MCP Server Builder

Build an MCP server that extends Claude Code with new tools, resources, or prompts.

Server Purpose: $ARGUMENTS

STEP 1: Define the Server Scope

Before writing code, define what the MCP server will expose:

1.1 Identify Capabilities

1.2 Agent-Centric Design Principles

MCP servers are consumed by AI agents, not humans. Design accordingly:

1. Build for workflows, not endpoints — Group related operations into tools that match how an agent thinks about a task. One tool that "creates a PR with tests" beats three tools for "create branch", "commit files", "open PR".

2. Optimize for limited context — Return only what the agent needs. A tool that returns a 10,000-line log is worse than one that returns the 20 relevant lines with context.

3. Make errors actionable — Instead of "Error: 403", return "Permission denied: the API token lacks 'write:issues' scope. Add this scope at https://...". The agent should be able to fix the problem from the error message alone.

4. Provide discovery — Include a tool that lists available resources or explains what the server can do. Agents need to understand capabilities at runtime.

5. Idempotent where possible — Agents may retry tools. Design create/update operations to be safe to call multiple times with the same input.

1.3 Define Tool Signatures

For each tool, define:

Tool: <name>
  Description: <what it does — this is what Claude reads to decide whether to use it>
  Input: <parameters with types and descriptions>
  Output: <what it returns>
  Side effects: <what it changes in the external system>
  Error cases: <what can go wrong and what the error message should say>

Description quality matters. Claude selects tools based on their descriptions. A vague description like "manage issues" produces poor tool selection. Be specific: "Create a new GitHub issue with title, body, labels, and assignee. Returns the issue URL and number."

STEP 2: Choose SDK and Scaffold

Option A: Python with FastMCP

FastMCP is the recommended Python SDK — minimal boilerplate, decorator-based.

# Initialize project
mkdir mcp-server-<name> && cd mcp-server-<name>
python -m venv .venv && source .venv/bin/activate  # or .venv/Scripts/activate on Windows
pip install fastmcp

Scaffold:

# server.py
from fastmcp import FastMCP

mcp = FastMCP(
    name="<server-name>",
    description="<what this server does>",
)

@mcp.tool()
def example_tool(param: str) -> str:
    """Description that Claude reads to decide when to use this tool.

    Args:
        param: What this parameter controls
    """
    # Implementation here
    return "result"

@mcp.resource("resource://{name}")
def example_resource(name: str) -> str:
    """Provides access to <what>."""
    return "resource content"

if __name__ == "__main__":
    mcp.run()

Option B: Node.js / TypeScript SDK

mkdir mcp-server-<name> && cd mcp-server-<name>
npm init -y
npm install @modelcontextprotocol/sdk
npm install -D typescript @types/node
npx tsc --init

Scaffold:

// src/index.ts
import { McpServer, ResourceTemplate } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";

const server = new McpServer({
  name: "<server-name>",
  version: "1.0.0",
});

server.tool(
  "example-tool",
  "Description that Claude reads to decide when to use this tool",
  { param: { type: "string", description: "What this parameter controls" } },
  async ({ param }) => {
    // Implementation here
    return { content: [{ type: "text", text: "result" }] };
  }
);

const transport = new StdioServerTransport();
await server.connect(transport);

STEP 3: Implement Tools

3.1 Tool Implementation Checklist

For each tool:

• Description is specific and tells Claude WHEN to use it (not just what it does)
• All parameters have types, descriptions, and sensible defaults where applicable
• Return values are concise — only what the agent needs for next steps
• Errors include actionable remediation instructions
• Side effects are documented in the description
• Tool is idempotent or clearly marked as non-idempotent

3.2 Input Validation

Validate inputs and return helpful messages:

@mcp.tool()
def create_issue(title: str, body: str, labels: list[str] | None = None) -> str:
    """Create a GitHub issue in the current repository.

    Args:
        title: Issue title (required, max 256 chars)
        body: Issue body in markdown
        labels: Optional list of label names (must already exist in repo)
    """
    if not title.strip():
        return "Error: title is required and cannot be empty."
    if len(title) > 256:
        return f"Error: title is {len(title)} chars, max is 256. Shorten the title."
    # ... implementation

3.3 Error Handling Pattern

try:
    result = external_api.call(params)
    return format_result(result)
except AuthenticationError:
    return (
        "Error: Authentication failed. Check that your API token is set in the "
        "environment variable EXAMPLE_API_TOKEN and has the required scopes: "
        "read:data, write:data. Generate a token at https://example.com/settings/tokens"
    )
except RateLimitError as e:
    return f"Error: Rate limited. Retry after {e.retry_after} seconds."
except Exception as e:
    return f"Error: Unexpected failure — {type(e).__name__}: {e}"

3.4 Output Formatting

Return structured, scannable output:

# BAD: Wall of text
return json.dumps(full_api_response)

# GOOD: Curated summary
return f"""Issue created successfully.
- URL: {issue.html_url}
- Number: #{issue.number}
- Labels: {', '.join(issue.labels)}

Next: assign the issue with the assign-issue tool, or link it to a PR."""

STEP 4: Implement Resources (if needed)

Resources provide read-only data that Claude can reference:

@mcp.resource("config://settings")
def get_settings() -> str:
    """Current project settings including API endpoints and feature flags."""
    settings = load_settings()
    # Return only what's relevant, not the entire config
    return yaml.dump({
        "api_base": settings["api_base"],
        "features": settings["features"],
        "environment": settings["environment"],
    })

Resource Design Rules

1. URI scheme matters — Use descriptive schemes: docs://, config://, schema://
2. Keep payloads small — Resources are loaded into context. A 50KB resource wastes tokens.
3. Provide templates for parameterized access — docs://{topic} is better than dumping all docs at once

STEP 5: Configure for Claude Code

5.1 Register in .mcp.json

Add the server to the project's .mcp.json:

{
  "mcpServers": {
    "<server-name>": {
      "command": "python",
      "args": ["path/to/server.py"],
      "env": {
        "EXAMPLE_API_TOKEN": "${EXAMPLE_API_TOKEN}"
      }
    }
  }
}

For Node.js:

{
  "mcpServers": {
    "<server-name>": {
      "command": "node",
      "args": ["path/to/dist/index.js"]
    }
  }
}

5.2 Environment Variables

NEVER hardcode credentials in the server. Use environment variables:

import os

API_TOKEN = os.environ.get("EXAMPLE_API_TOKEN")
if not API_TOKEN:
    raise RuntimeError(
        "EXAMPLE_API_TOKEN environment variable is required. "
        "Set it in .mcp.json env block or your shell profile."
    )

STEP 6: Test the Server

6.1 Manual Testing

# Test with MCP Inspector (if available)
npx @modelcontextprotocol/inspector python server.py

# Or test directly by running the server and sending JSON-RPC
echo '{"jsonrpc":"2.0","id":1,"method":"tools/list"}' | python server.py

6.2 Automated Tests

Write tests for each tool:

# test_server.py
import pytest
from server import mcp

@pytest.fixture
def client():
    """Create a test client for the MCP server."""
    return mcp.test_client()

def test_example_tool_happy_path(client):
    result = client.call_tool("example-tool", {"param": "test"})
    assert "expected output" in result

def test_example_tool_missing_param(client):
    result = client.call_tool("example-tool", {})
    assert "Error:" in result

def test_example_tool_error_is_actionable(client):
    result = client.call_tool("example-tool", {"param": "invalid"})
    # Error messages MUST tell the agent how to fix the problem
    assert any(word in result.lower() for word in ["try", "check", "use", "set"])

6.3 Evaluation-Driven Development

Beyond unit tests, evaluate how well Claude uses your tools:

1. Discovery test — Ask Claude "what tools do you have for X?" Does it find your tool?
2. Selection test — Give Claude a task that should use your tool. Does it pick the right one?
3. Error recovery test — Trigger an error. Can Claude fix the problem from the error message?
4. Workflow test — Give Claude a multi-step task. Does it chain your tools correctly?

If Claude struggles with any of these, improve the tool descriptions and error messages — don't blame the model.

STEP 7: Document and Ship

7.1 README

Create a README with:

• What the server does (one paragraph)
• Prerequisites (API keys, services, permissions)
• Installation and configuration steps
• List of tools with descriptions
• Example usage scenarios

7.2 Version and Maintain

• Pin dependency versions in requirements.txt or package.json
• Follow semantic versioning — breaking tool signature changes = major version bump
• Test after Claude Code updates — MCP protocol may evolve

MUST DO

• Always validate all tool inputs and return actionable error messages
• Always keep tool descriptions specific enough for Claude to select correctly
• Always use environment variables for credentials — never hardcode
• Always test each tool with both happy path and error cases
• Always keep resource payloads small — curate, don't dump
• Always make tools idempotent where the external system allows it

MUST NOT DO

• MUST NOT return raw API responses — curate the output for agent consumption
• MUST NOT use vague tool descriptions — "manages data" tells Claude nothing
• MUST NOT expose credentials in error messages or logs
• MUST NOT create tools with overlapping purposes — Claude will pick randomly between them
• MUST NOT return more than ~2KB per tool call unless the agent specifically needs bulk data
• MUST NOT skip error handling — an unhandled exception crashes the MCP server

Source: abhayla/claude-best-practices — distributed by TomeVault.