MCP Builder

mcp-builder

Author MCP servers that LLMs can drive end-to-end. The quality bar is can the agent finish the workflow, not does the endpoint return 200. This skill is the server-author counterpart to the existing mcp consumer skill.

When to use

• Wrapping an external API or service as MCP tools for an LLM client.
• Adding tools to an existing MCP server (Python FastMCP or TypeScript SDK).
• Reviewing an MCP server before shipping — Phase 4 evaluation gate below.

Do NOT use when:

• You only need to call an MCP server — route to mcp.
• The integration belongs in the host process — write a regular service, not an MCP server.
• The "server" wraps one endpoint with no workflow — a CLI wrapper is enough.

Procedure: Four phases, one tool at a time

Phase 1 — Research & plan

1. Agent-centric design. Tools encode workflows, not raw endpoints. Consolidate (schedule_event checks availability and creates the event). Default to human-readable names over IDs. Errors are educational, not just diagnostic ("retry with filter='active_only' to reduce results").
2. Load the protocol. Fetch https://modelcontextprotocol.io/llms-full.txt once into context — the canonical spec.
3. Load the SDK README for the chosen language:
   • Python: https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/main/README.md
   • TypeScript: https://raw.githubusercontent.com/modelcontextprotocol/typescript-sdk/main/README.md
4. Read the target service's API docs in full — auth, rate limits, pagination, error codes, schemas. Skipping this produces incomplete mocks (see testing-anti-patterns § Anti-Pattern 4).
5. Write the plan: tool list with priority, shared utilities (request helper, pagination, formatter), input/output schemas, error strategy, response-detail levels (concise vs detailed), character limits (default 25 000 tokens).

Phase 2 — Implement

1. Project layout. Python: single .py or modular package; Pydantic v2 with model_config. TypeScript: standard package.json + tsconfig.json strict mode; Zod schemas with .strict().
2. Shared utilities first. API request helper with retry/timeout, error formatter, JSON-vs-Markdown response builder, pagination cursor handling, auth/token cache.
3. Per tool:
   • Input schema (Pydantic / Zod) with constraints, descriptions, and examples.
   • One-line summary + detailed docstring covering purpose, parameters, return shape, when-to-use, when-NOT-to-use, error handling.
   • Tool annotations: readOnlyHint, destructiveHint, idempotentHint, openWorldHint.
   • Async/await for all I/O. Honor pagination. Truncate to the character limit and signal truncation in the response.

Phase 3 — Review & test

1. Code-quality pass: DRY across tools, shared helpers extracted, consistent response shapes, all external calls have error handling, full type coverage.
2. Build & syntax:
   • Python: python -m py_compile server.py.
   • TypeScript: npm run build; verify dist/index.js.
3. Run the server safely. MCP servers block on stdio. Either run inside tmux and drive from the harness, or wrap with timeout 5s python server.py for a smoke check. Do NOT block your own session by running it in-process.

Phase 4 — Evaluations (10-question harness)

Each evaluation is a question the agent must answer using only the new tools.

Requirements per question — independent, read-only, complex (multiple tool calls), realistic, verifiable (string-comparable answer), stable (answer does not drift over time).

<evaluation>
  <qa_pair>
    <question>...</question>
    <answer>...</answer>
  </qa_pair>
  <!-- 9 more -->
</evaluation>

Process: enumerate the tools, explore READ-ONLY data, draft 10 questions, solve each yourself first to confirm the answer is reachable and stable.

Output format

1. The server source plus the 10-question evaluation XML.
2. A README with: install, env vars, transport mode (stdio / sse / http), example tool call.
3. A line in agents/settings/contexts/skills-provenance.yml if the server was forked from an upstream, or a note that it was authored from scratch.

Gotcha

• "Wrap every endpoint" is the failure mode — agents cannot orchestrate 60 thin tools as well as 12 workflow tools.
• Returning the full upstream payload blows the agent's context. Default to a concise shape with an opt-in detailed mode.
• Pydantic / Zod descriptions are the only documentation the LLM sees at runtime — write them like usage docs, not comments.
• A server that hangs your session usually means stdio transport ran in the main process — move it under tmux or use a timeout.
• Inflated token claims are not credible without an evaluation harness — Phase 4 is the validation gate, not optional.

Do NOT

• Do NOT mirror REST routes 1:1.
• Do NOT use any (TypeScript) or untyped dict (Python) in tool I/O.
• Do NOT skip the 10-question evaluation — Phase 4 IS the quality bar.
• Do NOT run the MCP server in your main process during testing — it will block.
• Do NOT log tokens, API keys, or full request bodies — sanitize before logging.

Auto-trigger keywords

• mcp server
• model context protocol
• fastmcp
• mcp builder
• agent-centric tools

Provenance

• Upstream protocol: https://modelcontextprotocol.io
• Upstream SDKs: https://github.com/modelcontextprotocol/python-sdk · https://github.com/modelcontextprotocol/typescript-sdk
• Adopted from: an external reference (internal provenance, redacted) — external ./reference/*.md file links replaced with inline guidance + upstream URLs.
• Cross-linked: mcp, testing-anti-patterns, api-design.
• Provenance registry: agents/settings/contexts/skills-provenance.yml (entry: mcp-builder).
• Iron-Law floor: verify-before-complete, tool-safety, skill-quality.

Encode usage policy in the description

Workflow sequencing, preconditions, ID/output provenance ("copy ids verbatim, never from memory"), a mandatory "why" intent field, and turn-end contracts belong INSIDE this artifact's description/frontmatter — where they fire at the decision point — not in always-on prose. See tool-description-as-policy.