diff --git a/apps/api/src/__tests__/portal.test.ts b/apps/api/src/__tests__/portal.test.ts
new file mode 100644
index 0000000..3fed8ee
--- /dev/null
+++ b/apps/api/src/__tests__/portal.test.ts
@@ -0,0 +1,257 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { Hono } from "hono";
+
+const CLIENT_ID = "550e8400-e29b-41d4-a716-446655440001";
+const APPOINTMENT_ID = "660e8400-e29b-41d4-a716-446655440002";
+const SESSION_ID = "770e8400-e29b-41d4-a716-446655440003";
+
+const futureDate = () => new Date(Date.now() + 30 * 60 * 1000);
+const pastDate = () => new Date(Date.now() - 5 * 60 * 1000);
+
+const ACTIVE_SESSION = {
+  id: SESSION_ID,
+  clientId: CLIENT_ID,
+  status: "active" as const,
+  expiresAt: futureDate(),
+  createdAt: new Date(),
+};
+
+const EXPIRED_SESSION = {
+  id: SESSION_ID,
+  clientId: CLIENT_ID,
+  status: "active" as const,
+  expiresAt: pastDate(),
+  createdAt: new Date(),
+};
+
+const ENDED_SESSION = {
+  id: SESSION_ID,
+  clientId: CLIENT_ID,
+  status: "ended" as const,
+  expiresAt: futureDate(),
+  createdAt: new Date(),
+};
+
+const APPOINTMENT = {
+  id: APPOINTMENT_ID,
+  clientId: CLIENT_ID,
+  startTime: futureDate(),
+  endTime: futureDate(),
+  customerNotes: null,
+  confirmationToken: "secret-token-leak-test",
+};
+
+let selectSessionRow: Record<string, unknown> | null = null;
+let selectAppointmentRow: Record<string, unknown> | null = null;
+let updatedValues: Record<string, unknown>[] = [];
+
+function resetMock() {
+  selectSessionRow = null;
+  selectAppointmentRow = null;
+  updatedValues = [];
+}
+
+vi.mock("@groombook/db", () => {
+  function makeChainable(data: unknown[]): unknown {
+    const arr = [...data];
+    const chain = new Proxy(arr, {
+      get(target, prop) {
+        if (prop === "where" || prop === "orderBy" || prop === "limit") {
+          return () => chain;
+        }
+        // @ts-expect-error proxy
+        return target[prop];
+      },
+    });
+    return chain;
+  }
+
+  const impersonationSessions = new Proxy(
+    { _name: "impersonationSessions" },
+    { get: (t, p) => (p === "_name" ? "impersonationSessions" : { table: "impersonationSessions", column: p }) }
+  );
+
+  const appointments = new Proxy(
+    { _name: "appointments" },
+    { get: (t, p) => (p === "_name" ? "appointments" : { table: "appointments", column: p }) }
+  );
+
+  return {
+    getDb: () => ({
+      select: () => ({
+        from: (table: { _name: string }) => {
+          if (table._name === "impersonationSessions") {
+            return makeChainable(selectSessionRow ? [selectSessionRow] : []);
+          }
+          if (table._name === "appointments") {
+            return makeChainable(selectAppointmentRow ? [selectAppointmentRow] : []);
+          }
+          return makeChainable([]);
+        },
+      }),
+      update: () => ({
+        set: (vals: Record<string, unknown>) => ({
+          where: () => ({
+            returning: () => {
+              if (selectAppointmentRow) {
+                const updated = { ...selectAppointmentRow, ...vals };
+                updatedValues.push(vals);
+                return [updated];
+              }
+              return [];
+            },
+          }),
+        }),
+      }),
+    }),
+    impersonationSessions,
+    appointments,
+    eq: vi.fn(),
+    and: vi.fn(),
+  };
+});
+
+const { portalRouter } = await import("../routes/portal.js");
+
+const app = new Hono();
+app.route("/portal", portalRouter);
+
+function jsonPatch(path: string, body: unknown, headers?: Record<string, string>) {
+  return app.request(path, {
+    method: "PATCH",
+    headers: {
+      "Content-Type": "application/json",
+      ...headers,
+    },
+    body: JSON.stringify(body),
+  });
+}
+
+beforeEach(() => resetMock());
+
+describe("PATCH /portal/appointments/:id/notes", () => {
+  it("returns updated appointment with safe fields only", async () => {
+    selectSessionRow = ACTIVE_SESSION;
+    selectAppointmentRow = { ...APPOINTMENT };
+    const res = await jsonPatch(
+      `/portal/appointments/${APPOINTMENT_ID}/notes`,
+      { customerNotes: "Please be gentle with Fido" },
+      { "X-Impersonation-Session-Id": SESSION_ID }
+    );
+    expect(res.status).toBe(200);
+    const body = await res.json();
+    expect(body).toHaveProperty("id");
+    expect(body).toHaveProperty("customerNotes", "Please be gentle with Fido");
+    expect(body).toHaveProperty("updatedAt");
+    expect(body).not.toHaveProperty("confirmationToken");
+    expect(body).not.toHaveProperty("clientId");
+  });
+
+  it("returns 401 without X-Impersonation-Session-Id header", async () => {
+    const res = await jsonPatch(
+      `/portal/appointments/${APPOINTMENT_ID}/notes`,
+      { customerNotes: "Test note" }
+    );
+    expect(res.status).toBe(401);
+    const body = await res.json();
+    expect(body.error).toBe("Unauthorized");
+  });
+
+  it("returns 401 with expired session", async () => {
+    selectSessionRow = EXPIRED_SESSION;
+    const res = await jsonPatch(
+      `/portal/appointments/${APPOINTMENT_ID}/notes`,
+      { customerNotes: "Test note" },
+      { "X-Impersonation-Session-Id": SESSION_ID }
+    );
+    expect(res.status).toBe(401);
+    const body = await res.json();
+    expect(body.error).toBe("Unauthorized");
+  });
+
+  it("returns 401 with ended session", async () => {
+    selectSessionRow = null;
+    const res = await jsonPatch(
+      `/portal/appointments/${APPOINTMENT_ID}/notes`,
+      { customerNotes: "Test note" },
+      { "X-Impersonation-Session-Id": SESSION_ID }
+    );
+    expect(res.status).toBe(401);
+    const body = await res.json();
+    expect(body.error).toBe("Unauthorized");
+  });
+
+  it("returns 403 when appointment belongs to different client", async () => {
+    selectSessionRow = { ...ACTIVE_SESSION, clientId: "different-client-id" };
+    selectAppointmentRow = { ...APPOINTMENT };
+    const res = await jsonPatch(
+      `/portal/appointments/${APPOINTMENT_ID}/notes`,
+      { customerNotes: "Test note" },
+      { "X-Impersonation-Session-Id": SESSION_ID }
+    );
+    expect(res.status).toBe(403);
+    const body = await res.json();
+    expect(body.error).toBe("Forbidden");
+  });
+
+  it("returns 422 for past appointment", async () => {
+    selectSessionRow = ACTIVE_SESSION;
+    selectAppointmentRow = { ...APPOINTMENT, startTime: pastDate() };
+    const res = await jsonPatch(
+      `/portal/appointments/${APPOINTMENT_ID}/notes`,
+      { customerNotes: "Test note" },
+      { "X-Impersonation-Session-Id": SESSION_ID }
+    );
+    expect(res.status).toBe(422);
+    const body = await res.json();
+    expect(body.error).toMatch(/past|in-progress|cannot edit/i);
+  });
+
+  it("returns 422 when appointment is in progress", async () => {
+    selectSessionRow = ACTIVE_SESSION;
+    selectAppointmentRow = { ...APPOINTMENT, startTime: new Date(Date.now() - 2 * 60 * 1000) };
+    const res = await jsonPatch(
+      `/portal/appointments/${APPOINTMENT_ID}/notes`,
+      { customerNotes: "Test note" },
+      { "X-Impersonation-Session-Id": SESSION_ID }
+    );
+    expect(res.status).toBe(422);
+  });
+
+  it("returns 404 when appointment not found", async () => {
+    selectSessionRow = ACTIVE_SESSION;
+    selectAppointmentRow = null;
+    const res = await jsonPatch(
+      `/portal/appointments/nonexistent-id/notes`,
+      { customerNotes: "Test note" },
+      { "X-Impersonation-Session-Id": SESSION_ID }
+    );
+    expect(res.status).toBe(404);
+  });
+
+  it("accepts notes at exactly 500 characters", async () => {
+    selectSessionRow = ACTIVE_SESSION;
+    selectAppointmentRow = { ...APPOINTMENT };
+    const longNote = "a".repeat(500);
+    const res = await jsonPatch(
+      `/portal/appointments/${APPOINTMENT_ID}/notes`,
+      { customerNotes: longNote },
+      { "X-Impersonation-Session-Id": SESSION_ID }
+    );
+    expect(res.status).toBe(200);
+    const body = await res.json();
+    expect(body.customerNotes).toBe(longNote);
+  });
+
+  it("rejects notes exceeding 500 characters", async () => {
+    selectSessionRow = ACTIVE_SESSION;
+    selectAppointmentRow = { ...APPOINTMENT };
+    const longNote = "a".repeat(501);
+    const res = await jsonPatch(
+      `/portal/appointments/${APPOINTMENT_ID}/notes`,
+      { customerNotes: longNote },
+      { "X-Impersonation-Session-Id": SESSION_ID }
+    );
+    expect(res.status).toBe(400);
+  });
+});
\ No newline at end of file
diff --git a/apps/api/src/routes/portal.ts b/apps/api/src/routes/portal.ts
index 792d62a..cd0f6ec 100644
--- a/apps/api/src/routes/portal.ts
+++ b/apps/api/src/routes/portal.ts
@@ -64,6 +64,10 @@ portalRouter.patch(
       .where(eq(appointments.id, id))
       .returning();
 
-    return c.json(updated);
+    return c.json({
+      id: updated.id,
+      customerNotes: updated.customerNotes,
+      updatedAt: updated.updatedAt,
+    });
   }
 );