---

name: mcp-development description: Build production-grade MCP (Model Context Protocol) servers with observability, correlation ID tracing, and dual logging. Use when creating new MCP servers, adding tools to existing servers, implementing file logging, debugging MCP issues, wrapping CLI tools with spawnSyncCollect, or following Side Quest marketplace patterns. Covers @sidequest/core/mcp declarative API, @sidequest/core/spawn CLI wrapper patterns, Zod schemas, Bun runtime, and 9 gold standard patterns validated across Kit plugin (18 tools). Includes error handling, response format switching, MCP annotations, and graceful degradation.

MCP Development Skill

Build production-grade MCP servers using @sidequest/core/mcp and Bun.

---

Quick Start

/plugin-template:create my-plugin
cd plugins/my-plugin

Create mcp/index.ts:

#!/usr/bin/env bun
import { createCorrelationId, log, startServer, tool, z } from "@sidequest/core/mcp";

tool("hello", {
  description: "A simple greeting tool",
  inputSchema: {
    name: z.string().describe("Name to greet"),
    response_format: z.enum(["markdown", "json"]).optional()
      .describe("Output format: 'markdown' (default) or 'json'"),
  },
  annotations: { readOnlyHint: true, destructiveHint: false, idempotentHint: true, openWorldHint: false },
}, async (args: Record<string, unknown>) => {
  const { name, response_format } = args as { name: string; response_format?: string };

  const cid = createCorrelationId();
  log.info({ cid, tool: "hello", args: { name } }, "greeting");

  const greeting = `Hello, ${name}!`;
  const text = response_format === "json" ? JSON.stringify({ greeting }) : `# Hello\n\n${greeting}`;

  log.info({ cid, tool: "hello", success: true }, "greeting");
  return { content: [{ type: "text" as const, text }] };
});

startServer("my-plugin", {
  version: "1.0.0",
  fileLogging: { enabled: true, subsystems: ["greeting"], level: "debug" },
});

Create .mcp.json:

{
  "mcpServers": {
    "my-plugin": {
      "command": "bun",
      "args": ["run", "${CLAUDE_PLUGIN_ROOT}/mcp/index.ts"]
    }
  }
}

Test: bun run mcp/index.ts

---

6-Step Handler Pattern

Every handler follows this pattern:

async (args: Record<string, unknown>) => {
  const { query, response_format } = args as { query: string; response_format?: string };

  // 1. Generate correlation ID
  const cid = createCorrelationId();
  const startTime = Date.now();

  // 2. Log request
  log.info({ cid, tool: "my_tool", args: { query } }, "search");

  try {
    // 3. Execute business logic
    const result = await doSomething(query);

    // 4. Format output
    const text = response_format === "json" ? JSON.stringify(result) : formatAsMarkdown(result);

    // 5. Log response
    log.info({ cid, tool: "my_tool", success: true, durationMs: Date.now() - startTime }, "search");

    // 6. Return MCP response
    return { content: [{ type: "text" as const, text }] };
  } catch (error) {
    const errorMessage = error instanceof Error ? error.message : String(error);
    log.error({ cid, tool: "my_tool", error: errorMessage }, "search");

    return {
      content: [{ type: "text" as const, text: JSON.stringify({ error: errorMessage, hint: "Recovery action", isError: true }) }],
      isError: true
    };
  }
}

---

Gold Standard Patterns

9 production-validated patterns from Kit plugin. See @./references/gold-standard-patterns.md for details.

#	Pattern	Purpose
1	Declarative Tool Registration	Type-safe Zod schemas + MCP annotations
2	Correlation ID Tracing	Request tracking across logs
3	Structured Error Responses	{ error, hint, isError: true }
4	Response Format Switching	Markdown (default) + JSON
5	File Logging Configuration	Subsystem-based hierarchical logs
6	MCP Annotations	readOnlyHint, destructiveHint, etc.
7	No Nested Package.json	Avoid MCP discovery failures
8	Dual Logging	MCP protocol + file persistence
9	Separation of Concerns	Business logic vs MCP layer

---

Tool Registration

tool("my_tool", {
  description: "What this tool does",
  inputSchema: {
    query: z.string().describe("Search query"),
    limit: z.number().optional().describe("Max results"),
    response_format: z.enum(["markdown", "json"]).optional()
      .describe("Output format: 'markdown' (default) or 'json'"),
  },
  annotations: {
    readOnlyHint: true,      // No side effects
    destructiveHint: false,  // Doesn't delete data
    idempotentHint: true,    // Safe to retry
    openWorldHint: false,    // Local only
  },
}, handler);

---

File Logging

startServer("my-plugin", {
  version: "1.0.0",
  fileLogging: {
    enabled: true,
    subsystems: ["search", "index", "api"],
    level: "debug",  // "debug" | "info" | "warning" | "error"
    maxSize: 10_000_000,  // 10MB
    maxFiles: 5,
  },
});

Logs: ~/.claude/logs/<plugin>.jsonl

View: tail -f ~/.claude/logs/my-plugin.jsonl | jq

Filter: jq 'select(.cid == "abc123")'

---

CLI Wrapper Pattern

For tools wrapping external CLIs:

import { buildEnhancedPath, spawnSyncCollect } from "@sidequest/core/spawn";

const result = spawnSyncCollect(
  ["bun", "run", `${pluginRoot}/src/cli.ts`, "search", query],
  { env: { PATH: buildEnhancedPath() } }
);

return {
  ...(result.exitCode !== 0 ? { isError: true } : {}),
  content: [{ type: "text" as const, text: result.exitCode === 0 ? result.stdout : result.stderr }],
};

---

Error Handling

For detailed error patterns, see @./references/error-handling.md.

Quick pattern:

enum ToolError {
  InvalidInput = "INVALID_INPUT",
  NotFound = "NOT_FOUND",
  Timeout = "TIMEOUT",
  InternalError = "INTERNAL_ERROR",
}

// Return structured error
return {
  content: [{ type: "text", text: JSON.stringify({ error: msg, errorType, hint, isError: true }) }],
  isError: true
};

Type guard:

type Result<T> = { ok: true; data: T } | { ok: false; error: string; hint?: string };
const isSuccess = <T>(r: Result<T>): r is { ok: true; data: T } => r.ok;

---

Advanced Features

For complete examples, see @./references/core-mcp-api.md.

Graceful degradation:

try {
  return await semanticSearch(query);
} catch (error) {
  if (error.message.includes("not available")) {
    log.warning({ cid, fallback: "grep" }, "search");
    return await grepSearch(query);
  }
  throw error;
}

Timeout:

const withTimeout = <T>(promise: Promise<T>, ms: number) =>
  Promise.race([promise, new Promise<T>((_, reject) => setTimeout(() => reject(new Error("Timeout")), ms))]);

ResponseFormat enum:

enum ResponseFormat { MARKDOWN = "markdown", JSON = "json" }
const format = response_format === "json" ? ResponseFormat.JSON : ResponseFormat.MARKDOWN;

---

Testing

import { describe, expect, test } from "bun:test";

describe("my-server", () => {
  test("executes tool", async () => {
    const result = await callTool("mcp__my-plugin_my-server__hello", { name: "Nathan" });
    expect(result.content[0].text).toContain("Hello");
  });

  test("handles errors", async () => {
    const result = await callTool("mcp__my-plugin_my-server__search", { query: "" });
    expect(result.isError).toBe(true);
  });
});

---

Troubleshooting

Problem	Solution
Server won't start	bun run mcp/index.ts to check errors
Tool not appearing	Check .mcp.json path, verify tool name pattern
Handler crashes	Add try/catch, log args before processing
Nested package.json	Delete it — breaks MCP discovery

---

Reference Links

Essential:

• @./references/gold-standard-patterns.md — 9 patterns with pitfalls + best practices
• @./references/kit-case-study.md — Kit's 18-tool production implementation

API:

• @./references/core-mcp-api.md — @sidequest/core/mcp + @sidequest/core/spawn
• @./references/error-handling.md — Error taxonomy and recovery strategies
• @./references/mcp-protocol.md — MCP protocol and marketplace conventions

CLI:

• /plugin-template:create — Generate plugin scaffold
• /review-mcp — Validate against checklist

---

Summary

1. Import: tool, startServer, log, createCorrelationId, z from @sidequest/core/mcp
2. Register: Tools with Zod schemas and annotations
3. Log: With subsystems — log.info(data, "subsystem")
4. Trace: Correlation IDs on every log entry
5. Format: Support markdown (default) + JSON
6. Errors: Return { error, hint, isError: true }
7. Files: Logs to ~/.claude/logs/<plugin>.jsonl

Production example: @../../kit/CLAUDE.md