Update openapi.mdx

Author: jlowin

docs/patterns/openapi.mdx

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 If you have existing REST APIs documented with the OpenAPI Specification (OAS), FastMCP can automatically generate MCP tools, resources, and resource templates directly from that specification. This provides a quick way to make your existing HTTP APIs accessible to MCP clients and LLMs.
 
+FastMCP supports both OpenAPI 3.0 and 3.1 specifications for maximum compatibility with existing API definitions.
+
 ## The Goal: API -> MCP Server
 
 The core idea is to map OpenAPI paths and operations (like `GET /users/{id}` or `POST /orders`) to their corresponding MCP components: