diff --git a/src/renderer/a2ui/components/A2UIText.tsx b/src/renderer/a2ui/components/A2UIText.tsx
index f9d7c89..c37061c 100644
--- a/src/renderer/a2ui/components/A2UIText.tsx
+++ b/src/renderer/a2ui/components/A2UIText.tsx
@@ -1,4 +1,4 @@
-import React from 'react';
+import React, { type ReactNode } from 'react';
 import Markdown from 'marked-react';
 import type { A2UIResolvedComponent, A2UIClientAction } from '../../../main/a2ui/types';
 
@@ -10,7 +10,16 @@ interface A2UIComponentProps {
   renderChildren?: (children: A2UIResolvedComponent[]) => React.ReactNode;
 }
 
+const safeRenderer = {
+  image(src: string, alt: string): ReactNode {
+    if (/^https?:\/\//i.test(src)) {
+      return <a href={src} key={src} title={alt}>{alt || src}</a>;
+    }
+    return <img src={src} alt={alt} key={src} />;
+  },
+};
+
 export const A2UIText: React.FC<A2UIComponentProps> = ({ component }) => {
   const text = String(component.properties.text ?? '');
-  return <Markdown>{text}</Markdown>;
+  return <Markdown renderer={safeRenderer}>{text}</Markdown>;
 };
diff --git a/src/renderer/components/ChatSurface/ChatTranscript.tsx b/src/renderer/components/ChatSurface/ChatTranscript.tsx
index f13e040..10c059e 100644
--- a/src/renderer/components/ChatSurface/ChatTranscript.tsx
+++ b/src/renderer/components/ChatSurface/ChatTranscript.tsx
@@ -1,4 +1,4 @@
-import React from 'react';
+import React, { type ReactNode } from 'react';
 import Markdown from 'marked-react';
 import type { ChatMessage } from '../../types/electron';
 import type { ChatToolEvent } from '../../navigation/useChatSurfaceState';
@@ -49,6 +49,17 @@ export const ChatTranscript: React.FC<ChatTranscriptProps> = ({
   onSurfaceDataChange,
   currentTurnIndex,
 }) => {
+  // Block external images — CSP only allows self/data/file/blob/bds-media/bds-thumb
+  const safeRenderer = {
+    image(src: string, alt: string): ReactNode {
+      if (/^https?:\/\//i.test(src)) {
+        // Show alt text as a link instead of trying to load the image
+        return <a href={src} key={src} title={alt}>{alt || src}</a>;
+      }
+      return <img src={src} alt={alt} key={src} />;
+    },
+  };
+
   const renderToolMarkers = (events: ChatToolEvent[]) => {
     if (events.length === 0) {
       return null;
@@ -160,7 +171,7 @@ export const ChatTranscript: React.FC<ChatTranscriptProps> = ({
             </div>
           )}
           <div className="chat-message-text">
-            {message.role === 'assistant' ? <Markdown gfm>{message.content}</Markdown> : message.content}
+            {message.role === 'assistant' ? <Markdown gfm renderer={safeRenderer}>{message.content}</Markdown> : message.content}
           </div>
         </div>
       </div>
@@ -199,7 +210,7 @@ export const ChatTranscript: React.FC<ChatTranscriptProps> = ({
               {showToolMarkers ? renderToolMarkers(toolEvents) : null}
               {streamingContent && (
                 <div className="chat-message-text">
-                  <Markdown gfm>{streamingContent}</Markdown>
+                  <Markdown gfm renderer={safeRenderer}>{streamingContent}</Markdown>
                 </div>
               )}
             </div>
diff --git a/tests/renderer/components/ChatTranscript.externalImages.test.ts b/tests/renderer/components/ChatTranscript.externalImages.test.ts
new file mode 100644
index 0000000..f5d2fcf
--- /dev/null
+++ b/tests/renderer/components/ChatTranscript.externalImages.test.ts
@@ -0,0 +1,53 @@
+/**
+ * Verify that ChatTranscript and A2UIText block external images
+ * by providing a custom marked-react renderer that converts them to links.
+ */
+
+import { describe, expect, it } from 'vitest';
+import * as fs from 'node:fs';
+import * as path from 'node:path';
+
+const chatTranscriptPath = path.resolve(
+  __dirname,
+  '../../../src/renderer/components/ChatSurface/ChatTranscript.tsx',
+);
+const a2uiTextPath = path.resolve(
+  __dirname,
+  '../../../src/renderer/a2ui/components/A2UIText.tsx',
+);
+
+describe('External image blocking', () => {
+  it('ChatTranscript defines a safeRenderer that intercepts image tags', () => {
+    const source = fs.readFileSync(chatTranscriptPath, 'utf8');
+
+    // Must have a renderer that handles images
+    expect(source).toContain('safeRenderer');
+    expect(source).toContain('image(src');
+    expect(source).toContain('https?:');
+
+    // Both Markdown usages must pass the renderer
+    const markdownUsages = source.match(/<Markdown[^>]*>/g) ?? [];
+    expect(markdownUsages.length).toBeGreaterThanOrEqual(2);
+    for (const usage of markdownUsages) {
+      expect(usage).toContain('renderer={safeRenderer}');
+    }
+  });
+
+  it('A2UIText defines a safeRenderer that intercepts image tags', () => {
+    const source = fs.readFileSync(a2uiTextPath, 'utf8');
+
+    expect(source).toContain('safeRenderer');
+    expect(source).toContain('image(src');
+    expect(source).toContain('https?:');
+    expect(source).toContain('renderer={safeRenderer}');
+  });
+
+  it('safeRenderer converts external URLs to links, not img tags', () => {
+    const source = fs.readFileSync(chatTranscriptPath, 'utf8');
+
+    // The renderer should return <a> for external URLs, not <img>
+    expect(source).toContain('https?:');
+    expect(source).toContain('.test(src)');
+    expect(source).toContain('<a href={src}');
+  });
+});