diff --git a/src/__tests__/portalClientsFromAuth.test.ts b/src/__tests__/portalClientsFromAuth.test.ts
new file mode 100644
index 0000000..dd2e899
--- /dev/null
+++ b/src/__tests__/portalClientsFromAuth.test.ts
@@ -0,0 +1,201 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { Hono } from "hono";
+import { getAuth } from "../lib/auth.js";
+
+const NEW_USER_EMAIL = "new-sso-user@example.com";
+const NEW_USER_NAME = "New SSO User";
+const NEW_USER_ID = "11111111-2222-3333-4444-555555555555";
+
+const BETTER_AUTH_SESSION = {
+  user: {
+    id: "auth-user-new",
+    email: NEW_USER_EMAIL,
+    name: NEW_USER_NAME,
+  },
+  session: {
+    id: "ba-session-new",
+    expiresAt: new Date(Date.now() + 60 * 60 * 1000),
+  },
+};
+
+let mockGetAuth: ReturnType<typeof vi.fn>;
+let mockGetSession: ReturnType<typeof vi.fn>;
+let existingClientRow: Record<string, unknown> | null = null;
+let insertedClientValues: Record<string, unknown> | null = null;
+let insertShouldThrow: { code?: string } | null = null;
+
+function makeChainable(data: unknown[]): unknown {
+  const arr = [...data];
+  return new Proxy(arr, {
+    get(target, prop) {
+      if (prop === "where" || prop === "orderBy" || prop === "limit") {
+        return () => makeChainable(target);
+      }
+      // @ts-expect-error proxy
+      return target[prop];
+    },
+  });
+}
+
+vi.mock("@groombook/db", () => {
+  const clients = new Proxy(
+    { _name: "clients" },
+    { get: (t, p) => (p === "_name" ? "clients" : { table: "clients", column: p }) }
+  );
+
+  return {
+    getDb: () => ({
+      select: () => ({
+        from: (table: { _name: string }) => {
+          if (table._name === "clients") {
+            return makeChainable(existingClientRow ? [existingClientRow] : []);
+          }
+          return makeChainable([]);
+        },
+      }),
+      insert: (table: { _name: string }) => ({
+        values: (vals: Record<string, unknown>) => {
+          if (insertShouldThrow) {
+            const err = new Error("unique violation") as Error & { code?: string };
+            err.code = insertShouldThrow.code;
+            throw err;
+          }
+          return {
+            returning: () => {
+              if (table._name === "clients") {
+                insertedClientValues = { id: NEW_USER_ID, ...vals };
+                return [insertedClientValues];
+              }
+              return [];
+            },
+          };
+        },
+      }),
+    }),
+    clients,
+    eq: vi.fn(),
+    and: vi.fn(),
+    inArray: vi.fn(),
+  };
+});
+
+vi.mock("../lib/auth.js", () => ({
+  getAuth: vi.fn(),
+}));
+
+const { portalRouter } = await import("../routes/portal.js");
+
+const app = new Hono();
+app.route("/portal", portalRouter);
+
+describe("POST /portal/clients-from-auth (GRO-2359)", () => {
+  beforeEach(() => {
+    existingClientRow = null;
+    insertedClientValues = null;
+    insertShouldThrow = null;
+    mockGetSession = vi.fn();
+    mockGetAuth = vi.fn(() => ({
+      api: {
+        getSession: mockGetSession,
+      },
+    }));
+    vi.mocked(getAuth).mockImplementation(mockGetAuth);
+  });
+
+  it("returns 401 when no Better Auth session is present", async () => {
+    mockGetSession.mockResolvedValue(null);
+    const res = await app.request("/portal/clients-from-auth", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ name: "Test User" }),
+    });
+    expect(res.status).toBe(401);
+    const body = await res.json();
+    expect(body.error).toBe("Unauthorized");
+  });
+
+  it("returns 400 when body fails zod validation (empty name)", async () => {
+    mockGetSession.mockResolvedValue(BETTER_AUTH_SESSION);
+    const res = await app.request("/portal/clients-from-auth", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ name: "" }),
+    });
+    expect(res.status).toBe(400);
+  });
+
+  it("creates a new client row bound to the auth user's email and returns 201", async () => {
+    mockGetSession.mockResolvedValue(BETTER_AUTH_SESSION);
+    const res = await app.request("/portal/clients-from-auth", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({
+        name: " New SSO User ",
+        phone: "555-1234",
+        address: "1 Main St",
+        notes: "test note",
+      }),
+    });
+    expect(res.status).toBe(201);
+    const body = await res.json();
+    expect(body).toMatchObject({
+      id: NEW_USER_ID,
+      name: "New SSO User",
+      email: NEW_USER_EMAIL,
+    });
+    // Trim must be applied to the persisted values.
+    expect(insertedClientValues).not.toBeNull();
+    expect((insertedClientValues as Record<string, unknown>).name).toBe("New SSO User");
+    expect((insertedClientValues as Record<string, unknown>).email).toBe(NEW_USER_EMAIL);
+    expect((insertedClientValues as Record<string, unknown>).phone).toBe("555-1234");
+  });
+
+  it("normalizes empty optional fields to null on insert", async () => {
+    mockGetSession.mockResolvedValue(BETTER_AUTH_SESSION);
+    await app.request("/portal/clients-from-auth", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ name: "Test", phone: "", address: "   " }),
+    });
+    expect(insertedClientValues).not.toBeNull();
+    expect((insertedClientValues as Record<string, unknown>).phone).toBeNull();
+    expect((insertedClientValues as Record<string, unknown>).address).toBeNull();
+  });
+
+  it("returns 409 when a client row already exists for this email", async () => {
+    mockGetSession.mockResolvedValue(BETTER_AUTH_SESSION);
+    existingClientRow = { id: "existing-client-id", email: NEW_USER_EMAIL };
+    const res = await app.request("/portal/clients-from-auth", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ name: "Test" }),
+    });
+    expect(res.status).toBe(409);
+    const body = await res.json();
+    expect(body.error).toMatch(/already exists/i);
+    expect(insertedClientValues).toBeNull();
+  });
+
+  it("returns 409 on unique constraint race (23505)", async () => {
+    mockGetSession.mockResolvedValue(BETTER_AUTH_SESSION);
+    insertShouldThrow = { code: "23505" };
+    const res = await app.request("/portal/clients-from-auth", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ name: "Test" }),
+    });
+    expect(res.status).toBe(409);
+  });
+
+  it("returns 503 when auth is not configured", async () => {
+    mockGetAuth.mockImplementation(() => {
+      throw new Error("Auth not initialized");
+    });
+    const res = await app.request("/portal/clients-from-auth", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ name: "Test" }),
+    });
+    expect(res.status).toBe(503);
+  });
+});
diff --git a/src/routes/portal.ts b/src/routes/portal.ts
index 487861d..17425a3 100644
--- a/src/routes/portal.ts
+++ b/src/routes/portal.ts
@@ -147,6 +147,114 @@ portalRouter.post("/session-from-auth", async (c) => {
   );
 });
 
+// GRO-2359 — register a brand-new SSO user. The post-auth handler in the
+// web portal redirects here when `session-from-auth` returns 404, so the
+// OOBE can complete a customer record for the new user. Auth is via the
+// Better Auth session (same shape as `session-from-auth`), so this is
+// registered BEFORE the `validatePortalSession` middleware.
+//
+// Contract:
+//   POST /api/portal/clients-from-auth
+//   Body: { name: string; phone?: string|null; address?: string|null; notes?: string|null }
+//   201:  { id, name, email }
+//   400:  invalid body (zod failure)
+//   401:  no Better Auth session
+//   409:  a `clients` row already exists for this email (portal selection case)
+//   500:  insert failed
+//
+// We do NOT auto-link the user's auth account to the new client row; the
+// existing `session-from-auth` endpoint re-resolves the row by email on the
+// next call, so the OOBE's success path just navigates the user back to
+// `/` and lets the bridge mint a portal session.
+const createClientFromAuthSchema = z.object({
+  name: z.string().min(1).max(200),
+  phone: z.string().max(50).nullish(),
+  address: z.string().max(500).nullish(),
+  notes: z.string().max(2000).nullish(),
+});
+
+portalRouter.post(
+  "/clients-from-auth",
+  zValidator("json", createClientFromAuthSchema),
+  async (c) => {
+    let auth;
+    try {
+      auth = getAuth();
+    } catch {
+      return c.json({ error: "Authentication not configured" }, 503);
+    }
+
+    const session = await auth.api.getSession({
+      headers: c.req.raw.headers,
+    });
+
+    if (!session) {
+      return c.json({ error: "Unauthorized" }, 401);
+    }
+
+    const body = c.req.valid("json");
+    const db = getDb();
+
+    // Pre-check: if a client already exists for this email, return 409 so
+    // the OOBE can render the "portal selection" message (the user needs
+    // to contact their groomer to link the new SSO identity to the
+    // pre-existing customer record). We don't return the existing row to
+    // avoid leaking PII about other accounts.
+    const [existing] = await db
+      .select({ id: clients.id })
+      .from(clients)
+      .where(eq(clients.email, session.user.email))
+      .limit(1);
+
+    if (existing) {
+      return c.json(
+        { error: "A customer record with this email already exists" },
+        409,
+      );
+    }
+
+    let row;
+    try {
+      [row] = await db
+        .insert(clients)
+        .values({
+          name: body.name.trim(),
+          email: session.user.email,
+          phone: body.phone?.trim() || null,
+          address: body.address?.trim() || null,
+          notes: body.notes?.trim() || null,
+        })
+        .returning();
+    } catch (err) {
+      // Concurrent insert from a parallel OOBE submit — treat as 409.
+      if (
+        err instanceof Error &&
+        "code" in err &&
+        (err as { code?: string }).code === "23505"
+      ) {
+        return c.json(
+          { error: "A customer record with this email already exists" },
+          409,
+        );
+      }
+      throw err;
+    }
+
+    if (!row) {
+      return c.json({ error: "Failed to create client" }, 500);
+    }
+
+    return c.json(
+      {
+        id: row.id,
+        name: row.name,
+        email: row.email,
+      },
+      201,
+    );
+  },
+);
+
 // Apply middleware to all portal routes
 portalRouter.use("/*", validatePortalSession, portalAudit);