo.client.readResource({ uri });
 
@@ -114,9 +116,17 @@ async function getUiResourceHtml(serverInfo: ServerInfo, uri: string): Promise<s
     throw new Error(`Unsupported MIME type: ${content.mimeType}`);
   }
 
-  html = "blob" in content ? atob(content.blob) : content.text;
+  const html = "blob" in content ? atob(content.blob) : content.text;
-  serverInfo.appHtmlCache.set(uri, html);
+
-  return html;
+  // Extract CSP metadata from resource content._meta.ui.csp (or content.meta for Python SDK)
+  log.info("Resource content keys:", Object.keys(content));
+  log.info("Resource content._meta:", (content as any)._meta);
+
+  // Try both _meta (spec) and meta (Python SDK quirk)
+  const contentMeta = (content as any)._meta || (content as any).meta;
+  const csp = contentMeta?.ui?.csp;
+
+  return { html, csp };
 }
 
 
@@ -150,7 +160,7 @@ export function loadSandboxProxy(iframe: HTMLIFrameElement): Promise<boolean> {
 export async function initializeApp(
   iframe: HTMLIFrameElement,
   appBridge: AppBridge,
-  { input, resultPromise, appHtmlPromise }: Required<ToolCallInfo>,
+  { input, resultPromise, appResourcePromise }: Required<ToolCallInfo>,
 ): Promise<void> {
   const appInitializedPromise = hookInitializedCallback(appBridge);
 
@@ -162,9 +172,10 @@ export async function initializeApp(
     new PostMessageTransport(iframe.contentWindow!, iframe.contentWindow!),
   );
 
-  // Load inner iframe HTML
+  // Load inner iframe HTML with CSP metadata
-  log.info("Sending UI resource HTML to MCP App");
+  const { html, csp } = await appResourcePromise;
-  await appBridge.sendSandboxResourceReady({ html: await appHtmlPromise });
+  log.info("Sending UI resource HTML to MCP App", csp ? `(CSP: ${JSON.stringify(csp)})` : "");
+  await appBridge.sendSandboxResourceReady({ html, csp });
 
   // Wait for inner iframe to be ready
   log.info("Waiting for MCP App to initialize...");

examples/basic-host/src/sandbox.ts

@@ -56,17 +56,55 @@ const PROXY_READY_NOTIFICATION: McpUiSandboxProxyReadyNotification["method"] =
 // Special case: The "ui/notifications/sandbox-proxy-ready" message is
 // intercepted here (not relayed) because the Sandbox uses it to configure and
 // load the inner iframe with the Guest UI HTML content.
+// Build CSP meta tag from domains
+function buildCspMetaTag(csp?: { connectDomains?: string[]; resourceDomains?: string[] }): string {
+  const resourceDomains = csp?.resourceDomains?.join(" ") ?? "";
+  const connectDomains = csp?.connectDomains?.join(" ") ?? "";
+
+  // Base CSP directives
+  const directives = [
+    "default-src 'self'",
+    `script-src 'self' 'unsafe-inline' 'unsafe-eval' blob: data: ${resourceDomains}`.trim(),
+    `style-src 'self' 'unsafe-inline' blob: data: ${resourceDomains}`.trim(),
+    `img-src 'self' data: blob: ${resourceDomains}`.trim(),
+    `font-src 'self' data: blob: ${resourceDomains}`.trim(),
+    `connect-src 'self' ${connectDomains}`.trim(),
+    "frame-src 'none'",
+    "object-src 'none'",
+    "base-uri 'self'",
+  ];
+
+  return `<meta http-equiv="Content-Security-Policy" content="${directives.join("; ")}">`;
+}
+
 window.addEventListener("message", async (event) => {
   if (event.source === window.parent) {
     // NOTE: In production you'll also want to validate `event.origin` against
     // your Host domain.
     if (event.data && event.data.method === RESOURCE_READY_NOTIFICATION) {
-      const { html, sandbox } = event.data.params;
+      const { html, sandbox, csp } = event.data.params;
       if (typeof sandbox === "string") {
         inner.setAttribute("sandbox", sandbox);
       }
       if (typeof html === "string") {
-        inner.srcdoc = html;
+        // Inject CSP meta tag at the start of <head> if CSP is provided
+        console.log("[Sandbox] Received CSP:", csp);
+        let modifiedHtml = html;
+        if (csp) {
+          const cspMetaTag = buildCspMetaTag(csp);
+          console.log("[Sandbox] Injecting CSP meta tag:", cspMetaTag);
+          // Insert after <head> tag if present, otherwise prepend
+          if (modifiedHtml.includes("<head>")) {
+            modifiedHtml = modifiedHtml.replace("<head>", `<head>\n${cspMetaTag}`);
+          } else if (modifiedHtml.includes("<head ")) {
+            modifiedHtml = modifiedHtml.replace(/<head[^>]*>/, `$&\n${cspMetaTag}`);
+          } else {
+            modifiedHtml = cspMetaTag + modifiedHtml;
+          }
+        } else {
+          console.log("[Sandbox] No CSP provided, using default");
+        }
+        inner.srcdoc = modifiedHtml;
       }
     } else {
       if (inner && inner.contentWindow) {

examples/qr-server/requirements.txt

@@ -1,2 +1,2 @@
-mcp[cli]>=1.0.0
+mcp[cli]>=1.23.3
 qrcode[pil]>=7.4

examples/qr-server/server.py

@@ -10,7 +10,7 @@ from pathlib import Path
 import qrcode
 import uvicorn
 from mcp.server.fastmcp import FastMCP
-from mcp.types import ImageContent
+from mcp import types
 from starlette.middleware.cors import CORSMiddleware
 
 WIDGET_URI = "ui://qr-server/widget.html"
@@ -28,7 +28,7 @@ def generate_qr(
     error_correction: str = "M",
     fill_color: str = "black",
     back_color: str = "white",
-) -> list[ImageContent]:
+) -> list[types.ImageContent]:
     """Generate a QR code from text.
 
     Args:
@@ -59,31 +59,55 @@ def generate_qr(
     buffer = io.BytesIO()
     img.save(buffer, format="PNG")
     b64 = base64.b64encode(buffer.getvalue()).decode()
-    return [ImageContent(type="image", data=b64, mimeType="image/png")]
+    return [types.ImageContent(type="image", data=b64, mimeType="image/png")]
 
 
-# IMPORTANT: resourceDomains needed for CSP to allow loading SDK from unpkg.com
+# Register widget resource using FastMCP decorator (returns HTML string)
-# Without this, hosts enforcing CSP will block the external script import
+@mcp.resource(WIDGET_URI, mime_type="text/html;profile=mcp-app")
-@mcp.resource(WIDGET_URI, mime_type="text/html")
+def widget() -> str:
-def widget() -> dict:
+    return Path(__file__).parent.joinpath("widget.html").read_text()
+
+
+# Override the read_resource handler to inject _meta into the response
+# This is needed because FastMCP doesn't support custom _meta on resources
+_low_level_server = mcp._mcp_server
+
+
+async def _read_resource_with_meta(req: types.ReadResourceRequest):
+    """Custom handler that injects CSP metadata for the widget resource."""
+    uri = str(req.params.uri)
     html = Path(__file__).parent.joinpath("widget.html").read_text()
-    return {
-        "text": html,
-        "_meta": {
-            "ui": {
-                "csp": {
-                    "resourceDomains": ["https://unpkg.com"]
-                }
-            }
-        }
-    }
 
-# HACK: Bypass SDK's restrictive mime_type validation
+    if uri == WIDGET_URI:
-# The SDK pattern doesn't allow ";profile=mcp-app" but MCP spec requires it for widgets
+        # NOTE: Must use model_validate with '_meta' key (not 'meta') due to Pydantic alias behavior
-# https://github.com/modelcontextprotocol/python-sdk/pull/1755
+        content = types.TextResourceContents.model_validate({
-for resource in mcp._resource_manager._resources.values():
+            "uri": WIDGET_URI,
-    if str(resource.uri) == WIDGET_URI:
+            "mimeType": "text/html;profile=mcp-app",
-        object.__setattr__(resource, 'mime_type', 'text/html;profile=mcp-app')
+            "text": html,
+            # IMPORTANT: all the external domains used by app must be listed
+            # in the _meta.ui.csp.resourceDomains - otherwise they will be blocked by CSP policy
+            "_meta": {"ui": {"csp": {"resourceDomains": ["https://unpkg.com"]}}}
+        })
+        return types.ServerResult(
+            types.ReadResourceResult(contents=[content])
+        )
+
+    # Fallback for other resources (shouldn't happen for this server)
+    return types.ServerResult(
+        types.ReadResourceResult(
+            contents=[
+                types.TextResourceContents(
+                    uri=uri,
+                    mimeType="text/plain",
+                    text="Resource not found"
+                )
+            ]
+        )
+    )
+
+
+# Replace the handler after FastMCP has registered its own
+_low_level_server.request_handlers[types.ReadResourceRequest] = _read_resource_with_meta
 
 if __name__ == "__main__":
     if "--stdio" in sys.argv:

examples/qr-server/widget.html

@@ -26,7 +26,7 @@
 <body>
   <div id="qr"></div>
   <script type="module">
-    import { App, PostMessageTransport } from "https://unpkg.com/@modelcontextprotocol/ext-apps@0.0.4";
+    import { App, PostMessageTransport } from "https://unpkg.com/@modelcontextprotocol/ext-apps@0.0.1";
 
     const app = new App({ name: "QR Widget", version: "1.0.0" });
 

src/app.ts

@@ -79,7 +79,7 @@ export const RESOURCE_URI_META_KEY = "ui/resourceUri";
 /**
  * MIME type for MCP UI resources.
  */
-export const RESOURCE_MIME_TYPE = "text/html;profile=mcp";
+export const RESOURCE_MIME_TYPE = "text/html;profile=mcp-app";
 
 /**
  * Options for configuring App behavior.

src/types.ts

@@ -248,6 +248,13 @@ export interface McpUiSandboxResourceReadyNotification {
     html: string;
     /** Optional override for the inner iframe's sandbox attribute */
     sandbox?: string;
+    /** CSP configuration from resource metadata */
+    csp?: {
+      /** Origins for network requests (fetch/XHR/WebSocket) */
+      connectDomains?: string[];
+      /** Origins for static resources (scripts, images, styles, fonts) */
+      resourceDomains?: string[];
+    };
   };
 }
 
@@ -260,6 +267,12 @@ export const McpUiSandboxResourceReadyNotificationSchema = z.object({
   params: z.object({
     html: z.string(),
     sandbox: z.string().optional(),
+    csp: z
+      .object({
+        connectDomains: z.array(z.string()).optional(),
+        resourceDomains: z.array(z.string()).optional(),
+      })
+      .optional(),
   }),
 });