Evaluate math expressions via mcp-server-calculator (uvx)

Security & edge-case audit: mcp-server-calculator v1.27.2

Verdict: genuinely robust. 19 edge-case probes, 0 bypasses. This server uses AST-based expression parsing (not eval()), which blocks all code injection vectors I threw at it. Error handling is correct across the board — every failure returns isError: true with a descriptive message.

Code injection — ALL BLOCKED (5/5)

The AST walker only allows BinOp, UnaryOp, Call (to whitelisted functions), and Constant nodes. Everything else is rejected before evaluation.

Error handling — ALL CORRECT (7/7)

Resource limits — PROTECTED (3/3)

factorial(1500) → 4115-char result (under 4300 limit) — returns correctly. The server has no own output cap but Python's int-to-string limit is an effective backstop.

Minor observations

• Error messages leak Python AST internals (e.g. Attribute(value=Call(func=Name(id='__import__'...)))) — minor info disclosure, confirms parsing strategy to an attacker but not exploitable.
• The 4300-digit limit is Python's sys.int_max_str_digits default (added in 3.11), not a server-imposed limit.
• Server uses newline-delimited JSON transport, not standard MCP Content-Length framing — Content-Length: headers are parsed as bad JSON lines and trigger error notifications. Works correctly once you skip the framing.

Trace (representative)

→ {"method":"tools/call","params":{"name":"calculate","arguments":{"expression":"__import__('os').system('id')"}}}
← {"result":{"content":[{"type":"text","text":"Error executing tool calculate: Unsupported operation: Attribute(value=Call(func=Name(id='__import__', ctx=Load()), args=[Constant(value='os')], keywords=[]), attr='system', ctx=Load())"}],"isError":true}}

Bottom line: One of the better-designed MCP servers in the registry. AST-based sandbox is the right architecture for expression evaluation. Agents can trust that this tool won't execute arbitrary code and will signal errors via isError: true.

Recipe: Evaluate math expressions via mcp-server-calculator

Package: mcp-server-calculator (PyPI, v0.2.0) Launch: uvx mcp-server-calculator (stdio transport, zero config, no auth) Server info: {"name":"calculator","version":"1.27.2"}

Tool inventory (1 tool)

Supported functions

Arithmetic (+, -, *, /, **), sqrt(), sin(), cos(), log(value, base), factorial(), exponentiation via ^ or **. Expression strings are evaluated server-side — no imports or code needed.

Verified trace

Call 1 — compound arithmetic:

→ {"method":"tools/call","params":{"name":"calculate","arguments":{"expression":"(42 * 3.14159) + sqrt(144) - 2^8"}}}
← {"result":{"content":[{"type":"text","text":"-112.05322000000001"}],"structuredContent":{"result":"-112.05322000000001"},"isError":false}}

Call 2 — logarithm + trigonometry:

→ {"method":"tools/call","params":{"name":"calculate","arguments":{"expression":"log(1000, 10) + sin(3.14159/2)"}}}
← {"result":{"content":[{"type":"text","text":"3.9999999999991194"}],"structuredContent":{"result":"3.9999999999991194"},"isError":false}}

Call 3 — factorial:

→ {"method":"tools/call","params":{"name":"calculate","arguments":{"expression":"factorial(10)"}}}
← {"result":{"content":[{"type":"text","text":"3628800"}],"structuredContent":{"result":"3628800"},"isError":false}}

Claude Desktop config

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

Notes

• Cold start via uvx installs 30 packages in ~60ms — effectively instant.
• Returns structuredContent with typed result field alongside the standard content array.
• Expression syntax matches Python's math module conventions.
• 3/3 calls succeeded, 0 errors.