medical-codes-mcp-server: Instructions file for Claude Code

Developer Protocol

Server: medical-codes-mcp-server Version: 0.2.8 Framework: @cyanheads/mcp-ts-core ^0.12.3 Engines: Bun ≥1.3.0, Node ≥24.0.0 MCP SDK: @modelcontextprotocol/server ^2.0.0 Zod: ^4.4.3

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.

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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. No handler here does — every tool answers from the bundled index in one round.
• Secrets in env vars only — never hardcoded.
• 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.

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Patterns

This server is tool-only (six medcode_* tools, no resources or prompts) and reads a bundled read-only SQLite index — handlers compose synchronous query methods on the code-index service.

Tool

import { tool, z } from '@cyanheads/mcp-ts-core';
import { JsonRpcErrorCode } from '@cyanheads/mcp-ts-core/errors';
import { getCodeIndexService } from '@/services/code-index/code-index-service.js';
import { SYSTEM_IDS } from '@/services/code-index/types.js';

export const checkCodeTool = tool('medcode_check_code', {
  title: 'medical-codes-mcp-server',
  description: 'Validate whether a US medical code exists, is current, and is billable …',
  annotations: { readOnlyHint: true, idempotentHint: true, openWorldHint: false },

  input: z.object({
    code: z.string().min(1).describe('The code to validate, with or without dots.'),
    system: z.enum(SYSTEM_IDS).optional().describe('Force the lookup into this system. Omit to auto-detect.'),
  }),
  output: z.object({
    system: z.string().describe('The system the code was resolved in, echoed for chaining.'),
    code: z.string().describe('The code in display form (ICD-10-CM carries the dot).'),
    status: z.enum(['valid_billable', 'valid_not_billable', 'valid_header', 'terminated'])
      .describe('Discriminated validity status.'),
    billable: z.boolean().describe('True only when status is valid_billable.'),
    whyNot: z.string().nullable().describe('Explanation for non-billable/terminated, or null.'),
  }),

  // Typed per-tool error contract — ctx.fail is type-checked against these reasons.
  errors: [
    { reason: 'unknown_code', code: JsonRpcErrorCode.NotFound,
      when: 'The code does not exist in the named or detected system.',
      recovery: 'Check the code, or search by description with medcode_search_codes.' },
  ],

  handler(input, ctx) {
    const outcome = getCodeIndexService().checkCode(input.code, input.system);
    // … discriminate on outcome.kind; a non-billable/terminated code is a SUCCESS
    // result with a whyNot, NOT an error — only an absent code throws unknown_code.
    if (outcome.kind === 'resolved' && outcome.result.status === 'unknown') {
      throw ctx.fail('unknown_code', `Unknown code "${input.code}".`, { ...ctx.recoveryFor('unknown_code') });
    }
    ctx.log.info('Checked code', { code: input.code });
    return { /* … */ } as never;
  },

  // format() populates content[] — the markdown twin of structuredContent.
  // Different clients read different surfaces (Claude Code → structuredContent,
  // Claude Desktop → content[]); both must carry the same data.
  format: (result) => [{ type: 'text', text: `${result.code}: ${result.status}` }],
});