Google Cloud API Gateway can now serve REST APIs as MCP tools

Google Cloud API Gateway can now act as a remote MCP server, so REST operations already defined in an OpenAPI spec become MCP tools. The gateway accepts MCP JSON-RPC requests on one endpoint, converts each tools/call into the matching REST request, applies existing policies, and translates the response back. Teams enable this by setting mcp to true at the document level and adding x-google-mcp-tool annotations where they want to customize or skip an operation.

The gateway requires OpenAPI 3.0.x or 3.1.x, since OpenAPI 2.0 is not supported. Each exposed operation needs a backend and a non-empty description. Discovery through tools/list is unauthenticated by default, and Google recommends requiring a JWT for production, because API keys cannot secure that method. Agents such as those built with the Agent Development Kit connect to the gateway's /mcp endpoint.

Google says the Public Preview covers REST and OpenAPI 3.x backends with current authentication, while MCP resources and prompts, response streaming and Model Armor payload inspection are on the roadmap. Operations that return empty bodies such as HTTP 204 are not exposed, and a gateway serves up to 1,000 tools. MCP and model routing cannot be enabled in the same API config.

Teams with REST services behind API Gateway can expose them to agents without writing and operating a separate MCP server, and MCP calls reuse the same JWT or API-key authentication, quotas and logging as REST traffic.

Tool descriptions drive when an LLM calls a tool, so they should say when and why to use it. Leaving tools/list unauthenticated publishes tool names and input schemas, so production setups should require a JWT for discovery.