Cal MCP

What pwh-pwh/cal-mcp MCP server does

The pwh-pwh/cal-mcp MCP server exposes a single calculation tool named cal. It accepts one string parameter, exp, containing a mathematical expression, then returns the evaluated result as a string. Supported examples include 2 + 2, 2 + 3 * PI, and E ^ 2 + 1.

The expression environment includes the constants PI and E, mapped to Bun's Math.PI and Math.E. Boolean values true and false are also available. This makes the server suitable for arithmetic, expressions that use common mathematical constants, and simple boolean expressions supported by the parser.

How it works

The service is implemented with the FastMCP framework and uses the expr-eval library to parse and evaluate the supplied expression. zod is used for parameter validation. Communication takes place through stdio, so an MCP client starts the process locally and exchanges requests and responses through its standard input and output streams.

The server does not describe a database, file store, or third-party API integration. Its scope is expression evaluation. Invalid input must be a valid expression; otherwise, evaluation may raise an error.

Setup and configuration

Running the pwh-pwh/cal-mcp MCP server requires the Bun runtime, with the README recommending a current version. The repository's manual setup consists of cloning the project and running bun install from its directory. Start the implementation with bun run index.ts; the process then serves MCP messages over stdio.

An MCP client configuration can use bunx with the package name cal-mcp:

{
  "command": "bunx",
  "args": ["cal-mcp"]
}

The README also documents installation through Smithery for Claude Desktop. A hosted deployment is listed through Fronteir AI, but the project notes that its own service currently supports stdio interaction.

Tools and capabilities

The pwh-pwh/cal-mcp MCP server provides:

• The cal tool for evaluating a supplied expression.
• A required string argument named exp.
• Basic arithmetic expression support through expr-eval.
• Built-in PI and E constants.
• Boolean constants named true and false.
• String-form calculation results.

For example, PI * 2 evaluates to approximately 6.283185307179586. The available behavior is determined by the expression parser and the expressions it accepts; the README does not document additional domain-specific functions or tools.

Limitations and notes

The server currently supports stdio only when run from the project itself, and it depends on a Bun-compatible environment. Expressions that are malformed or unsupported can produce errors, so callers should validate or handle failures around tool requests. No authentication settings or environment variables are documented. The README identifies FastMCP, expr-eval, and zod as dependencies, but does not state a project license or describe access controls for the hosted deployment.