I don't think any of the mainstream vendor APIs require MCP for tool use - they all supported functions (generally defined using a chunk of OpenAPI JSON schema) before the MCP spec gained widespread acceptance and continue to do so today.

Yep, the Anthropic API supported tool use well before an MCP-related construct was added to the API (MCP connector in May of this year).

While it's not an API, Anthropic's Agent SDK does require MCP to use custom tools.