GPT MCP App – User & Loan Info Tools

Simple GPT app backend that exposes two MCP tools to GPT via a server running on AWS:

get_user_info – Retrieve user profile by user_id
retrieve_loan_info – Retrieve loan details by loan_id

The backend is an MCP server on AWS Lambda behind API Gateway (HTTP API). GPT (OpenAI Responses API) talks to it using the Streamable HTTP MCP transport.

Project layout

gpt_mcp/
├── src/
│   ├── handler.py          # Lambda entrypoint
│   ├── requirements.txt    # Lambda deps (for sam build)
│   └── mcp_server/
│       ├── __init__.py
│       └── server.py       # MCP server + get_user_info, retrieve_loan_info
├── template.yaml           # AWS SAM template (Lambda + HTTP API)
├── requirements.txt        # Local dev deps
└── README.md

Run locally (optional)

Create a venv and install deps:

python -m venv .venv
.venv\Scripts\activate   # Windows
# source .venv/bin/activate  # macOS/Linux
pip install -r requirements.txt

The tools use in-memory mock data in server.py. For local testing without Lambda, you can use the MCP Python SDK with stdio or run the Lambda handler via a local Lambda runtime (e.g. SAM CLI).

Deploy to AWS

Install AWS SAM CLI Install the AWS SAM CLI.
Build and deploy
```
cd c:\Users\Andrew\Documents\gpt_mcp
sam build
sam deploy --guided
```
Use the default stack name (or choose one), set Stage (e.g. dev), and accept defaults for the rest unless you need a different region or bucket.
Get the MCP server URL

After deploy, SAM prints the stack outputs. Use the McpApiUrl value, e.g.:
```
https://<api-id>.execute-api.<region>.amazonaws.com/mcp
```
If your HTTP API uses the $default stage, the URL might be:
```
https://<api-id>.execute-api.<region>.amazonaws.com/$default/mcp
```
Use the URL that works when you call it from the OpenAI API (see below).

Connect GPT to your MCP server

Use the OpenAI Responses API with the mcp tool type and your deployed URL as server_url. GPT will discover and call get_user_info and retrieve_loan_info from your backend.

Example (Python)

from openai import OpenAI

client = OpenAI()

resp = client.responses.create(
    model="gpt-4o",  # or another MCP-capable model
    tools=[
        {
            "type": "mcp",
            "server_label": "gpt-app",
            "server_description": "User and loan info for the GPT app.",
            "server_url": "https://<your-api-id>.execute-api.<region>.amazonaws.com/mcp",
            "require_approval": "never",
        }
    ],
    input="What is the balance for loan_001?",
)
print(resp.output_text)

Example (curl)

curl https://api.openai.com/v1/responses \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $OPENAI_API_KEY" \
  -d '{
    "model": "gpt-4o",
    "tools": [{
      "type": "mcp",
      "server_label": "gpt-app",
      "server_description": "User and loan info.",
      "server_url": "https://<your-api-id>.execute-api.<region>.amazonaws.com/mcp",
      "require_approval": "never"
    }],
    "input": "Get user info for user_001"
  }'

Replace https://<your-api-id>.execute-api.<region>.amazonaws.com/mcp with your McpApiUrl from the SAM deploy output.

ChatGPT / GPT in the OpenAI UI Remote MCP is used via the Responses API (or products built on it). In the ChatGPT UI you typically use Actions/connectors; for a custom backend like this you’d integrate via your own app that calls the Responses API with the mcp tool and this server_url.
Require approval Set "require_approval": "always" if you want to approve each tool call; use "never" for automatic calls (only if you trust the MCP server).
Auth If you add auth (e.g. API key or OAuth), pass it in the authorization field of the MCP tool config and protect your API Gateway (e.g. Lambda authorizer or API key).

Tools

Tool	Description	Parameters
`get_user_info`	Get user profile by user ID	`user_id`
`retrieve_loan_info`	Get loan details (balance, terms…)	`loan_id`

Mock data in src/mcp_server/server.py includes user_001, user_002, loan_001, and loan_002. Replace _get_user_from_store and _get_loan_from_store with DynamoDB, RDS, or your internal APIs for production.

Security and production

Auth: Add API Gateway authorization (e.g. IAM, Lambda authorizer, or API key) and/or validate tokens inside the Lambda.
Data: Do not rely on in-memory data in production; use a real store and restrict access by identity.
HTTPS: API Gateway provides HTTPS; keep the MCP server URL on HTTPS when configuring GPT.

GPT MCP App – User & Loan Info Tools