diff --git a/extension/sidepanel.css b/extension/sidepanel.css
index 5b99b7bf..0939a350 100644
--- a/extension/sidepanel.css
+++ b/extension/sidepanel.css
@@ -87,6 +87,138 @@
   flex: 1;
 }
 
+/* ─── Security Banner ─────────────────────────────────────────────
+   Variant A approved in /plan-design-review 2026-04-19. Centered
+   alert-heavy. Fires on security_event — canary leaks + ML BLOCK
+   verdicts. Trust UX: layer names + confidence scores in mono so
+   the user can see exactly WHY the session was terminated.
+*/
+
+.security-banner {
+  position: relative;
+  padding: 20px 16px;
+  text-align: center;
+  background: rgba(20, 20, 20, 0.98);
+  border-bottom: 1px solid rgba(239, 68, 68, 0.3);
+  animation: securityBannerEnter 250ms cubic-bezier(0.16, 1, 0.3, 1);
+}
+
+@keyframes securityBannerEnter {
+  from { opacity: 0; transform: translateY(-8px); }
+  to { opacity: 1; transform: translateY(0); }
+}
+
+.security-banner-close {
+  position: absolute;
+  top: 6px;
+  right: 6px;
+  width: 28px;
+  height: 28px;
+  background: transparent;
+  border: none;
+  color: var(--zinc-500, #71717A);
+  font-size: 20px;
+  line-height: 1;
+  cursor: pointer;
+  border-radius: var(--radius-md, 8px);
+  padding: 0;
+}
+.security-banner-close:hover {
+  background: rgba(255, 255, 255, 0.05);
+  color: var(--zinc-300, #D4D4D8);
+}
+.security-banner-close:focus-visible {
+  outline: 2px solid var(--amber-500);
+  outline-offset: 2px;
+}
+
+.security-banner-icon {
+  color: var(--error);
+  display: flex;
+  justify-content: center;
+  margin-bottom: 8px;
+}
+
+.security-banner-title {
+  font-family: var(--font-display, 'Satoshi', sans-serif);
+  font-weight: 700;
+  font-size: 18px;
+  color: var(--error);
+  margin-bottom: 2px;
+}
+
+.security-banner-subtitle {
+  font-family: var(--font-body, 'DM Sans', sans-serif);
+  font-size: 13px;
+  color: var(--zinc-400, #A1A1AA);
+  margin-bottom: 12px;
+}
+
+.security-banner-expand {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+  background: transparent;
+  border: 1px solid rgba(255, 255, 255, 0.08);
+  border-radius: var(--radius-md, 8px);
+  padding: 6px 12px;
+  color: var(--zinc-300, #D4D4D8);
+  font-family: var(--font-body, 'DM Sans', sans-serif);
+  font-size: 12px;
+  cursor: pointer;
+}
+.security-banner-expand:hover {
+  background: rgba(255, 255, 255, 0.04);
+}
+.security-banner-expand:focus-visible {
+  outline: 2px solid var(--amber-500);
+  outline-offset: 2px;
+}
+.security-banner-chevron {
+  transition: transform 200ms ease-out;
+}
+
+.security-banner-details {
+  margin-top: 12px;
+  padding-top: 12px;
+  border-top: 1px solid rgba(255, 255, 255, 0.06);
+  text-align: left;
+}
+
+.security-banner-section-label {
+  font-family: var(--font-mono, 'JetBrains Mono', monospace);
+  font-size: 10px;
+  letter-spacing: 0.08em;
+  color: var(--zinc-500, #71717A);
+  margin-bottom: 6px;
+}
+
+.security-banner-layers {
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+}
+
+.security-banner-layer {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  padding: 4px 8px;
+  background: rgba(255, 255, 255, 0.02);
+  border-radius: var(--radius-sm, 4px);
+  font-family: var(--font-mono, 'JetBrains Mono', monospace);
+  font-size: 12px;
+}
+
+.security-banner-layer-name {
+  color: var(--zinc-300, #D4D4D8);
+}
+
+.security-banner-layer-score {
+  color: var(--amber-400);
+  font-variant-numeric: tabular-nums;
+}
+
 .conn-btn {
   font-size: 9px;
   font-family: var(--font-mono);
diff --git a/extension/sidepanel.html b/extension/sidepanel.html
index 33c77f1f..1824cda4 100644
--- a/extension/sidepanel.html
+++ b/extension/sidepanel.html
@@ -14,6 +14,32 @@
     </div>
   </div>
 
+  <!-- Security event banner — fires on prompt injection detection.
+       Variant A from /plan-design-review 2026-04-19: centered alert-heavy,
+       big red error icon, mono layer scores in expandable details. -->
+  <div class="security-banner" id="security-banner" role="alert" aria-live="assertive" style="display:none">
+    <button class="security-banner-close" id="security-banner-close" aria-label="Dismiss">&times;</button>
+    <div class="security-banner-icon" aria-hidden="true">
+      <svg width="28" height="28" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+        <circle cx="12" cy="12" r="10"></circle>
+        <line x1="12" y1="8" x2="12" y2="12"></line>
+        <line x1="12" y1="16" x2="12.01" y2="16"></line>
+      </svg>
+    </div>
+    <div class="security-banner-title" id="security-banner-title">Session terminated</div>
+    <div class="security-banner-subtitle" id="security-banner-subtitle">prompt injection detected</div>
+    <button class="security-banner-expand" id="security-banner-expand" aria-expanded="false" aria-controls="security-banner-details">
+      <span>What happened</span>
+      <svg class="security-banner-chevron" width="12" height="12" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+        <polyline points="6 9 12 15 18 9"></polyline>
+      </svg>
+    </button>
+    <div class="security-banner-details" id="security-banner-details" hidden>
+      <div class="security-banner-section-label">SECURITY LAYERS</div>
+      <div class="security-banner-layers" id="security-banner-layers"></div>
+    </div>
+  </div>
+
   <!-- Browser tab bar -->
   <div class="browser-tabs" id="browser-tabs" style="display:none"></div>