diff --git a/frontend/package-lock.json b/frontend/package-lock.json
index 487d6ff..1e5ac8f 100644
--- a/frontend/package-lock.json
+++ b/frontend/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "wiggleverse-ecomm-frontend",
-  "version": "0.2.0",
+  "version": "0.4.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "wiggleverse-ecomm-frontend",
-      "version": "0.2.0",
+      "version": "0.4.0",
       "dependencies": {
         "react": "^18.3.1",
         "react-dom": "^18.3.1"
diff --git a/frontend/src/cooldown.test.ts b/frontend/src/cooldown.test.ts
new file mode 100644
index 0000000..10405d1
--- /dev/null
+++ b/frontend/src/cooldown.test.ts
@@ -0,0 +1,24 @@
+import { describe, expect, it } from "vitest";
+import { cooldownAppliesTo } from "./cooldown";
+
+describe("resend cooldown keying (SD-0001 §5.2, INV-3; bug #20)", () => {
+  it("no cooldown running -> send not blocked", () => {
+    expect(cooldownAppliesTo("a@example.com", null, 0)).toBe(false);
+  });
+
+  it("cooldown running for the same address -> blocked", () => {
+    expect(cooldownAppliesTo("a@example.com", "a@example.com", 31)).toBe(true);
+  });
+
+  it("same address modulo case/whitespace -> still blocked", () => {
+    expect(cooldownAppliesTo("  A@Example.COM ", "a@example.com", 31)).toBe(true);
+  });
+
+  it("cooldown running but the input is a different address -> not blocked", () => {
+    expect(cooldownAppliesTo("right@example.com", "wrong@example.com", 31)).toBe(false);
+  });
+
+  it("cooldown expired -> not blocked even for the same address", () => {
+    expect(cooldownAppliesTo("a@example.com", "a@example.com", 0)).toBe(false);
+  });
+});
diff --git a/frontend/src/cooldown.ts b/frontend/src/cooldown.ts
new file mode 100644
index 0000000..4e25b48
--- /dev/null
+++ b/frontend/src/cooldown.ts
@@ -0,0 +1,17 @@
+// Resend-cooldown keying (SD-0001 §5.2, INV-3; bug #20). The server's cooldown is
+// per-address, so the client countdown must be too: it blocks sending only while it
+// is running AND the current input is the address it was set for. Normalization
+// mirrors the backend's normalize_email (lowercase + strip, INV-2).
+
+function normalize(email: string): string {
+  return email.trim().toLowerCase();
+}
+
+export function cooldownAppliesTo(
+  input: string,
+  cooldownEmail: string | null,
+  secondsLeft: number,
+): boolean {
+  if (secondsLeft <= 0 || cooldownEmail === null) return false;
+  return normalize(input) === normalize(cooldownEmail);
+}
diff --git a/frontend/src/screens/SignIn.tsx b/frontend/src/screens/SignIn.tsx
index 47712e0..b00e7b2 100644
--- a/frontend/src/screens/SignIn.tsx
+++ b/frontend/src/screens/SignIn.tsx
@@ -3,6 +3,7 @@
 // is handled by App on success. Visuals per the ui/designs export (hf-signin).
 import { useEffect, useState } from "react";
 import { requestCode, verifyCode, type ApiError, type VerifyResult } from "../api";
+import { cooldownAppliesTo } from "../cooldown";
 import CodeInput from "../ui/CodeInput";
 import { AuthCard, Banner, Field, Footer, PrimaryButton, Screen, TopBar, Wordmark } from "../ui/kit";
 
@@ -34,6 +35,9 @@ export default function SignIn({ door, onAuthed, onBack }: Props) {
   const [codeError, setCodeError] = useState<string | null>(null);
   const [bannerError, setBannerError] = useState<ApiError | null>(null);
   const [cooldown, setCooldown] = useCooldown();
+  // The address the running cooldown belongs to — the server's cooldown is per-address
+  // (INV-3), so a different address must not be blocked by it (bug #20).
+  const [cooldownFor, setCooldownFor] = useState<string | null>(null);
 
   function applyError(err: ApiError) {
     setFieldError(null);
@@ -53,9 +57,11 @@ export default function SignIn({ door, onAuthed, onBack }: Props) {
     setBusy(false);
     if (err) {
       applyError(err);
+      if (err.code === "resend_cooldown") setCooldownFor(email);
       return err.code === "resend_cooldown"; // a cooldown still means a code is out there
     }
     setCooldown(60);
+    setCooldownFor(email);
     return true;
   }
 
@@ -131,8 +137,12 @@ export default function SignIn({ door, onAuthed, onBack }: Props) {
                 }}
               />
               <div style={{ display: "flex", flexDirection: "column", gap: 12 }}>
-                <PrimaryButton busy={busy} disabled={cooldown > 0}>
-                  {busy ? "Sending…" : cooldown > 0 ? `Resend in ${cooldown}s` : "Send code"}
+                <PrimaryButton busy={busy} disabled={cooldownAppliesTo(email, cooldownFor, cooldown)}>
+                  {busy
+                    ? "Sending…"
+                    : cooldownAppliesTo(email, cooldownFor, cooldown)
+                      ? `Resend in ${cooldown}s`
+                      : "Send code"}
                 </PrimaryButton>
                 <p className="note">
                   We'll email you a one-time code. That's all we need — no password, ever.