API Gateway Remote MCP Server (Public Preview)

This skill converts new or existing REST APIs on Google Cloud API Gateway into remote Model Context Protocol (MCP) servers (POST https://{default_hostname}/mcp, Public Preview). By annotating an OpenAPI 3.x (3.0.x or 3.1.x) specification with x-google-api-management.mcp and x-google-mcp-tool, API Gateway translates MCP JSON-RPC 2.0 requests (initialize, notifications/initialized, tools/list, tools/call) into standard HTTP REST requests to your backend while enforcing existing authentication and quota policies.

Prerequisites

1. Install the Google Cloud CLI and the validator dependency:
   • Install the Google Cloud CLI. Then update it to the latest version with gcloud components update. An old CLI can fail when it uploads an OpenAPI 3.x API config.
   • Install Python 3.10 or later and PyYAML for the local OpenAPI MCP validator script:

     pip install "PyYAML>=6.0,<7.0"
     

2. Authenticate your environment:
   • Local CLI session: gcloud auth login
   • Application Default Credentials (SDKs / ADK): gcloud auth application-default login
3. Select or create a project, confirm billing, and pick a supported gateway location:
   • Select an existing project (gcloud config set project {project_id}) or create one (gcloud projects create {project_id}).
   • Confirm that an active billing account is linked to {project_id}. API Gateway has a free monthly allowance of API calls for each billing account, but you still need an active billing account to deploy. Calls above the allowance and network data transfer out are charged. For current prices, see API Gateway pricing.
   • Use us-central1 as the default --location for gateway commands. For the other supported regions, see API Gateway deployment model.
4. Enable the required Google Cloud APIs:

   gcloud services enable \
     apigateway.googleapis.com \
     servicemanagement.googleapis.com \
     servicecontrol.googleapis.com \
     --project={project_id}
   

   For API hub discovery (optional), enable the API hub API in the API hub host project: gcloud services enable apihub.googleapis.com --project={hub_project_id}. To query Agent Registry (optional), also run gcloud services enable agentregistry.googleapis.com --project={project_id}.
5. Verify the IAM roles (use the most restrictive role for each step):
   • To create and update the API, API configs, and gateways: API Gateway Admin (roles/apigateway.admin) on {project_id}. To run the gcloud services enable commands: Service Usage Admin (roles/serviceusage.serviceUsageAdmin) on that project.
   • To reference the gateway service account in an API config: Service Account User (roles/iam.serviceAccountUser) on {gateway_sa_name}@{project_id}.iam.gserviceaccount.com.
   • For a Cloud Run backend: grant the gateway service account the Cloud Run Invoker role (roles/run.invoker) on the service.
   • To mint test ID tokens as a caller service account: Service Account Token Creator (roles/iam.serviceAccountTokenCreator) on {caller_sa_email}.
   • Reference: Configure the development environment.

Decision Matrix: MCP Server Configuration and Client Integration

Task	Required Mechanism	Critical Guardrail	Detailed Reference
Migrate OpenAPI 2.0 (swagger: "2.0") or author OpenAPI 3.x for MCP	Root x-google-api-management.mcp + named backends + operation-level x-google-mcp-tool	NEVER add MCP extensions to an OpenAPI 2.0 spec. Rationale: MCP is not supported for OpenAPI 2.0. Use OpenAPI 3.0.x or 3.1.x, and deploy the spec with gcloud api-gateway api-configs create. Keep all MCP operations in one OpenAPI document: if an API config has several documents, API Gateway serves the MCP tools of only one of them.	OpenAPI 2.0 to 3.x Migration
Optimize tool names, descriptions, and schemas for LLM tool selection	x-google-mcp-tool (name, description, false) + described requestBody properties	Every operation MUST have an operationId, with or without MCP. Every exposed tool MUST resolve to a unique name that matches [A-Za-z0-9_.-]{1,128} (x-google-mcp-tool.name, or else the operationId), a non-empty description, and a non-empty response body (HTTP 204 is unsupported). Only GET, POST, PUT, PATCH, and DELETE operations can be tools, and a gateway serves at most 1,000 tools. API Gateway rejects complex nested schemas when you create the API config, but a simple nested object body works. In OpenAPI 3.1, give each schema one type, because API Gateway also rejects a type list such as type: ["string", "null"]. Describe each property, and check each tool's inputSchema in tools/list after you deploy.	OpenAPI Agent Tool Optimizer
Secure tools/list and hide internal/admin routes	x-google-api-management.mcp.tools-list.security (exactly one JWT or API key scheme; a JWT scheme is also listed in root security) + x-google-mcp-tool: false	tools/list is unauthenticated by default. It accepts one JWT scheme, or one API key scheme sent in the x-api-key header. API Gateway rejects a config whose JWT tools-list.security scheme is missing from the root security list, and that root entry also requires the JWT on every REST operation without its own security. If the root list already has an API key scheme, add the JWT to the same requirement object (AND): API Gateway does not support alternative (OR) requirements with an API key. An mcp object (for tools-list) also enables MCP for every eligible operation, so opt out each operation that agents must not use. x-google-mcp-tool: false hides a route from MCP only; keep REST security on admin routes.	OpenAPI 2.0 to 3.x Migration
Connect Google ADK (McpToolset) and discover in API hub / Agent Registry	McpToolset (StreamableHTTPConnectionParams) + API hub project attachment	No separate registration step is needed. After you attach the API Gateway project to API hub, the MCP server appears in API hub as {your_api} (MCP) and in Agent Registry automatically.	ADK and API Hub Discovery
Verify /mcp JSON-RPC lifecycle and troubleshoot -32601 / -32602 / -32000 errors	POST https://{default_hostname}/mcp with MCP-Protocol-Version: 2025-11-25	Keep /mcp at the host root: a servers base path (such as /v2) applies to REST paths only, and {basepath}/mcp returns HTTP 404. Nest JSON request bodies under params.arguments.body ({"body": {...}}). NEVER combine x-google-api-management.mcp with x-google-model-router in one config. Rationale: API Gateway does not allow MCP and model routing in the same API config.	MCP Verification and Troubleshooting

Execution Workflow and Validation Loop

1. Upgrade and annotate the OpenAPI 3.x specification:
   • The references in this skill contain the conversion rules: the Swagger 2.0 Construct Conversion Map covers each Swagger 2.0 construct, and the Complete Converted Example shows a finished spec with a Google ID token scheme. Use them before you search other sources.
   • Follow OpenAPI 2.0 to 3.x Migration and OpenAPI Agent Tool Optimizer to set openapi to a 3.0.x or 3.1.x version (the examples use 3.0.4), remove unsupported OpenAPI 2.0 fields (x-google-allow), define x-google-api-management.backends (with jwtAudience or disableAuth), configure x-google-api-management.mcp.tools-list.security with exactly one JWT or API key scheme (add a JWT scheme to the root security list too, in the same requirement object as any API key scheme), author x-google-mcp-tool descriptions stating when and why to invoke each tool, and opt out administrative routes (x-google-mcp-tool: false).
   • Before you deploy, give the user a short summary that states each of these points, and get approval:
     • MCP is not supported for OpenAPI 2.0, so you converted the spec to OpenAPI 3.x.
     • Which operations you exposed as tools, and which you opted out with x-google-mcp-tool: false. Opted-out operations stay on REST.
     • If tools/list uses a JWT scheme, the root security entry makes every REST operation without its own security require the JWT. The gateway rejects REST callers without it.
     • The MCP endpoint is https://{default_hostname}/mcp, at the host root even when the spec sets a servers base path.
     • The limits that apply: only GET, POST, PUT, PATCH, and DELETE operations can be tools, a gateway serves at most 1,000 tools, request parameters must be primitive types, API Gateway rejects complex nested schemas and OpenAPI 3.1 type lists (such as type: ["string", "null"]) when you create the API config, tools cannot return HTTP 204, and MCP is stateless with UTF-8 text responses only.
   • Give the same points when the user only asks for the converted spec or for advice, without a deploy.
   • Preserve exact quota metric names in x-google-api-management.metrics across API configs so existing gateways do not fail with HTTP 500 Failed to call Service Control Quota.
2. Run the deterministic MCP OpenAPI validation loop before creating the API config:
   • Execute scripts/validate_mcp_openapi.py on {candidate_openapi_spec}:

     python3 scripts/validate_mcp_openapi.py --spec={candidate_openapi_spec}
     

   • If scripts/validate_mcp_openapi.py exits with code 1 (status: "FAIL"), fix every reported error in {candidate_openapi_spec} (such as OpenAPI 2.0 syntax, x-google-allow, path-level x-google-mcp-tool, undefined backend names, an operation without an operationId, missing descriptions, invalid or duplicate tool names, a header parameter with the reserved x-google- prefix, a tools-list.security scheme that is neither an oauth2 JWT nor an apiKey scheme, a JWT tools-list.security scheme missing from the root security list, alternative (OR) security requirements with an API key, or an x-google-model-router operation in an MCP spec) and re-run the script until it exits with code 0 (status: "PASS"). If it exits with code 2, fix the file path or YAML syntax error printed to stderr.
   • Read every warning (such as an HTTP 204 response, a request parameter that is not a primitive type, or operations that an mcp object exposes by default). Fix it, or tell the user why you keep it.
3. Create the API (if new), create the API config, deploy the gateway, and enable the managed service (request human approval before updating a production gateway):

   gcloud api-gateway apis create {api_id} --project={project_id}
   
   gcloud api-gateway api-configs create {config_id} \
     --api={api_id} \
     --openapi-spec={candidate_openapi_spec} \
     --backend-auth-service-account={gateway_sa_name}@{project_id}.iam.gserviceaccount.com \
     --project={project_id}
   
   gcloud api-gateway gateways update {gateway_id} \
     --api={api_id} \
     --api-config={config_id} \
     --location={region} \
     --project={project_id}
   
   gcloud services enable $(gcloud api-gateway apis describe {api_id} --project={project_id} --format="value(managedService)") --project={project_id}
   

   (For a brand-new gateway, run gcloud api-gateway gateways create {gateway_id} ... instead of gateways update).
   • Run one api-configs create at a time per API. Wait for it to finish before you start the next one, because API Gateway creates only one config for an API at a time. A complex API config can take up to ten minutes to create.
   • If clients use API keys from other projects, enable the managed service in each of those projects too. API Gateway rejects an API key unless the API is enabled in the project that created the key.
4. Verify the JSON-RPC 2.0 lifecycle and connect ADK / API hub:
   • Send every probe to https://{default_hostname}/mcp, even when the spec sets a servers base path.
   • An HTML 404 Page not found means that the gateway may not be deployed correctly. Check that the gateway state is ACTIVE and that the last create or update finished, and retry before you change the config. During a gateway update, some requests can still reach the previous API config. A JSON 404 (The current request is not defined by this API.) means that the active config does not define the path.
   • If tools/list requires a JWT, mint a Google ID token whose audience is in x-google-auth.audiences: gcloud auth print-identity-token --impersonate-service-account={caller_sa_email} --audiences={jwt_audience}. If tools/list requires an API key, send it in the x-api-key header.
   • Run the initialize, notifications/initialized, tools/list, and tools/call verification probes in MCP Verification and Troubleshooting.
   • Connect Google ADK (McpToolset) and verify automatic catalog discovery in Apigee API hub ({your_api} (MCP)) and Agent Registry using ADK and API Hub Discovery.

Pre-Deployment Checklist

• Specification version is openapi: 3.0.x or 3.1.x (not swagger: "2.0"), omits x-google-allow, and does not contain x-google-model-router.
• python3 scripts/validate_mcp_openapi.py --spec={candidate_openapi_spec} returns status: "PASS" (exit code 0).
• Every operation has an operationId, and all MCP operations are in one OpenAPI document.
• x-google-api-management.mcp.tools-list.security names exactly one scheme: an oauth2 JWT scheme with x-google-auth that the root security list also references, or an apiKey scheme with in: header and name: x-api-key.
• If tools/list uses a JWT scheme, the user knows that REST operations without their own security now require that JWT.
• Administrative, health-check, and HTTP 204 endpoints are opted out with x-google-mcp-tool: false, and opted-out admin paths remain protected on REST.
• MCP clients call https://{default_hostname}/mcp (not {basepath}/mcp), nest tools/call JSON request bodies inside params.arguments.body, and send MCP-Protocol-Version: 2025-11-25.

Resource Cleanup

To avoid ongoing charges when tearing down a test or staging environment, request human approval and delete the gateway, API config, API, and service account in dependency order:

gcloud api-gateway gateways delete {gateway_id} --location={region} --project={project_id}
gcloud api-gateway api-configs delete {config_id} --api={api_id} --project={project_id}
gcloud api-gateway apis delete {api_id} --project={project_id}
gcloud iam service-accounts delete {gateway_sa_name}@{project_id}.iam.gserviceaccount.com --project={project_id}