What is an MCP server, and when do you need a custom one?
An MCP server is a program that exposes specific actions and data from one of your systems to AI applications, using the open Model Context Protocol. You need a custom one when the system is your own, when no trustworthy server exists for it, or when an existing server exposes more than you are willing to allow.
The short definition
The Model Context Protocol is an open standard for connecting AI applications to external systems. It defines three roles. The host is the AI application a person uses, such as a chat assistant or a coding tool. The host runs one client per connection. Each client talks to one server, and the server is the part that knows how to reach your CRM, database or internal API.
Messages follow JSON-RPC 2.0. Because the protocol is shared, a server written once can be used from any host that supports MCP, which is the main reason to build one instead of a bespoke plugin for each assistant.
What a server exposes
A server offers three kinds of things, which the specification calls primitives.
| Primitive | What it is | Example |
|---|---|---|
| Tools | Functions the AI model can call to take an action or fetch something | create_invoice, search_orders |
| Resources | Data the application can read to give the model context | A product catalogue, a policy document |
| Prompts | Reusable templates a user can pick to start a task | A weekly report template |
Local and remote servers
The specification defines two standard transports. With stdio, the host launches the server as a local process on the user's machine and exchanges messages over its standard input and output. This suits developer tools and anything that works on local files. With Streamable HTTP, the server runs at a web address and each message is an HTTP request, so many users can share one deployment.
The choice matters for security. A local server usually reads credentials from its environment. A remote server should follow the protocol's authorisation rules, which are based on OAuth 2.1: users sign in, the server receives a token issued for that server specifically, and it must not forward that token to the systems behind it.
When an existing server is enough
Many software vendors now publish an official MCP server for their product. If the vendor of your ticketing system, code host or database offers one, start there. It is maintained by the people who own the API, and a custom rebuild would only add cost.
Check two things before you connect it: who publishes it, and what it can do once connected. The protocol's own security guidance treats tool descriptions from untrusted servers as untrusted input, so an unknown community server deserves the same care as an unknown browser extension.
When a custom server is justified
If one of these applies to you, a custom server is worth building. BlackCard builds custom MCP servers for exactly these cases, and a free 15-minute call is enough to work out which tools your first version needs.
- The system is yours. Internal applications, in-house databases and private APIs have no public server, so someone has to write one.
- You need less than the official server offers. A custom server can expose read-only access, or three safe actions, where the vendor's version exposes everything.
- The task spans several systems. One tool that checks stock, creates the order and notifies the warehouse is easier for a model to use than three separate servers.
- You sell software. If customers want to use your product from their AI assistant, a remote server with proper sign-in becomes part of the product.
- Rules apply to the data. Audit logging, per-user permissions and data residency are easier to enforce in a server you control.
What changed in the 2026 specification
The current revision is dated 2026-07-28, and its changelog matters to anyone commissioning a server now. The protocol became stateless: the old initialisation handshake and server-side sessions were removed, and each request carries the protocol version and client capabilities. Several older features were deprecated, including the original HTTP with SSE transport, with a transition period before removal.
The practical point for a buyer is simple. Ask which revision a developer builds against, and whether a server built last year needs migration. A developer who cannot answer has probably not read the specification recently. BlackCard builds against the current revision.
Common questions
Is an MCP server the same as an API?
No. An API is written for programmers and their code. An MCP server sits in front of one or more APIs and describes a small set of actions in a form an AI model can choose from and call. It also handles the protocol's rules for discovery and authorisation. Most custom servers are a thin, carefully designed layer over APIs you already have.
Do I need a server for every system?
Not necessarily. Start with the one or two systems where staff lose the most time moving information by hand. A server per system is common, but a single server that covers one workflow across several systems can be simpler to secure and easier for the model to use.