You might be interested in the dark factory work here https://factory.strongdm.ai/

They do something very similar for some of their work. It’s hard to use external services so they replicate them and the cost of doing so has come down from “don’t be daft, we can’t reimplement slack and google drive this sprint just to make testing faster” to realistic. They run the sdks against the live services and their own implementations until they don’t see behaviour differences. Now they have a fast slack and drive and more (that do everything they need for their testing) accelerating other work. I’m dramatically shifting my concept of what’s expensive and not for development. What you’re describing could have been done by someone before, but the difficulty of building that backend has dropped enormously. Even if the application was closed you could probably either now or soon start to do the same thing starting with building back to core user stories and building the app as well.

You can view some of this as having things like the application as a very precise specification.

Really fascinating moment of change.

> It’s hard to use external services

I think it's interesting to add what they use it for and why its hard.

What they use it for:

- It's about automated testing against third party services.

- It's not about replicating the product for end users

Why using external services is hard/problematic

- Performance: They want to have super fast feedback cycles in the agentic loop: In-Memory tests. So they let the AI write full in-memory simulations of (for example) the slack api that are behaviorally equivalent for their use cases.

- Feasiblity: The sandboxes offered by these services usually have performance limits (= number of requests per month, etc) that would easily be exhausted if attached to a test harness that runs every other minute in an automated BDD loop.