mcp-resource-patterns

MCP Resource Patterns

Resources are read-only, URI-addressable data. Get them right and your server feels like a filesystem the agent can browse.

When to Use

• Deciding whether a read operation should be a tool or a resource
• Exposing hierarchical data (files, tickets, logs) to an agent
• Implementing live-updating data via subscriptions
• Designing URI schemes for a new MCP server

Tool vs Resource vs Prompt

Aspect	Tool	Resource	Prompt
Invocation	Agent calls it	Agent reads it	User selects it
Side effects	Allowed	No	No
Addressing	Name + args	URI	Name
Result	Agent-chosen	User/agent-attached	Template expansion
Example	create_issue	github://issues/123	/summarize-pr

Rule of thumb: if the agent needs to decide whether to read it, it's a tool. If the user or agent needs to attach specific known data to context, it's a resource.

Static Resources

server.resource(
  "changelog",
  "file://changelog.md",
  async () => ({
    contents: [{
      uri: "file://changelog.md",
      mimeType: "text/markdown",
      text: await fs.readFile("CHANGELOG.md", "utf-8"),
    }],
  }),
);

Templated Resources

Use URI templates (RFC 6570) for addressable collections:

server.resource(
  "issue",
  "github://repos/{owner}/{repo}/issues/{number}",
  async (uri, { owner, repo, number }) => {
    const issue = await gh.issues.get({ owner, repo, issue_number: +number });
    return {
      contents: [{
        uri: uri.href,
        mimeType: "application/json",
        text: JSON.stringify(issue.data),
      }],
    };
  },
);

The client auto-discovers the template and can fetch any matching URI.

Listing Resources

server.setRequestHandler(ListResourcesRequestSchema, async () => ({
  resources: [
    { uri: "github://repos/acme/api/issues", name: "Open issues", mimeType: "application/json" },
    { uri: "github://repos/acme/api/prs", name: "Open PRs", mimeType: "application/json" },
  ],
}));