add-resource— agent skill

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. The standard registration pattern uses a definitions/index.ts barrel that collects all resources into an allResourceDefinitions array for createApp(). Fresh scaffolds start with direct imports in src/index.ts — the barrel is introduced as definitions grow. Match the pattern already used by the project you're editing.

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.

Steps

1. Gather the resource's URI template, purpose, and data shape from the user's request — ask only if genuinely absent
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, add the export to that barrel instead:

export { {{RESOURCE_EXPORT}} } from './{{resource-name}}.resource.js';

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.',
      recovery: 'Use pubmed_search_articles to discover valid PMIDs first.' },
    { reason: 'withdrawn', code: JsonRpcErrorCode.NotFound,
      when: 'Article was withdrawn upstream.',
      recovery: 'Check PubMed directly for retraction or withdrawal notices.' },
    { reason: 'upstream_throttled', code: JsonRpcErrorCode.RateLimited,
      when: 'Upstream PubMed quota hit.', retryable: true,
      recovery: 'Wait a few seconds and retry the request.' },
  ],
  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.

URI template variable completion

Add a complete map to enable autocompletion for URI template variables. The SDK auto-installs completion/complete handling and advertises the completions capability when a registered resource template has completion callbacks — no other changes needed.

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

const ITEM_IDS = ['item-001', 'item-002', 'item-abc'];

export const itemResource = resource('items://{itemId}', {
  description: 'Retrieve an item by ID.',
  params: z.object({ itemId: z.string().describe('Item identifier') }),
  handler: (params) => ({ id: params.itemId }),
  list: () => ({ resources: ITEM_IDS.map((id) => ({ uri: `items://${id}`, name: id })) }),
  // Per-variable completion callbacks — keys must match URI template variable names.
  complete: {
    itemId: async (partial) => ITEM_IDS.filter((id) => id.startsWith(partial)),
  },
});

Only applies to templated resources (URI templates with {variable} syntax). Static URIs don't support completion.

Other resource() options

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

Checklist

• [ ] File created at src/mcp-server/resources/definitions/{{resource-name}}.resource.ts
• [ ] Resource name passed to resource() uses a valid URI template with {paramName} syntax
• [ ] All Zod params fields have .describe() annotations
• [ ] output schema added if the handler returns structured data that benefits from runtime validation
• [ ] JSDoc @fileoverview and @module header present
• [ ] handler(params, ctx) is pure — throws on failure, no try/catch
• [ ] If errors[] contract declared: every entry has a recovery field (≥5 words, lint-enforced)
• [ ] Data is reachable via the tool surface — confirm by checking src/mcp-server/tools/definitions/ for a tool that exposes this data, or document why this resource is resources-only
• [ ] list() function provided if the resource is discoverable
• [ ] Pagination used for large result sets (extractCursor/paginateArray) — applies to both handler data and list() catalogs with many entries
• [ ] 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)