UPDATE — Got it working. Here's everything we found. Thanks to those who replied. We eventually solved it through extensive debugging. Posting the full findings here in case it helps anyone else — macOS 26 changed some things that aren't documented anywhere yet. The symptom: Extension showed "Uninstall" in Mail → Settings → Extensions (meaning it was registered), but decideAction(for:completionHandler:) never fired. No errors, no logs, nothing. Root cause #1: The Mail checkbox Mail keeps its own enable state for extensions, separate from the OS-level pluginkit state. Even with pluginkit showing + (enabled), Mail internally tracks userEnabled: Not Enabled. The fix is simply ticking the checkbox next to your extension in Mail → Settings → Extensions. It's unchecked by default. Every new user must enable it manually, and it resets every time you replace the app binary during development. Root cause #2: Missing XPCService block in Info.plist Your extension's Info.plist needs both the standard NSExtension block AND an XPCService block: XPCService JoinExistingSession RunLoopType _NSApplicationMain ServiceType Application Without this, the extension process can't start correctly on macOS 26. macOS 26 change: ExtensionFoundation MailKit switched its internal discovery mechanism to ExtensionFoundation/ExtensionKit on macOS 26. Your Swift code doesn't change — NSObject + MEExtension + MEMessageActionHandler still works exactly as documented. But the enable toggle moved: it's no longer in System Settings → Privacy & Security. It's only inside Mail's own Settings panel. EXSettingsEnablementUIHidden = true in MailKit's SDK definition is why it doesn't appear in System Settings. The app must be in /Applications Mail won't discover extensions from apps anywhere else. Not your home folder, not the Desktop. /Applications only. Verifying registration: pluginkit -m -A -i com.your.bundle.extension Should show + prefix. If it shows -, force-enable with: pluginkit -e use -i com.your.bundle.extension Testing: The extension XPC process only spawns when Mail processes incoming mail — not at launch. Send yourself a test email, then check ps aux | grep YourExtensionName to confirm it's running. The actual Swift code is fine as documented. All the pain is in the plumbing above. Hope this saves someone else the same weeks of debugging.