How to add a Companies House connector to ChatGPT

Before you start

You need a free UK Company Data key and a ChatGPT plan that allows custom MCP servers. Steps checked against OpenAI's help centre on 19 October 2026. OpenAI has renamed this feature several times (connectors, then apps, now plugins) and removed the developer mode switch on 1 October 2026, so older guides that tell you to enable developer mode are out of date.

1. Get a key from the button on the developer docs page, or with curl -X POST https://data.globalmindsetmarketing.co.uk/v1/keys/free -H "Content-Type: application/json" -d '{}'. It starts with ukcd_ and is shown once.
2. Check your plan:

Custom MCP servers work in ChatGPT on the web, not in the mobile apps.

Add the server step by step

1. Open Plugins in ChatGPT (chatgpt.com/plugins).
2. Click +, then Add custom MCP server. Some accounts still show Create MCP App instead.
3. Enter a name, for example UK Company Data, and an optional description such as "UK company register and filed accounts".
4. Under Connection, enter the server URL with your key:

https://data.globalmindsetmarketing.co.uk/mcp?key=YOUR_KEY

1. For authentication choose No authentication. ChatGPT offers OAuth or no authentication and has no field for an API key header, which is why the key goes in the URL.
2. Confirm the warning with I understand and want to continue, then click Create as a plugin.
3. Review the tools ChatGPT discovered and install the plugin from your personal plugins.

On Business, Enterprise and Edu, an admin can then publish it to the workspace. Everyone using a published plugin shares the key in its URL, and therefore its monthly quota.

Use it in a chat

Type @ in the message box, select UK Company Data and ask your question. The selection applies to that message, so mention the plugin again in follow-ups that need fresh data.

Example prompts:

• "@UK Company Data find TESCO PLC and give me its company number, status and next accounts due date."
• "@UK Company Data show the last three balance sheets for company 10623966 and explain the change in net assets."
• "@UK Company Data compare 10623966 with 00445790 on cash and net assets."
• "@UK Company Data which companies published new accounts in the last 7 days?"

ChatGPT asks you to confirm actions that change data by default. In this server the only tools that change anything are the watchlist tools, watch_add and watch_remove; everything else reads. On Pro, where OpenAI limits custom servers to read and fetch, expect the watchlist tools to be unavailable.

What the tools return

Figures are what each company filed in machine-readable iXBRL accounts since 2008. About 69.2% of active companies have recent figures; companies that file only PDF accounts, including many large groups, return register details only. Overdue filers are easy to spot: 93,785 active companies were past their accounts due date on 1 October 2026 (overdue accounts statistics).

Call the same server from the OpenAI API

If you are building your own agent, the Responses API can call a remote MCP server directly and send the key as a header:

{
  "model": "YOUR_MODEL",
  "tools": [{
    "type": "mcp",
    "server_label": "uk_company_data",
    "server_description": "UK Companies House register and filed accounts",
    "server_url": "https://data.globalmindsetmarketing.co.uk/mcp",
    "headers": { "Authorization": "Bearer YOUR_KEY" },
    "require_approval": "never",
    "allowed_tools": ["search_by_name", "lookup_company", "compare_companies"]
  }],
  "input": "What were the net assets of company 10623966 at its last two balance sheet dates?"
}

allowed_tools keeps the model to the read-only tools you need, and require_approval can be "always" if you want to approve each call. The API does not store the key, so send it with every request, from your server rather than from browser code. For plain REST calls without a model, see the API quickstart.

Troubleshooting

Our server lists its tools without a key so that directories can index it, which means the plugin can be created successfully even with a missing key. The first real question is the test.

Treat the plugin URL as a secret: anyone with it can spend your quota. If it leaks, create a new key, recreate the plugin and email info@globalmindsetmarketing.co.uk to revoke the old key. Setup for other clients is in the Claude guide and the Cursor guide.

Sources

• OpenAI Help Center: connecting custom MCP servers in ChatGPT
• OpenAI API docs: connectors and MCP servers
• OpenAI API docs: building a custom MCP server

Sources: Companies House register and accounts data (Open Government Licence v3.0), legislation.gov.uk and GOV.UK guidance where linked. This page is general information, not legal, tax or credit advice. Spotted an error? Email info@globalmindsetmarketing.co.uk.