add-resource

name	add-resource
description	Scaffold a new MCP resource definition. Use when the user asks to add a resource, expose data via URI, or create a readable endpoint.
metadata	{"author":"cyanheads","version":"1.3","audience":"external","type":"reference"}

Context

Resources use the resource() builder from @cyanheads/mcp-ts-core. Each resource lives in src/mcp-server/resources/definitions/ with a .resource.ts suffix and is registered into createApp() in src/index.ts. Some repos later add definitions/index.ts barrels; follow the pattern already used by the project.

Tool coverage. Not all MCP clients expose resources — many are tool-only (Claude Code, Cursor, most chat UIs). Before adding a resource, verify the same data is reachable via the tool surface — either through a dedicated tool, included in another tool's output, or bundled into a broader tool. A resource whose data has no tool path is invisible to a large share of agents.

For the full resource() API, pagination utilities, and Context interface, read node_modules/@cyanheads/mcp-ts-core/CLAUDE.md.

Steps

1. Ask the user for the resource's URI template, purpose, and data shape
2. Design the URI — use {paramName} for path parameters (e.g., myscheme://{itemId}/data)
3. Create the file at src/mcp-server/resources/definitions/{{resource-name}}.resource.ts
4. Register the resource in the project's existing createApp() resource list (directly in src/index.ts for fresh scaffolds, or via a barrel if the repo already has one)
5. Run bun run devcheck to verify
6. Smoke-test with bun run rebuild && bun run start:stdio (or start:http)

Template

/**
 * @fileoverview {{RESOURCE_DESCRIPTION}}
 * @module mcp-server/resources/definitions/{{RESOURCE_NAME}}
 */

import { resource, z } from '@cyanheads/mcp-ts-core';

export const {{RESOURCE_EXPORT}} = resource('{{scheme}}://{{{paramName}}}/data', {
  description: '{{RESOURCE_DESCRIPTION}}',
  mimeType: 'application/json',
  // size: 1024,  // optional: content size in bytes, if known
  params: z.object({
    {{paramName}}: z.string().describe('{{PARAM_DESCRIPTION}}'),
  }),
  // auth: ['resource:{{resource_name}}:read'],

  async handler(params, ctx) {
    ctx.log.debug('Fetching resource', { {{paramName}}: params.{{paramName}} });
    // Pure logic — throw on failure, no try/catch
    return { /* resource data */ };
  },

  list: async (extra) => ({
    resources: [
      {
        uri: '{{scheme}}://all',
        name: '{{RESOURCE_LIST_NAME}}',
        mimeType: 'application/json',
      },
    ],
  }),
});

With pagination

For resources that return large result sets, include cursor in the URI template params and use opaque cursor pagination in the handler. The cursor arrives as a validated URI param. paginateArray requires a RequestContext for logging — create one from requestContextService:

import { extractCursor, paginateArray, requestContextService } from '@cyanheads/mcp-ts-core/utils';

// URI template: '{{scheme}}://{{{paramName}}}/items'
params: z.object({
  {{paramName}}: z.string().describe('{{PARAM_DESCRIPTION}}'),
  cursor: z.string().optional().describe('Opaque pagination cursor'),
}),

async handler(params, ctx) {
  const allItems = await fetchAllItems(params.{{paramName}});
  const cursor = extractCursor({ cursor: params.cursor });
  const reqCtx = requestContextService.createRequestContext({
    operation: 'list-{{paramName}}',
    parentContext: { requestId: ctx.requestId, traceId: ctx.traceId },
  });
  const page = paginateArray(allItems, cursor, 20, 100, reqCtx);
  return {
    items: page.items,
    nextCursor: page.nextCursor,
  };
},

Registration

// src/index.ts (fresh scaffold default)
import { createApp } from '@cyanheads/mcp-ts-core';
import { {{RESOURCE_EXPORT}} } from './mcp-server/resources/definitions/{{resource-name}}.resource.js';

await createApp({
  tools: [/* existing tools */],
  resources: [{{RESOURCE_EXPORT}}],
  prompts: [/* existing prompts */],
});

If the repo already uses src/mcp-server/resources/definitions/index.ts, update that barrel instead of changing the registration style.

Optional: declarative errors[] contract

Resources can opt into the same typed error contract as tools — bound to a typed ctx.fail(reason, …) keyed by the declared reason union:

import { JsonRpcErrorCode } from '@cyanheads/mcp-ts-core/errors';

export const articleResource = resource('article://{pmid}', {
  description: 'Read an article by PMID.',
  errors: [
    { reason: 'no_pmid_match', code: JsonRpcErrorCode.NotFound,
      when: 'PMID not found in the index.' },
    { reason: 'withdrawn', code: JsonRpcErrorCode.NotFound,
      when: 'Article was withdrawn upstream.' },
    { reason: 'upstream_throttled', code: JsonRpcErrorCode.RateLimited,
      when: 'Upstream PubMed quota hit.', retryable: true },
  ],
  params: z.object({ pmid: z.string().describe('PubMed ID') }),
  async handler(params, ctx) {
    const article = await fetchOne(params.pmid);
    if (!article) throw ctx.fail('no_pmid_match', `PMID ${params.pmid} not indexed`);
    if (article.withdrawn) throw ctx.fail('withdrawn');
    return article;
  },
});

Without errors[], the handler receives plain Context (no fail method) and throws via error factories (notFound, serviceUnavailable, …) directly. The contract is opt-in. See skills/api-errors/SKILL.md for the full pattern, baseline codes, and conformance rules.

Other resource() options

Beyond description, params, handler, and list, the builder also supports:

Field	Purpose
output	Optional Zod schema for runtime validation of the handler return value (parity with tool()'s output).
format	Optional formatter mapping the handler's return to the ReadResourceResult.contents[] shape. Default: string passthrough; objects serialized to JSON. Override when you need to attach permissions, custom encodings, or split into multiple content items.
annotations	Resource annotations (e.g., audience, priority) — see ResourceAnnotations.
title	Human-readable display title (defaults to name).

Checklist

• File created at src/mcp-server/resources/definitions/{{resource-name}}.resource.ts
• URI template uses {paramName} syntax for path parameters
• All Zod params fields have .describe() annotations
• JSDoc @fileoverview and @module header present
• handler(params, ctx) is pure — throws on failure, no try/catch
• Data is reachable via the tool surface (dedicated tool, another tool's output, or not needed for tool-only agents)
• list() function provided if the resource is discoverable
• Pagination used for large result sets (extractCursor/paginateArray)
• Registered in the project's existing createApp() resource list (directly or via barrel)
• bun run devcheck passes
• Smoke-tested with bun run rebuild && bun run start:stdio (or start:http)