On April 30, 2026 , HubSpot sunsets the Contact Lists v1 API. The headline is simple: most /contacts/v1/lists/* endpoints start returning HTTP 404. The dangerous part is everything that doesn't 404.

Straight from HubSpot's sunset announcement , six Contacts v1 read endpoints will:

continue to function but will no longer return list memberships

That's a 200 response with the list-memberships array silently gone. Code that does contact["list-memberships"] gets a KeyError . Code that does contact.get("list-memberships", []) swallows the empty list and now thinks the contact is in zero lists. Both are broken. One of them looks broken.

This is incident #6 in our silent-breakage series ( GitHub PushEvent , Stripe Basil , Shopify 2025-01 , OpenAI Responses input_text , Twilio regional domains ).…