the api i want to build on bans 'substantially similar' products in its terms - has anyone actually been cut off

Politely disagreeing with the framing rather than the advice. That clause is in basically every developer agreement I have read, including from companies who cheerfully let hundreds of tools do the forbidden thing. Enforcement is a business decision, not a legal one, and it happens when you are either large enough to matter or annoying enough to come up in a sales call. At 40 customers on a $19 plan you are neither.

The thing that actually kills you is them shipping your feature for free in the base tier, and no contract clause protects you from that either way.

Built on two platform APIs, got squeezed on one. Nobody read the terms and served me a letter. What happened was the OAuth verification review - we asked for a scope that let us read attendee data, the reviewer sat on it for six weeks, then approved it with a restriction. Six months later our per-minute quota got cut roughly in half with an email that said it was to ensure platform stability. No appeal that went anywhere.

That's the shape it takes. Not a ban, just slow strangulation until the product stops feeling good. Build so a user can export everything, and so that at least part of your value is what you do with the data rather than having access to it.

Ask them. There is usually a partner or technology program with a human on the other end. Write four sentences describing what you would build and ask whether it is in scope. A no costs you an email. A yes in writing is worth more than any amount of forum speculation.

Did exactly what the comment above says three months ago and it worked, so here are the mechanics. Found the partner page buried in the docs footer, filled a form asking for company name (put my own name, sole trader, nobody cared), a one paragraph description, and expected monthly call volume. Estimated high on purpose. Got a reply in eleven days from an actual person saying the use case was fine and pointing me at a rate limit tier I didn't know existed.

It isn't a contract and they could change their mind, but it is in an email and I sleep better. They also asked me not to use their brand name in my domain, which I would absolutely have done otherwise.

Price the downside before you decide. Assume the API dies in 18 months. If your product is 90% 'we display their data nicer', salvage value is near zero and you've spent a year of weekends. If it's 'we ingest from four sources and theirs is one connector', losing it costs you three weekends of connector work and some churn. I'd spend one weekend building the second-worst source first, precisely because it tells you how portable the core actually is.

One practical thing: read what the terms say about caching and data retention, not just the competition clause. Most of these agreements cap how long you may keep their data, and an analytics layer is exactly the product that breaks that cap. That's the clause people actually get caught by, because it's testable from the outside.