---
epic: 12
story: 12.1
title: "Plugin API (Rust Trait)"
status: draft
---

## Epic 12 — Plugin & Extension System
**Goal:** Let power users extend CalcPad.

### Story 12.1: Plugin API (Rust Trait)

As a Rust developer,
I want to implement a `CalcPadPlugin` trait to extend CalcPad with custom functions, units, and variables,
So that I can add domain-specific capabilities to the engine.

**Acceptance Criteria:**

**Given** the `CalcPadPlugin` trait is defined in the calcpad-engine crate
**When** a developer implements the trait
**Then** they can provide implementations for `register_functions()`, `register_units()`, and `register_variables()`
**And** each method receives a registry to add new capabilities

**Given** a developer compiles their plugin as a dynamic library (.dylib on macOS, .dll on Windows, .so on Linux)
**When** CalcPad loads the plugin at startup
**Then** the registered functions, units, and variables are available in expressions
**And** conflicts with built-in names are reported as warnings

**Given** a developer compiles their plugin as a WASM module
**When** CalcPad loads the WASM plugin
**Then** the registered functions, units, and variables are available in expressions
**And** the WASM plugin runs in a sandboxed environment with no filesystem or network access

**Given** a plugin registers a custom function (e.g., `bmi(weight, height)`)
**When** a user types `bmi(80kg, 1.8m)` in a sheet
**Then** the plugin function is invoked with the provided arguments
**And** the result is displayed in the answer column