openfoodfacts-mcp-server: Instructions file for Codex

Developer Protocol

Server: openfoodfacts-mcp-server Version: 0.4.0 Framework: @cyanheads/mcp-ts-core ^0.13.18 Engines: Bun ≥1.4.0, Node ≥24.0.0 MCP SDK: @modelcontextprotocol/server ^2.4.0 Zod: ^4.6.7

Read the framework docs first: node_modules/@cyanheads/mcp-ts-core/CLAUDE.md contains the full API reference — builders, Context, error codes, exports, patterns. This file covers server-specific conventions only.

Core Rules

• Logic throws, framework catches. Tool/resource handlers are pure — throw on failure, no try/catch. Plain Error is fine; the framework catches, classifies, and formats. Use error factories (notFound(), validationError(), etc.) when the error code matters.
• Use ctx.log for request-scoped logging. No console calls.
• Use ctx.state for tenant-scoped storage. Never access persistence directly.
• Need input the caller didn't supply? return ctx.requestInput(...) and read ctx.inputs when the handler is re-entered. Never await for user input mid-handler.
• Secrets in env vars only — never hardcoded.
• Cut noise. Add only what earns its place: no speculative generality, no guards for states the framework already prevents (Zod-validated params, classified errors), no abstraction until a third caller proves it, no option nothing sets.
• Close the loop on issues. When implementing work tracked by a GitHub issue, comment on the issue with what landed and close it. Do both — a comment without a close leaves stale issues open; a close without a comment leaves no record of what shipped. The comment is for future readers — state the concrete changes, not the conversation that produced them.

Patterns

Tool

import { tool, z } from '@cyanheads/mcp-ts-core';
import { JsonRpcErrorCode } from '@cyanheads/mcp-ts-core/errors';
import { getOpenFoodFactsService } from '@/services/openfoodfacts/openfoodfacts-service.js';
import { BARCODE_PATTERN } from '@/services/openfoodfacts/types.js';

export const offGetProduct = tool('off_get_product', {
  description: 'Fetch a packaged food product by barcode (4–40 digits).',
  annotations: { readOnlyHint: true },
  input: z.object({
    barcode: z.string().regex(BARCODE_PATTERN).describe('Product barcode, digits only: 4–40 digits after any leading zeros.'),
    fields: z.array(z.enum(['product_name', 'brands', 'nutriscore_grade', 'nutriments'])).optional()
      .describe('Subset of fields to return. Omitting returns all standard fields.'),
  }),
  output: z.object({
    barcode: z.string().describe('The input barcode, echoed back unchanged.'),
    // No `found` flag: a missing record is the thrown not_found error, so a boolean here could
    // only ever be true. Don't advertise a state the handler cannot return — a caller that
    // branches on it never handles the path that actually fires.
    product: z.object({ /* ... */ }).describe('Product data. Always present on success.'),
  }),
  errors: [
    {
      reason: 'not_found',
      code: JsonRpcErrorCode.NotFound,
      when: 'Open Food Facts answers HTTP 404 or status 0 for the barcode — no contributor has recorded it',
      recovery: 'Try off_search_products with the product name or brand to find the correct barcode.',
    },
  ],

  async handler(input, ctx) {
    const svc = getOpenFoodFactsService();
    const product = await svc.getProduct(input.barcode, ctx);
    if (!product) {
      throw ctx.fail('not_found', `Barcode ${input.barcode} not found`);
    }
    return { barcode: input.barcode, product };
  },

  format: (result) => [{
    type: 'text',
    text: `**${result.product.product_name ?? 'Unknown'}** (${result.barcode})`,
  }],
});