Most tokyo thai restaurants are adapted for japanese tastes (too mild, too sweet). But there are authentic spots if you know where to look!

Thonglor in shibuya serves real som tam that'll burn your mouth off. The chef is from isaan and doesn't hold back on chilies. Their larb gai is spot-on too.

Krua thai in yoyogi has lunch sets around ¥1200 that taste like bangkok street food. The pad kra pao is properly spicy with crispy pork and a fried egg that's still runny.

Both places have thai staff who get excited when you order in thai or ask for extra spice. Just say 'pet pet' for proper heat level!