Resources & Prompts

MCP has three primitives: Tools, Resources, and Prompts. You already know Tools. This lesson covers the other two — data the AI can read (Resources) and interaction templates the user can select (Prompts) — with working code for both.

The Three MCP Primitives

Think of MCP primitives as three different directions of information flow:

⚙️

Tools

Actions the AI can take

📄

Resources

Data the AI can read

📝

Prompts

Templates the user selects

Side-by-Side Comparison

Aspect	Tools	Resources	Prompts
Direction	AI → World	World → AI	User → AI
Triggered by	AI model decides	Client/user requests	User selects
Purpose	Execute actions	Provide context	Structure interactions
Example	Write a file	Read config data	Code review template
State change	Yes (side effects)	No (read-only)	No (templates only)
Discovery	tools/list	resources/list	prompts/list
Who initiates	The AI autonomously	The app or user	The user from a menu

Resources: Giving AI Read-Only Data

A Resource is read-only data your server exposes to the AI through a URI. Unlike tools, resources do not execute actions or cause side effects. They are for providing context — configuration files, documentation, database schemas, API specs.

When to use a Resource instead of a Tool: If the data is read-only and the AI does not need to decide when to fetch it — use a Resource. If the AI needs to take an action or make a decision about when to fetch data — use a Tool. Example: A project README is a Resource. A database query is a Tool.

Resource ExampleTypeScript
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { readFileSync } from "fs";

const server = new McpServer({ name: "project-context", version: "1.0.0" });

// ── Static Resource: project README ──────────────────────
// URI scheme is up to you — file://, docs://, project:// all work.
// The URI is how the client requests this specific resource.
server.resource(
  "project-readme",                        // unique name
  "file:///project/README.md",              // URI
  async () => ({
    contents: [{
      uri: "file:///project/README.md",
      text: readFileSync("./README.md", "utf-8"),
      mimeType: "text/markdown",
    }],
  })
);

// ── Dynamic Resource: database schema ────────────────────
// Resources can be dynamic — the handler runs each time
// the resource is requested, returning fresh data.
server.resource(
  "db-schema",
  "db://schema/tables",
  async () => {
    const tables = await db.query("SELECT table_name FROM information_schema.tables WHERE table_schema = 'public'");
    return {
      contents: [{
        uri: "db://schema/tables",
        text: tables.rows.map(t => t.table_name).join("\n"),
        mimeType: "text/plain",
      }],
    };
  }
);

name A human-readable identifier. Shows up in resource listings.

URI The address the client uses to request this resource. You define the scheme (file://, db://, etc.).

handler Async function returning { contents: [{ uri, text, mimeType }] }. Runs when the resource is read.

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