diff --git a/apps/web/src/App.tsx b/apps/web/src/App.tsx
index 6988b24..30fd3c1 100644
--- a/apps/web/src/App.tsx
+++ b/apps/web/src/App.tsx
@@ -30,6 +30,7 @@ import { useBulkImport } from "./hooks/use-bulk-import";
 import { useEncounter } from "./hooks/use-encounter";
 import { usePlayerCharacters } from "./hooks/use-player-characters";
 import { useSidePanelState } from "./hooks/use-side-panel-state";
+import { cn } from "./lib/utils";
 
 function rollDice(): number {
 	return Math.floor(Math.random() * 20) + 1;
@@ -53,12 +54,12 @@ function useActionBarAnimation(combatantCount: number) {
 	}, [combatantCount]);
 
 	const empty = combatantCount === 0;
-	const risingClass = rising ? " animate-rise-to-center" : "";
-	const settlingClass = settling ? " animate-settle-to-bottom" : "";
+	const risingClass = rising ? "animate-rise-to-center" : "";
+	const settlingClass = settling ? "animate-settle-to-bottom" : "";
 	const exitingClass = topBarExiting
-		? " absolute inset-x-0 top-0 z-10 px-4 animate-slide-up-out"
+		? "absolute inset-x-0 top-0 z-10 px-4 animate-slide-up-out"
 		: "";
-	const topBarClass = settling ? " animate-slide-down-in" : exitingClass;
+	const topBarClass = settling ? "animate-slide-down-in" : exitingClass;
 	const showTopBar = !empty || topBarExiting;
 
 	return {
@@ -200,7 +201,7 @@ export function App() {
 			<div className="relative mx-auto flex min-h-0 w-full max-w-2xl flex-1 flex-col gap-3 px-4">
 				{!!actionBarAnim.showTopBar && (
 					<div
-						className={`shrink-0 pt-8${actionBarAnim.topBarClass}`}
+						className={cn("shrink-0 pt-8", actionBarAnim.topBarClass)}
 						onAnimationEnd={actionBarAnim.onTopBarExitEnd}
 					>
 						<TurnNavigation
@@ -216,7 +217,7 @@ export function App() {
 					/* Empty state — ActionBar centered */
 					<div className="flex min-h-0 flex-1 items-center justify-center pt-8 pb-[15%]">
 						<div
-							className={`w-full${actionBarAnim.risingClass}`}
+							className={cn("w-full", actionBarAnim.risingClass)}
 							onAnimationEnd={actionBarAnim.onRiseEnd}
 						>
 							<ActionBar
@@ -275,7 +276,7 @@ export function App() {
 
 						{/* Action Bar — fixed at bottom */}
 						<div
-							className={`shrink-0 pb-8${actionBarAnim.settlingClass}`}
+							className={cn("shrink-0 pb-8", actionBarAnim.settlingClass)}
 							onAnimationEnd={actionBarAnim.onSettleEnd}
 						>
 							<ActionBar
diff --git a/apps/web/src/components/action-bar.tsx b/apps/web/src/components/action-bar.tsx
index 0a24e1e..6270674 100644
--- a/apps/web/src/components/action-bar.tsx
+++ b/apps/web/src/components/action-bar.tsx
@@ -137,12 +137,14 @@ function AddModeSuggestions({
 								<li key={key}>
 									<button
 										type="button"
-										className={`flex w-full items-center justify-between px-3 py-1.5 text-left text-sm ${(() => {
-											if (isQueued) return "bg-accent/30 text-foreground";
-											if (i === suggestionIndex)
-												return "bg-accent/20 text-foreground";
-											return "text-foreground hover:bg-hover-neutral-bg";
-										})()}`}
+										className={cn(
+											"flex w-full items-center justify-between px-3 py-1.5 text-left text-foreground text-sm",
+											isQueued && "bg-accent/30",
+											!isQueued && i === suggestionIndex && "bg-accent/20",
+											!isQueued &&
+												i !== suggestionIndex &&
+												"hover:bg-hover-neutral-bg",
+										)}
 										onMouseDown={(e) => e.preventDefault()}
 										onClick={() => onClickSuggestion(result)}
 										onMouseEnter={() => onSetSuggestionIndex(i)}
@@ -502,11 +504,12 @@ export function ActionBar({
 										<li key={creatureKey(result)}>
 											<button
 												type="button"
-												className={`flex w-full items-center justify-between px-3 py-1.5 text-left text-sm ${
+												className={cn(
+													"flex w-full items-center justify-between px-3 py-1.5 text-left text-sm",
 													i === suggestionIndex
 														? "bg-accent/20 text-foreground"
-														: "text-foreground hover:bg-hover-neutral-bg"
-												}`}
+														: "text-foreground hover:bg-hover-neutral-bg",
+												)}
 												onMouseDown={(e) => e.preventDefault()}
 												onClick={() => handleBrowseSelect(result)}
 												onMouseEnter={() => setSuggestionIndex(i)}
diff --git a/apps/web/src/components/condition-tags.tsx b/apps/web/src/components/condition-tags.tsx
index becd332..da0418c 100644
--- a/apps/web/src/components/condition-tags.tsx
+++ b/apps/web/src/components/condition-tags.tsx
@@ -18,6 +18,7 @@ import {
 	Sparkles,
 	ZapOff,
 } from "lucide-react";
+import { cn } from "../lib/utils.js";
 
 const ICON_MAP: Record<string, LucideIcon> = {
 	EyeOff,
@@ -75,7 +76,10 @@ export function ConditionTags({
 						type="button"
 						title={def.label}
 						aria-label={`Remove ${def.label}`}
-						className={`inline-flex items-center rounded p-0.5 transition-colors hover:bg-hover-neutral-bg ${colorClass}`}
+						className={cn(
+							"inline-flex items-center rounded p-0.5 transition-colors hover:bg-hover-neutral-bg",
+							colorClass,
+						)}
 						onClick={(e) => {
 							e.stopPropagation();
 							onRemove(condId);
diff --git a/apps/web/src/components/stat-block-panel.tsx b/apps/web/src/components/stat-block-panel.tsx
index 36474cb..a832c3b 100644
--- a/apps/web/src/components/stat-block-panel.tsx
+++ b/apps/web/src/components/stat-block-panel.tsx
@@ -5,6 +5,7 @@ import { useEffect, useState } from "react";
 import { getSourceDisplayName } from "../adapters/bestiary-index-adapter.js";
 import type { BulkImportState } from "../hooks/use-bulk-import.js";
 import { useSwipeToDismiss } from "../hooks/use-swipe-to-dismiss.js";
+import { cn } from "../lib/utils.js";
 import { BulkImportPrompt } from "./bulk-import-prompt.js";
 import { SourceFetchPrompt } from "./source-fetch-prompt.js";
 import { SourceManager } from "./source-manager.js";
@@ -55,9 +56,10 @@ function CollapsedTab({
 		<button
 			type="button"
 			onClick={onToggleCollapse}
-			className={`flex h-full w-[40px] cursor-pointer items-center justify-center text-muted-foreground hover:text-hover-neutral ${
-				side === "right" ? "self-start" : "self-end"
-			}`}
+			className={cn(
+				"flex h-full w-[40px] cursor-pointer items-center justify-center text-muted-foreground hover:text-hover-neutral",
+				side === "right" ? "self-start" : "self-end",
+			)}
 			aria-label="Expand stat block panel"
 		>
 			<span className="writing-vertical-rl font-medium text-sm">
@@ -152,7 +154,11 @@ function DesktopPanel({
 
 	return (
 		<div
-			className={`fixed top-0 bottom-0 flex w-[400px] flex-col border-border bg-card transition-slide-panel ${sideClasses} ${isCollapsed ? collapsedTranslate : "translate-x-0"}`}
+			className={cn(
+				"fixed top-0 bottom-0 flex w-[400px] flex-col border-border bg-card transition-slide-panel",
+				sideClasses,
+				isCollapsed ? collapsedTranslate : "translate-x-0",
+			)}
 		>
 			{isCollapsed ? (
 				<CollapsedTab
@@ -194,7 +200,10 @@ function MobileDrawer({
 				aria-label="Close stat block"
 			/>
 			<div
-				className={`absolute top-0 right-0 bottom-0 w-[85%] max-w-md border-border border-l bg-card shadow-xl ${isSwiping ? "" : "animate-slide-in-right"}`}
+				className={cn(
+					"absolute top-0 right-0 bottom-0 w-[85%] max-w-md border-border border-l bg-card shadow-xl",
+					!isSwiping && "animate-slide-in-right",
+				)}
 				style={
 					isSwiping ? { transform: `translateX(${offsetX}px)` } : undefined
 				}
diff --git a/package.json b/package.json
index d3a3a77..a859ae8 100644
--- a/package.json
+++ b/package.json
@@ -30,6 +30,7 @@
 		"jscpd": "jscpd",
 		"oxlint": "oxlint --tsconfig apps/web/tsconfig.json --type-aware",
 		"check:ignores": "node scripts/check-lint-ignores.mjs",
-		"check": "pnpm audit --audit-level=high && knip && biome check . && oxlint --tsconfig apps/web/tsconfig.json --type-aware && node scripts/check-lint-ignores.mjs && tsc --build && vitest run && jscpd"
+		"check:classnames": "node scripts/check-cn-classnames.mjs",
+		"check": "pnpm audit --audit-level=high && knip && biome check . && oxlint --tsconfig apps/web/tsconfig.json --type-aware && node scripts/check-lint-ignores.mjs && node scripts/check-cn-classnames.mjs && tsc --build && vitest run && jscpd"
 	}
 }
diff --git a/scripts/check-cn-classnames.mjs b/scripts/check-cn-classnames.mjs
new file mode 100644
index 0000000..7f82a60
--- /dev/null
+++ b/scripts/check-cn-classnames.mjs
@@ -0,0 +1,47 @@
+/**
+ * Ban template-literal classNames in TSX files.
+ *
+ * Tailwind v4's production content extractor does static analysis on source
+ * files to discover utility classes. Template literals like
+ *   className={`foo ${bar}`}
+ * can cause the extractor to miss classes adjacent to `${`, leading to
+ * styles that work in dev (JIT) but break in production.
+ *
+ * Rule: always use cn() for dynamic class composition instead.
+ */
+
+import { execSync } from "node:child_process";
+import { readFileSync } from "node:fs";
+
+const PATTERN = /className\s*=\s*\{`/;
+
+function findFiles() {
+	return execSync("git ls-files -- '*.tsx'", { encoding: "utf-8" })
+		.trim()
+		.split("\n")
+		.filter(Boolean);
+}
+
+let errors = 0;
+
+for (const file of findFiles()) {
+	const lines = readFileSync(file, "utf-8").split("\n");
+
+	for (let i = 0; i < lines.length; i++) {
+		if (PATTERN.test(lines[i])) {
+			console.error(
+				`${file}:${i + 1}: className uses template literal — use cn() instead`,
+			);
+			errors++;
+		}
+	}
+}
+
+if (errors > 0) {
+	console.error(
+		`\n${errors} template-literal className(s) found. Use cn() for dynamic classes.`,
+	);
+	process.exit(1);
+} else {
+	console.log("No template-literal classNames found.");
+}