API and developers

Connect SI agents with the hosted MCP server

Our hosted Model Context Protocol (MCP) server lets SI agents and assistants such as Claude Code and Cursor look up UK companies with your API key. Any API plan can use it.

The CompanyStack MCP server gives SI agents and assistants tools to search UK companies, read their records and manage your lists. It works with clients that support the Model Context Protocol over Streamable HTTP with a custom header.

What you need

An API key. Every account has one on the API Free plan. Find it on the API keys page. See API keys and authentication.
The server address: https://api.companystack.co.uk/mcp

Nothing needs installing.

Set it up

Claude Code:

claude mcp add --transport http companystack https://api.companystack.co.uk/mcp --header "X-Api-Key: cs_your_key"

Cursor: add this to ~/.cursor/mcp.json (for every project) or .cursor/mcp.json (for one project), and set COMPANYSTACK_API_KEY in your environment:

{
  "mcpServers": {
    "companystack": {
      "url": "https://api.companystack.co.uk/mcp",
      "headers": { "X-Api-Key": "${env:COMPANYSTACK_API_KEY}" }
    }
  }
}

Other clients: point them at the address above and send your key in an X-Api-Key header, or as Authorization: Bearer cs_your_key.

Clients that can only connect with OAuth, such as some chat app connectors, cannot use the server yet.

The tools

Tool	What it does	Lowest API plan
`search_companies`	Find companies and their numbers	Free
`get_company`	Register profile and filing due dates	Free
`get_lists`	Your lists, with grades and alerts	Free
`get_list_companies`	The companies on one of your lists	Free
`add_companies_to_list`	Add companies to a list to monitor them	Free
`get_officers`	Directors and secretaries	Starter
`get_persons_with_significant_control`	Who owns or controls the company	Starter
`get_filing_history`	Companies House filings	Starter
`get_financial_history`	Figures from filed accounts	Starter
`get_health_score`	The health score and its reasons	Starter
`run_kyb_check`	A KYB check, stored on your account	Starter
`list_kyb_checks`	KYB checks already run	Starter
`get_changes`	The register change feed	Starter

With a Free key, Starter tools reply with an upgrade link instead of data.

Usage and limits

Each tool call is an ordinary API request. It counts against your plan's per-minute and per-day limits, like any other call. See API rate limits and error codes.
The connection messages themselves are free, up to 120 a minute per key.
Use an API key, not your website sign-in.

Common problems

401: the key is missing, mistyped, or has been revoked. Check the header in your client's settings.
Upgrade messages: the tool needs Starter or above.
429: you have reached a rate limit. Wait and try again, or upgrade.

Keep your key safe

Your client stores your API key. Treat it like a password. Use an environment variable where your client allows it, and do not commit configuration files containing keys to shared repositories.

Last updated 2 October 2026

Still need help? Contact us, or if you have an account, open a support request.