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+---
+epic: 1
+story: 1.6
+title: "WASM Bindings (for Web)"
+status: draft
+---
+
+## Epic 1 — Core Calculation Engine (Rust Crate)
+**Goal:** Build `calcpad-engine` as a standalone Rust crate that powers all platforms. This is the foundation.
+
+### Story 1.6: WASM Bindings (for Web)
+
+As a web developer integrating CalcPad,
+I want `wasm-bindgen` exports that return JSON-compatible results and fit within a 500KB gzipped bundle,
+So that the engine runs performantly in the browser without large download overhead.
+
+**Acceptance Criteria:**
+
+**Given** a JavaScript caller invoking the `eval_line` WASM export with a string input
+**When** the function executes
+**Then** it returns a `JsValue` that can be deserialized to a JavaScript object
+**And** the object includes `type`, `display`, `rawValue`, and `error` fields
+
+**Given** a JavaScript caller invoking `eval_sheet` with an array of lines
+**When** the function executes
+**Then** it returns an array of result objects corresponding to each line
+**And** variable context flows from earlier lines to later lines
+
+**Given** the WASM crate is built with `wasm-pack build --release`
+**When** the output `.wasm` file is gzip-compressed
+**Then** the compressed size is under 500KB
+
+**Given** the WASM module is loaded in a browser environment
+**When** `eval_line` is called with `0.1 + 0.2`
+**Then** the result is `0.3` (arbitrary precision works in WASM)
+**And** performance is comparable to native for typical expressions (under 1ms per line)
+
+**Given** the WASM module is loaded in a Web Worker
+**When** `eval_sheet` is called
+**Then** it does not block the main thread
+**And** results can be posted back via standard `postMessage`