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+---
+epic: 13
+story: 13.3
+title: "Web Worker Offloading (Web Only)"
+status: draft
+---
+
+## Epic 13 — Performance & Reliability
+**Goal:** Instant evaluation, never lose work.
+
+### Story 13.3: Web Worker Offloading (Web Only)
+
+As a web CalcPad user,
+I want the calculation engine to run in a Web Worker,
+So that the main thread remains free for rendering and the UI never freezes during heavy computation.
+
+**Acceptance Criteria:**
+
+**Given** the web version of CalcPad is loaded in a browser
+**When** the application initializes
+**Then** the calcpad-engine WASM module is instantiated inside a dedicated Web Worker
+**And** the main thread contains no evaluation logic
+
+**Given** a user types an expression on the main thread
+**When** the input is sent to the Web Worker for evaluation
+**Then** the result is returned via postMessage to the main thread
+**And** the main thread renders the result without blocking on computation
+
+**Given** the Web Worker is processing a large sheet (500+ lines)
+**When** the user continues typing on the main thread
+**Then** keystrokes are captured and rendered with zero perceptible lag
+**And** pending evaluation requests are queued or debounced so the latest input is always processed
+
+**Given** the Web Worker crashes or becomes unresponsive
+**When** the main thread detects the worker has not responded within 5 seconds
+**Then** the worker is terminated and a new worker is spawned
+**And** the current sheet state is re-sent to the new worker for evaluation
+**And** the user sees a brief non-blocking notification that the engine was restarted