What is MCP and why should I care?

The Model Context Protocol (MCP) is an open standard for piping external data and tools into large-language-model hosts. Think of it as USB-C for LLMs—a single plug that lets any compliant client talk to any compliant server and swap in new data sources or models without rewiring everything.

If you to do custom action with our MCP, read the guide here.

CustomGPT.ai now ships a Hosted MCP Server for every agent you create:

Top-ranked RAG built-in. Retrieval layer independently benchmarked #1 for business-document accuracy and minimal hallucination.
Fully managed. We run the container, watch the graphs, patch the OS, and keep TLS fresh.
Zero extra cost for existing plans. Free-trial users can test it too.
Standard endpoint. Works with any client that understands MCP (Claude Desktop, Cursor, Windsurf, n8n, Zapier, etc.).

Read our Hosted MCP launch blog post here

Architecture at a glance

         ┌──────────────────────────┐
 ───────▶│  CustomGPT Hosted MCP    │  SSE / HTTPS
         │  (RAG + auth, scaled)    │──────────┐
         └──────────────────────────┘          │
                   ▲                           ▼
           Private knowledge base      MCP-aware clients
          (PDF, Drive, Notion …)   (Claude, Cursor, Windsurf …)

Internally we expose an SSE (Server-Sent Events) stream at https://mcp.customgpt.ai/projects/<PROJECT_ID>/sse?token=<TOKEN>

The stream carries JSON events defined by the MCP spec. You authenticate with an MCP token (see below).

Prerequisites

Item	Notes
CustomGPT.ai account	Free trial or paid
A project / agent with data ingested	PDFs, Google Drive, Notion, Confluence, Web, etc.
One of the supported clients	Claude Desktop, Cursor IDE, Windsurf, Trae, n8n, Zapier, or your own code
Desktop host only (for now)	MCP currently runs only on desktop hosts; browser-based hosts are in progress.(Model Context Protocol)
Node.js ≥ 18	Required by several clients (Claude, Trae) to spawn the gateway

Enable the Hosted MCP Server

Log in to app.customgpt.ai.
Create / open a project and ingest your data.
Test it in the ASK tab to confirm answers look good.
Click Deploy ➞ MCP Server (Beta).
Hit Generate MCP Token. A toast will confirm creation; the token is embedded in the sample JSON.
Copy the server configuration shown on-screen.

Security note – the token is a bearer secret. Keep it out of repos and rotate it from the same panel if it leaks.

Supported Clients

Get more info here:

Deep Dive into Model Context Protocol (MCP)

Key Concept	What it means in practice
Purpose	Standard I/O contract so any LLM client (IDE, chat, workflow engine) can call any back-end “context provider” the same way.
Transport	Server-Sent Events (SSE) over HTTPS. One long-lived stream; the server pushes JSON blobs that match the official schema.
Message types	`tool_list`, `tool_call`, `content_chunk`, `error`, `done`. Clients send JSON; servers answer in-stream.
Discovery	Each server advertises one or more tools. In our case you’ll always see `send_message` (RAG search) plus, for advanced plans, `upload_file`, `list_sources`, etc.
Security	Bearer-token header or query-string param. No cookies. Tokens are opaque 256-bit secrets that you rotate in the UI.
Why SSE, not WebSocket?	Simpler proxy support, out-of-the-box with `curl`, no extra heartbeat logic, and Anthropic picked it for the reference gateway (`supergateway`).

Where CustomGPT fits?

Client (Cursor / Claude / n8n)
        │  JSON request
        ▼
┌─────────────────────────┐
│ npx supergateway        │  ← optional local sidecar; some clients spawn this
└─────────────────────────┘
        │  SSE
        ▼
┌─────────────────────────┐
│ Hosted MCP Server (CustomGPT.ai)   ← you enable this
│  • Auth + rate-limit    │
│  • RAG retrieval layer  │
│  • Streaming responder  │
└─────────────────────────┘
        │
        ▼
Knowledge base (PDF, Drive, Notion …)

If a client can hit SSE directly (Cursor, Windsurf, Zapier), you just feed it the URL. If it needs a local sidecar (Claude Desktop, Trae), you install Node.js; the sidecar opens the SSE and the client talks Unix pipes.