All the model companies except kind of Anthropic (and even they half-assedly do) implement the OpenAI API. It's not an open standard but, like the S3 API, it effectively is.

And, to answer your question, no. The existence of a common API makes it trivial to change zero code and send requests to a different model.

It's more complicated than that. Lots of providers use an "OpenAI-ish" API, but many of them have subtle differences in things like tool calling or thinking blocks. OpenRouter normalizes the wire format.

OpenRouter does more than just proxying; they also aggregate providers for open-weight models, which has a stabilizing effect on pricing and gives you protection against a single provider's downtime.

no. i don't know if openrouter even guarantees consistent protocl across models.

the value of openrouter is it offers centralized billing. you can route your calls to any provider you want, test a whole bunch of models against each other, and you just get one bill from openrouter. switching to a new model, or a new provider of the same model, doesn't mean setting up a new billing account with a new provider.