Calculator MCP server

Calculator MCP Server

<!-- mcp-name: io.github.slettmayer/calc-mcp-server -->

An MCP server that does arithmetic, exactly — so an LLM does not have to do it mentally.

One tool, calculate, evaluating expressions against a hand-rolled AST allowlist. No eval(), no sympy, no unbounded dependencies.

2 + 3 * (4 - 1) / 2 ** 2   ->  4.25
sqrt(16) + sin(pi/2)       ->  5.0
123456789 * 987654321      ->  121932631112635269

Why this exists

The widely-used mcp-server-calculator package is abandoned (last commit May 2025) and declares mcp>=1.4.1 with no upper bound. When the MCP Python SDK released v2 and removed mcp.server.fastmcp, every uvx …@latest install of it started crashing on import — which took down the entire MCP proxy hosting it, and every other server alongside it.

No maintained Python replacement exists. The one actively-maintained npm calculator computes in float64, so 123456789 * 987654321 comes back as 121932631112635260 — off by 9. That is a poor trait in a tool whose whole purpose is that the model should not be doing the arithmetic itself.

So this server:

• bounds its one dependency (mcp[cli]>=2,<3) — the failure above cannot recur here;
• keeps integers exact at any size, never coercing to float;
• bounds resource use, not just code execution — see below.

Safety

Two problems, and most calculator servers only solve the first.

Code execution. Expressions are parsed with ast.parse and walked against an explicit allowlist of node types. Attribute is not on it, so (1).class.bases is rejected. A Call is only evaluated when its target is a bare name in the function allowlist, so import('os').system(…) is rejected before any argument is even evaluated.

Resource exhaustion. An allowlist alone still lets 999 occupy the process for minutes on unbounded bignum exponentiation — the incumbent hangs for over five seconds on it. Four caps close that: expression length (500 chars), nesting depth (32), result size (4300 digits, checked on the operands before exponentiating), and factorial argument (1000).

Full detail in docs/tech/SAFE-EVALUATION.md.

Install

uvx calc-mcp-server

Pin it. Do not add @latest — that is how the package this replaces broke.

Configure

As a stdio MCP server:

{
  "mcpServers": {
    "calculator": {
      "command": "uvx",
      "args": ["calc-mcp-server"]
    }
  }
}

The calculate tool

Returns the result as a string, or a line starting with Error: explaining why the expression was rejected. The tool never raises, so a bad expression is an answer the agent can read back rather than a tool-call failure.

Operators — + - / // % , parentheses, unary +/-. ^ is accepted as a power operator, and × · ÷ − are accepted as their ASCII equivalents (speech-to-text produces them).

Constants — pi, e, tau.

Functions — abs round min max sqrt exp log log2 log10 sin cos tan asin acos atan atan2 degrees radians hypot floor ceil factorial gcd lcm.

Results — integer arithmetic returns an exact integer of any size. Floats are rendered at 12 significant digits, which removes IEEE-754 representation noise (0.1 + 0.2 reads 0.3, not 0.30000000000000004) while keeping far more precision than a calculator result is used at. A whole float keeps its .0, so 8 / 2 reads 4.0 and stays distinct from the exact integer 4.

Development

Shortened. The full README is on GitHub.