We put an MCP endpoint in 49 business apps. Here is what a read-only key can and cannot do to an invoice register.

We build small self-hosted business tools, and since our 3.0 release every one of them except our AI client answers the Model Context Protocol at POST /mcp . Forty-nine of them. That was a large enough change, applied uniformly enough, that the interesting engineering question stopped being "how do we add MCP" and became "what should a language model be allowed to do to a live invoice register." This post is about the second question, because it is the one that actually matters and the one most MCP integrations answer by accident. The boring part first: the handshake There is nothing vendor-specific in it. Three facts: The address. https://your-install/mcp - your server, your domain. The key. An Authorization: Bearer apk_... header. The transport. MCP over streamable HTTP, stateless. One request in, one response out. That is the whole contract. In Claude it is one CLI line: claude mcp add --transport http invora https://your-install/mcp \ --header "Authorization: Bearer apk_xxxx" In the OpenAI Responses API it is one entry in the tools array: { "type" : "mcp" , "server_label" : "invora" , "server_url" : "https://your-install/mcp" , "authorization" : "apk_xxxx" , "require_approval" : "never" } Clients that keep servers in a config file take the same three fields under a different set of key names. n8n's MCP Client node takes the URL and the same Authorization header. And if you would rather not use a client at all, it is plain JSON-RPC 2.0 over one POST: curl -X POST https://your-install/mcp \ -H "Authorization: Bearer apk_xxxx" -H "Content-Type: application/json" \ -d '{"jsonrpc":"2.0","id":1,"method":"tools/list"}' We implemented the protocol rather than an integration with a particular vendor, which means clients that do not exist yet will work too. That is the main argument for MCP over building N bespoke connectors, and it is a good one, but it is not what this post is about. The part that took the actual thinking Once your invoice register speaks a protocol tha