Summary

Where a model's own arithmetic stops being trustworthy.

Large factorials, exact rationals and 256-bit powers are precisely the cases language models get subtly wrong, and this returns them exact rather than plausible. The `~` marker is a nice touch: you can tell at a glance whether a result was rendered exactly or rounded to the session epsilon.

What it is

A Rust port of `calc`, Landon Curt Noll's arbitrary-precision calculator, that speaks JSON-RPC 2.0 over stdio as an MCP server. The numeric core is exact rational arithmetic, which is the same model calc uses natively, so results are not floating-point approximations.

What you get

calc_eval — evaluate an expression; `expression` is required, with `mode` (`real`, `frac` or `int`), `digits` and `epsilon` as per-call overrides
calc_config — read or change the session's precision and display settings, including input and output base from 2 to 36
calc_functions — list the builtins, optionally filtered by substring
483 builtin names, with variables, user-defined functions, `if`/`while`/`for` control flow, lists, complex numbers and string operations
A leading `~` in real-mode output marks a rounded or non-terminating rendering, the same convention calc uses
The same binary also runs as the `rcalc` command-line calculator and as a browser REPL

Requirements

Rust 1.75 or later to build; `cargo build --release` produces `toRustCalcMCP`, which auto-detects MCP versus CLI mode, and `rcalc`, which is always the calculator. Start it with `toRustCalcMCP --mcp` to speak JSON-RPC over stdio. `digits` defaults to 20 in real mode and accepts up to 10000; the default epsilon is exactly 1/10^20. Licensed LGPL-2.1.