And that's assuming you even have access to said source code. In many cases, you just receive a semi-documented JSON object and a prayer. Anecdotal story: We r

And that's assuming you even have access to said source code. In many cases, you just receive a semi-documented JSON object and a prayer. Anecdotal story: We recently went through an effort to introduce strict-type checking for our typescript backend. As part of that effort, we introduced concrete types for every API accessible across a service boundary, which caught 6 mismatches between the documentation and the implementation (e.g. return Promise<bool> vs Promise<Instance>) and several hundred issues with missing null or type checks that would result in an "InternalServerError" instead of a meaningful error message. We started rolling the strict version out this week, and we can already see a meaningful improvement in the sentry logs.