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Setting up the Fergus MCP Server with ChatGPT

How to connect the Fergus MCP Server to ChatGPT so you can manage your Fergus data directly in a conversation.

Written by Alyssa Smith

Overview

ChatGPT supports custom MCP servers through its Developer Mode, which is currently in beta. Setup involves three steps: enabling Developer Mode, adding the Fergus connector, and activating it in a chat.

Please Note: If you're new to Fergus MCP Server, Claude is the recommended starting point - it's quicker to set up and the connection persists across conversations automatically. See Setting up the Fergus MCP Server with Claude.

Before you start

You'll need:

  • The Fergus MCP server URL: https://mcp.fergus.com/mcp

  • A Fergus account with a user type of Full User and Master Admin permissions

  • A ChatGPT plan that supports Developer Mode: Plus, Pro, Team, or Enterprise.

If you're not sure whether your Fergus account qualifies, see How to access the Fergus MCP Server.

Step 1 - Enable Developer Mode

This is a one-time setup step.

  • Go to chat.openai.com and sign in.

  • Click your profile icon and select Settings.

  • Navigate to Apps & Connectors → Advanced settings.

  • Toggle Developer Mode to ON.

  • Confirm the warning prompt to proceed.

Please Note: The warning advises that Developer Mode allows unverified connectors that could modify or erase data. This is a standard ChatGPT prompt for any custom MCP server - it's expected.

Step 2 - Add the Fergus Connector

  1. Go to Settings → Apps & Connectors

  2. Click Create in the top-right corner.

  3. Fill in the following details:

    • Name: Fergus

    • Description: Fergus job management - manage jobs, quotes, customers, invoices, calendar events, and more.

    • MCP Server URL: https://mcp.fergus.com/mcp

    • Authentication: OAuth

  4. check the box confirming you trust this application, then click Create.

  5. You'll be redirected to the Fergus login screen. Sign in with your Fergus credentials to authorise the connection.

Step 3 - Use Fergus in a chat

Developer Mode needs to be activated in each new conversation.

  1. Open a new chat in ChatGPT.

  2. Click the + button in the lower-left of the message bar.

  3. Click More, then Developer Mode.

  4. Click Add sources and enable the Fergus connector.

  5. Try "What jobs are currently active?" to confirm it's working.

How ChatGPT behaves differently to Claude

  • Developer Mode must be enabled per chat. It doesn't carry over to new conversations automatically.

  • Memory is disabled when Developer Mode is active. This is a ChatGPT safety feature to prevent MCP servers from accessing data from other conversations.

  • Write actions require confirmation. Before creating or updating anything in Fergus, ChatGPT will show you the details and ask you to confirm. This is normal and expected.

Troubleshooting

  • Don't see the "Create button in Apps & Connectors? Developer Mode isn't enabled. Go back to Advanced settings and toggle it on first.

  • OAuth login fails? Confirm you're using valid Fergus credentials and that your account has Full User type and Master Admin permissions.

  • Tools not being invoked? Try phrasing your request more explicitly, e.g. "Use the Fergus tools to show me all active jobs" rather than just asking the question.

  • Connector stuck on "Authorising"? Try removing the connector and re-adding it to start a fresh OAuth flow.

Related Articles
What is the Fergus MCP Server?
Setting up the Fergus MCP Server with Claude
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