devkit-mcp

A Model Context Protocol server that gives an AI assistant seven everyday developer utilities: sending HTTP requests, inspecting JWTs, hashing, generating UUIDs, testing regular expressions, explaining cron schedules, and encoding text.

No API keys, no accounts, no configuration. Clone it, build it, point a client at it.

┌────────────┐   MCP over stdio   ┌──────────────┐
│  AI client │ ◄────────────────► │  devkit-mcp  │
│  (Claude)  │    JSON-RPC 2.0    │   7 tools    │
└────────────┘                    └──────────────┘

Why this exists

Language models are bad at exactly the things these tools are good at: computing a SHA-256 by hand, predicting when 0 9 * * 1-5 next fires in America/New_York, or deciding whether a base64 string round-trips. Each tool here replaces a plausible-sounding guess with a real answer.

Tools

Tool	What it does
http_request	Sends an HTTP request and returns status, headers, timing, and body. Parses JSON bodies automatically. Redirects are followed manually and re-checked at every hop.
decode_jwt	Decodes a JWT header and payload, and reports iat/nbf/exp in epoch and ISO form with expiry status. Does not verify signatures — and says so in every response.
hash_text	MD5, SHA-1, SHA-256, SHA-384, SHA-512 digests or keyed HMACs, in hex, base64, or base64url. Optionally verifies against an expected digest in constant time.
generate_uuid	UUIDv4 (random) or UUIDv7 (time-ordered, RFC 9562) from a cryptographically secure source.
regex_test	Runs a regex and returns every match with its offset, positional groups, and named groups. Can preview a replacement.
cron_explain	Translates a cron expression to plain English and lists its next runs in a given timezone. Handles 5-field and 6-field syntax.
transform_text	base64, base64url, hex, and URL encoding in both directions, plus JSON formatting and minification. Decoders reject bad input instead of returning garbage.

Install

Requires Node.js 20 or newer.

git clone https://github.com/lllNuggetslll/devkit-mcp.git
cd devkit-mcp
npm install
npm run build

Connect it to a client

Claude Code

claude mcp add devkit -- node /absolute/path/to/devkit-mcp/dist/index.js

Claude Desktop

Add this to claude_desktop_config.json:

• macOS — ~/Library/Application Support/Claude/claude_desktop_config.json
• Windows — %APPDATA%\Claude\claude_desktop_config.json

{
  "mcpServers": {
    "devkit": {
      "command": "node",
      "args": ["/absolute/path/to/devkit-mcp/dist/index.js"]
    }
  }
}

Restart the client, and the seven tools appear.

Anything else

The server speaks MCP over stdio, so any compliant client works:

node dist/index.js

Try it

Once connected, these all work in plain language:

• "What's the SHA-256 of hunter2?"
• "When does 0 */4 * * * next run in Tokyo time?"
• "Decode this JWT and tell me if it's expired."
• "Does ^\d{3}-\d{4}$ match 555-1234?"
• "Generate 5 UUIDv7s for my database seeds."
• "GET https://api.github.com/repos/anthropics/claude-code and show me the star count."

Network access and SSRF

http_request refuses requests to loopback, link-local, and RFC 1918 addresses by default.

This matters more for an MCP server than for an ordinary HTTP client. The server takes its instructions from a model, and a model can be steered by whatever text it just read — a web page, an issue comment, a log file. An unrestricted fetch tool is therefore a server-side request forgery primitive aimed at everything the host machine can reach, including cloud metadata endpoints like 169.254.169.254.

The guard resolves hostnames before deciding, so a DNS record pointing at 127.0.0.1 does not get through, and it re-checks every redirect hop rather than trusting the first URL.

Testing a local API is the legitimate case for this, so it is one variable away:

{
  "mcpServers": {
    "devkit": {
      "command": "node",
      "args": ["/absolute/path/to/devkit-mcp/dist/index.js"],
      "env": { "DEVKIT_ALLOW_PRIVATE_HOSTS": "1" }
    }
  }
}

Other limits: responses are capped at 256 KB, redirects at 5 hops, and requests time out after 15 seconds by default (60 seconds maximum).

Development

npm test          # run the suite
npm run test:watch
npm run typecheck
npm run dev       # tsc --watch

The suite has 38 tests in two layers. tests/tools.test.ts covers the pure functions, including published test vectors — the RFC 4231 HMAC-SHA256 vector, the known SHA-256 of the empty string — plus the edge cases worth pinning down: zero-length regex matches that would otherwise loop forever, IPv4-mapped IPv6 addresses like ::ffff:127.0.0.1 that would slip past a naive private-range check, and encodings that must round-trip exactly.

tests/server.test.ts runs the real server against a real MCP client over an in-memory transport, so tool registration, schema generation, defaults, and error results are verified through the protocol rather than around it.

How it is put together

src/
  index.ts          entry point; stdio transport and signal handling
  server.ts         builds the McpServer and registers every tool
  tools/
    types.ts        ToolDefinition contract and result helpers
    index.ts        the tool registry
    http.ts         http_request, plus the SSRF host policy
    jwt.ts  hash.ts  uuid.ts  regex.ts  cron.ts  text.ts

Every tool is a ToolDefinition: a name, a description, a Zod input schema, MCP annotations, and a handler. server.ts iterates the registry and registers each one, so adding a tool means writing a module and adding a line to tools/index.ts — nothing else changes.

Two conventions are worth calling out:

The logic is separate from the protocol. Each module exports a plain function — hashText, decodeJwt, explainCron — that knows nothing about MCP. The tool wrapper handles argument parsing and result formatting. That is what lets the unit tests call the real logic directly, with no transport in the way.

Failures are results, not exceptions. MCP models a tool failure as an ordinary response with isError: true, which lets the model read the message and correct itself. A thrown protocol error would just look like a broken server. Every handler catches and returns fail(message) instead.

Tools also declare their side effects through MCP annotations, so a host can decide what needs approval: everything is readOnlyHint: true except http_request, which is the only one marked openWorldHint: true.

License

MIT