name: mcp-builder description: "Create MCP (Model Context Protocol) servers that enable LLMs to interact with external services through well-designed tools, resources, and prompts. The quality of an MCP server is measured by how well it enables LLMs to accomplish real-world tasks safely, reliably, and with predictable outputs.. Use when Use this skill when the task matches its description and triggers.."

MCP Server Development Guide (Gold Standard, Dec 2025)

Compliance

Check against GOLD Industry Standards guide in ~/.codex/AGENTS.override.md

Overview

Create MCP (Model Context Protocol) servers that enable LLMs to interact with external services through well-designed tools, resources, and prompts. The quality of an MCP server is measured by how well it enables LLMs to accomplish real-world tasks safely, reliably, and with predictable outputs.

Gold Standard Checklist (Compact)

Protocol compliance: latest spec, JSON Schema 2020-12, streamable HTTP or stdio
Structured outputs: outputSchema + structuredContent + text JSON fallback
Discoverability: consistent tool names, concise descriptions, title/icons
Safety: read-only defaults, precise annotations, clear errors, strict auth
Data quality: stable field names, pagination, filtering, and resource links
Testability: schema contract tests, golden snapshots, inspector validation

Process

🚀 High-Level Workflow

Creating a high-quality MCP server involves four main phases:

Phase 0: Review & Fix Existing Implementations

Use this phase when the user asks to audit an MCP server, identify bugs, or propose fixes.

Scope first:

Identify the stack: TypeScript SDK or FastMCP (https://github.com/punkpeye/fastmcp)
Confirm transport (streamable HTTP vs stdio) and deployment target
List current tools, resources, and prompts and compare to intended use cases

Common bug patterns to check:

Missing/invalid inputSchema or outputSchema (not JSON Schema 2020-12)
structuredContent missing or not matching outputSchema
Tool annotations incorrect (readOnly/destructive/idempotent/openWorld)
Pagination and filtering inconsistencies across list tools
Auth bypasses (tokens accepted without aud/iss validation)
Widget rendering issues (wrong mimeType, missing template URI, CSP blocked)
Stale UI bundles due to cache and unchanged template URI

Fix workflow:

Reproduce with MCP Inspector or a minimal tool call
Add or correct schema contracts and structured outputs
Align tool metadata for discoverability and safety
Add regression tests (schema contract + golden snapshots)

Gold standard checklist:

Use 🧪 Review & Fix Checklist as the baseline for 2025 compliance.
Use 🛠 Review & Fix Recipes for diagnosis and fixes.
Use 🧭 Common Fixes Matrix for quick triage.
Use 🧱 Apps SDK Requirements for ChatGPT Apps-specific compliance.
If Apps SDK is in scope, run an explicit Apps SDK audit (see below).

Apps SDK audit (quick):

/mcp public HTTPS, Streamable HTTP preferred, SSE legacy only
text/html+skybridge templates and _meta["openai/outputTemplate"]
CSP set and minimal; widget data split (structuredContent vs _meta)
Tool handlers idempotent and safe on retry

Phase 1: Deep Research and Planning

1.1 Understand Modern MCP Design (2025+)

API Coverage vs. Workflow Tools: Balance comprehensive API endpoint coverage with specialized workflow tools. Workflow tools can be more convenient for specific tasks, while comprehensive coverage gives agents flexibility to compose operations. Performance varies by client—some clients benefit from code execution that combines basic tools, while others work better with higher-level workflows. When uncertain, prioritize comprehensive API coverage.

Tool Naming and Discoverability: Clear, descriptive tool names help agents find the right tools quickly. Use consistent prefixes (e.g., github_create_issue, github_list_repos) and action-oriented naming.

Context Management: Agents benefit from concise tool descriptions and the ability to filter/paginate results. Design tools that return focused, relevant data. Some clients support code execution which can help agents filter and process data efficiently.

Actionable Error Messages: Error messages should guide agents toward solutions with specific suggestions and next steps.

First-class Outputs: Prefer structured outputs with schemas and provide a text fallback for compatibility. Favor stable, machine-consumable fields over free-form text when possible.

Resources and Prompts: Use resources for read-only data and prompts for reusable interaction patterns. Tools should do the minimum work needed and delegate context to resources where possible.

1.2 Study MCP Protocol Documentation (Latest Spec)

Navigate the MCP specification:

Start with the sitemap to find relevant pages: https://modelcontextprotocol.io/sitemap.xml

Then fetch specific pages with .md suffix for markdown format (e.g., https://modelcontextprotocol.io/specification/draft.md).

Key pages to review (latest revision first):

Specification overview and architecture
Transport mechanisms (streamable HTTP, stdio)
Tool, resource, and prompt definitions
Authorization (OAuth 2.1, Protected Resource Metadata, Resource Indicators, PKCE)

1.3 Study Framework Documentation

Recommended stack:

Language: TypeScript (high-quality SDK support and good compatibility in many execution environments e.g. MCPB. Plus AI models are good at generating TypeScript code, benefiting from its broad usage, static typing and good linting tools)
Transport: Streamable HTTP for remote servers, using stateless JSON (simpler to scale and maintain, as opposed to stateful sessions and streaming responses). stdio for local servers. Use SSE only for backwards compatibility.

Load framework documentation:

MCP Best Practices: 📋 View Best Practices - Core guidelines (updated for 2025 spec)

For TypeScript (recommended):

TypeScript SDK: Use WebFetch to load https://raw.githubusercontent.com/modelcontextprotocol/typescript-sdk/main/README.md
⚡ TypeScript Guide - TypeScript patterns and examples

For Python:

Python SDK: Use WebFetch to load https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/main/README.md
🐍 Python Guide - Python patterns and examples

UI (optional, separate from Apps SDK):

MCP UI: https://github.com/MCP-UI-Org/mcp-ui.git (optional UI components/patterns, not required for Apps SDK)
🧭 MCP UI vs Apps SDK - when to use each

1.4 Plan Your Implementation

Understand the API: Review the service's API documentation to identify key endpoints, authentication requirements, and data models. Use web search and WebFetch as needed.

Tool Selection: Prioritize comprehensive API coverage. List endpoints to implement, starting with the most common operations.

Auth Plan (HTTP servers): If the server is HTTP-based and requires auth, design for OAuth 2.1 with Protected Resource Metadata discovery and Resource Indicators. Plan for PKCE, short-lived tokens, and strict audience validation. Avoid token passthrough.

Auth0 Implementation: Use 🔐 Auth + Security (Auth0) for concrete setup steps, validation rules, and token handling patterns.

Phase 2: Implementation

2.1 Set Up Project Structure

See language-specific guides for project setup:

⚡ TypeScript Guide - Project structure, package.json, tsconfig.json
🐍 Python Guide - Module organization, dependencies

2.2 Implement Core Infrastructure

Create shared utilities:

API client with authentication
Error handling helpers
Response formatting (JSON/Markdown)
Pagination support

2.3 Implement Tools

For each tool:

Input Schema:

Use Zod (TypeScript) or Pydantic (Python)
Include constraints and clear descriptions
Add examples in field descriptions
Ensure inputSchema is a valid JSON Schema object. For tools with no params, use { "type": "object", "additionalProperties": false }.

Output Schema:

Define outputSchema where possible for structured data
Use structuredContent in tool responses (TypeScript SDK feature)
Helps clients understand and process tool outputs
For compatibility, return serialized JSON in a TextContent block alongside structuredContent

Tool Description:

Concise summary of functionality
Parameter descriptions
Return type schema
Consider title and icons for display in UIs

Implementation:

Async/await for I/O operations
Proper error handling with actionable messages
Support pagination where applicable
Return both text content and structured data when using modern SDKs
Use resource links or embedded resources when a tool naturally returns documents or files

Annotations:

readOnlyHint: true/false
destructiveHint: true/false
idempotentHint: true/false
openWorldHint: true/false

Capabilities (Optional but Modern):

Sampling: server-side tools can request client LLM completions for assistive workflows
Elicitation: form and URL-based user input flows for secure data capture
Tasks: long-running operations with resumable/pollable execution

Phase 3: Review and Test

3.1 Code Quality

Review for:

No duplicated code (DRY principle)
Consistent error handling
Full type coverage
Clear tool descriptions

3.2 Build and Test

TypeScript:

Run npm run build to verify compilation
Test with MCP Inspector: npx @modelcontextprotocol/inspector

Python:

Verify syntax: python -m py_compile your_server.py
Test with MCP Inspector

See language-specific guides for detailed testing approaches and quality checklists. Add contract tests for JSON Schema input/output and golden snapshots for structuredContent.

Phase 4: Create Evaluations

After implementing your MCP server, create comprehensive evaluations to test its effectiveness.

Load ✅ Evaluation Guide for complete evaluation guidelines.

4.1 Understand Evaluation Purpose

Use evaluations to test whether LLMs can effectively use your MCP server to answer realistic, complex questions.

4.2 Create 10 Evaluation Questions

To create effective evaluations, follow the process outlined in the evaluation guide:

Tool Inspection: List available tools and understand their capabilities
Content Exploration: Use READ-ONLY operations to explore available data
Question Generation: Create 10 complex, realistic questions
Answer Verification: Solve each question yourself to verify answers

4.3 Evaluation Requirements

Ensure each question is:

Independent: Not dependent on other questions
Read-only: Only non-destructive operations required
Complex: Requiring multiple tool calls and deep exploration
Realistic: Based on real use cases humans would care about
Verifiable: Single, clear answer that can be verified by string comparison
Stable: Answer won't change over time

4.4 Output Format

Create an XML file with this structure:

<evaluation>
  <qa_pair>
    <question>Find discussions about AI model launches with animal codenames. One model needed a specific safety designation that uses the format ASL-X. What number X was being determined for the model named after a spotted wild cat?</question>
    <answer>3</answer>
  </qa_pair>
<!-- More qa_pairs... -->
</evaluation>

Reference Files

📚 Documentation Library

Load these resources as needed during development:

Core MCP Documentation (Load First)

MCP Protocol: Start with sitemap at https://modelcontextprotocol.io/sitemap.xml, then fetch specific pages with .md suffix
Tools spec: Pay attention to JSON Schema 2020-12, outputSchema, structuredContent, tool title/icons, and resource links
Authorization spec: OAuth 2.1, Protected Resource Metadata, Resource Indicators, PKCE, token handling
📋 MCP Best Practices - Universal MCP guidelines including:
- Server and tool naming conventions
- Response format guidelines (JSON vs Markdown)
- Pagination best practices
- Transport selection (streamable HTTP vs stdio)
- Security and error handling standards
- Structured outputs and schema contract testing

SDK Documentation (Load During Phase 1/2)

Python SDK: Fetch from https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/main/README.md
TypeScript SDK: Fetch from https://raw.githubusercontent.com/modelcontextprotocol/typescript-sdk/main/README.md

Language-Specific Implementation Guides (Load During Phase 2)

🐍 Python Implementation Guide - Complete Python/FastMCP guide with:
- Server initialization patterns
- Pydantic model examples
- Tool registration with @mcp.tool
- Complete working examples
- Quality checklist
⚡ TypeScript Implementation Guide - Complete TypeScript guide with:
- Project structure
- Zod schema patterns
- Tool registration with server.registerTool
- Complete working examples
- Quality checklist

Evaluation Guide (Load During Phase 4)

✅ Evaluation Guide - Complete evaluation creation guide with:
- Question creation guidelines
- Answer verification strategies
- XML format specifications
- Example questions and answers
- Running an evaluation with the provided scripts

Additional Reference Patterns

🔐 Auth + Security (Auth0) - OAuth 2.1 setup and validation
🧩 Tool Result Patterns - errors, pagination, resources, structured output
🧪 Review & Fix Checklist - gold-standard audit checklist (Dec 31 2025)
🛠 Review & Fix Recipes - diagnosis and fixes (TS SDK + FastMCP)
🧭 Common Fixes Matrix - symptom to fix map
🧷 FastMCP vs TS SDK Parity - consistency checklist
🧪 Test Command Recipes - quick verification commands
🚀 Deployment & Distribution - npm, tunnels, Workers, prod hosting
🧱 Apps SDK Requirements - OpenAI Apps SDK compliance
🛡 Reliability & Ops Runbook - SLOs, metrics, incident response
🧾 Spec vs SEP Notes - resolve conflicts between spec and proposals

When to use

Use this skill when the task matches its description and triggers.
If the request is outside scope, route to the referenced skill.

Inputs

User request details and any relevant files/links.

Outputs

A structured response or artifact appropriate to the skill.
Include schema_version: 1 if outputs are contract-bound.

Constraints

Redact secrets/PII by default.
Avoid destructive operations without explicit user direction.

Validation

Run any relevant checks or scripts when available.
Fail fast and report errors before proceeding.

Philosophy

Favor clarity, explicit tradeoffs, and verifiable outputs.

Anti-patterns

Avoid vague guidance without concrete steps.
Do not invent results or commands.

Procedure

Clarify scope and inputs.
Execute the core workflow.
Summarize outputs and next steps.

Antipatterns

Do not add features outside the agreed scope.

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