diff --git a/apps/shell/src/auth/AuthWizard.tsx b/apps/shell/src/auth/AuthWizard.tsx
index 93d8cbe..c410b2f 100644
--- a/apps/shell/src/auth/AuthWizard.tsx
+++ b/apps/shell/src/auth/AuthWizard.tsx
@@ -36,6 +36,7 @@ import {
   shouldShowRegisterChooser,
   type ChooserAction,
 } from "./accountChooser.js";
+import { buildHandleCheckUrl, friendlyHandleWriteError } from "./signupHandle.js";
 import { saveAccountType } from "../accountType.js";
 import { AccountTypeSelection } from "./accountTypeSelection.js";
 import {
@@ -400,19 +401,42 @@ export function AuthWizard({ mode, onClose, embedded = false }: AuthWizardProps)
       setHandleStatus(validationError);
       return;
     }
+    const emailForCheck = email.trim().toLowerCase();
+    if (!emailForCheck.includes("@")) {
+      setHandleStatus("Enter your email before choosing a username.");
+      return;
+    }
+    // Clear stale “available” immediately when handle or email changes (diff F-1).
+    setHandleStatus("Checking username…");
     let cancelled = false;
     const timer = setTimeout(() => {
       void (async () => {
         try {
-          const resp = await fetch(
-            `${CONTROL_PLANE_URL.replace(/\/$/, "")}/handles/check?handle=${encodeURIComponent(normalizeOwnerHandle(handle))}`,
-          );
-          const data = (await resp.json()) as { available?: boolean; handle?: string };
+          const url = buildHandleCheckUrl({
+            controlPlaneBase: CONTROL_PLANE_URL,
+            handle: normalizeOwnerHandle(handle),
+            email: emailForCheck,
+          });
+          const headers: Record<string, string> = { Accept: "application/json" };
+          try {
+            const { data } = await getSupabaseClient().auth.getSession();
+            const token = data.session?.access_token;
+            if (token) headers.Authorization = `Bearer ${token}`;
+          } catch {
+            /* pre-session Profile step */
+          }
+          const resp = await fetch(url, { headers });
+          const data = (await resp.json()) as {
+            available?: boolean;
+            handle?: string;
+            error?: string;
+            code?: string;
+          };
           if (cancelled) return;
           if (data.available) {
             setHandleStatus(`${data.handle ?? normalizeOwnerHandle(handle)} is available`);
           } else {
-            setHandleStatus("That username is already taken");
+            setHandleStatus(data.error ?? "That username is already taken");
           }
         } catch {
           if (!cancelled) {
@@ -425,7 +449,7 @@ export function AuthWizard({ mode, onClose, embedded = false }: AuthWizardProps)
       cancelled = true;
       clearTimeout(timer);
     };
-  }, [handle, hosting]);
+  }, [handle, hosting, email]);
 
   useEffect(() => {
     if (!usesSupabaseHostedAuth()) {
@@ -1219,8 +1243,11 @@ export function AuthWizard({ mode, onClose, embedded = false }: AuthWizardProps)
               accountType: selection.primaryAccountType(),
               handle,
             });
-          } catch {
-            /* handle uniqueness may wait until bootstrap */
+          } catch (err) {
+            setError(
+              friendlyHandleWriteError(err instanceof Error ? err.message : String(err)),
+            );
+            return;
           }
           if (pendingLane === "standard" || pendingLane === "byok") {
             goTo("pay");
@@ -1242,8 +1269,11 @@ export function AuthWizard({ mode, onClose, embedded = false }: AuthWizardProps)
               accountType: selection.primaryAccountType(),
               handle,
             });
-          } catch {
-            /* optional until session stable */
+          } catch (err) {
+            setError(
+              friendlyHandleWriteError(err instanceof Error ? err.message : String(err)),
+            );
+            return;
           }
           if (pendingLane === "standard" || pendingLane === "byok") {
             goTo("pay");
@@ -1466,8 +1496,16 @@ export function AuthWizard({ mode, onClose, embedded = false }: AuthWizardProps)
           return false;
         }
       }
-      if (handleStatus?.includes("taken")) {
-        setError("Choose a different handle.");
+      if (!handleStatus?.includes("is available")) {
+        setError(
+          !handleStatus || handleStatus === "Checking username…"
+            ? "Wait for the username check to finish."
+            : handleStatus.includes("Enter your email")
+              ? handleStatus
+              : handleStatus.includes("Couldn’t check")
+                ? handleStatus
+                : "Choose a different handle.",
+        );
         return false;
       }
     } else if (!adminUrl.trim() || !adminToken.trim()) {
@@ -1541,13 +1579,44 @@ export function AuthWizard({ mode, onClose, embedded = false }: AuthWizardProps)
       const planReadiness = locked
         ? businessHostingDefaults().readinessSkuId
         : clampReadinessForAccount(selection.primaryAccountType(), readinessSkuId);
+
+      // SIGNUP-HANDLE-01: re-check handle vs register email before writing profile / Checkout.
+      if (handle.trim() && email.trim().includes("@")) {
+        try {
+          const url = buildHandleCheckUrl({
+            controlPlaneBase: CONTROL_PLANE_URL,
+            handle: normalizeOwnerHandle(handle),
+            email: email.trim(),
+          });
+          const headers: Record<string, string> = { Accept: "application/json" };
+          try {
+            const { data } = await getSupabaseClient().auth.getSession();
+            const token = data.session?.access_token;
+            if (token) headers.Authorization = `Bearer ${token}`;
+          } catch {
+            /* ignore */
+          }
+          const resp = await fetch(url, { headers });
+          const data = (await resp.json()) as { available?: boolean; error?: string };
+          if (!data.available) {
+            setError(data.error ?? "That username is already taken");
+            return;
+          }
+        } catch {
+          setError("Couldn’t check that username right now — try again in a moment.");
+          return;
+        }
+      }
+
       try {
         await persistSignupProfileIntent({
           accountType: selection.primaryAccountType(),
           handle,
         });
       } catch (err) {
-        setError(err instanceof Error ? err.message : String(err));
+        setError(
+          friendlyHandleWriteError(err instanceof Error ? err.message : String(err)),
+        );
         return;
       }
       savePendingHostedAuth({
diff --git a/apps/shell/src/auth/hostedAccount.ts b/apps/shell/src/auth/hostedAccount.ts
index e29ad50..e2e4679 100644
--- a/apps/shell/src/auth/hostedAccount.ts
+++ b/apps/shell/src/auth/hostedAccount.ts
@@ -1,6 +1,7 @@
 import { createClient, type SupabaseClient } from "@supabase/supabase-js";
 import { SUPABASE_ANON_KEY, SUPABASE_URL, isSupabaseConfigured } from "../hostConfig.js";
 import { bareOwnerHandle } from "../ownerHandle.js";
+import { friendlyHandleWriteError, maySetProfileHandle } from "./signupHandle.js";
 
 let client: SupabaseClient | null = null;
 
@@ -389,13 +390,28 @@ export async function persistSignupProfileIntent(input: {
   const supabase = getSupabaseClient();
   const { data: auth, error: authError } = await supabase.auth.getUser();
   if (authError || !auth.user?.id) throw new Error("Sign in required");
+  const { data: existing, error: readError } = await supabase
+    .from("profiles")
+    .select("handle")
+    .eq("id", auth.user.id)
+    .maybeSingle();
+  if (readError) throw new Error(friendlyHandleWriteError(readError.message));
+
   const patch: Record<string, string> = { account_type: input.accountType };
   const handle = input.handle?.trim();
   if (handle) {
-    patch.handle = bareOwnerHandle(handle);
+    const gate = maySetProfileHandle({
+      existingHandle: existing?.handle ? String(existing.handle) : null,
+      requestedHandle: handle,
+    });
+    if (!gate.ok) throw new Error(friendlyHandleWriteError("handle_immutable"));
+    const existingBare = (existing?.handle ? String(existing.handle) : "").trim();
+    if (!existingBare) {
+      patch.handle = bareOwnerHandle(handle);
+    }
   }
   const { error } = await supabase.from("profiles").update(patch).eq("id", auth.user.id);
-  if (error) throw new Error(error.message);
+  if (error) throw new Error(friendlyHandleWriteError(error.message));
 }
 
 /** True only when CP reports Stripe-backed provisionable (ignore bare `active`). */
diff --git a/apps/shell/src/auth/signupHandle.test.ts b/apps/shell/src/auth/signupHandle.test.ts
new file mode 100644
index 0000000..df98a79
--- /dev/null
+++ b/apps/shell/src/auth/signupHandle.test.ts
@@ -0,0 +1,41 @@
+import { describe, expect, it } from "vitest";
+import {
+  buildHandleCheckUrl,
+  friendlyHandleWriteError,
+  maySetProfileHandle,
+} from "./signupHandle.js";
+
+describe("maySetProfileHandle", () => {
+  it("blocks overwrite luke → testb", () => {
+    const r = maySetProfileHandle({ existingHandle: "luke", requestedHandle: "testb" });
+    expect(r.ok).toBe(false);
+  });
+
+  it("allows first set", () => {
+    expect(maySetProfileHandle({ existingHandle: null, requestedHandle: "fresh" }).ok).toBe(true);
+  });
+});
+
+describe("friendlyHandleWriteError", () => {
+  it("maps postgres unique to taken copy", () => {
+    expect(
+      friendlyHandleWriteError('duplicate key value violates unique constraint "profiles_handle_key"'),
+    ).toBe("That username is already taken");
+  });
+
+  it("maps immutable", () => {
+    expect(friendlyHandleWriteError("handle_immutable")).toBe("Your username can’t be changed.");
+  });
+});
+
+describe("buildHandleCheckUrl", () => {
+  it("includes email and handle", () => {
+    const url = buildHandleCheckUrl({
+      controlPlaneBase: "https://cp.example",
+      handle: "@TestB",
+      email: "Support@Qwixl.com",
+    });
+    expect(url).toContain("email=support%40qwixl.com");
+    expect(url).toContain("handle=");
+  });
+});
diff --git a/apps/shell/src/auth/signupHandle.ts b/apps/shell/src/auth/signupHandle.ts
new file mode 100644
index 0000000..0006c43
--- /dev/null
+++ b/apps/shell/src/auth/signupHandle.ts
@@ -0,0 +1,42 @@
+import { bareOwnerHandle } from "../ownerHandle.js";
+
+/** SIGNUP-HANDLE-01 — mirror CP maySetProfileHandle. */
+export function maySetProfileHandle(opts: {
+  existingHandle: string | null | undefined;
+  requestedHandle: string;
+}): { ok: true } | { ok: false; code: "handle_immutable" } {
+  const existing = (opts.existingHandle ?? "").trim();
+  if (!existing) return { ok: true };
+  if (bareOwnerHandle(existing) === bareOwnerHandle(opts.requestedHandle)) {
+    return { ok: true };
+  }
+  return { ok: false, code: "handle_immutable" };
+}
+
+export function friendlyHandleWriteError(message: string): string {
+  const lower = message.toLowerCase();
+  if (lower.includes("handle_immutable") || lower.includes("p0001")) {
+    return "Your username can’t be changed.";
+  }
+  if (
+    lower.includes("profiles_handle_key") ||
+    lower.includes("duplicate key") ||
+    lower.includes("already taken")
+  ) {
+    return "That username is already taken";
+  }
+  return message;
+}
+
+export function buildHandleCheckUrl(opts: {
+  controlPlaneBase: string;
+  handle: string;
+  email: string;
+}): string {
+  const base = opts.controlPlaneBase.replace(/\/$/, "");
+  const params = new URLSearchParams({
+    handle: opts.handle,
+    email: opts.email.trim().toLowerCase(),
+  });
+  return `${base}/handles/check?${params.toString()}`;
+}