mcp-tool-implementation

A set of rules for implementing MCP tools, which are functions an MCP server makes available to other software.

In plain words
What is it for?
Use it when adding or changing TypeScript tools, including their names, aliases, parameters, API calls, and error responses.
Why use it?
It helps keep tool registration, input checking, error handling, typing, and documentation consistent.

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/dxheroes/mcp-devtools/mcp-tool-implementation
Clone the repo
git clone --depth 1 https://github.com/DXHeroes/mcp-devtools

Made for: Cursor.

Per session 318 This file is loaded in full into every session.
When invoked 318 The same file — it is already loaded in full.
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 $0.00318 $0.00318
Opus 5 $0.00159 $0.00159
Sonnet 5 $0.00064 $0.00064
Haiku 4.5 $0.00032 $0.00032

Measured yesterday against content hash b4800aac0e7b, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

mcp-tool-implementation 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/mcp-tool-implementation.mdc · 60 lines

What it actually says

MCP Tool Implementation Guidelines

When implementing a new MCP tool, follow these patterns:

@file packages/jira/src/tools/getTicket.ts @file .cursor/rules/typescript-style.mdc

Tool Registration

export const registerToolName = (server: McpServer): void => {
  const toolParams = {
    // Define parameters according to JSON schema
  };

  // For tools with aliases, use an array of tool names
  const toolNames = ["primary_tool_name", "alias_1", "alias_2"];

  // Register all tool names with the same handler
  for (const tool of toolNames) {
    server.tool(tool, toolParams, async (params) => {
      try {
        // Implementation
        return {
          message: "Success message",
        };
      } catch (error) {
        return {
          error: `Error message: ${error.message}`,
        };
      }
    });
  }
};

Parameter Validation

  • Always validate required parameters early in the function
  • Use TypeScript types/interfaces for parameter typing
  • Document parameters with clear descriptions

Error Handling

  • Use try/catch blocks for all API calls
  • Return meaningful error messages
  • Log errors with appropriate context

Documentation

  • Update the README.md with your new tool
  • Add the tool to the Tool Reference table with all parameters
  • Include any aliases in the documentation
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 · 60 lines · 318 tokens per session scan A b4800aac0e7b

Subscribe to this mod's changes

mcp-tool-implementation is a cursor rule published in the GitHub repository DXHeroes/mcp-devtools (13 stars, last pushed 1y ago), licensed MIT. It adds 318 tokens to every session, about $0.0016 per session on Opus 5. 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 2026-08-30.

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