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An MCP server is the server-side program that makes tools, data, or reusable prompts available to an AI application through the Model Context Protocol (MCP). The AI host connects an MCP client to that server, discovers the capabilities it declares, and then asks it to perform an allowed action or return context. The server is not the language model and does not have to be a complete AI application.
This guide explains the three MCP primitives, transport choices, protocol-version issues, and a TypeScript example you can adapt. It also shows how a screenshot service such as ScreenshotNeo can be exposed to an agent as an MCP tool.
How an MCP server fits into an AI application
MCP is an open standard that connects AI applications to the systems where your data and tools live, according to the official TypeScript SDK documentation. A typical connection has four parts:
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problems- Host: the AI application, such as an editor or chat product, that the user operates.
- Client: the host-side MCP component that opens a protocol connection and presents available capabilities to the model.
- Server: your program. It declares tools, resources and prompts, validates requests, performs work and returns protocol messages.
- Backend: the APIs, files, databases or services that the server is authorized to access.
The model proposes a tool call; the client sends it to the server; the server executes its handler and returns text or structured data. A server can therefore be a small local process or a network service. It does not “run the model.”
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The three server primitives
The MCP server overview separates capabilities by purpose:
| Primitive | What it does | Typical example | Who normally invokes it |
|---|---|---|---|
| Tools | Request an action, calculation or retrieval. The handler runs on the server. | Query an issue tracker, resize an image, or take a screenshot. | The model, through the client, after the user allows it. |
| Resources | Expose contextual data managed by the application. | A document, database record or generated report identified by a URI. | The host or client, often to provide context. |
| Prompts | Reusable message templates and workflows. | A “summarize this incident” template with arguments. | Usually the user selects one; it is not an autonomous action. |
Keep the boundary intentional. A read-only resource is safer than a tool that can modify the same data. Tool descriptions and input schemas are part of the interface the model sees, so state side effects, required permissions and limits there.
Transport: local process or remote service?
stdio for local integrations
With stdio, the host launches your server as a child process and exchanges protocol messages over standard input and output. It is a practical choice for a developer tool running on the same machine: no public port is required, and the host controls process lifetime. Log diagnostics to stderr, not stdout, because stdout carries protocol traffic.
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Streamable HTTP for remote connections
Streamable HTTP is designed for a server reached over a network. You must handle deployment, authentication, authorization, TLS and concurrent requests. The host you choose must support the transport and its authentication method; there is no universal host compatibility matrix.
SSE and older examples
The v1 TypeScript SDK guide describes HTTP plus Server-Sent Events (SSE) as a backward-compatibility path. Do not copy an SSE example into a v2 deployment without checking the host and migration documentation.
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Protocol and SDK versions matter
The stable TypeScript SDK v2 documentation states that its v2 line implements the 2026-07-28 MCP specification. That specification announcement describes several changes, including:
- Retirement of the
initialize/initializedexchange. - Retirement of the
Mcp-Session-Idheader. - An optional
server/discoverRPC. - Self-contained requests.
ttlMsandcacheScopemetadata on list and resource-read responses.
These are version-specific statements, not properties of every MCP server. Older clients and the v1 SDK examples use different assumptions. Keep the SDK package, server, client and protocol revision aligned, and read the migration guide before combining snippets.
Working TypeScript example (SDK v2 baseline)
The following one-file pattern follows the v2 documentation’s McpServer and tool-registration model. It demonstrates a safe, deterministic tool; replace the handler with your own operation. Confirm current package names and transport helpers in the v2 guide before production use, because SDK APIs can change.
1. Create the project
mkdir mcp-demo
cd mcp-demo
npm init -y
npm install @modelcontextprotocol/sdk zod
npm install -D typescript tsx
Set your project to run TypeScript with npx tsx server.ts. A host that supports local MCP servers should be configured to launch that command using stdio.
2. Register a tool
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import { z } from "zod";
const server = new McpServer({
name: "text-tools",
version: "1.0.0"
});
server.registerTool(
"word_count",
{
title: "Count words",
description: "Count non-empty whitespace-separated words in supplied text.",
inputSchema: {
text: z.string().min(1).max(100000)
}
},
async ({ text }) => {
const words = text.trim() ? text.trim().split(/s+/).length : 0;
return {
content: [{ type: "text", text: JSON.stringify({ words }) }],
structuredContent: { words }
};
}
);
const transport = new StdioServerTransport();
await server.connect(transport);
The important pieces are the name, human-readable description, schema and callback. When a client lists tools, it receives that declaration. A conforming call supplies a text value; the SDK validates it before your handler runs. The callback returns normal content plus structured output for clients that understand it.
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3. Connect it to a host
In your host’s MCP configuration, select a local/stdio server and set the command to npx with arguments tsx /absolute/path/to/server.ts (the exact UI labels vary by host). Restart or reload the host, inspect the discovered tool list, and ask the model to count a short sample. If discovery succeeds but calls fail, run the command directly and check stderr.
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End-to-end v1 quickstart option
If you need a documented runnable client/server pair rather than adapting the v2 pattern, use the v1 quickstart in the official v1 SDK guide. It instructs you to install dependencies, start simpleStreamableHttp.ts, then run its matching interactive client in another terminal. That sample covers tools, resources and prompts. Treat it as a v1 example: use its package names and protocol behavior together, and do not mix them with v2 snippets without checking migration notes.
Designing a production MCP server
Validate every argument
Use a schema with bounds, enumerations and required fields. Reject unknown or dangerous values before touching a backend. Never let a model-supplied path, URL or SQL fragment become an unrestricted filesystem or database operation.
Make permissions explicit
Give each tool the smallest credential scope possible. Separate read and write tools, require confirmation for destructive actions, redact secrets from returned content and log request IDs rather than tokens.
Return useful, bounded results
Prefer structured fields for values a client may need to reuse. Truncate huge documents, paginate lists and include actionable error text. A tool should report whether an operation was completed, not merely return an empty success message.
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Plan for latency and retries
Set backend timeouts, cancel work when the client disconnects and make mutating operations idempotent where possible. Remote Streamable HTTP deployments need TLS, authentication, rate limits and observability. For stdio, ensure only protocol messages use stdout and that the host can restart a crashed process.
Troubleshooting
The host shows no tools
- Confirm the executable path, working directory and environment variables.
- Run the exact launch command manually.
- Check that diagnostics go to stderr, not stdout.
- Verify the host supports your selected transport and protocol revision.
Initialization or session errors
You may be mixing a client that expects an older revision with a server targeting the 2026-07-28 specification. Align versions or use the matching v1 example. In particular, do not add Mcp-Session-Id or an initialize exchange to a flow that explicitly targets the newer revision.
Tool calls fail schema validation
Inspect the advertised schema and the actual JSON arguments. Enforce string lengths, numeric ranges and required properties at the server boundary. Return a clear error and avoid performing partial writes.
Remote calls time out
Check TLS and proxy buffering, server logs, backend deadlines and host support for Streamable HTTP. Test a trivial health-oriented tool first, then add the slow backend operation.
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- Use one when an AI host needs a consistent, discoverable interface to your private systems.
- Choose tools for controlled actions, resources for application-managed context and prompts for user-selected workflows.
- Choose stdio for a locally spawned integration; choose Streamable HTTP for a network service.
- Pin compatible SDK and protocol versions, especially when moving between v1 examples and the 2026-07-28/v2 baseline.
Frequently Asked Questions
Is an MCP server an AI model?
No. It is server-side software that exposes declared capabilities to an MCP client. The host supplies the AI model and user interface.
Can one server expose tools and resources together?
Yes. A server may register multiple primitive types, provided each has a clear purpose and suitable authorization.
Should I use stdio or Streamable HTTP?
Use stdio when the host launches a local process. Use Streamable HTTP when clients connect over a network, subject to the host’s current transport and authentication support.
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