It's interesting engineering problem, I wouldn't imagine LLMs as they currently are could work directly on the whole codebase without breaking it just as often as the APIs. But perhaps you could have it maintain connectors/interfaces for each individual API, such that it can get one very wrong and not ruin the whole program.
You could even have its success depend on a test suite, so that it iterates until the tests pass.
For “API list” that only has tests to be fixed, something like shifting endpoint should be fixable with tests + LLM.
So that’s the idea I proposed that single dev with automation and LLM should be able to maintain “API list” but maintaining any code that depends on the API I expect is above LLMs.
You could even have its success depend on a test suite, so that it iterates until the tests pass.