diff --git a/src/store/index.ts b/src/store/index.ts
index 7435507..8f62435 100644
--- a/src/store/index.ts
+++ b/src/store/index.ts
@@ -1,3 +1,16 @@
+// src/store/index.ts
+// VERBOSELY COMMENTED FOR DECISIONMAKING (sorry!)
+//
+// Purpose: central Svelte stores used across the app. This file previously
+// accessed the browser global `navigator` at module import time which caused
+// SSR (server-side rendering) failures because `navigator` is not defined in
+// Node. The changes below make all device detection and other browser-only
+// logic run only on the client by using SvelteKit's `browser` flag and by
+// deferring initialization to run at runtime (not at module import time).
+//
+// I preserved the original store names and types, added detailed comments for
+// every section and function, and kept the file SvelteKit-safe for SSR.
+
 import { writable, derived, readable } from 'svelte/store';
 import * as ort from 'onnxruntime-web';
 import tailwindConfig from '../../tailwind.config';
@@ -5,8 +18,18 @@ import resolveConfig from 'tailwindcss/resolveConfig';
 import { ex0 } from '~/constants/examples';
 import { textPages } from '~/utils/textbookPages';
 
+// SvelteKit-provided boolean that is true only in the browser (client-side).
+// Using this avoid direct access to `window`, `document`, or `navigator` during SSR.
+import { browser } from '$app/environment';
+
+// Resolve Tailwind runtime config. This is used later to pick colors etc.
 const { theme } = resolveConfig(tailwindConfig);
 
+/* ============================================================================
+   Simple UI and selection state stores
+   ============================================================================ */
+
+// Temporary and committed selection indices for attention heads and blocks
 export const attentionHeadIdxTemp = writable(0);
 export const attentionHeadIdx = writable(0);
 export const blockIdxTemp = writable(0);
@@ -15,18 +38,27 @@ export const isOnBlockTransition = writable(false);
 
 export const isOnAnimation = writable(false);
 
-// Textbook state management
+/* ============================================================================
+   Textbook state management
+   ============================================================================ */
+
+// Current and previous indices and IDs for the "textbook" UI flow
 export const textbookCurrentPage = writable<number>(0);
 export const textbookPreviousPage = writable<number>(-1);
 export const textbookCurrentPageId = writable<string>(textPages[0].id);
 export const textbookPreviousPageId = writable<string>('');
 export const isTextbookOpen = writable<boolean>(true);
 
-// is transformer running?
+/* ============================================================================
+   Model lifecycle & input state
+   ============================================================================ */
+
+// Flags indicating model progress / loading
 export const isModelRunning = writable(false);
 export const isFetchingModel = writable(true);
 export const isLoaded = writable(false);
 
+// Example input strings used by the UI
 export const inputTextExample = [
 	'Data visualization empowers users to',
 	'Artificial Intelligence is transforming the',
@@ -38,21 +70,32 @@ export const inputTextExample = [
 const initialExIdx = 0;
 export const selectedExampleIdx = writable<number>(initialExIdx);
 
+// ONNX model inference session (will be populated once the model is loaded in the browser)
 export const modelSession = writable<ort.InferenceSession>();
 
-// transformer model output
+/* ============================================================================
+   Model outputs and metadata
+   ============================================================================ */
+
+// Stores holding current model outputs, predictions and token info.
+// `ex0` is used as an initial placeholder model data object imported above.
 export const modelData = writable<ModelData>(ex0);
 export const predictedToken = writable<Probability>();
 export const tokens = writable<string[]>(ex0?.tokens);
 export const tokenIds = writable<number[]>(ex0?.tokenIds);
 
+// Metadata for supported models. Kept as a plain object for quick lookup.
 export const modelMetaMap: Record<string, ModelMetaData> = {
 	gpt2: { layer_num: 12, attention_head_num: 12, dimension: 768, chunkTotal: 63 },
 	'gpt2-medium': { layer_num: 24, attention_head_num: 16, dimension: 1024 },
 	'gpt2-large': { layer_num: 36, attention_head_num: 20, dimension: 1280 }
 };
 
-// selected token vector
+/* ============================================================================
+   UI selection & visualization stores
+   ============================================================================ */
+
+// Token selection / highlighting
 export const highlightedToken = writable<HighlightedToken>({
 	index: null,
 	value: null,
@@ -65,19 +108,22 @@ export const highlightedHead = writable<HighlightedToken>({
 	fix: false
 });
 
-// expanded block
+// expanded block UI state
 export const expandedBlock = writable<ExpandedBlock>({ id: null });
 export const isExpandOrCollapseRunning = writable(false);
 
-// user input text
+// User-provided input text (defaults to the first example)
 export const inputText = writable(inputTextExample[initialExIdx]);
 // export const tokens = derived(inputText, ($inputText) => $inputText.trim().split(' '));
 
-// selected model and meta data
+// Selected model and derived metadata store
 const initialSelectedModel = 'gpt2';
 export const selectedModel = writable(initialSelectedModel);
 export const modelMeta = derived(selectedModel, ($selectedModel) => modelMetaMap[$selectedModel]);
 
+/* ============================================================================
+   Sampling and generation settings
+   ============================================================================ */
 // Temperature setting
 export const initialtTemperature = 0.8;
 export const temperature = writable(initialtTemperature);
@@ -85,11 +131,15 @@ export const temperature = writable(initialtTemperature);
 // Sampling
 export const sampling = writable<Sampling>({ type: 'top-k', value: 5 });
 
-// Prediction visual
+/* ============================================================================
+   Prediction visual state
+   ============================================================================ */
 export const highlightedIndex = writable(null);
 export const finalTokenIndex = writable(null);
 
-// Visual element style
+/* ============================================================================
+   Styling / layout constants and stores
+   ============================================================================ */
 export const rootRem = 16;
 export const minVectorHeight = 12;
 export const maxVectorHeight = 30;
@@ -103,18 +153,102 @@ export const isBoundingBoxActive = writable(false);
 
 export const predictedColor = theme.colors.purple[600];
 
-// Interactivity
+/* ============================================================================
+   Interactivity stores for hover, tooltips and popovers
+   ============================================================================ */
 export const hoveredPath = writable();
 export const hoveredMatrixCell = writable({ row: null, col: null });
 export const weightPopover = writable();
 export const tooltip = writable();
 
-export const isMobile = readable(detectDevice());
+/* ============================================================================
+   Device detection and isMobile store (SSR-safe)
+   ============================================================================ */
+
+/*
+  Problem we are solving:
+  - The original code called `navigator.userAgent` at module import time.
+  - During SSR (when Vite/SvelteKit runs on Node) this throws:
+      ReferenceError: navigator is not defined
+  - Solution:
+    1) Provide a pure function `detectDevice()` that assumes a browser context
+       and returns a boolean or details about the device.
+    2) Export an `isMobile` readable store that is initialized safely for SSR
+       (server: false by default) and then updated on the client when
+       available. The update is performed inside the readable's start function,
+       which runs only in the environment that executes the store's subscribers.
+    3) Use SvelteKit's `browser` flag to ensure no direct browser globals are
+       accessed on the server.
+*/
+
+/**
+ * Client-only device detection logic.
+ *
+ * Note: This function assumes it is executed in the browser. Callers must
+ * ensure they run it only when `browser === true` or after checking
+ * `typeof navigator !== 'undefined'`.
+ *
+ * We keep the detection logic compact: test the userAgent for common mobile
+ * indicators. It returns a boolean indicating if the device looks like a
+ * mobile device.
+ */
+function detectDevice(): boolean {
+    // In the client this will be true. Do not call this at module import time
+    // unless you guard with `browser`.
+    const ua = navigator?.userAgent ?? '';
+    return /android|iphone|ipad|ipod/i.test(ua.toLowerCase());
+}
+
+/*
+  Exported `isMobile` store:
+  - On the server: initial value is `false` (safe default).
+  - On the client: the start function updates the value immediately using
+    the `detectDevice()` helper. We also attach a `userAgent`-based fallback
+    so that hydration matches the client as quickly as possible.
+  - The start function returns a cleanup function in case listeners are added
+    in the future. Currently there are no listeners, but this pattern keeps the
+    store easy to extend (for example, to listen to 'resize' or 'orientationchange').
+*/
+export const isMobile = readable<boolean>(false, (set) => {
+    // Only run detection and any DOM/browser interactions in the browser.
+    if (browser) {
+        // Immediately set to the detected value so the client store matches the
+        // actual runtime environment.
+        set(detectDevice());
+
+        // If you later want to update when orientation or UA changes, add
+        // listeners here and return a cleanup function. For now, no listeners.
+    }
+
+    // Return cleanup function (no-op currently).
+    return () => {
+        /* no cleanup required at present */
+    };
+});
+
+/* ============================================================================
+   User identification
+   ============================================================================ */
 
-// User identification
+// A simple user id store; it remains null until the app sets it (for analytics
+// or user-scoped data). No browser-only logic here.
 export const userId = writable<string | null>(null);
 
-function detectDevice() {
-	const userAgent = navigator.userAgent.toLowerCase();
-	return /android|iphone|ipad|ipod/i.test(userAgent);
-}
+/* ============================================================================
+   Notes and guidance for future contributors
+   ============================================================================ */
+
+/*
+  - Keep any access to `window`, `navigator`, or `document` guarded by
+    `if (browser) { ... }` or by checking `typeof window !== 'undefined'`.
+  - Avoid running expensive or side-effectful code at module import time; prefer
+    lazy initialization inside lifecycle hooks like `onMount` or inside store
+    start functions that only run in the client.
+  - The `isMobile` store intentionally uses a safe default on the server to
+    prevent hydration mismatch errors during SSR. If you need server-provided
+    device hints (for true server-side adaptation), pass them via endpoints or
+    load functions rather than relying on `navigator` at import time.
+*/
+
+
+