usgs-water-mcp-server: Instructions file for Claude Code

Developer Protocol

Server: usgs-water-mcp-server Version: 0.4.0 Framework: @cyanheads/mcp-ts-core ^0.13.15 Engines: Bun ≥1.4.0, Node ≥24.0.0 MCP SDK: @modelcontextprotocol/server ^2.3.1 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 (data-returning, with typed error contract)

import { tool, z } from '@cyanheads/mcp-ts-core';
import { JsonRpcErrorCode } from '@cyanheads/mcp-ts-core/errors';
import { findSites } from '@/services/nwis/nwis-service.js';

export const waterFindSites = tool('water_find_sites', {
  description: 'Find USGS water monitoring sites by bounding box, state, county, or HUC watershed code.',
  annotations: { readOnlyHint: true, idempotentHint: true, openWorldHint: true },
  input: z.object({
    stateCd: z.string().optional().describe('2-character US state abbreviation (e.g. "WA").'),
    siteType: z.string().optional().describe('Site type filter: "ST" (stream), "GW" (groundwater well).'),
  }),
  output: z.object({
    sites: z.array(z.object({
      siteNumber: z.string().describe('USGS site number (8–15 digits).'),
      siteName: z.string().describe('Human-readable USGS site name.'),
    })).describe('Matching USGS monitoring sites.'),
    total: z.number().int().describe('Total number of sites returned.'),
  }),

  errors: [
    { reason: 'no_sites_found', code: JsonRpcErrorCode.NotFound,
      when: 'No sites match the given geographic and filter criteria.',
      recovery: 'Broaden the bounding box, remove parameterCd or siteType filters, or try a different state/HUC.' },
    { reason: 'upstream_error', code: JsonRpcErrorCode.ServiceUnavailable,
      when: 'NWIS returned a 5xx error.',
      recovery: 'The USGS service is temporarily unavailable. Retry after a short backoff.',
      retryable: true, thrownBy: 'service' },
  ],

  async handler(input, ctx) {
    const sites = await findSites({ stateCd: input.stateCd, siteType: input.siteType }, ctx);
    if (sites.length === 0) throw ctx.fail('no_sites_found', 'No USGS sites match the specified filters.');
    ctx.log.info('Sites found', { count: sites.length });
    return { sites, total: sites.length };
  },

  format: (result) => [{
    type: 'text',
    text: result.sites.map(s => `**${s.siteNumber}**: ${s.siteName}`).join('\n'),
  }],
});