Rebuild a custom GPT action as a remote MCP server

{ "openapi": "3.1.0", "info": { "title": "Exchange rates", "description": "Daily reference exchange rates from Frankfurter (frankfurter.dev). No API key.", "version": "1.0.0" }, "servers": [{ "url": "https://api.frankfurter.dev/v1" }], "paths": { "/currencies": { "get": { "operationId": "listCurrencies", "summary": "List the currency codes this API supports, with their names.", "responses": { "200": { "description": "Currency codes and names." } } } }, "/latest": { "get": { "operationId": "getLatestRates", "summary": "Latest daily exchange rates for one base currency.", "parameters": [ { "name": "base", "in": "query", "description": "Three-letter base currency code, like USD. EUR if left out.", "schema": { "type": "string", "pattern": "^[A-Z]{3}$" } }, { "name": "symbols", "in": "query", "description": "Comma-separated codes to return, like EUR,GBP. All if left out.", "schema": { "type": "string", "pattern": "^[A-Z]{3}(,[A-Z]{3}){0,40}$" } }, { "name": "amount", "in": "query", "description": "Amount of the base currency to convert. 1 if left out.", "schema": { "type": "number", "exclusiveMinimum": 0, "maximum": 1000000000 } } ], "responses": { "200": { "description": "Rates for the base currency." } } } }, "/{date}": { "get": { "operationId": "getHistoricalRates", "summary": "Exchange rates on one past date (working days, back to 1999-01-04).", "parameters": [ { "name": "date", "in": "path", "required": true, "description": "The date as YYYY-MM-DD.", "schema": { "type": "string", "pattern": "^\\d{4}-\\d{2}-\\d{2}$" } }, { "name": "base", "in": "query", "description": "Three-letter base currency code, like USD. EUR if left out.", "schema": { "type": "string", "pattern": "^[A-Z]{3}$" } }, { "name": "symbols", "in": "query", "description": "Comma-separated codes to return, like EUR,GBP. All if left out.", "schema": { "type": "string", "pattern": "^[A-Z]{3}(,[A-Z]{3}){0,40}$" } } ], "responses": { "200": { "description": "Rates on that date." } } } } } } 

mkdir gpt-action-mcp && cd gpt-action-mcp npm init -y && npm pkg set type=module npm install @modelcontextprotocol/sdk@1.32.2 @cfworker/json-schema@4.1.1 npm install -D wrangler@4.152.0 

name = "gpt-action-mcp-demo" main = "src/index.js" compatibility_date = "2026-10-01" workers_dev = true preview_urls = false # At most 60 MCP requests a minute from one IP address, so nobody can use this Worker to flood the API. [[ratelimits]] name = "LIMITER" namespace_id = "6001" simple = { limit = 60, period = 60 } 

// A custom GPT action's OpenAPI schema -> MCP tools, one per operation. // Scope: GET operations with path and query parameters. Anything else throws when the Worker loads, // so a schema this file cannot map fails at deploy time, never halfway through a call. import { McpError, ErrorCode } from "@modelcontextprotocol/sdk/types.js"; import { CfWorkerJsonSchemaValidator } from "@modelcontextprotocol/sdk/validation/cfworker"; const validator = new CfWorkerJsonSchemaValidator(); const MAX_ANSWER = 100_000; // characters of API answer passed back to the model // getLatestRates -> get_latest_rates export const toolName = (operationId) => operationId.replace(/([a-z0-9])([A-Z])/g, "$1_$2").toLowerCase(); export function toolsFromSchema(doc) { const base = new URL(doc.servers?.[0]?.url); if (base.protocol !== "https:") throw new Error("The server URL must be https."); const tools = []; for (const [path, item] of Object.entries(doc.paths ?? {})) { for (const [method, op] of Object.entries(item)) { if (method !== "get") throw new Error(`${method.toUpperCase()} ${path}: only GET is mapped.`); if (!/^[A-Za-z][A-Za-z0-9]*$/.test(op.operationId ?? "")) throw new Error(`GET ${path}: needs a plain operationId.`); const properties = {}; const required = []; const where = {}; for (const p of op.parameters ?? []) { if (p.in !== "query" && p.in !== "path") throw new Error( `${op.operationId}: ${p.in} parameters are not mapped.`, ); properties[p.name] = { ...p.schema, description: p.description }; if (p.required || p.in === "path") required.push(p.name); where[p.name] = p.in; } const inputSchema = { type: "object", properties, required, additionalProperties: false, }; tools.push({ name: toolName(op.operationId), description: op.summary, inputSchema, annotations: { readOnlyHint: true, openWorldHint: true }, base, path, where, check: validator.getValidator(inputSchema), }); } } if (new Set(tools.map((t) => t.name)).size !== tools.length) throw new Error("Two operations map to the same tool name."); return tools; } // What tools/list shows: never the base URL or the validator. export const listing = ({ name, description, inputSchema, annotations }) => ({ name, description, inputSchema, annotations, }); // Checked arguments -> the API URL. The host and base path come only from the schema. export function requestUrl(tool, args) { const path = tool.path.replace(/\{(\w+)\}/g, (_, k) => encodeURIComponent(args[k]), ); const url = new URL(tool.base.href.replace(/\/$/, "") + path); for (const [k, where] of Object.entries(tool.where)) if (where === "query" && args[k] !== undefined) url.searchParams.set(k, String(args[k])); if ( url.origin !== tool.base.origin || !url.pathname.startsWith(tool.base.pathname) ) throw new Error("The request left the API's base URL."); return url; } const fail = (text) => ({ content: [{ type: "text", text }], isError: true }); export async function callTool(tools, name, args = {}, fetcher = fetch) { const tool = tools.find((t) => t.name === name); if (!tool) throw new McpError(ErrorCode.InvalidParams, "Unknown tool."); const v = tool.check(args); if (!v.valid) return fail(`Bad arguments: ${v.errorMessage}`); let res, text; try { res = await fetcher(requestUrl(tool, args), { headers: { accept: "application/json" }, redirect: "manual", signal: AbortSignal.timeout(10_000), }); text = await res.text(); } catch { return fail("The API could not be reached. Try again in a minute."); } if (!res.ok) return fail(`The API answered ${res.status}: ${text.slice(0, 300)}`); if (text.length > MAX_ANSWER) return fail("The API answer is too large to pass on."); try { JSON.parse(text); } catch { return fail("The API did not answer with JSON."); } return { content: [{ type: "text", text }] }; } 

// A remote MCP server on Cloudflare Workers: stateless Streamable HTTP, plain JSON answers, no sessions. // Each POST to /mcp gets a fresh server and transport, so no state is shared between callers. import { Server } from "@modelcontextprotocol/sdk/server/index.js"; import { WebStandardStreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/webStandardStreamableHttp.js"; import { CallToolRequestSchema, ListToolsRequestSchema, } from "@modelcontextprotocol/sdk/types.js"; import { CfWorkerJsonSchemaValidator } from "@modelcontextprotocol/sdk/validation/cfworker"; import schema from "../openapi.json" with { type: "json" }; import { callTool, listing, toolsFromSchema } from "./tools.js"; const tools = toolsFromSchema(schema); export default { async fetch(request, env) { const url = new URL(request.url); if (url.pathname !== "/mcp") return new Response("MCP server. Connect your client to /mcp.\n", { status: url.pathname === "/" ? 200 : 404, }); if (request.method !== "POST") return new Response("POST JSON-RPC to /mcp.\n", { status: 405, headers: { allow: "POST" }, }); const ip = request.headers.get("cf-connecting-ip") ?? ""; if (!(await env.LIMITER.limit({ key: ip })).success) return new Response("Too many requests. Wait a minute.\n", { status: 429, }); const server = new Server( { name: "gpt-action-mcp-demo", version: "1.0.0" }, { capabilities: { tools: {} }, jsonSchemaValidator: new CfWorkerJsonSchemaValidator(), }, ); server.setRequestHandler(Source：https://dev.to/brasspage/rebuild-a-custom-gpt-action-as-a-remote-mcp-server-1d2g
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