diff --git a/browse/src/browser-manager.ts b/browse/src/browser-manager.ts
index 033ed8747..cbce2bdad 100644
--- a/browse/src/browser-manager.ts
+++ b/browse/src/browser-manager.ts
@@ -9,6 +9,7 @@
 
 import { chromium, type Browser, type BrowserContext, type Page, type Locator } from 'playwright';
 import { addConsoleEntry, addNetworkEntry, networkBuffer, type LogEntry, type NetworkEntry } from './buffers';
+import { validateUrl } from './url-validation';
 
 export class BrowserManager {
   private browser: Browser | null = null;
@@ -66,6 +67,7 @@ export class BrowserManager {
     this.wirePageEvents(page);
 
     if (url) {
+      validateUrl(url);
       await page.goto(url, { waitUntil: 'domcontentloaded', timeout: 15000 });
     }
 
diff --git a/browse/src/meta-commands.ts b/browse/src/meta-commands.ts
index 0fbe9aea4..5f7608b0a 100644
--- a/browse/src/meta-commands.ts
+++ b/browse/src/meta-commands.ts
@@ -4,6 +4,7 @@
 
 import type { BrowserManager } from './browser-manager';
 import { handleSnapshot } from './snapshot';
+import { validateUrl } from './url-validation';
 import * as Diff from 'diff';
 import * as fs from 'fs';
 
@@ -152,6 +153,8 @@ export async function handleMetaCommand(
     case 'diff': {
       const [url1, url2] = args;
       if (!url1 || !url2) throw new Error('Usage: browse diff <url1> <url2>');
+      validateUrl(url1);
+      validateUrl(url2);
 
       // Get text from URL1
       const page = bm.getPage();
diff --git a/browse/src/url-validation.ts b/browse/src/url-validation.ts
new file mode 100644
index 000000000..d2001b56e
--- /dev/null
+++ b/browse/src/url-validation.ts
@@ -0,0 +1,75 @@
+/**
+ * URL validation to prevent SSRF and local resource access.
+ *
+ * Blocks file:// URLs, private/internal IPs, and non-HTTP schemes.
+ * Set BROWSE_ALLOW_PRIVATE=1 to bypass for local development.
+ */
+
+const ALLOWED_SCHEMES = new Set(['http:', 'https:']);
+
+/**
+ * Check if a hostname resolves to a private/internal IP address.
+ */
+function isPrivateHost(hostname: string): boolean {
+  // IPv6 loopback
+  if (hostname === '[::1]' || hostname === '::1') return true;
+
+  // Strip brackets from IPv6
+  const bare = hostname.replace(/^\[|\]$/g, '');
+
+  // IPv6 unique-local (fc00::/7)
+  if (/^f[cd][0-9a-f]{2}:/i.test(bare)) return true;
+
+  // IPv4 checks
+  const parts = bare.split('.').map(Number);
+  if (parts.length === 4 && parts.every(p => !isNaN(p))) {
+    const [a, b] = parts;
+    // 127.0.0.0/8
+    if (a === 127) return true;
+    // 10.0.0.0/8
+    if (a === 10) return true;
+    // 172.16.0.0/12
+    if (a === 172 && b >= 16 && b <= 31) return true;
+    // 192.168.0.0/16
+    if (a === 192 && b === 168) return true;
+    // 169.254.0.0/16 (link-local / cloud metadata)
+    if (a === 169 && b === 254) return true;
+    // 0.0.0.0
+    if (a === 0 && b === 0 && parts[2] === 0 && parts[3] === 0) return true;
+  }
+
+  // "localhost" variants
+  if (bare === 'localhost' || bare.endsWith('.localhost')) return true;
+
+  return false;
+}
+
+/**
+ * Validate a URL before navigating. Throws if the URL is not allowed.
+ */
+export function validateUrl(url: string): void {
+  // Bypass when explicitly opted in for local development
+  if (process.env.BROWSE_ALLOW_PRIVATE === '1') return;
+
+  let parsed: URL;
+  try {
+    parsed = new URL(url);
+  } catch {
+    throw new Error(
+      `Invalid URL: "${url}". Only http: and https: URLs are allowed.`
+    );
+  }
+
+  if (!ALLOWED_SCHEMES.has(parsed.protocol)) {
+    throw new Error(
+      `Blocked URL scheme "${parsed.protocol}" in "${url}". Only http: and https: are allowed.`
+    );
+  }
+
+  if (isPrivateHost(parsed.hostname)) {
+    throw new Error(
+      `Blocked navigation to private/internal address "${parsed.hostname}". ` +
+      `Set BROWSE_ALLOW_PRIVATE=1 to allow local development URLs.`
+    );
+  }
+}
diff --git a/browse/src/write-commands.ts b/browse/src/write-commands.ts
index e1c91942b..3bae0c73c 100644
--- a/browse/src/write-commands.ts
+++ b/browse/src/write-commands.ts
@@ -6,6 +6,7 @@
  */
 
 import type { BrowserManager } from './browser-manager';
+import { validateUrl } from './url-validation';
 
 export async function handleWriteCommand(
   command: string,
@@ -18,6 +19,7 @@ export async function handleWriteCommand(
     case 'goto': {
       const url = args[0];
       if (!url) throw new Error('Usage: browse goto <url>');
+      validateUrl(url);
       const response = await page.goto(url, { waitUntil: 'domcontentloaded', timeout: 15000 });
       const status = response?.status() || 'unknown';
       return `Navigated to ${url} (${status})`;