Building Mcp Servers

MCP Server Development Guide

Overview

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

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High-Level Workflow

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

Phase 1: Deep Research and Planning

1.1 Understand Modern MCP Design

API Coverage vs. Workflow Tools: Balance comprehensive API endpoint coverage with specialized workflow tools. When uncertain, prioritize comprehensive API coverage.

Tool Naming and Discoverability: Use consistent prefixes (e.g., github_create_issue, github_list_repos) and action-oriented naming.

Context Management: Design tools that return focused, relevant data. Support filtering/pagination.

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

1.2 Study MCP Protocol Documentation

Start with the sitemap: https://modelcontextprotocol.io/sitemap.xml

Fetch pages with .md suffix (e.g., https://modelcontextprotocol.io/specification/draft.md).

Key pages: Specification overview, transport mechanisms, tool/resource/prompt definitions.

1.3 Study Framework Documentation

Recommended stack:

• Language: TypeScript (high-quality SDK, good AI code generation)
• Transport: Streamable HTTP for remote servers, stdio for local servers

Load framework documentation:

• MCP Best Practices - Core guidelines
• TypeScript Guide - TypeScript patterns
• Python Guide - Python/FastMCP patterns

SDK Documentation:

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

1.4 Plan Your Implementation

Review the service's API documentation. List endpoints to implement, starting with most common operations.

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Phase 2: Implementation

2.1 Set Up Project Structure

See language-specific guides:

• 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

Output Schema:

• Define outputSchema where possible
• Use structuredContent in responses

Tool Description:

• Concise summary, parameter descriptions, return type

Annotations:

• readOnlyHint, destructiveHint, idempotentHint, openWorldHint

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Phase 3: Review and Test

3.1 Code Quality

Review for: DRY principle, consistent error handling, full type coverage, clear descriptions.

3.2 Build and Test

TypeScript:

npm run build
npx @modelcontextprotocol/inspector

Python:

python -m py_compile your_server.py
# Test with MCP Inspector

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Phase 4: Create Evaluations

Create 10 evaluation questions to test LLM effectiveness with your server.

Requirements for each question:

• Independent, read-only, complex, realistic, verifiable, stable

Output Format:

<evaluation>
  <qa_pair>
    <question>Your question here</question>
    <answer>Expected answer</answer>
  </qa_pair>
</evaluation>

See Evaluation Guide for complete guidelines.

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Docker/Containerization

Transport Security (allowed_hosts)

FastMCP validates Host headers. For Docker, configure:

from mcp.server.fastmcp import FastMCP
from mcp.server.transport_security import TransportSecuritySettings

transport_security = TransportSecuritySettings(
    allowed_hosts=[
        "127.0.0.1:*", "localhost:*", "[::1]:*",
        "mcp-server:*",  # Docker container name
        "0.0.0.0:*",
    ],
)
mcp = FastMCP("my_server", transport_security=transport_security)

Health Check Endpoint

Add /health endpoint via middleware (see references for full example).

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Verification

Run: python3 scripts/verify.py

Expected: ✓ building-mcp-servers skill ready

If Verification Fails

1. Run diagnostic: Check references/ folder exists 2. Check: All reference files present 3. Stop and report if still failing

References

• MCP Best Practices - Universal guidelines
• Python Guide - Python/FastMCP patterns
• TypeScript Guide - TypeScript patterns
• TaskFlow Patterns - Internal server patterns
• Evaluation Guide - Creating evaluations

MCP Server Evaluation Guide

Overview

This document provides guidance on creating comprehensive evaluations for MCP servers. Evaluations test whether LLMs can effectively use your MCP server to answer realistic, complex questions using only the tools provided.

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Quick Reference

Evaluation Requirements

• Create 10 human-readable questions
• Questions must be READ-ONLY, INDEPENDENT, NON-DESTRUCTIVE
• Each question requires multiple tool calls (potentially dozens)
• Answers must be single, verifiable values
• Answers must be STABLE (won't change over time)

Output Format

<evaluation>
   <qa_pair>
      <question>Your question here</question>
      <answer>Single verifiable answer</answer>
   </qa_pair>
</evaluation>

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Purpose of Evaluations

The measure of quality of an MCP server is NOT how well or comprehensively the server implements tools, but how well these implementations (input/output schemas, docstrings/descriptions, functionality) enable LLMs with no other context and access ONLY to the MCP servers to answer realistic and difficult questions.

Evaluation Overview

Create 10 human-readable questions requiring ONLY READ-ONLY, INDEPENDENT, NON-DESTRUCTIVE, and IDEMPOTENT operations to answer. Each question should be:

• Realistic
• Clear and concise
• Unambiguous
• Complex, requiring potentially dozens of tool calls or steps
• Answerable with a single, verifiable value that you identify in advance

Question Guidelines

Core Requirements

1. Questions MUST be independent

• Each question should NOT depend on the answer to any other question
• Should not assume prior write operations from processing another question

2. Questions MUST require ONLY NON-DESTRUCTIVE AND IDEMPOTENT tool use

• Should not instruct or require modifying state to arrive at the correct answer

3. Questions must be REALISTIC, CLEAR, CONCISE, and COMPLEX

• Must require another LLM to use multiple (potentially dozens of) tools or steps to answer

Complexity and Depth

4. Questions must require deep exploration

• Consider multi-hop questions requiring multiple sub-questions and sequential tool calls
• Each step should benefit from information found in previous questions

5. Questions may require extensive paging

• May need paging through multiple pages of results
• May require querying old data (1-2 years out-of-date) to find niche information
• The questions must be DIFFICULT

6. Questions must require deep understanding

• Rather than surface-level knowledge
• May pose complex ideas as True/False questions requiring evidence
• May use multiple-choice format where LLM must search different hypotheses

7. Questions must not be solvable with straightforward keyword search

• Do not include specific keywords from the target content
• Use synonyms, related concepts, or paraphrases
• Require multiple searches, analyzing multiple related items, extracting context, then deriving the answer

Tool Testing

8. Questions should stress-test tool return values

• May elicit tools returning large JSON objects or lists, overwhelming the LLM
• Should require understanding multiple modalities of data:
• IDs and names
• Timestamps and datetimes (months, days, years, seconds)
• File IDs, names, extensions, and mimetypes
• URLs, GIDs, etc.
• Should probe the tool's ability to return all useful forms of data

9. Questions should MOSTLY reflect real human use cases

• The kinds of information retrieval tasks that HUMANS assisted by an LLM would care about

10. Questions may require dozens of tool calls

• This challenges LLMs with limited context
• Encourages MCP server tools to reduce information returned

11. Include ambiguous questions

• May be ambiguous OR require difficult decisions on which tools to call
• Force the LLM to potentially make mistakes or misinterpret
• Ensure that despite AMBIGUITY, there is STILL A SINGLE VERIFIABLE ANSWER

Stability

12. Questions must be designed so the answer DOES NOT CHANGE

• Do not ask questions that rely on "current state" which is dynamic
• For example, do not count:
• Number of reactions to a post
• Number of replies to a thread
• Number of members in a channel

13. DO NOT let the MCP server RESTRICT the kinds of questions you create

• Create challenging and complex questions
• Some may not be solvable with the available MCP server tools
• Questions may require specific output formats (datetime vs. epoch time, JSON vs. MARKDOWN)
• Questions may require dozens of tool calls to complete

Answer Guidelines

Verification

1. Answers must be VERIFIABLE via direct string comparison

• If the answer can be re-written in many formats, clearly specify the output format in the QUESTION
• Examples: "Use YYYY/MM/DD.", "Respond True or False.", "Answer A, B, C, or D and nothing else."
• Answer should be a single VERIFIABLE value such as:
• User ID, user name, display name, first name, last name
• Channel ID, channel name
• Message ID, string
• URL, title
• Numerical quantity
• Timestamp, datetime
• Boolean (for True/False questions)
• Email address, phone number
• File ID, file name, file extension
• Multiple choice answer
• Answers must not require special formatting or complex, structured output
• Answer will be verified using DIRECT STRING COMPARISON

Readability

2. Answers should generally prefer HUMAN-READABLE formats

• Examples: names, first name, last name, datetime, file name, message string, URL, yes/no, true/false, a/b/c/d
• Rather than opaque IDs (though IDs are acceptable)
• The VAST MAJORITY of answers should be human-readable

Stability

3. Answers must be STABLE/STATIONARY

• Look at old content (e.g., conversations that have ended, projects that have launched, questions answered)
• Create QUESTIONS based on "closed" concepts that will always return the same answer
• Questions may ask to consider a fixed time window to insulate from non-stationary answers
• Rely on context UNLIKELY to change
• Example: if finding a paper name, be SPECIFIC enough so answer is not confused with papers published later

4. Answers must be CLEAR and UNAMBIGUOUS

• Questions must be designed so there is a single, clear answer
• Answer can be derived from using the MCP server tools

Diversity

5. Answers must be DIVERSE

• Answer should be a single VERIFIABLE value in diverse modalities and formats
• User concept: user ID, user name, display name, first name, last name, email address, phone number
• Channel concept: channel ID, channel name, channel topic
• Message concept: message ID, message string, timestamp, month, day, year

6. Answers must NOT be complex structures

• Not a list of values
• Not a complex object
• Not a list of IDs or strings
• Not natural language text
• UNLESS the answer can be straightforwardly verified using DIRECT STRING COMPARISON
• And can be realistically reproduced
• It should be unlikely that an LLM would return the same list in any other order or format

Evaluation Process

Step 1: Documentation Inspection

Read the documentation of the target API to understand:

• Available endpoints and functionality
• If ambiguity exists, fetch additional information from the web
• Parallelize this step AS MUCH AS POSSIBLE
• Ensure each subagent is ONLY examining documentation from the file system or on the web

Step 2: Tool Inspection

List the tools available in the MCP server:

• Inspect the MCP server directly
• Understand input/output schemas, docstrings, and descriptions
• WITHOUT calling the tools themselves at this stage

Step 3: Developing Understanding

Repeat steps 1 & 2 until you have a good understanding:

• Iterate multiple times
• Think about the kinds of tasks you want to create
• Refine your understanding
• At NO stage should you READ the code of the MCP server implementation itself
• Use your intuition and understanding to create reasonable, realistic, but VERY challenging tasks

Step 4: Read-Only Content Inspection

After understanding the API and tools, USE the MCP server tools:

• Inspect content using READ-ONLY and NON-DESTRUCTIVE operations ONLY
• Goal: identify specific content (e.g., users, channels, messages, projects, tasks) for creating realistic questions
• Should NOT call any tools that modify state
• Will NOT read the code of the MCP server implementation itself
• Parallelize this step with individual sub-agents pursuing independent explorations
• Ensure each subagent is only performing READ-ONLY, NON-DESTRUCTIVE, and IDEMPOTENT operations
• BE CAREFUL: SOME TOOLS may return LOTS OF DATA which would cause you to run out of CONTEXT
• Make INCREMENTAL, SMALL, AND TARGETED tool calls for exploration
• In all tool call requests, use the limit parameter to limit results (<10)
• Use pagination

Step 5: Task Generation

After inspecting the content, create 10 human-readable questions:

• An LLM should be able to answer these with the MCP server
• Follow all question and answer guidelines above

Output Format

Each QA pair consists of a question and an answer. The output should be an XML file with this structure:

<evaluation>
   <qa_pair>
      <question>Find the project created in Q2 2024 with the highest number of completed tasks. What is the project name?</question>
      <answer>Website Redesign</answer>
   </qa_pair>
   <qa_pair>
      <question>Search for issues labeled as "bug" that were closed in March 2024. Which user closed the most issues? Provide their username.</question>
      <answer>sarah_dev</answer>
   </qa_pair>
   <qa_pair>
      <question>Look for pull requests that modified files in the /api directory and were merged between January 1 and January 31, 2024. How many different contributors worked on these PRs?</question>
      <answer>7</answer>
   </qa_pair>
   <qa_pair>
      <question>Find the repository with the most stars that was created before 2023. What is the repository name?</question>
      <answer>data-pipeline</answer>
   </qa_pair>
</evaluation>

Evaluation Examples

Good Questions

Example 1: Multi-hop question requiring deep exploration (GitHub MCP)

<qa_pair>
   <question>Find the repository that was archived in Q3 2023 and had previously been the most forked project in the organization. What was the primary programming language used in that repository?</question>
   <answer>Python</answer>
</qa_pair>

This question is good because:

• Requires multiple searches to find archived repositories
• Needs to identify which had the most forks before archival
• Requires examining repository details for the language
• Answer is a simple, verifiable value
• Based on historical (closed) data that won't change

Example 2: Requires understanding context without keyword matching (Project Management MCP)

<qa_pair>
   <question>Locate the initiative focused on improving customer onboarding that was completed in late 2023. The project lead created a retrospective document after completion. What was the lead's role title at that time?</question>
   <answer>Product Manager</answer>
</qa_pair>

This question is good because:

• Doesn't use specific project name ("initiative focused on improving customer onboarding")
• Requires finding completed projects from specific timeframe
• Needs to identify the project lead and their role
• Requires understanding context from retrospective documents
• Answer is human-readable and stable
• Based on completed work (won't change)

Example 3: Complex aggregation requiring multiple steps (Issue Tracker MCP)

<qa_pair>
   <question>Among all bugs reported in January 2024 that were marked as critical priority, which assignee resolved the highest percentage of their assigned bugs within 48 hours? Provide the assignee's username.</question>
   <answer>alex_eng</answer>
</qa_pair>

This question is good because:

• Requires filtering bugs by date, priority, and status
• Needs to group by assignee and calculate resolution rates
• Requires understanding timestamps to determine 48-hour windows
• Tests pagination (potentially many bugs to process)
• Answer is a single username
• Based on historical data from specific time period

Example 4: Requires synthesis across multiple data types (CRM MCP)

<qa_pair>
   <question>Find the account that upgraded from the Starter to Enterprise plan in Q4 2023 and had the highest annual contract value. What industry does this account operate in?</question>
   <answer>Healthcare</answer>
</qa_pair>

This question is good because:

• Requires understanding subscription tier changes
• Needs to identify upgrade events in specific timeframe
• Requires comparing contract values
• Must access account industry information
• Answer is simple and verifiable
• Based on completed historical transactions

Poor Questions

Example 1: Answer changes over time

<qa_pair>
   <question>How many open issues are currently assigned to the engineering team?</question>
   <answer>47</answer>
</qa_pair>

This question is poor because:

• The answer will change as issues are created, closed, or reassigned
• Not based on stable/stationary data
• Relies on "current state" which is dynamic

Example 2: Too easy with keyword search

<qa_pair>
   <question>Find the pull request with title "Add authentication feature" and tell me who created it.</question>
   <answer>developer123</answer>
</qa_pair>

This question is poor because:

• Can be solved with a straightforward keyword search for exact title
• Doesn't require deep exploration or understanding
• No synthesis or analysis needed

Example 3: Ambiguous answer format

<qa_pair>
   <question>List all the repositories that have Python as their primary language.</question>
   <answer>repo1, repo2, repo3, data-pipeline, ml-tools</answer>
</qa_pair>

This question is poor because:

• Answer is a list that could be returned in any order
• Difficult to verify with direct string comparison
• LLM might format differently (JSON array, comma-separated, newline-separated)
• Better to ask for a specific aggregate (count) or superlative (most stars)

Verification Process

After creating evaluations:

1. Examine the XML file to understand the schema 2. Load each task instruction and in parallel using the MCP server and tools, identify the correct answer by attempting to solve the task YOURSELF 3. Flag any operations that require WRITE or DESTRUCTIVE operations 4. Accumulate all CORRECT answers and replace any incorrect answers in the document 5. Remove any `<qa_pair>` that require WRITE or DESTRUCTIVE operations

Remember to parallelize solving tasks to avoid running out of context, then accumulate all answers and make changes to the file at the end.

Tips for Creating Quality Evaluations

1. Think Hard and Plan Ahead before generating tasks 2. Parallelize Where Opportunity Arises to speed up the process and manage context 3. Focus on Realistic Use Cases that humans would actually want to accomplish 4. Create Challenging Questions that test the limits of the MCP server's capabilities 5. Ensure Stability by using historical data and closed concepts 6. Verify Answers by solving the questions yourself using the MCP server tools 7. Iterate and Refine based on what you learn during the process

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Running Evaluations

After creating your evaluation file, you can use the provided evaluation harness to test your MCP server.

Setup

1. Install Dependencies

   pip install -r scripts/requirements.txt

Or install manually:

   pip install anthropic mcp

2. Set API Key

   export ANTHROPIC_API_KEY=your_api_key_here

Evaluation File Format

Evaluation files use XML format with <qa_pair> elements:

<evaluation>
   <qa_pair>
      <question>Find the project created in Q2 2024 with the highest number of completed tasks. What is the project name?</question>
      <answer>Website Redesign</answer>
   </qa_pair>
   <qa_pair>
      <question>Search for issues labeled as "bug" that were closed in March 2024. Which user closed the most issues? Provide their username.</question>
      <answer>sarah_dev</answer>
   </qa_pair>
</evaluation>

Running Evaluations

The evaluation script (scripts/evaluation.py) supports three transport types:

Important:

• stdio transport: The evaluation script automatically launches and manages the MCP server process for you. Do not run the server manually.
• sse/http transports: You must start the MCP server separately before running the evaluation. The script connects to the already-running server at the specified URL.

1. Local STDIO Server

For locally-run MCP servers (script launches the server automatically):

python scripts/evaluation.py \
  -t stdio \
  -c python \
  -a my_mcp_server.py \
  evaluation.xml

With environment variables:

python scripts/evaluation.py \
  -t stdio \
  -c python \
  -a my_mcp_server.py \
  -e API_KEY=abc123 \
  -e DEBUG=true \
  evaluation.xml

2. Server-Sent Events (SSE)

For SSE-based MCP servers (you must start the server first):

python scripts/evaluation.py \
  -t sse \
  -u https://example.com/mcp \
  -H "Authorization: Bearer token123" \
  -H "X-Custom-Header: value" \
  evaluation.xml

3. HTTP (Streamable HTTP)

For HTTP-based MCP servers (you must start the server first):

python scripts/evaluation.py \
  -t http \
  -u https://example.com/mcp \
  -H "Authorization: Bearer token123" \
  evaluation.xml

Command-Line Options

usage: evaluation.py [-h] [-t {stdio,sse,http}] [-m MODEL] [-c COMMAND]
                     [-a ARGS [ARGS ...]] [-e ENV [ENV ...]] [-u URL]
                     [-H HEADERS [HEADERS ...]] [-o OUTPUT]
                     eval_file

positional arguments:
  eval_file             Path to evaluation XML file

optional arguments:
  -h, --help            Show help message
  -t, --transport       Transport type: stdio, sse, or http (default: stdio)
  -m, --model           Claude model to use (default: claude-3-7-sonnet-20250219)
  -o, --output          Output file for report (default: print to stdout)

stdio options:
  -c, --command         Command to run MCP server (e.g., python, node)
  -a, --args            Arguments for the command (e.g., server.py)
  -e, --env             Environment variables in KEY=VALUE format

sse/http options:
  -u, --url             MCP server URL
  -H, --header          HTTP headers in 'Key: Value' format

Output

The evaluation script generates a detailed report including:

• Summary Statistics:
• Accuracy (correct/total)
• Average task duration
• Average tool calls per task
• Total tool calls

• Per-Task Results:
• Prompt and expected response
• Actual response from the agent
• Whether the answer was correct (✅/❌)
• Duration and tool call details
• Agent's summary of its approach
• Agent's feedback on the tools

Save Report to File

python scripts/evaluation.py \
  -t stdio \
  -c python \
  -a my_server.py \
  -o evaluation_report.md \
  evaluation.xml

Complete Example Workflow

Here's a complete example of creating and running an evaluation:

1. Create your evaluation file (my_evaluation.xml):

<evaluation>
   <qa_pair>
      <question>Find the user who created the most issues in January 2024. What is their username?</question>
      <answer>alice_developer</answer>
   </qa_pair>
   <qa_pair>
      <question>Among all pull requests merged in Q1 2024, which repository had the highest number? Provide the repository name.</question>
      <answer>backend-api</answer>
   </qa_pair>
   <qa_pair>
      <question>Find the project that was completed in December 2023 and had the longest duration from start to finish. How many days did it take?</question>
      <answer>127</answer>
   </qa_pair>
</evaluation>

2. Install dependencies:

pip install -r scripts/requirements.txt
export ANTHROPIC_API_KEY=your_api_key

3. Run evaluation:

python scripts/evaluation.py \
  -t stdio \
  -c python \
  -a github_mcp_server.py \
  -e GITHUB_TOKEN=ghp_xxx \
  -o github_eval_report.md \
  my_evaluation.xml

4. Review the report in github_eval_report.md to:

• See which questions passed/failed
• Read the agent's feedback on your tools
• Identify areas for improvement
• Iterate on your MCP server design

Troubleshooting

Connection Errors

If you get connection errors:

• STDIO: Verify the command and arguments are correct
• SSE/HTTP: Check the URL is accessible and headers are correct
• Ensure any required API keys are set in environment variables or headers

Low Accuracy

If many evaluations fail:

• Review the agent's feedback for each task
• Check if tool descriptions are clear and comprehensive
• Verify input parameters are well-documented
• Consider whether tools return too much or too little data
• Ensure error messages are actionable

Timeout Issues

If tasks are timing out:

• Use a more capable model (e.g., claude-3-7-sonnet-20250219)
• Check if tools are returning too much data
• Verify pagination is working correctly
• Consider simplifying complex questions

MCP Server Best Practices

Quick Reference

Server Naming

• Python: {service}_mcp (e.g., slack_mcp)
• Node/TypeScript: {service}-mcp-server (e.g., slack-mcp-server)

Tool Naming

• Use snake_case with service prefix
• Format: {service}_{action}_{resource}
• Example: slack_send_message, github_create_issue

Response Formats

• Support both JSON and Markdown formats
• JSON for programmatic processing
• Markdown for human readability

Pagination

• Always respect limit parameter
• Return has_more, next_offset, total_count
• Default to 20-50 items

Transport

• Streamable HTTP: For remote servers, multi-client scenarios
• stdio: For local integrations, command-line tools
• Avoid SSE (deprecated in favor of streamable HTTP)

---

Server Naming Conventions

Follow these standardized naming patterns:

Python: Use format {service}_mcp (lowercase with underscores)

• Examples: slack_mcp, github_mcp, jira_mcp

Node/TypeScript: Use format {service}-mcp-server (lowercase with hyphens)

• Examples: slack-mcp-server, github-mcp-server, jira-mcp-server

The name should be general, descriptive of the service being integrated, easy to infer from the task description, and without version numbers.

---

Tool Naming and Design

Tool Naming

1. Use snake_case: search_users, create_project, get_channel_info 2. Include service prefix: Anticipate that your MCP server may be used alongside other MCP servers

• Use slack_send_message instead of just send_message
• Use github_create_issue instead of just create_issue

3. Be action-oriented: Start with verbs (get, list, search, create, etc.) 4. Be specific: Avoid generic names that could conflict with other servers

Tool Design

• Tool descriptions must narrowly and unambiguously describe functionality
• Descriptions must precisely match actual functionality
• Provide tool annotations (readOnlyHint, destructiveHint, idempotentHint, openWorldHint)
• Keep tool operations focused and atomic

---

Response Formats

All tools that return data should support multiple formats:

JSON Format (response_format="json")

• Machine-readable structured data
• Include all available fields and metadata
• Consistent field names and types
• Use for programmatic processing

Markdown Format (response_format="markdown", typically default)

• Human-readable formatted text
• Use headers, lists, and formatting for clarity
• Convert timestamps to human-readable format
• Show display names with IDs in parentheses
• Omit verbose metadata

---

Pagination

For tools that list resources:

• Always respect the `limit` parameter
• Implement pagination: Use offset or cursor-based pagination
• Return pagination metadata: Include has_more, next_offset/next_cursor, total_count
• Never load all results into memory: Especially important for large datasets
• Default to reasonable limits: 20-50 items is typical

Example pagination response:

{
  "total": 150,
  "count": 20,
  "offset": 0,
  "items": [...],
  "has_more": true,
  "next_offset": 20
}

---

Transport Options

Streamable HTTP

Best for: Remote servers, web services, multi-client scenarios

Characteristics:

• Bidirectional communication over HTTP
• Supports multiple simultaneous clients
• Can be deployed as a web service
• Enables server-to-client notifications

Use when:

• Serving multiple clients simultaneously
• Deploying as a cloud service
• Integration with web applications

stdio

Best for: Local integrations, command-line tools

Characteristics:

• Standard input/output stream communication
• Simple setup, no network configuration needed
• Runs as a subprocess of the client

Use when:

• Building tools for local development environments
• Integrating with desktop applications
• Single-user, single-session scenarios

Note: stdio servers should NOT log to stdout (use stderr for logging)

Transport Selection

---

Security Best Practices

Authentication and Authorization

OAuth 2.1:

• Use secure OAuth 2.1 with certificates from recognized authorities
• Validate access tokens before processing requests
• Only accept tokens specifically intended for your server

API Keys:

• Store API keys in environment variables, never in code
• Validate keys on server startup
• Provide clear error messages when authentication fails

Input Validation

• Sanitize file paths to prevent directory traversal
• Validate URLs and external identifiers
• Check parameter sizes and ranges
• Prevent command injection in system calls
• Use schema validation (Pydantic/Zod) for all inputs

Error Handling

• Don't expose internal errors to clients
• Log security-relevant errors server-side
• Provide helpful but not revealing error messages
• Clean up resources after errors

DNS Rebinding Protection

For streamable HTTP servers running locally:

• Enable DNS rebinding protection
• Validate the Origin header on all incoming connections
• Bind to 127.0.0.1 rather than 0.0.0.0

---

Tool Annotations

Provide annotations to help clients understand tool behavior:

Important: Annotations are hints, not security guarantees. Clients should not make security-critical decisions based solely on annotations.

---

Error Handling

• Use standard JSON-RPC error codes
• Report tool errors within result objects (not protocol-level errors)
• Provide helpful, specific error messages with suggested next steps
• Don't expose internal implementation details
• Clean up resources properly on errors

Example error handling:

try {
  const result = performOperation();
  return { content: [{ type: "text", text: result }] };
} catch (error) {
  return {
    isError: true,
    content: [{
      type: "text",
      text: `Error: ${error.message}. Try using filter='active_only' to reduce results.`
    }]
  };
}

---

Testing Requirements

Comprehensive testing should cover:

• Functional testing: Verify correct execution with valid/invalid inputs
• Integration testing: Test interaction with external systems
• Security testing: Validate auth, input sanitization, rate limiting
• Performance testing: Check behavior under load, timeouts
• Error handling: Ensure proper error reporting and cleanup

---

Documentation Requirements

• Provide clear documentation of all tools and capabilities
• Include working examples (at least 3 per major feature)
• Document security considerations
• Specify required permissions and access levels
• Document rate limits and performance characteristics

Node/TypeScript MCP Server Implementation Guide

Overview

This document provides Node/TypeScript-specific best practices and examples for implementing MCP servers using the MCP TypeScript SDK. It covers project structure, server setup, tool registration patterns, input validation with Zod, error handling, and complete working examples.

---

Quick Reference

Key Imports

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/streamableHttp.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import express from "express";
import { z } from "zod";

Server Initialization

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

Tool Registration Pattern

server.registerTool(
  "tool_name",
  {
    title: "Tool Display Name",
    description: "What the tool does",
    inputSchema: { param: z.string() },
    outputSchema: { result: z.string() }
  },
  async ({ param }) => {
    const output = { result: `Processed: ${param}` };
    return {
      content: [{ type: "text", text: JSON.stringify(output) }],
      structuredContent: output // Modern pattern for structured data
    };
  }
);

---

MCP TypeScript SDK

The official MCP TypeScript SDK provides:

• McpServer class for server initialization
• registerTool method for tool registration
• Zod schema integration for runtime input validation
• Type-safe tool handler implementations

IMPORTANT - Use Modern APIs Only:

• DO use: server.registerTool(), server.registerResource(), server.registerPrompt()
• DO NOT use: Old deprecated APIs such as server.tool(), server.setRequestHandler(ListToolsRequestSchema, ...), or manual handler registration
• The register* methods provide better type safety, automatic schema handling, and are the recommended approach

See the MCP SDK documentation in the references for complete details.

Server Naming Convention

Node/TypeScript MCP servers must follow this naming pattern:

• Format: {service}-mcp-server (lowercase with hyphens)
• Examples: github-mcp-server, jira-mcp-server, stripe-mcp-server

The name should be:

• General (not tied to specific features)
• Descriptive of the service/API being integrated
• Easy to infer from the task description
• Without version numbers or dates

Project Structure

Create the following structure for Node/TypeScript MCP servers:

{service}-mcp-server/
├── package.json
├── tsconfig.json
├── README.md
├── src/
│   ├── index.ts          # Main entry point with McpServer initialization
│   ├── types.ts          # TypeScript type definitions and interfaces
│   ├── tools/            # Tool implementations (one file per domain)
│   ├── services/         # API clients and shared utilities
│   ├── schemas/          # Zod validation schemas
│   └── constants.ts      # Shared constants (API_URL, CHARACTER_LIMIT, etc.)
└── dist/                 # Built JavaScript files (entry point: dist/index.js)

Tool Implementation

Tool Naming

Use snake_case for tool names (e.g., "search_users", "create_project", "get_channel_info") with clear, action-oriented names.

Avoid Naming Conflicts: Include the service context to prevent overlaps:

• Use "slack_send_message" instead of just "send_message"
• Use "github_create_issue" instead of just "create_issue"
• Use "asana_list_tasks" instead of just "list_tasks"

Tool Structure

Tools are registered using the registerTool method with the following requirements:

• Use Zod schemas for runtime input validation and type safety
• The description field must be explicitly provided - JSDoc comments are NOT automatically extracted
• Explicitly provide title, description, inputSchema, and annotations
• The inputSchema must be a Zod schema object (not a JSON schema)
• Type all parameters and return values explicitly

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { z } from "zod";

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

// Zod schema for input validation
const UserSearchInputSchema = z.object({
  query: z.string()
    .min(2, "Query must be at least 2 characters")
    .max(200, "Query must not exceed 200 characters")
    .describe("Search string to match against names/emails"),
  limit: z.number()
    .int()
    .min(1)
    .max(100)
    .default(20)
    .describe("Maximum results to return"),
  offset: z.number()
    .int()
    .min(0)
    .default(0)
    .describe("Number of results to skip for pagination"),
  response_format: z.nativeEnum(ResponseFormat)
    .default(ResponseFormat.MARKDOWN)
    .describe("Output format: 'markdown' for human-readable or 'json' for machine-readable")
}).strict();

// Type definition from Zod schema
type UserSearchInput = z.infer<typeof UserSearchInputSchema>;

server.registerTool(
  "example_search_users",
  {
    title: "Search Example Users",
    description: `Search for users in the Example system by name, email, or team.

This tool searches across all user profiles in the Example platform, supporting partial matches and various search filters. It does NOT create or modify users, only searches existing ones.

Args:
  - query (string): Search string to match against names/emails
  - limit (number): Maximum results to return, between 1-100 (default: 20)
  - offset (number): Number of results to skip for pagination (default: 0)
  - response_format ('markdown' | 'json'): Output format (default: 'markdown')

Returns:
  For JSON format: Structured data with schema:
  {
    "total": number,           // Total number of matches found
    "count": number,           // Number of results in this response
    "offset": number,          // Current pagination offset
    "users": [
      {
        "id": string,          // User ID (e.g., "U123456789")
        "name": string,        // Full name (e.g., "John Doe")
        "email": string,       // Email address
        "team": string,        // Team name (optional)
        "active": boolean      // Whether user is active
      }
    ],
    "has_more": boolean,       // Whether more results are available
    "next_offset": number      // Offset for next page (if has_more is true)
  }

Examples:
  - Use when: "Find all marketing team members" -> params with query="team:marketing"
  - Use when: "Search for John's account" -> params with query="john"
  - Don't use when: You need to create a user (use example_create_user instead)

Error Handling:
  - Returns "Error: Rate limit exceeded" if too many requests (429 status)
  - Returns "No users found matching '<query>'" if search returns empty`,
    inputSchema: UserSearchInputSchema,
    annotations: {
      readOnlyHint: true,
      destructiveHint: false,
      idempotentHint: true,
      openWorldHint: true
    }
  },
  async (params: UserSearchInput) => {
    try {
      // Input validation is handled by Zod schema
      // Make API request using validated parameters
      const data = await makeApiRequest<any>(
        "users/search",
        "GET",
        undefined,
        {
          q: params.query,
          limit: params.limit,
          offset: params.offset
        }
      );

      const users = data.users || [];
      const total = data.total || 0;

      if (!users.length) {
        return {
          content: [{
            type: "text",
            text: `No users found matching '${params.query}'`
          }]
        };
      }

      // Prepare structured output
      const output = {
        total,
        count: users.length,
        offset: params.offset,
        users: users.map((user: any) => ({
          id: user.id,
          name: user.name,
          email: user.email,
          ...(user.team ? { team: user.team } : {}),
          active: user.active ?? true
        })),
        has_more: total > params.offset + users.length,
        ...(total > params.offset + users.length ? {
          next_offset: params.offset + users.length
        } : {})
      };

      // Format text representation based on requested format
      let textContent: string;
      if (params.response_format === ResponseFormat.MARKDOWN) {
        const lines = [`# User Search Results: '${params.query}'`, "",
          `Found ${total} users (showing ${users.length})`, ""];
        for (const user of users) {
          lines.push(`## ${user.name} (${user.id})`);
          lines.push(`- **Email**: ${user.email}`);
          if (user.team) lines.push(`- **Team**: ${user.team}`);
          lines.push("");
        }
        textContent = lines.join("\n");
      } else {
        textContent = JSON.stringify(output, null, 2);
      }

      return {
        content: [{ type: "text", text: textContent }],
        structuredContent: output // Modern pattern for structured data
      };
    } catch (error) {
      return {
        content: [{
          type: "text",
          text: handleApiError(error)
        }]
      };
    }
  }
);

Zod Schemas for Input Validation

Zod provides runtime type validation:

import { z } from "zod";

// Basic schema with validation
const CreateUserSchema = z.object({
  name: z.string()
    .min(1, "Name is required")
    .max(100, "Name must not exceed 100 characters"),
  email: z.string()
    .email("Invalid email format"),
  age: z.number()
    .int("Age must be a whole number")
    .min(0, "Age cannot be negative")
    .max(150, "Age cannot be greater than 150")
}).strict();  // Use .strict() to forbid extra fields

// Enums
enum ResponseFormat {
  MARKDOWN = "markdown",
  JSON = "json"
}

const SearchSchema = z.object({
  response_format: z.nativeEnum(ResponseFormat)
    .default(ResponseFormat.MARKDOWN)
    .describe("Output format")
});

// Optional fields with defaults
const PaginationSchema = z.object({
  limit: z.number()
    .int()
    .min(1)
    .max(100)
    .default(20)
    .describe("Maximum results to return"),
  offset: z.number()
    .int()
    .min(0)
    .default(0)
    .describe("Number of results to skip")
});

Response Format Options

Support multiple output formats for flexibility:

enum ResponseFormat {
  MARKDOWN = "markdown",
  JSON = "json"
}

const inputSchema = z.object({
  query: z.string(),
  response_format: z.nativeEnum(ResponseFormat)
    .default(ResponseFormat.MARKDOWN)
    .describe("Output format: 'markdown' for human-readable or 'json' for machine-readable")
});

Markdown format:

• Use headers, lists, and formatting for clarity
• Convert timestamps to human-readable format
• Show display names with IDs in parentheses
• Omit verbose metadata
• Group related information logically

JSON format:

• Return complete, structured data suitable for programmatic processing
• Include all available fields and metadata
• Use consistent field names and types

Pagination Implementation

For tools that list resources:

const ListSchema = z.object({
  limit: z.number().int().min(1).max(100).default(20),
  offset: z.number().int().min(0).default(0)
});

async function listItems(params: z.infer<typeof ListSchema>) {
  const data = await apiRequest(params.limit, params.offset);

  const response = {
    total: data.total,
    count: data.items.length,
    offset: params.offset,
    items: data.items,
    has_more: data.total > params.offset + data.items.length,
    next_offset: data.total > params.offset + data.items.length
      ? params.offset + data.items.length
      : undefined
  };

  return JSON.stringify(response, null, 2);
}

Character Limits and Truncation

Add a CHARACTER_LIMIT constant to prevent overwhelming responses:

// At module level in constants.ts
export const CHARACTER_LIMIT = 25000;  // Maximum response size in characters

async function searchTool(params: SearchInput) {
  let result = generateResponse(data);

  // Check character limit and truncate if needed
  if (result.length > CHARACTER_LIMIT) {
    const truncatedData = data.slice(0, Math.max(1, data.length / 2));
    response.data = truncatedData;
    response.truncated = true;
    response.truncation_message =
      `Response truncated from ${data.length} to ${truncatedData.length} items. ` +
      `Use 'offset' parameter or add filters to see more results.`;
    result = JSON.stringify(response, null, 2);
  }

  return result;
}

Error Handling

Provide clear, actionable error messages:

import axios, { AxiosError } from "axios";

function handleApiError(error: unknown): string {
  if (error instanceof AxiosError) {
    if (error.response) {
      switch (error.response.status) {
        case 404:
          return "Error: Resource not found. Please check the ID is correct.";
        case 403:
          return "Error: Permission denied. You don't have access to this resource.";
        case 429:
          return "Error: Rate limit exceeded. Please wait before making more requests.";
        default:
          return `Error: API request failed with status ${error.response.status}`;
      }
    } else if (error.code === "ECONNABORTED") {
      return "Error: Request timed out. Please try again.";
    }
  }
  return `Error: Unexpected error occurred: ${error instanceof Error ? error.message : String(error)}`;
}

Shared Utilities

Extract common functionality into reusable functions:

// Shared API request function
async function makeApiRequest<T>(
  endpoint: string,
  method: "GET" | "POST" | "PUT" | "DELETE" = "GET",
  data?: any,
  params?: any
): Promise<T> {
  try {
    const response = await axios({
      method,
      url: `${API_BASE_URL}/${endpoint}`,
      data,
      params,
      timeout: 30000,
      headers: {
        "Content-Type": "application/json",
        "Accept": "application/json"
      }
    });
    return response.data;
  } catch (error) {
    throw error;
  }
}

Async/Await Best Practices

Always use async/await for network requests and I/O operations:

// Good: Async network request
async function fetchData(resourceId: string): Promise<ResourceData> {
  const response = await axios.get(`${API_URL}/resource/${resourceId}`);
  return response.data;
}

// Bad: Promise chains
function fetchData(resourceId: string): Promise<ResourceData> {
  return axios.get(`${API_URL}/resource/${resourceId}`)
    .then(response => response.data);  // Harder to read and maintain
}

TypeScript Best Practices

1. Use Strict TypeScript: Enable strict mode in tsconfig.json 2. Define Interfaces: Create clear interface definitions for all data structures 3. Avoid `any`: Use proper types or unknown instead of any 4. Zod for Runtime Validation: Use Zod schemas to validate external data 5. Type Guards: Create type guard functions for complex type checking 6. Error Handling: Always use try-catch with proper error type checking 7. Null Safety: Use optional chaining (?.) and nullish coalescing (??)

// Good: Type-safe with Zod and interfaces
interface UserResponse {
  id: string;
  name: string;
  email: string;
  team?: string;
  active: boolean;
}

const UserSchema = z.object({
  id: z.string(),
  name: z.string(),
  email: z.string().email(),
  team: z.string().optional(),
  active: z.boolean()
});

type User = z.infer<typeof UserSchema>;

async function getUser(id: string): Promise<User> {
  const data = await apiCall(`/users/${id}`);
  return UserSchema.parse(data);  // Runtime validation
}

// Bad: Using any
async function getUser(id: string): Promise<any> {
  return await apiCall(`/users/${id}`);  // No type safety
}

Package Configuration

package.json

{
  "name": "{service}-mcp-server",
  "version": "1.0.0",
  "description": "MCP server for {Service} API integration",
  "type": "module",
  "main": "dist/index.js",
  "scripts": {
    "start": "node dist/index.js",
    "dev": "tsx watch src/index.ts",
    "build": "tsc",
    "clean": "rm -rf dist"
  },
  "engines": {
    "node": ">=18"
  },
  "dependencies": {
    "@modelcontextprotocol/sdk": "^1.6.1",
    "axios": "^1.7.9",
    "zod": "^3.23.8"
  },
  "devDependencies": {
    "@types/node": "^22.10.0",
    "tsx": "^4.19.2",
    "typescript": "^5.7.2"
  }
}

tsconfig.json

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "Node16",
    "moduleResolution": "Node16",
    "lib": ["ES2022"],
    "outDir": "./dist",
    "rootDir": "./src",
    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "declaration": true,
    "declarationMap": true,
    "sourceMap": true,
    "allowSyntheticDefaultImports": true
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules", "dist"]
}

Complete Example

#!/usr/bin/env node
/**
 * MCP Server for Example Service.
 *
 * This server provides tools to interact with Example API, including user search,
 * project management, and data export capabilities.
 */

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
import { z } from "zod";
import axios, { AxiosError } from "axios";

// Constants
const API_BASE_URL = "https://api.example.com/v1";
const CHARACTER_LIMIT = 25000;

// Enums
enum ResponseFormat {
  MARKDOWN = "markdown",
  JSON = "json"
}

// Zod schemas
const UserSearchInputSchema = z.object({
  query: z.string()
    .min(2, "Query must be at least 2 characters")
    .max(200, "Query must not exceed 200 characters")
    .describe("Search string to match against names/emails"),
  limit: z.number()
    .int()
    .min(1)
    .max(100)
    .default(20)
    .describe("Maximum results to return"),
  offset: z.number()
    .int()
    .min(0)
    .default(0)
    .describe("Number of results to skip for pagination"),
  response_format: z.nativeEnum(ResponseFormat)
    .default(ResponseFormat.MARKDOWN)
    .describe("Output format: 'markdown' for human-readable or 'json' for machine-readable")
}).strict();

type UserSearchInput = z.infer<typeof UserSearchInputSchema>;

// Shared utility functions
async function makeApiRequest<T>(
  endpoint: string,
  method: "GET" | "POST" | "PUT" | "DELETE" = "GET",
  data?: any,
  params?: any
): Promise<T> {
  try {
    const response = await axios({
      method,
      url: `${API_BASE_URL}/${endpoint}`,
      data,
      params,
      timeout: 30000,
      headers: {
        "Content-Type": "application/json",
        "Accept": "application/json"
      }
    });
    return response.data;
  } catch (error) {
    throw error;
  }
}

function handleApiError(error: unknown): string {
  if (error instanceof AxiosError) {
    if (error.response) {
      switch (error.response.status) {
        case 404:
          return "Error: Resource not found. Please check the ID is correct.";
        case 403:
          return "Error: Permission denied. You don't have access to this resource.";
        case 429:
          return "Error: Rate limit exceeded. Please wait before making more requests.";
        default:
          return `Error: API request failed with status ${error.response.status}`;
      }
    } else if (error.code === "ECONNABORTED") {
      return "Error: Request timed out. Please try again.";
    }
  }
  return `Error: Unexpected error occurred: ${error instanceof Error ? error.message : String(error)}`;
}

// Create MCP server instance
const server = new McpServer({
  name: "example-mcp",
  version: "1.0.0"
});

// Register tools
server.registerTool(
  "example_search_users",
  {
    title: "Search Example Users",
    description: `[Full description as shown above]`,
    inputSchema: UserSearchInputSchema,
    annotations: {
      readOnlyHint: true,
      destructiveHint: false,
      idempotentHint: true,
      openWorldHint: true
    }
  },
  async (params: UserSearchInput) => {
    // Implementation as shown above
  }
);

// Main function
// For stdio (local):
async function runStdio() {
  if (!process.env.EXAMPLE_API_KEY) {
    console.error("ERROR: EXAMPLE_API_KEY environment variable is required");
    process.exit(1);
  }

  const transport = new StdioServerTransport();
  await server.connect(transport);
  console.error("MCP server running via stdio");
}

// For streamable HTTP (remote):
async function runHTTP() {
  if (!process.env.EXAMPLE_API_KEY) {
    console.error("ERROR: EXAMPLE_API_KEY environment variable is required");
    process.exit(1);
  }

  const app = express();
  app.use(express.json());

  app.post('/mcp', async (req, res) => {
    const transport = new StreamableHTTPServerTransport({
      sessionIdGenerator: undefined,
      enableJsonResponse: true
    });
    res.on('close', () => transport.close());
    await server.connect(transport);
    await transport.handleRequest(req, res, req.body);
  });

  const port = parseInt(process.env.PORT || '3000');
  app.listen(port, () => {
    console.error(`MCP server running on http://localhost:${port}/mcp`);
  });
}

// Choose transport based on environment
const transport = process.env.TRANSPORT || 'stdio';
if (transport === 'http') {
  runHTTP().catch(error => {
    console.error("Server error:", error);
    process.exit(1);
  });
} else {
  runStdio().catch(error => {
    console.error("Server error:", error);
    process.exit(1);
  });
}

---

Advanced MCP Features

Resource Registration

Expose data as resources for efficient, URI-based access:

import { ResourceTemplate } from "@modelcontextprotocol/sdk/types.js";

// Register a resource with URI template
server.registerResource(
  {
    uri: "file://documents/{name}",
    name: "Document Resource",
    description: "Access documents by name",
    mimeType: "text/plain"
  },
  async (uri: string) => {
    // Extract parameter from URI
    const match = uri.match(/^file:\/\/documents\/(.+)$/);
    if (!match) {
      throw new Error("Invalid URI format");
    }

    const documentName = match[1];
    const content = await loadDocument(documentName);

    return {
      contents: [{
        uri,
        mimeType: "text/plain",
        text: content
      }]
    };
  }
);

// List available resources dynamically
server.registerResourceList(async () => {
  const documents = await getAvailableDocuments();
  return {
    resources: documents.map(doc => ({
      uri: `file://documents/${doc.name}`,
      name: doc.name,
      mimeType: "text/plain",
      description: doc.description
    }))
  };
});

When to use Resources vs Tools:

• Resources: For data access with simple URI-based parameters
• Tools: For complex operations requiring validation and business logic
• Resources: When data is relatively static or template-based
• Tools: When operations have side effects or complex workflows

Transport Options

The TypeScript SDK supports two main transport mechanisms:

Streamable HTTP (Recommended for Remote Servers)

import { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/streamableHttp.js";
import express from "express";

const app = express();
app.use(express.json());

app.post('/mcp', async (req, res) => {
  // Create new transport for each request (stateless, prevents request ID collisions)
  const transport = new StreamableHTTPServerTransport({
    sessionIdGenerator: undefined,
    enableJsonResponse: true
  });

  res.on('close', () => transport.close());

  await server.connect(transport);
  await transport.handleRequest(req, res, req.body);
});

app.listen(3000);

stdio (For Local Integrations)

import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";

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

Transport selection:

• Streamable HTTP: Web services, remote access, multiple clients
• stdio: Command-line tools, local development, subprocess integration

Notification Support

Notify clients when server state changes:

// Notify when tools list changes
server.notification({
  method: "notifications/tools/list_changed"
});

// Notify when resources change
server.notification({
  method: "notifications/resources/list_changed"
});

Use notifications sparingly - only when server capabilities genuinely change.

---

Code Best Practices

Code Composability and Reusability

Your implementation MUST prioritize composability and code reuse:

1. Extract Common Functionality:

• Create reusable helper functions for operations used across multiple tools
• Build shared API clients for HTTP requests instead of duplicating code
• Centralize error handling logic in utility functions
• Extract business logic into dedicated functions that can be composed
• Extract shared markdown or JSON field selection & formatting functionality

2. Avoid Duplication:

• NEVER copy-paste similar code between tools
• If you find yourself writing similar logic twice, extract it into a function
• Common operations like pagination, filtering, field selection, and formatting should be shared
• Authentication/authorization logic should be centralized

Building and Running

Always build your TypeScript code before running:

# Build the project
npm run build

# Run the server
npm start

# Development with auto-reload
npm run dev

Always ensure npm run build completes successfully before considering the implementation complete.

Quality Checklist

Before finalizing your Node/TypeScript MCP server implementation, ensure:

Strategic Design

• [ ] Tools enable complete workflows, not just API endpoint wrappers
• [ ] Tool names reflect natural task subdivisions
• [ ] Response formats optimize for agent context efficiency
• [ ] Human-readable identifiers used where appropriate
• [ ] Error messages guide agents toward correct usage

Implementation Quality

• [ ] FOCUSED IMPLEMENTATION: Most important and valuable tools implemented
• [ ] All tools registered using registerTool with complete configuration
• [ ] All tools include title, description, inputSchema, and annotations
• [ ] Annotations correctly set (readOnlyHint, destructiveHint, idempotentHint, openWorldHint)
• [ ] All tools use Zod schemas for runtime input validation with .strict() enforcement
• [ ] All Zod schemas have proper constraints and descriptive error messages
• [ ] All tools have comprehensive descriptions with explicit input/output types
• [ ] Descriptions include return value examples and complete schema documentation
• [ ] Error messages are clear, actionable, and educational

TypeScript Quality

• [ ] TypeScript interfaces are defined for all data structures
• [ ] Strict TypeScript is enabled in tsconfig.json
• [ ] No use of any type - use unknown or proper types instead
• [ ] All async functions have explicit Promise<T> return types
• [ ] Error handling uses proper type guards (e.g., axios.isAxiosError, z.ZodError)

Advanced Features (where applicable)

• [ ] Resources registered for appropriate data endpoints
• [ ] Appropriate transport configured (stdio or streamable HTTP)
• [ ] Notifications implemented for dynamic server capabilities
• [ ] Type-safe with SDK interfaces

Project Configuration

• [ ] Package.json includes all necessary dependencies
• [ ] Build script produces working JavaScript in dist/ directory
• [ ] Main entry point is properly configured as dist/index.js
• [ ] Server name follows format: {service}-mcp-server
• [ ] tsconfig.json properly configured with strict mode

Code Quality

• [ ] Pagination is properly implemented where applicable
• [ ] Large responses check CHARACTER_LIMIT constant and truncate with clear messages
• [ ] Filtering options are provided for potentially large result sets
• [ ] All network operations handle timeouts and connection errors gracefully
• [ ] Common functionality is extracted into reusable functions
• [ ] Return types are consistent across similar operations

Testing and Build

• [ ] npm run build completes successfully without errors
• [ ] dist/index.js created and executable
• [ ] Server runs: node dist/index.js --help
• [ ] All imports resolve correctly
• [ ] Sample tool calls work as expected

Python MCP Server Implementation Guide

Overview

This document provides Python-specific best practices and examples for implementing MCP servers using the MCP Python SDK. It covers server setup, tool registration patterns, input validation with Pydantic, error handling, and complete working examples.

---

Quick Reference

Key Imports

from mcp.server.fastmcp import FastMCP
from pydantic import BaseModel, Field, field_validator, ConfigDict
from typing import Optional, List, Dict, Any
from enum import Enum
import httpx

Server Initialization

mcp = FastMCP("service_mcp")

Tool Registration Pattern

@mcp.tool(name="tool_name", annotations={...})
async def tool_function(params: InputModel) -> str:
    # Implementation
    pass

---

MCP Python SDK and FastMCP

The official MCP Python SDK provides FastMCP, a high-level framework for building MCP servers. It provides:

• Automatic description and inputSchema generation from function signatures and docstrings
• Pydantic model integration for input validation
• Decorator-based tool registration with @mcp.tool

For complete SDK documentation, use WebFetch to load: https://raw.githubusercontent.com/modelcontextprotocol/python-sdk/main/README.md

Server Naming Convention

Python MCP servers must follow this naming pattern:

• Format: {service}_mcp (lowercase with underscores)
• Examples: github_mcp, jira_mcp, stripe_mcp

The name should be:

• General (not tied to specific features)
• Descriptive of the service/API being integrated
• Easy to infer from the task description
• Without version numbers or dates

Tool Implementation

Tool Naming

Use snake_case for tool names (e.g., "search_users", "create_project", "get_channel_info") with clear, action-oriented names.

Avoid Naming Conflicts: Include the service context to prevent overlaps:

• Use "slack_send_message" instead of just "send_message"
• Use "github_create_issue" instead of just "create_issue"
• Use "asana_list_tasks" instead of just "list_tasks"

Tool Structure with FastMCP

Tools are defined using the @mcp.tool decorator with Pydantic models for input validation:

from pydantic import BaseModel, Field, ConfigDict
from mcp.server.fastmcp import FastMCP

# Initialize the MCP server
mcp = FastMCP("example_mcp")

# Define Pydantic model for input validation
class ServiceToolInput(BaseModel):
    '''Input model for service tool operation.'''
    model_config = ConfigDict(
        str_strip_whitespace=True,  # Auto-strip whitespace from strings
        validate_assignment=True,    # Validate on assignment
        extra='forbid'              # Forbid extra fields
    )

    param1: str = Field(..., description="First parameter description (e.g., 'user123', 'project-abc')", min_length=1, max_length=100)
    param2: Optional[int] = Field(default=None, description="Optional integer parameter with constraints", ge=0, le=1000)
    tags: Optional[List[str]] = Field(default_factory=list, description="List of tags to apply", max_items=10)

@mcp.tool(
    name="service_tool_name",
    annotations={
        "title": "Human-Readable Tool Title",
        "readOnlyHint": True,     # Tool does not modify environment
        "destructiveHint": False,  # Tool does not perform destructive operations
        "idempotentHint": True,    # Repeated calls have no additional effect
        "openWorldHint": False     # Tool does not interact with external entities
    }
)
async def service_tool_name(params: ServiceToolInput) -> str:
    '''Tool description automatically becomes the 'description' field.

    This tool performs a specific operation on the service. It validates all inputs
    using the ServiceToolInput Pydantic model before processing.

    Args:
        params (ServiceToolInput): Validated input parameters containing:
            - param1 (str): First parameter description
            - param2 (Optional[int]): Optional parameter with default
            - tags (Optional[List[str]]): List of tags

    Returns:
        str: JSON-formatted response containing operation results
    '''
    # Implementation here
    pass

Pydantic v2 Key Features

• Use model_config instead of nested Config class
• Use field_validator instead of deprecated validator
• Use model_dump() instead of deprecated dict()
• Validators require @classmethod decorator
• Type hints are required for validator methods

from pydantic import BaseModel, Field, field_validator, ConfigDict

class CreateUserInput(BaseModel):
    model_config = ConfigDict(
        str_strip_whitespace=True,
        validate_assignment=True
    )

    name: str = Field(..., description="User's full name", min_length=1, max_length=100)
    email: str = Field(..., description="User's email address", pattern=r'^[\w\.-]+@[\w\.-]+\.\w+$')
    age: int = Field(..., description="User's age", ge=0, le=150)

    @field_validator('email')
    @classmethod
    def validate_email(cls, v: str) -> str:
        if not v.strip():
            raise ValueError("Email cannot be empty")
        return v.lower()

Response Format Options

Support multiple output formats for flexibility:

from enum import Enum

class ResponseFormat(str, Enum):
    '''Output format for tool responses.'''
    MARKDOWN = "markdown"
    JSON = "json"

class UserSearchInput(BaseModel):
    query: str = Field(..., description="Search query")
    response_format: ResponseFormat = Field(
        default=ResponseFormat.MARKDOWN,
        description="Output format: 'markdown' for human-readable or 'json' for machine-readable"
    )

Markdown format:

• Use headers, lists, and formatting for clarity
• Convert timestamps to human-readable format (e.g., "2024-01-15 10:30:00 UTC" instead of epoch)
• Show display names with IDs in parentheses (e.g., "@john.doe (U123456)")
• Omit verbose metadata (e.g., show only one profile image URL, not all sizes)
• Group related information logically

JSON format:

• Return complete, structured data suitable for programmatic processing
• Include all available fields and metadata
• Use consistent field names and types

Pagination Implementation

For tools that list resources:

class ListInput(BaseModel):
    limit: Optional[int] = Field(default=20, description="Maximum results to return", ge=1, le=100)
    offset: Optional[int] = Field(default=0, description="Number of results to skip for pagination", ge=0)

async def list_items(params: ListInput) -> str:
    # Make API request with pagination
    data = await api_request(limit=params.limit, offset=params.offset)

    # Return pagination info
    response = {
        "total": data["total"],
        "count": len(data["items"]),
        "offset": params.offset,
        "items": data["items"],
        "has_more": data["total"] > params.offset + len(data["items"]),
        "next_offset": params.offset + len(data["items"]) if data["total"] > params.offset + len(data["items"]) else None
    }
    return json.dumps(response, indent=2)

Error Handling

Provide clear, actionable error messages:

def _handle_api_error(e: Exception) -> str:
    '''Consistent error formatting across all tools.'''
    if isinstance(e, httpx.HTTPStatusError):
        if e.response.status_code == 404:
            return "Error: Resource not found. Please check the ID is correct."
        elif e.response.status_code == 403:
            return "Error: Permission denied. You don't have access to this resource."
        elif e.response.status_code == 429:
            return "Error: Rate limit exceeded. Please wait before making more requests."
        return f"Error: API request failed with status {e.response.status_code}"
    elif isinstance(e, httpx.TimeoutException):
        return "Error: Request timed out. Please try again."
    return f"Error: Unexpected error occurred: {type(e).__name__}"

Shared Utilities

Extract common functionality into reusable functions:

# Shared API request function
async def _make_api_request(endpoint: str, method: str = "GET", **kwargs) -> dict:
    '''Reusable function for all API calls.'''
    async with httpx.AsyncClient() as client:
        response = await client.request(
            method,
            f"{API_BASE_URL}/{endpoint}",
            timeout=30.0,
            **kwargs
        )
        response.raise_for_status()
        return response.json()

Async/Await Best Practices

Always use async/await for network requests and I/O operations:

# Good: Async network request
async def fetch_data(resource_id: str) -> dict:
    async with httpx.AsyncClient() as client:
        response = await client.get(f"{API_URL}/resource/{resource_id}")
        response.raise_for_status()
        return response.json()

# Bad: Synchronous request
def fetch_data(resource_id: str) -> dict:
    response = requests.get(f"{API_URL}/resource/{resource_id}")  # Blocks
    return response.json()

Type Hints

Use type hints throughout:

from typing import Optional, List, Dict, Any

async def get_user(user_id: str) -> Dict[str, Any]:
    data = await fetch_user(user_id)
    return {"id": data["id"], "name": data["name"]}

Tool Docstrings

Every tool must have comprehensive docstrings with explicit type information:

async def search_users(params: UserSearchInput) -> str:
    '''
    Search for users in the Example system by name, email, or team.

    This tool searches across all user profiles in the Example platform,
    supporting partial matches and various search filters. It does NOT
    create or modify users, only searches existing ones.

    Args:
        params (UserSearchInput): Validated input parameters containing:
            - query (str): Search string to match against names/emails (e.g., "john", "@example.com", "team:marketing")
            - limit (Optional[int]): Maximum results to return, between 1-100 (default: 20)
            - offset (Optional[int]): Number of results to skip for pagination (default: 0)

    Returns:
        str: JSON-formatted string containing search results with the following schema:

        Success response:
        {
            "total": int,           # Total number of matches found
            "count": int,           # Number of results in this response
            "offset": int,          # Current pagination offset
            "users": [
                {
                    "id": str,      # User ID (e.g., "U123456789")
                    "name": str,    # Full name (e.g., "John Doe")
                    "email": str,   # Email address (e.g., "john@example.com")
                    "team": str     # Team name (e.g., "Marketing") - optional
                }
            ]
        }

        Error response:
        "Error: <error message>" or "No users found matching '<query>'"

    Examples:
        - Use when: "Find all marketing team members" -> params with query="team:marketing"
        - Use when: "Search for John's account" -> params with query="john"
        - Don't use when: You need to create a user (use example_create_user instead)
        - Don't use when: You have a user ID and need full details (use example_get_user instead)

    Error Handling:
        - Input validation errors are handled by Pydantic model
        - Returns "Error: Rate limit exceeded" if too many requests (429 status)
        - Returns "Error: Invalid API authentication" if API key is invalid (401 status)
        - Returns formatted list of results or "No users found matching 'query'"
    '''

Complete Example

See below for a complete Python MCP server example:

#!/usr/bin/env python3
'''
MCP Server for Example Service.

This server provides tools to interact with Example API, including user search,
project management, and data export capabilities.
'''

from typing import Optional, List, Dict, Any
from enum import Enum
import httpx
from pydantic import BaseModel, Field, field_validator, ConfigDict
from mcp.server.fastmcp import FastMCP

# Initialize the MCP server
mcp = FastMCP("example_mcp")

# Constants
API_BASE_URL = "https://api.example.com/v1"

# Enums
class ResponseFormat(str, Enum):
    '''Output format for tool responses.'''
    MARKDOWN = "markdown"
    JSON = "json"

# Pydantic Models for Input Validation
class UserSearchInput(BaseModel):
    '''Input model for user search operations.'''
    model_config = ConfigDict(
        str_strip_whitespace=True,
        validate_assignment=True
    )

    query: str = Field(..., description="Search string to match against names/emails", min_length=2, max_length=200)
    limit: Optional[int] = Field(default=20, description="Maximum results to return", ge=1, le=100)
    offset: Optional[int] = Field(default=0, description="Number of results to skip for pagination", ge=0)
    response_format: ResponseFormat = Field(default=ResponseFormat.MARKDOWN, description="Output format")

    @field_validator('query')
    @classmethod
    def validate_query(cls, v: str) -> str:
        if not v.strip():
            raise ValueError("Query cannot be empty or whitespace only")
        return v.strip()

# Shared utility functions
async def _make_api_request(endpoint: str, method: str = "GET", **kwargs) -> dict:
    '''Reusable function for all API calls.'''
    async with httpx.AsyncClient() as client:
        response = await client.request(
            method,
            f"{API_BASE_URL}/{endpoint}",
            timeout=30.0,
            **kwargs
        )
        response.raise_for_status()
        return response.json()

def _handle_api_error(e: Exception) -> str:
    '''Consistent error formatting across all tools.'''
    if isinstance(e, httpx.HTTPStatusError):
        if e.response.status_code == 404:
            return "Error: Resource not found. Please check the ID is correct."
        elif e.response.status_code == 403:
            return "Error: Permission denied. You don't have access to this resource."
        elif e.response.status_code == 429:
            return "Error: Rate limit exceeded. Please wait before making more requests."
        return f"Error: API request failed with status {e.response.status_code}"
    elif isinstance(e, httpx.TimeoutException):
        return "Error: Request timed out. Please try again."
    return f"Error: Unexpected error occurred: {type(e).__name__}"

# Tool definitions
@mcp.tool(
    name="example_search_users",
    annotations={
        "title": "Search Example Users",
        "readOnlyHint": True,
        "destructiveHint": False,
        "idempotentHint": True,
        "openWorldHint": True
    }
)
async def example_search_users(params: UserSearchInput) -> str:
    '''Search for users in the Example system by name, email, or team.

    [Full docstring as shown above]
    '''
    try:
        # Make API request using validated parameters
        data = await _make_api_request(
            "users/search",
            params={
                "q": params.query,
                "limit": params.limit,
                "offset": params.offset
            }
        )

        users = data.get("users", [])
        total = data.get("total", 0)

        if not users:
            return f"No users found matching '{params.query}'"

        # Format response based on requested format
        if params.response_format == ResponseFormat.MARKDOWN:
            lines = [f"# User Search Results: '{params.query}'", ""]
            lines.append(f"Found {total} users (showing {len(users)})")
            lines.append("")

            for user in users:
                lines.append(f"## {user['name']} ({user['id']})")
                lines.append(f"- **Email**: {user['email']}")
                if user.get('team'):
                    lines.append(f"- **Team**: {user['team']}")
                lines.append("")

            return "\n".join(lines)

        else:
            # Machine-readable JSON format
            import json
            response = {
                "total": total,
                "count": len(users),
                "offset": params.offset,
                "users": users
            }
            return json.dumps(response, indent=2)

    except Exception as e:
        return _handle_api_error(e)

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

---

Advanced FastMCP Features

Context Parameter Injection

FastMCP can automatically inject a Context parameter into tools for advanced capabilities like logging, progress reporting, resource reading, and user interaction:

from mcp.server.fastmcp import FastMCP, Context

mcp = FastMCP("example_mcp")

@mcp.tool()
async def advanced_search(query: str, ctx: Context) -> str:
    '''Advanced tool with context access for logging and progress.'''

    # Report progress for long operations
    await ctx.report_progress(0.25, "Starting search...")

    # Log information for debugging
    await ctx.log_info("Processing query", {"query": query, "timestamp": datetime.now()})

    # Perform search
    results = await search_api(query)
    await ctx.report_progress(0.75, "Formatting results...")

    # Access server configuration
    server_name = ctx.fastmcp.name

    return format_results(results)

@mcp.tool()
async def interactive_tool(resource_id: str, ctx: Context) -> str:
    '''Tool that can request additional input from users.'''

    # Request sensitive information when needed
    api_key = await ctx.elicit(
        prompt="Please provide your API key:",
        input_type="password"
    )

    # Use the provided key
    return await api_call(resource_id, api_key)

Context capabilities:

• ctx.report_progress(progress, message) - Report progress for long operations
• ctx.log_info(message, data) / ctx.log_error() / ctx.log_debug() - Logging
• ctx.elicit(prompt, input_type) - Request input from users
• ctx.fastmcp.name - Access server configuration
• ctx.read_resource(uri) - Read MCP resources

Resource Registration

Expose data as resources for efficient, template-based access:

@mcp.resource("file://documents/{name}")
async def get_document(name: str) -> str:
    '''Expose documents as MCP resources.

    Resources are useful for static or semi-static data that doesn't
    require complex parameters. They use URI templates for flexible access.
    '''
    document_path = f"./docs/{name}"
    with open(document_path, "r") as f:
        return f.read()

@mcp.resource("config://settings/{key}")
async def get_setting(key: str, ctx: Context) -> str:
    '''Expose configuration as resources with context.'''
    settings = await load_settings()
    return json.dumps(settings.get(key, {}))

When to use Resources vs Tools:

• Resources: For data access with simple parameters (URI templates)
• Tools: For complex operations with validation and business logic

Structured Output Types

FastMCP supports multiple return types beyond strings:

from typing import TypedDict
from dataclasses import dataclass
from pydantic import BaseModel

# TypedDict for structured returns
class UserData(TypedDict):
    id: str
    name: str
    email: str

@mcp.tool()
async def get_user_typed(user_id: str) -> UserData:
    '''Returns structured data - FastMCP handles serialization.'''
    return {"id": user_id, "name": "John Doe", "email": "john@example.com"}

# Pydantic models for complex validation
class DetailedUser(BaseModel):
    id: str
    name: str
    email: str
    created_at: datetime
    metadata: Dict[str, Any]

@mcp.tool()
async def get_user_detailed(user_id: str) -> DetailedUser:
    '''Returns Pydantic model - automatically generates schema.'''
    user = await fetch_user(user_id)
    return DetailedUser(**user)

Lifespan Management

Initialize resources that persist across requests:

from contextlib import asynccontextmanager

@asynccontextmanager
async def app_lifespan():
    '''Manage resources that live for the server's lifetime.'''
    # Initialize connections, load config, etc.
    db = await connect_to_database()
    config = load_configuration()

    # Make available to all tools
    yield {"db": db, "config": config}

    # Cleanup on shutdown
    await db.close()

mcp = FastMCP("example_mcp", lifespan=app_lifespan)

@mcp.tool()
async def query_data(query: str, ctx: Context) -> str:
    '''Access lifespan resources through context.'''
    db = ctx.request_context.lifespan_state["db"]
    results = await db.query(query)
    return format_results(results)

Transport Options

FastMCP supports two main transport mechanisms:

# stdio transport (for local tools) - default
if __name__ == "__main__":
    mcp.run()

# Streamable HTTP transport (for remote servers)
if __name__ == "__main__":
    mcp.run(transport="streamable_http", port=8000)

Transport selection:

• stdio: Command-line tools, local integrations, subprocess execution
• Streamable HTTP: Web services, remote access, multiple clients

---

Code Best Practices

Code Composability and Reusability

Your implementation MUST prioritize composability and code reuse:

1. Extract Common Functionality:

• Create reusable helper functions for operations used across multiple tools
• Build shared API clients for HTTP requests instead of duplicating code
• Centralize error handling logic in utility functions
• Extract business logic into dedicated functions that can be composed
• Extract shared markdown or JSON field selection & formatting functionality

2. Avoid Duplication:

• NEVER copy-paste similar code between tools
• If you find yourself writing similar logic twice, extract it into a function
• Common operations like pagination, filtering, field selection, and formatting should be shared
• Authentication/authorization logic should be centralized

Python-Specific Best Practices

1. Use Type Hints: Always include type annotations for function parameters and return values 2. Pydantic Models: Define clear Pydantic models for all input validation 3. Avoid Manual Validation: Let Pydantic handle input validation with constraints 4. Proper Imports: Group imports (standard library, third-party, local) 5. Error Handling: Use specific exception types (httpx.HTTPStatusError, not generic Exception) 6. Async Context Managers: Use async with for resources that need cleanup 7. Constants: Define module-level constants in UPPER_CASE

Quality Checklist

Before finalizing your Python MCP server implementation, ensure:

Strategic Design

• [ ] Tools enable complete workflows, not just API endpoint wrappers
• [ ] Tool names reflect natural task subdivisions
• [ ] Response formats optimize for agent context efficiency
• [ ] Human-readable identifiers used where appropriate
• [ ] Error messages guide agents toward correct usage

Implementation Quality

• [ ] FOCUSED IMPLEMENTATION: Most important and valuable tools implemented
• [ ] All tools have descriptive names and documentation
• [ ] Return types are consistent across similar operations
• [ ] Error handling is implemented for all external calls
• [ ] Server name follows format: {service}_mcp
• [ ] All network operations use async/await
• [ ] Common functionality is extracted into reusable functions
• [ ] Error messages are clear, actionable, and educational
• [ ] Outputs are properly validated and formatted

Tool Configuration

• [ ] All tools implement 'name' and 'annotations' in the decorator
• [ ] Annotations correctly set (readOnlyHint, destructiveHint, idempotentHint, openWorldHint)
• [ ] All tools use Pydantic BaseModel for input validation with Field() definitions
• [ ] All Pydantic Fields have explicit types and descriptions with constraints
• [ ] All tools have comprehensive docstrings with explicit input/output types
• [ ] Docstrings include complete schema structure for dict/JSON returns
• [ ] Pydantic models handle input validation (no manual validation needed)

Advanced Features (where applicable)

• [ ] Context injection used for logging, progress, or elicitation
• [ ] Resources registered for appropriate data endpoints
• [ ] Lifespan management implemented for persistent connections
• [ ] Structured output types used (TypedDict, Pydantic models)
• [ ] Appropriate transport configured (stdio or streamable HTTP)

Code Quality

• [ ] File includes proper imports including Pydantic imports
• [ ] Pagination is properly implemented where applicable
• [ ] Filtering options are provided for potentially large result sets
• [ ] All async functions are properly defined with async def
• [ ] HTTP client usage follows async patterns with proper context managers
• [ ] Type hints are used throughout the code
• [ ] Constants are defined at module level in UPPER_CASE

Testing

• [ ] Server runs successfully: python your_server.py --help
• [ ] All imports resolve correctly
• [ ] Sample tool calls work as expected
• [ ] Error scenarios handled gracefully

TaskFlow MCP Server Patterns

Overview

Patterns learned from building the TaskFlow MCP Server - an internal service that wraps REST APIs for AI agent consumption via stateless HTTP transport.

---

Architecture: Internal MCP Server

┌─────────────┐     JWT      ┌──────────────┐    user_id     ┌────────────┐    JWT/token   ┌──────────┐
│   User      │────────────▶│  Chat Server │──────────────▶│ MCP Server │──────────────▶│ REST API │
│ (Browser)   │             │ (validates)  │  access_token  │ (no auth)  │               │          │
└─────────────┘             └──────────────┘                └────────────┘               └──────────┘

Key Insight: MCP server is an internal service. Auth is handled upstream by Chat Server.

---

Stateless HTTP Transport Setup

FastMCP Singleton (app.py)

from mcp.server.fastmcp import FastMCP

def _create_mcp() -> FastMCP:
    return FastMCP(
        "taskflow_mcp",
        stateless_http=True,  # Stateless - no persistent sessions
        json_response=True,   # Pure JSON responses (no SSE streaming)
    )

mcp = _create_mcp()

Server Entry Point (server.py)

IMPORTANT: Use FastMCP's streamable_http_app() directly. Do NOT wrap in additional Starlette app.

from starlette.middleware.cors import CORSMiddleware
from .app import mcp

# Import tools to register decorators (side effects)
import taskflow_mcp.tools.tasks  # noqa: F401
import taskflow_mcp.tools.projects  # noqa: F401

# Use FastMCP's built-in app directly - it includes /mcp route and lifespan
_mcp_app = mcp.streamable_http_app()

# Only add CORS wrapper
streamable_http_app = CORSMiddleware(
    _mcp_app,
    allow_origins=["*"],
    allow_methods=["GET", "POST", "DELETE", "OPTIONS"],
    allow_headers=["*"],
    expose_headers=["Mcp-Session-Id"],
)

if __name__ == "__main__":
    import uvicorn
    uvicorn.run(
        "taskflow_mcp.server:streamable_http_app",
        host="0.0.0.0",
        port=8001,
        reload=True,
    )

Anti-Pattern: Double Wrapping

# DON'T DO THIS - causes session timeout issues
_starlette_app = Starlette(
    routes=[Mount("/", app=mcp.streamable_http_app())],
    lifespan=custom_lifespan,  # Conflicts with FastMCP's lifespan
)

---

Three Authentication Modes

Configuration

from pydantic_settings import BaseSettings

class Settings(BaseSettings):
    model_config = SettingsConfigDict(env_prefix="TASKFLOW_")

    api_url: str = "http://localhost:8000"
    api_timeout: float = 30.0
    dev_mode: bool = False           # Dev mode bypass
    service_token: str | None = None  # M2M service token

API Client Headers

def _get_headers(self, user_id: str, access_token: str | None = None) -> dict:
    headers: dict[str, str] = {}

    if self.service_token:
        # Mode 1: Service-to-service (M2M)
        headers["Authorization"] = f"Bearer {self.service_token}"
        headers["X-User-ID"] = user_id
    elif access_token:
        # Mode 2: Forward user's JWT
        headers["Authorization"] = f"Bearer {access_token}"
    elif self.dev_mode:
        # Mode 3: Dev mode bypass
        headers["X-User-ID"] = user_id
        headers["X-Service"] = "taskflow-mcp"

    return headers

When to Use Each Mode

---

Input Models with Auth

Base Model

from pydantic import BaseModel, Field

class AuthenticatedInput(BaseModel):
    """Base model with auth fields for all tools."""
    user_id: str = Field(..., description="User ID performing the action")
    access_token: str | None = Field(
        None,
        description="JWT access token (required in production, optional in dev mode)"
    )

Tool-Specific Models

class AddTaskInput(AuthenticatedInput):
    project_id: int = Field(..., description="Project ID to add task to")
    title: str = Field(..., min_length=1, max_length=200, description="Task title")
    description: str | None = Field(None, max_length=2000, description="Task description")

class TaskIdInput(AuthenticatedInput):
    """Reusable for single-task operations."""
    task_id: int = Field(..., description="Task ID to operate on")

class ProgressInput(AuthenticatedInput):
    task_id: int = Field(..., description="Task ID")
    progress_percent: int = Field(..., ge=0, le=100, description="Progress (0-100)")
    note: str | None = Field(None, max_length=500, description="Progress note")

---

Tool Response Patterns

Success Response Helper

def _format_task_result(task: dict, status: str) -> str:
    return json.dumps({
        "task_id": task.get("id"),
        "status": status,
        "title": task.get("title"),
    }, indent=2)

Error Response Helper

def _format_error(e: APIError, task_id: int | None = None) -> str:
    result = {
        "error": True,
        "message": e.detail or e.message,
        "status_code": e.status_code,
    }
    if task_id:
        result["task_id"] = task_id
    return json.dumps(result, indent=2)

Tool Implementation Pattern

@mcp.tool(
    name="taskflow_add_task",
    annotations={
        "title": "Add Task",
        "readOnlyHint": False,
        "destructiveHint": False,
        "idempotentHint": False,
        "openWorldHint": False,
    },
)
async def taskflow_add_task(params: AddTaskInput, ctx: Context) -> str:
    """Create a new task in a project.

    Args:
        params: AddTaskInput with user_id, project_id, title, optional description

    Returns:
        JSON with task_id, status="created", and title
    """
    try:
        client = get_api_client()
        result = await client.create_task(
            user_id=params.user_id,
            project_id=params.project_id,
            title=params.title,
            description=params.description,
            access_token=params.access_token,
        )
        return _format_task_result(result, "created")
    except APIError as e:
        return _format_error(e)
    except Exception as e:
        return json.dumps({"error": True, "message": str(e)})

---

Package Structure

packages/mcp-server/
├── src/taskflow_mcp/
│   ├── __init__.py         # Version
│   ├── app.py              # FastMCP singleton
│   ├── server.py           # Starlette + CORS (minimal)
│   ├── config.py           # Pydantic settings
│   ├── api_client.py       # httpx REST client
│   ├── models.py           # Input validation models
│   └── tools/
│       ├── __init__.py
│       ├── tasks.py        # Task tools
│       └── projects.py     # Project tools
├── tests/
│   └── test_models.py
├── pyproject.toml
├── .env.example
└── README.md

---

Dependencies

[project]
dependencies = [
    "mcp>=1.22.0",
    "httpx>=0.28.0",
    "pydantic>=2.12.0",
    "pydantic-settings>=2.0.0",
    "starlette>=0.45.0",
    "uvicorn>=0.34.0",
]

[tool.pytest.ini_options]
asyncio_mode = "auto"
testpaths = ["tests"]

---

Common Issues & Fixes

Issue: Session Timeout (ClosedResourceError)

Symptom: Timed out while waiting for response. Waited 5.0 seconds.

Cause: Double-wrapping FastMCP app in Starlette causes session management overhead.

Fix: Use mcp.streamable_http_app() directly without additional Starlette wrapper.

Issue: 401 Not Authenticated

Symptom: API returns auth errors despite passing user_id.

Cause: REST API expects JWT, not X-User-ID header.

Fix: 1. Enable dev mode on both services, OR 2. Pass access_token parameter from Chat Server

Issue: Tool Not Found

Symptom: MCP client can't find registered tools.

Cause: Tool modules not imported before streamable_http_app() is called.

Fix: Import tool modules at top of server.py (side effect imports).

---

Testing Commands

# Run tests
cd packages/mcp-server
uv run pytest -v

# Start server
uv run python -m taskflow_mcp.server

# Test with MCP Inspector
npx @modelcontextprotocol/inspector http://localhost:8001/mcp

---

Source

Learned from TaskFlow MCP Server implementation (005-mcp-server). PHRs: history/prompts/005-mcp-server/