server-development

Coding rules for building an MCP server, a service that lets AI tools access defined functions and data resources.

In plain words
What is it for?
Use them when adding MCP tools or resources, configuring a server, validating inputs, and returning resource content.
Why use it?
They provide common patterns for configuration, input checking, error handling, and clear descriptions that AI can understand.

Cursor rule for Cursor

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add rules/ekakit/lightfast-mcp/server-development
Clone the repo
git clone --depth 1 https://github.com/ekakit/lightfast-mcp

Made for: Cursor.

Per session 0 Nothing until a file matches its globs; then the whole rule loads.
When invoked 375 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00000 $0.00375
Opus 5 $0.00000 $0.00187
Sonnet 5 $0.00000 $0.00075
Haiku 4.5 $0.00000 $0.00038

Measured yesterday against content hash 6c38c1539b86, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

server-development scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured yesterday.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

.cursor/rules/server-development.mdc · 72 lines

What it actually says


description: MCP server development patterns and best practices
alwaysApply: false

MCP Server Development

Server Configuration Pattern

config = ServerConfig(
    name="MyServer",
    description="Description of what this server does",
    host="localhost", 
    port=8001,
    transport="streamable-http",  # or "stdio"
    path="/mcp",
    config={"type": "my_server", "custom_param": "value"}
)

Tool Implementation Pattern

@mcp.tool()
async def my_tool(param: str) -> str:
    """Tool description for AI to understand usage.
    
    Args:
        param: Description of the parameter
        
    Returns:
        Description of the return value
    """
    # Validate inputs
    if not param:
        raise ValueError("Parameter cannot be empty")
    
    # Implement tool logic
    result = await some_async_operation(param)
    return result

Resource Implementation Pattern

@mcp.resource("my-resource://path/{id}")
async def get_resource(id: str) -> str:
    """Resource description for AI context.
    
    Args:
        id: Resource identifier
        
    Returns:
        Resource content
    """
    # Validate resource ID
    # Return resource content
    return "content"

Best Practices

  • All servers inherit from BaseServer and use ServerConfig
  • Use @mcp.tool() decorator for tools, @mcp.resource() for resources
  • Support both stdio and streamable-http transports
  • Validate all inputs from AI models
  • Handle errors gracefully with meaningful messages
  • Use clear descriptions for AI consumption
  • Log security-relevant operations
Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. yesterday First seen · 72 lines · 0 tokens per session scan A 6c38c1539b86

Subscribe to this mod's changes

server-development is a cursor rule published in the GitHub repository ekakit/lightfast-mcp (2 stars, last pushed 1y ago), licensed MIT. It costs nothing until one of its globs matches a file; then it loads 375 tokens. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.