OpenAI, in addition to JSON schema, supports "context-free grammar"[0], i.e. regex and lark. Anthropic also supports JSON schema since a few weeks ago, but they don't support specifying the length of JSON array, so you still have to worry about the model producing invalid output.

[0]: https://platform.openai.com/docs/guides/function-calling#con...

One thing that pisses me off is this widespread misunderstanding that you can just fall back to function calling (Anthropic's function calling accepts JSON schema for arguments), and that it's the same as structured outputs. It is not. They just dump the JSON schema into the context without doing the actual structured outputs. Vercel's AI SDK does that and it pisses me off because doing that only confuses the model and prefilling works much better.