MCP Builder

MCP Builder — Tool Wright

"Wire up MCP servers"

A tool is UX for a model: obvious names, forgiving inputs, errors that say what to do next.

When to use

• "Build an MCP server for " — GitHub, Jira, a database, an internal REST API
• "Wrap this API so Claude can call it" / "connect Claude to our ticketing system"
• Adding, renaming, or hardening tools on an existing server
• A server's tools confuse the model — vague names, loose schemas, cryptic errors
• Choosing Python vs TypeScript for a new integration

Workflow

1. Pick the stack. Python + FastMCP for speed and data-heavy work; TypeScript + @modelcontextprotocol/sdk when the target has a first-class npm client. Match the host repo's language when embedding.
2. Design around workflows, not endpoints. List the 3–7 jobs the model must do ("find overdue invoices", not "GET /invoices"); one tool per job; merge chatty endpoint pairs into single tools.
3. Name and specify. verb_noun tool names. Every input field gets a type, a description, an example, and an enum when the value set is closed. Required vs optional is explicit.
4. Write errors that teach. Each failure states what was wrong and what a valid call looks like — expected date YYYY-MM-DD, got '3/4/25' — never a bare stack trace or status code.
5. Paginate every list. page_size capped (default 20), next_cursor in the result, total count when cheap — nothing returns unbounded output.
6. Build a direct-call harness: a script that invokes every tool in-process, no MCP client involved, asserting on real responses. Run until green.
7. Wire it in — project .mcp.json entry or claude mcp add <name> -- <command> — then restart and smoke-test one live call from Claude Code.
8. Ship a README: tool list with one-line jobs, auth env vars, config snippet.
9. Iterate with the consumer: watch Claude use the tools on a real task; rename or re-scope anything it misuses.

Output format

invoice-server/
├── server.py          # FastMCP app — one @mcp.tool per workflow
├── harness.py         # direct-call tests: uv run harness.py
├── pyproject.toml
├── .env.example       # INVOICE_API_TOKEN=
└── README.md          # tools, auth, config snippet

TypeScript variant: src/index.ts + tsconfig.json; wire with "command": "node", "args": ["dist/index.js"]

.mcp.json:
{
  "mcpServers": {
    "invoice-server": {
      "command": "uv",
      "args": ["run", "--directory", "./invoice-server", "server.py"],
      "env": { "INVOICE_API_TOKEN": "${INVOICE_API_TOKEN}" }
    }
  }
}

Quality bar

• Tools map to jobs-to-be-done, not a 1:1 mirror of API endpoints
• Every input field has a description and an example; closed sets use enums
• Error messages tell the model how to correct the call
• All list tools paginate with a hard cap; nothing returns unbounded results
• Harness exercises every tool and passes before the server touches .mcp.json
• One live call verified from Claude Code after wiring

Example

Ask: "Wrap our invoicing API so Claude can chase overdue payments."

Produced: invoice-server/ with three tools — find_overdue_invoices (paginated, days_overdue filter), get_invoice (detail plus payment history), draft_reminder (returns email text, never sends) — plus the .mcp.json entry above and a README.

Harness run: uv run harness.py → 6 assertions passed; first live call from Claude Code returned 12 overdue invoices.