@cyanheads/mcp-ts-core

What is this?

@cyanheads/mcp-ts-core is the infrastructure layer for TypeScript MCP servers. Install it as a dependency — don't fork it. Your agent collaborates with you to design and build the tools, resources, and prompts for your server.

The framework handles the plumbing: transports, auth, config, logging, telemetry, & more.

import { createApp, tool, z } from '@cyanheads/mcp-ts-core';
import { JsonRpcErrorCode } from '@cyanheads/mcp-ts-core/errors';

const search = tool('search', {
  description: 'Search the catalog and return ranked matches.',
  annotations: { readOnlyHint: true },
  input: z.object({
    query: z.string().describe('Search terms'),
    limit: z.number().default(10).describe('Max results'),
  }),
  output: z.object({
    items: z.array(z.string()).describe('Matching item names, best first'),
  }),
  enrichment: {
    effectiveQuery: z.string().describe('Query as the server parsed it'),
    totalCount: z.number().describe('Total matches before the limit'),
    notice: z.string().optional().describe('Guidance when nothing matched'),
  },
  errors: [
    {
      reason: 'index_unavailable',
      code: JsonRpcErrorCode.ServiceUnavailable,
      when: 'The upstream search index is unreachable.',
      retryable: true,
      recovery: 'Retry in a few seconds — the index may be briefly unavailable.',
    },
  ],
  handler: async (input, ctx) => {
    const res = await runSearch(input.query, input.limit);
    if (!res) throw ctx.fail('index_unavailable'); // genuine failure → typed error contract
    ctx.enrich({ effectiveQuery: res.parsed, totalCount: res.total });
    if (res.items.length === 0) {
      ctx.enrich({ notice: `No matches for "${input.query}". Try broader terms.` }); // empty result → notice, not a throw
    }
    return { items: res.items }; // enrichment never rides in the domain return
  },
});

await createApp({ tools: [search] });

That's a complete MCP server, and it shows both of the framework's core contracts.

enrichment carries the context an agent reasons with (the parsed query, the true total, an empty-result notice); the framework merges it into structuredContent and mirrors it into content[], so structuredContent-only clients (Claude Code) and content[]-only clients (Claude Desktop) both see it, no format() needed. The typed errors[] contract handles genuine failures (an empty result is a notice, not a throw), and the linter cross-checks both against the handler body. Both publish in tools/list, so clients preview a tool's success and failure shapes.

The rest is automatic: every tool call is logged with duration, payload sizes, and request correlation, and createApp() handles config parsing, logger init, transport startup, signal handlers, and graceful shutdown.

Related MCP server: FastMCP

Quick start

bunx @cyanheads/mcp-ts-core init my-mcp-server
cd my-mcp-server
bun install

You get a scaffolded project with CLAUDE.md/AGENTS.md, Agent Skills, plugin metadata (Codex + Claude Code), and a src/ tree ready for your tools. Infrastructure (transports, auth, storage, telemetry, lifecycle, linting) lives in node_modules. What's left is domain: which APIs to wrap, which workflows to expose.

Start your coding agent (e.g. Claude Code, Codex) and describe what you want. The agent knows what to do from there. The included Agent Skills cover the full cycle: setup, design-mcp-server, scaffolding, testing, security-pass, release-and-publish, maintenance, & more.

What you get

The headline tool returns structured output. Clients that read structuredContent (Claude Code) get it directly. To also render markdown for clients that read content[] (Claude Desktop), add a format(). The format-parity linter checks it renders every output field, so the two surfaces never drift:

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

export const itemSearch = tool('item_search', {
  description: 'Search for items by query.',
  input: z.object({
    query: z.string().describe('Search query'),
    limit: z.number().default(10).describe('Max results'),
  }),
  output: z.object({
    items: z.array(z.string()).describe('Search results'),
  }),
  async handler(input) {
    const results = await doSearch(input.query, input.limit);
    return { items: results };
  },
  format: (result) => [
    { type: 'text', text: result.items.map((name) => `- ${name}`).join('\n') },
  ],
});

And resources:

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

export const itemData = resource('items://{itemId}', {
  description: 'Retrieve item data by ID.',
  params: z.object({
    itemId: z.string().describe('Item ID'),
  }),
  async handler(params, ctx) {
    return await getItem(params.itemId);
  },
});

Everything registers through createApp() in your entry point:

await createApp({
  name: 'my-mcp-server',
  version: '0.1.0',
  tools: allToolDefinitions,
  resources: allResourceDefinitions,
  prompts: allPromptDefinitions,
  instructions: 'Brief composition hints for the model.', // optional, sent on every `initialize`
});

It also works on Cloudflare Workers with createWorkerHandler() — same definitions, different entry point.

Features

• Declarative definitions — tool(), resource(), prompt() builders with Zod schemas; appTool()/appResource() add interactive HTML UIs.

• Server-level orientation — instructions on createApp/createWorkerHandler rides every initialize for the model. Cross-tool composition hints, regional notes, scope guidance — without leaking text into every tool description.

• Server identity — optional title, websiteUrl, description, icons (SEP-973) on createApp/createWorkerHandler flow to initialize serverInfo, the /.well-known/mcp.json server card, and the landing page.

• Unified Context — one ctx for logging, tenant-scoped storage, elicitation, cancellation, and task progress.

• Auth — auth: ['scope'] on definitions, checked before dispatch (no wrapper code). Modes: none, jwt, or oauth (local secret or JWKS).

• Task tools — task: true for long-running ops; framework manages create/poll/progress/complete/cancel.

• Definition linter — validates names, schemas, auth scopes, annotations, format-parity, and cross-vendor JSON Schema portability at build time. Run via lint:mcp or devcheck — not invoked at server startup.

• Typed error contracts — declare errors: [{ reason, code, when, recovery, retryable? }] and handlers get a typed ctx.fail(reason, …). Contracts publish in tools/list so clients preview failure modes; the linter cross-checks the handler. Factories (notFound(), httpErrorFromResponse(), …) cover ad-hoc throws; plain Error auto-classifies.

• Multi-backend storage — in-memory, filesystem, Supabase, Cloudflare D1/KV/R2. Swap via env var; handlers don't change.

• DataCanvas (optional) — Tier 3 SQL/analytical workspace backed by DuckDB. Register tabular data from upstream APIs, run SQL across registered tables, export CSV/Parquet/JSON. Token-sharing model (opaque canvas_id) for multi-agent collaboration; sliding TTL + per-tenant scoping. Opt-in via CANVAS_PROVIDER_TYPE=duckdb; fails closed on Workers.

• Observability — Pino logging + optional OpenTelemetry traces/metrics. Request correlation and tool metrics automatic.

• Tiered dependencies — parsers, OTEL SDK, Supabase, OpenAI as optional peers. Install what you use.

• Agent-first DX — ships CLAUDE.md / AGENTS.md and Agent Skills that give your coding agent full framework knowledge — it can scaffold tools, write tests, run security audits, and ship releases without you writing the boilerplate.

Server structure

my-mcp-server/
  src/
    index.ts                              # createApp() entry point
    worker.ts                             # createWorkerHandler() (optional)
    config/
      server-config.ts                    # Server-specific env vars
    services/
      [domain]/                           # Domain services (init/accessor pattern)
    mcp-server/
      tools/definitions/                  # Tool definitions (.tool.ts)
      resources/definitions/              # Resource definitions (.resource.ts)
      prompts/definitions/                # Prompt definitions (.prompt.ts)
  package.json
  tsconfig.json                           # extends @cyanheads/mcp-ts-core/tsconfig.base.json
  CLAUDE.md / AGENTS.md                   # Point to core's CLAUDE.md / AGENTS.md for framework docs

No src/utils/, no src/storage/, no src/types-global/, no src/mcp-server/transports/ — infrastructure lives in node_modules.

Configuration

All core config is Zod-validated from environment variables. Server-specific config uses a separate Zod schema with lazy parsing.

See CLAUDE.md/AGENTS.md for the full configuration reference.

API overview

Entry points

Builders

Context

Handlers receive a unified Context object:

Subpath exports

import { createApp, tool, resource, prompt } from '@cyanheads/mcp-ts-core';
import { createWorkerHandler } from '@cyanheads/mcp-ts-core/worker';
import { McpError, JsonRpcErrorCode, notFound, serviceUnavailable } from '@cyanheads/mcp-ts-core/errors';
import { checkScopes } from '@cyanheads/mcp-ts-core/auth';
import { markdown, fetchWithTimeout } from '@cyanheads/mcp-ts-core/utils';
import { OpenRouterProvider, GraphService } from '@cyanheads/mcp-ts-core/services';
import type { DataCanvas, CanvasInstance } from '@cyanheads/mcp-ts-core/canvas';
import { validateDefinitions } from '@cyanheads/mcp-ts-core/linter';
import { createMockContext } from '@cyanheads/mcp-ts-core/testing';
import { fuzzTool, fuzzResource, fuzzPrompt } from '@cyanheads/mcp-ts-core/testing/fuzz';

See CLAUDE.md/AGENTS.md for the complete exports reference.

Examples

The examples/ directory contains a reference server consuming core through public exports, demonstrating all patterns:

Testing

import { createMockContext } from '@cyanheads/mcp-ts-core/testing';
import { myTool } from '@/mcp-server/tools/definitions/my-tool.tool.js';

const ctx = createMockContext({ tenantId: 'test-tenant' });
const input = myTool.input.parse({ query: 'test' });
const result = await myTool.handler(input, ctx);

createMockContext() provides stubbed log, state, and signal. Pass { tenantId } for state operations, { elicit } for elicitation mocking, { progress: true } for task tools.

Fuzz testing

Schema-aware fuzz testing via fast-check. Generates valid inputs from Zod schemas and adversarial payloads (prototype pollution, injection strings, type confusion) to verify handler invariants.

import { fuzzTool } from '@cyanheads/mcp-ts-core/testing/fuzz';

const report = await fuzzTool(myTool, { numRuns: 100 });
expect(report.crashes).toHaveLength(0);
expect(report.leaks).toHaveLength(0);
expect(report.prototypePollution).toBe(false);

Also exports fuzzResource, fuzzPrompt, zodToArbitrary, and ADVERSARIAL_STRINGS for custom property-based tests.

Documentation

• CLAUDE.md/AGENTS.md — Framework reference: exports catalog, patterns, Context interface, error codes, auth, config, testing. Ships in the npm package and is auto-accessible in your project after init.

• docs/telemetry/ — OpenTelemetry: full catalog of spans, metrics, and attributes the framework emits (observability.md), plus an example Grafana dashboard and vendor-agnostic query recipes for Datadog, New Relic, Honeycomb (dashboards.md).

• CHANGELOG.md — Version history. Each entry includes a summary, migration notes, and links to commits/issues.

Development

bun run rebuild        # clean + build (scripts/clean.ts + scripts/build.ts)
bun run devcheck       # full gate: lint/format, typecheck, MCP defs, framework antipatterns, docs/skills/changelog sync, tests, audit, outdated, secrets/TODO scan
bun run lint:mcp       # validate MCP definitions against spec
bun run test:all       # vitest: unit + Workers pool + integration

License

Apache 2.0 — see LICENSE.