diff --git a/src/App.svelte b/src/App.svelte
index 9ecb8a2..55473af 100644
--- a/src/App.svelte
+++ b/src/App.svelte
@@ -94,6 +94,7 @@
   let overlaySavedSignature = '';
   let overlayAutosaveAttemptedSignature = '';
   let pendingOverlayDraft: { overlayName: string; overlaySlug: string; overlayHtml: string; overlayCss: string; overlayJavascript: string; overlayEnabled: boolean; overlayDisabledDesign: string } | null = null;
+  let overlayDiagnosticsOpen = false;
   let overlayAutosaveTimer: number | undefined;
   let oauthScopes: Record<'streamer' | 'bot', { connected: boolean; missing_scopes: string[]; reauthorization_required: boolean }> = {
     streamer: { connected: false, missing_scopes: [], reauthorization_required: false },
@@ -724,6 +725,21 @@
     return JSON.stringify({ overlayName, overlaySlug, overlayHtml, overlayCss, overlayJavascript, overlayEnabled, overlayDisabledDesign });
   }
 
+  function overlayVariableWarnings() {
+    const warnings: string[] = [];
+    for (const [language, source] of [['HTML', overlayHtml], ['CSS', overlayCss], ['JAVASCRIPT', overlayJavascript]] as const) {
+      if (source.includes('${')) {
+        for (const match of source.match(/\$\{([^}]*)\}/g) || []) {
+          const name = match.slice(2, -1);
+          if (!/^[A-Za-z0-9_]+$/.test(name) || name.length === 0) warnings.push(`${language}: invalid variable ${match[0]}`);
+          else if (name !== 'username') warnings.push(`${language}: undefined variable ${match[0]}`);
+        }
+        if (source.split('${').length - 1 !== (source.match(/\$\{[^}]*\}/g) || []).length) warnings.push(`${language}: an opening variable marker has no closing }`);
+      }
+    }
+    return warnings;
+  }
+
   function overlayDraftCacheKey(id: string) {
     return `trixie.overlay.draft.${id}`;
   }
@@ -1026,5 +1042,11 @@
         <div class="settings-body"><span>A newer unsaved draft is available for this overlay.</span><div class="overlay-controls"><button class="primary" onclick={restorePendingOverlayDraft}>RESTORE LOCAL DRAFT</button><button class="secondary" onclick={discardPendingOverlayDraft}>DISCARD LOCAL DRAFT</button></div></div>
       </section>
     {/if}
+    {#if activeView === 'Overlays' && (selectedOverlayId || overlayName) && overlayVariableWarnings().length}
+      <section class="panel overlay-diagnostics-panel">
+        <div class="panel-header"><button class="diagnostics-toggle" onclick={() => (overlayDiagnosticsOpen = !overlayDiagnosticsOpen)} aria-expanded={overlayDiagnosticsOpen}>⚠ {overlayVariableWarnings().length} VARIABLE WARNING{overlayVariableWarnings().length === 1 ? '' : 'S'}</button></div>
+        {#if overlayDiagnosticsOpen}<div class="diagnostics-details">{#each overlayVariableWarnings() as warning}<div>{warning}</div>{/each}</div>{/if}
+      </section>
+    {/if}
   </main>
 </div>
diff --git a/src/styles.css b/src/styles.css
index 8709992..f51adac 100644
--- a/src/styles.css
+++ b/src/styles.css
@@ -931,3 +931,20 @@ label {
   margin: 6px 0 10px 18px;
   border-left: 2px solid var(--ui-line-focus);
 }
+.overlay-diagnostics-panel {
+  margin-top: 10px;
+}
+.diagnostics-toggle {
+  border: 0;
+  background: transparent;
+  color: var(--ui-error, #ff6b7a);
+  font: inherit;
+  letter-spacing: 0.08em;
+  text-transform: uppercase;
+}
+.diagnostics-details {
+  padding: 10px;
+  color: var(--ui-error, #ff6b7a);
+  font-family: var(--font-mono);
+  font-size: 0.8rem;
+}