MCP Server Builder

Use this skill when designing or implementing an MCP server.

When to Use

• Building a new MCP server from scratch.
• Refactoring a weak or over-thin MCP tool surface.
• Reviewing whether a server exposes the right workflows.
• Designing evaluation cases for MCP usability.

Core Principles

• Build workflow tools, not thin endpoint wrappers — one tool should complete a meaningful agent task, not expose a single API method.
• Keep input schemas narrow and typed — reject unknown fields; use enums over free strings where possible.
• Return high-signal, size-bounded outputs — default to concise; add detail or verbose flags when larger payloads are occasionally useful.
• Make error messages corrective — tell the agent what to do next, not just what went wrong.
• Prefer human-meaningful identifiers over opaque IDs when both are available.
• Design evaluation cases before declaring the server "done".

Workflow

1. Map the workflow

• Identify the real tasks an agent must complete, not the underlying API surface.
• Merge low-level steps into meaningful operations (e.g., one create_and_publish tool instead of separate create, validate, publish).

2. Design the tool surface

tool name:    stable, verb-noun, describes the workflow step
input schema: typed, narrow, required fields only + optional detail flags
output shape: consistent structure across all tools in the server
failure modes: named error codes + correction hint

3. Design for context limits

• Default response fits in ~500 tokens for list operations, ~1500 for detail operations.
• Add limit, page, or summary parameters for large result sets.
• Truncate deterministically (e.g., top N by recency) — never truncate randomly.

4. Design corrective errors

Bad error: "Error: 404 Not Found"

Good error: "Resource 'project-123' not found. Use list_projects to see available project IDs."

Every error should tell the agent its next valid action.

5. Implement shared infrastructure first

• Auth handling and token refresh.
• Retry logic with exponential backoff and rate-limit awareness.
• Pagination helpers.
• Output formatting helpers (consistent truncation, redaction of secrets).

6. Evaluate before shipping

• Write representative task scenarios (not unit tests for individual tools).
• Check whether an agent can complete the full workflow using only the exposed tools.
• Redesign weak tools before adding more tools — more tools is not always better.

Server Readiness Checklist

• [ ] Every tool completes a meaningful workflow step.
• [ ] All inputs are typed and schema-validated.
• [ ] Output size is bounded by default.
• [ ] All error messages include a correction hint.
• [ ] Auth and retry are handled in shared infrastructure.
• [ ] At least one end-to-end task scenario has been tested.
• [ ] No secrets appear in tool outputs or error messages.

Local References

• reference/mcp_best_practices.md
• reference/python_mcp_server.md
• reference/node_mcp_server.md
• reference/evaluation.md

Use the Python and Node references only for the stack you are actually shipping.

Bundled Scripts

• scripts/evaluation.py — evaluation scaffolding
• scripts/connections.py — connection-oriented examples

Use them as optional helpers, not mandatory runtime requirements.