diff --git a/src/main/security-policy.ts b/src/main/security-policy.ts
index 66e30f3..062a7ef 100644
--- a/src/main/security-policy.ts
+++ b/src/main/security-policy.ts
@@ -1,7 +1,6 @@
-export function isTrustedAppUrl(rawUrl: string): boolean {
+function isHarnessUrl(rawUrl: string): boolean {
   try {
     const url = new URL(rawUrl)
-    if (url.protocol === 'file:') return true
     return (
       url.protocol === 'http:' &&
       (url.hostname === '127.0.0.1' || url.hostname === 'localhost')
@@ -10,3 +9,25 @@ export function isTrustedAppUrl(rawUrl: string): boolean {
     return false
   }
 }
+
+export function isTrustedAppUrl(rawUrl: string): boolean {
+  try {
+    if (new URL(rawUrl).protocol === 'file:') return true
+  } catch {
+    return false
+  }
+  return isHarnessUrl(rawUrl)
+}
+
+export function canGrantWindowPermission(
+  permission: string,
+  requestingUrl: string | undefined,
+  isMainFrame: boolean
+): boolean {
+  return (
+    permission === 'clipboard-sanitized-write' &&
+    isMainFrame &&
+    requestingUrl !== undefined &&
+    isHarnessUrl(requestingUrl)
+  )
+}
diff --git a/src/main/security.ts b/src/main/security.ts
index 742acf1..fae4178 100644
--- a/src/main/security.ts
+++ b/src/main/security.ts
@@ -1,5 +1,5 @@
 import { shell, type BrowserWindow } from 'electron'
-import { isTrustedAppUrl } from './security-policy'
+import { canGrantWindowPermission, isTrustedAppUrl } from './security-policy'
 
 export function secureWindow(window: BrowserWindow): void {
   window.webContents.setWindowOpenHandler(({ url }) => {
@@ -15,7 +15,19 @@ export function secureWindow(window: BrowserWindow): void {
   })
 
   window.webContents.on('will-attach-webview', (event) => event.preventDefault())
-  window.webContents.session.setPermissionRequestHandler((_webContents, _permission, callback) => {
-    callback(false)
-  })
+  window.webContents.session.setPermissionCheckHandler(
+    (_webContents, permission, requestingOrigin, details) =>
+      canGrantWindowPermission(
+        permission,
+        details.requestingUrl ?? requestingOrigin,
+        details.isMainFrame
+      )
+  )
+  window.webContents.session.setPermissionRequestHandler(
+    (_webContents, permission, callback, details) => {
+      callback(
+        canGrantWindowPermission(permission, details.requestingUrl, details.isMainFrame)
+      )
+    }
+  )
 }
diff --git a/test/runtime.test.ts b/test/runtime.test.ts
index 2ece421..83f5331 100644
--- a/test/runtime.test.ts
+++ b/test/runtime.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest'
 import { buildHarnessArguments, buildNodeArguments } from '../src/main/runtime/harness-runtime'
-import { isTrustedAppUrl } from '../src/main/security-policy'
+import { canGrantWindowPermission, isTrustedAppUrl } from '../src/main/security-policy'
 import { shouldLoadHarnessUrl } from '../src/main/window-navigation'
 
 describe('Harness launch contract', () => {
@@ -36,6 +36,43 @@ describe('navigation trust boundary', () => {
     expect(isTrustedAppUrl('http://example.com')).toBe(false)
     expect(isTrustedAppUrl('javascript:alert(1)')).toBe(false)
   })
+
+  it('only grants clipboard writes from the trusted main frame', () => {
+    expect(
+      canGrantWindowPermission(
+        'clipboard-sanitized-write',
+        'http://127.0.0.1:43127/session',
+        true
+      )
+    ).toBe(true)
+    expect(
+      canGrantWindowPermission(
+        'clipboard-sanitized-write',
+        'http://localhost:43127/session',
+        true
+      )
+    ).toBe(true)
+    expect(
+      canGrantWindowPermission('clipboard-read', 'http://127.0.0.1:43127/session', true)
+    ).toBe(false)
+    expect(
+      canGrantWindowPermission(
+        'clipboard-sanitized-write',
+        'http://127.0.0.1:43127/session',
+        false
+      )
+    ).toBe(false)
+    expect(
+      canGrantWindowPermission(
+        'clipboard-sanitized-write',
+        'https://example.com/session',
+        true
+      )
+    ).toBe(false)
+    expect(
+      canGrantWindowPermission('clipboard-sanitized-write', 'file:///tmp/app.html', true)
+    ).toBe(false)
+  })
 })
 
 describe('Harness window activation', () => {