diff --git a/src/__tests__/shopping.test.ts b/src/__tests__/shopping.test.ts
index 47a28b3..04abbf7 100644
--- a/src/__tests__/shopping.test.ts
+++ b/src/__tests__/shopping.test.ts
@@ -2,6 +2,7 @@ import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
 import * as fs from 'node:fs';
 import * as os from 'node:os';
 import * as path from 'node:path';
+import { Command } from 'commander';
 import { mkClient } from './mkclient.js';
 import { renderRaw } from '../lib/ui.js';
 
@@ -46,25 +47,29 @@ let tmp: string;
 let prevXdg: string | undefined;
 let prevEmail: string | undefined;
 let prevPw: string | undefined;
+let previousExitCode: typeof process.exitCode;
 
 beforeEach(() => {
   prevXdg = process.env.XDG_CONFIG_HOME;
   prevEmail = process.env.TAKEALOT_EMAIL;
   prevPw = process.env.TAKEALOT_PASSWORD;
+  previousExitCode = process.exitCode;
   tmp = fs.mkdtempSync(path.join(os.tmpdir(), 'tk-shop-'));
   process.env.XDG_CONFIG_HOME = tmp;
   process.env.TAKEALOT_EMAIL = 'shopper@example.com';
   process.env.TAKEALOT_PASSWORD = 'pw';
+  process.exitCode = undefined;
 });
 afterEach(() => {
   process.env.XDG_CONFIG_HOME = prevXdg;
   process.env.TAKEALOT_EMAIL = prevEmail;
   process.env.TAKEALOT_PASSWORD = prevPw;
+  process.exitCode = previousExitCode;
   fs.rmSync(tmp, { recursive: true, force: true });
   vi.restoreAllMocks();
 });
 
-async function seededContext() {
+async function seededContext(verbose = false) {
   vi.resetModules();
   const cfg = await import('../lib/config.js');
   cfg.saveCredentials({
@@ -82,7 +87,7 @@ async function seededContext() {
     device: { profile: (await import('../lib/device.js')).resolveDeviceProfile({}), did: 'DID' },
   } as any);
   const { Context } = await import('../lib/context.js');
-  return new Context({ json: true, verbose: false });
+  return new Context({ json: true, verbose });
 }
 
 function captureStdout(): { chunks: string[]; restore: () => void } {
@@ -94,6 +99,141 @@ function captureStdout(): { chunks: string[]; restore: () => void } {
   return { chunks, restore: () => spy.mockRestore() };
 }
 
+function captureStderr(): { chunks: string[]; restore: () => void } {
+  const chunks: string[] = [];
+  const spy = vi.spyOn(process.stderr, 'write').mockImplementation((s: any) => {
+    chunks.push(String(s));
+    return true;
+  });
+  return { chunks, restore: () => spy.mockRestore() };
+}
+
+const cartBody = (lines: Array<{ skuId: number; quantity: number; title: string }>) => ({
+  products: lines.map((line) => ({ product_id: line.skuId, title: line.title, selling_price: 10 })),
+  cart_items: lines.map((line) => ({ product_id: line.skuId, quantity: line.quantity, sub_total: line.quantity * 10 })),
+});
+
+const response = (body: unknown): Response => ({
+  ok: true,
+  status: 200,
+  statusText: 'OK',
+  headers: new Headers({ 'content-type': 'application/json' }),
+  text: async () => JSON.stringify(body),
+  json: async () => body,
+}) as unknown as Response;
+
+const threeLines = [
+  { skuId: 101, quantity: 1, title: 'One' },
+  { skuId: 202, quantity: 2, title: 'Two' },
+  { skuId: 303, quantity: 3, title: 'Three' },
+];
+
+describe('cart mutation guard', () => {
+  it('detects a vanished duplicate line of the same non-target SKU', async () => {
+    const before = [...threeLines, { skuId: 202, quantity: 2, title: 'Two again' }];
+    const after = [threeLines[1]!, threeLines[2]!];
+    const replies = [cartBody(before), {}, cartBody(after)];
+    globalThis.fetch = vi.fn(async () => response(replies.shift())) as any;
+    const ctx = await seededContext(false);
+    const { cartRemove } = await import('../commands/cart.js');
+    const stdout = captureStdout();
+    const stderr = captureStderr();
+    await cartRemove(ctx, 101, { confirm: true, yes: true });
+    const result = JSON.parse(stdout.chunks.join(''));
+    stdout.restore();
+    stderr.restore();
+    expect(result.unexpectedRemovals).toEqual([{ skuId: 202, quantity: 2, title: 'Two' }]);
+    expect(process.exitCode).toBe(1);
+    process.exitCode = undefined;
+  });
+
+  it('removes one SKU while preserving the other lines and logs all three auth contexts', async () => {
+    const replies = [cartBody(threeLines), {}, cartBody(threeLines.slice(1))];
+    const fetchMock = vi.fn(async (_url: string | URL, _init?: RequestInit) => response(replies.shift()));
+    globalThis.fetch = fetchMock as any;
+    const ctx = await seededContext(true);
+    const { cartRemove } = await import('../commands/cart.js');
+    const stdout = captureStdout();
+    const stderr = captureStderr();
+    await cartRemove(ctx, 101, { confirm: true, yes: true });
+    const result = JSON.parse(stdout.chunks.join(''));
+    stdout.restore();
+    stderr.restore();
+    expect(result).toMatchObject({ removed: true, remaining: 2 });
+    expect(result).not.toHaveProperty('unexpectedRemovals');
+    expect(fetchMock.mock.calls.map((call) => (call[1] as RequestInit).method ?? 'GET')).toEqual(['GET', 'DELETE', 'GET']);
+    const verbose = stderr.chunks.join('');
+    expect(verbose.match(/customerId=12345 authGeneration=0/g)).toHaveLength(3);
+    expect(verbose).toContain('DELETE body {"products":[{"id":101}]}');
+    expect(process.exitCode).toBeUndefined();
+  });
+
+  it('reports unexpectedRemovals and exits 1 when the server wipes the cart', async () => {
+    const replies = [cartBody(threeLines), {}, cartBody([])];
+    const fetchMock = vi.fn(async (_url: string | URL, _init?: RequestInit) => response(replies.shift()));
+    globalThis.fetch = fetchMock as any;
+    const ctx = await seededContext();
+    const { cartRemove } = await import('../commands/cart.js');
+    const stdout = captureStdout();
+    const stderr = captureStderr();
+    await cartRemove(ctx, 101, { confirm: true, yes: true });
+    const result = JSON.parse(stdout.chunks.join(''));
+    stdout.restore();
+    stderr.restore();
+    expect(result.unexpectedRemovals).toEqual(threeLines.slice(1));
+    expect(stderr.chunks.join('')).toContain('CART SAFETY CHECK FAILED');
+    expect(process.exitCode).toBe(1);
+  });
+
+  it('guards set-qty against collateral cart changes', async () => {
+    const changed = [{ ...threeLines[0]!, quantity: 4 }, { ...threeLines[1]!, quantity: 1 }, threeLines[2]!];
+    const replies = [cartBody(threeLines), {}, cartBody(changed)];
+    const fetchMock = vi.fn(async (_url: string | URL, _init?: RequestInit) => response(replies.shift()));
+    globalThis.fetch = fetchMock as any;
+    const ctx = await seededContext();
+    const { cartSetQty } = await import('../commands/cart.js');
+    const stdout = captureStdout();
+    const stderr = captureStderr();
+    await cartSetQty(ctx, 101, 4, { confirm: true, yes: true });
+    const result = JSON.parse(stdout.chunks.join(''));
+    stdout.restore();
+    stderr.restore();
+    expect(result.unexpectedRemovals).toEqual([threeLines[1]]);
+    expect(process.exitCode).toBe(1);
+  });
+
+  it('rejects a missing SKU with exit 4 and sends no DELETE', async () => {
+    const fetchMock = vi.fn(async (_url: string | URL, _init?: RequestInit) => response(cartBody(threeLines)));
+    globalThis.fetch = fetchMock as any;
+    await seededContext();
+    const { run } = await import('../cli.js');
+    const { cartRemove } = await import('../commands/cart.js');
+    const command = new Command().option('--json');
+    command.setOptionValue('json', true);
+    const stdout = captureStdout();
+    await run(command, (ctx) => cartRemove(ctx, 404, { confirm: true, yes: true }));
+    const result = JSON.parse(stdout.chunks.join(''));
+    stdout.restore();
+    expect(result).toMatchObject({ code: 'usage_error', error: 'SKU 404 is not in the current cart' });
+    expect(process.exitCode).toBe(4);
+    expect(fetchMock).toHaveBeenCalledTimes(1);
+    expect((fetchMock.mock.calls[0]![1] as RequestInit).method ?? 'GET').toBe('GET');
+  });
+
+  it('lists the cart lines that would remain in a remove dry run', async () => {
+    const fetchMock = vi.fn(async (_url: string | URL, _init?: RequestInit) => response(cartBody(threeLines)));
+    globalThis.fetch = fetchMock as any;
+    const ctx = await seededContext();
+    const { cartRemove } = await import('../commands/cart.js');
+    const stdout = captureStdout();
+    await cartRemove(ctx, 101);
+    const result = JSON.parse(stdout.chunks.join(''));
+    stdout.restore();
+    expect(result.request.remaining).toEqual(threeLines.slice(1));
+    expect(fetchMock).toHaveBeenCalledTimes(1);
+  });
+});
+
 describe('mutation gating', () => {
   it('a mutating command is a NO-OP under default dry-run (no write fetch)', async () => {
     const fetchMock = vi.fn(async () => ({ ok: true, status: 200, headers: new Headers({ 'content-type': 'application/json' }), text: async () => '{}' } as unknown as Response));
diff --git a/src/commands/cart.ts b/src/commands/cart.ts
index 34f8fce..42191f4 100644
--- a/src/commands/cart.ts
+++ b/src/commands/cart.ts
@@ -2,6 +2,8 @@ import type { Context } from '../lib/context.js';
 import { c, rand } from '../lib/ui.js';
 import { gate } from './generic.js';
 import type { PreferenceMatch } from '../lib/preferences.js';
+import { UsageError } from '../lib/errors.js';
+import type { CartItem, CartResult } from '../types.js';
 
 export interface CartWriteFlags {
   confirm?: boolean;
@@ -15,6 +17,54 @@ const REASON_LABEL: Record<PreferenceMatch['reason'], string> = {
   'top-result': 'top result',
 };
 
+type CartLine = { skuId: number; quantity: number; title: string };
+
+const skuOf = (item: CartItem): number => item.skuId ?? item.productId;
+
+const cartLines = (cart: CartResult): CartLine[] =>
+  cart.items.map((item) => ({ skuId: skuOf(item), quantity: item.quantity, title: item.title }));
+
+const quantityBySku = (cart: CartResult): Map<number, number> => {
+  const totals = new Map<number, number>();
+  for (const item of cart.items) totals.set(skuOf(item), (totals.get(skuOf(item)) ?? 0) + item.quantity);
+  return totals;
+};
+
+const changedLines = (
+  before: CartResult,
+  after: CartResult,
+  targetSkuId: number,
+  targetQuantity: number | null,
+): CartLine[] => {
+  const beforeTotals = quantityBySku(before);
+  const afterTotals = quantityBySku(after);
+  const reported = new Set<number>();
+  return cartLines(before).filter((line) => {
+    if (line.skuId === targetSkuId || reported.has(line.skuId)) return false;
+    if ((afterTotals.get(line.skuId) ?? 0) === beforeTotals.get(line.skuId)) return false;
+    reported.add(line.skuId);
+    return true;
+  }).concat(
+    targetQuantity !== null && (afterTotals.get(targetSkuId) ?? 0) !== targetQuantity
+      ? cartLines(before).filter((line) => line.skuId === targetSkuId).slice(0, 1)
+      : [],
+  );
+};
+
+const previewLines = (lines: CartLine[]): string =>
+  lines.length ? lines.map((line) => `${line.quantity}× ${line.title} (SKU ${line.skuId})`).join('; ') : 'none';
+
+function warnRestoration(ctx: Context, unexpected: CartLine[], after: CartResult): void {
+  const remaining = new Set(after.items.map(skuOf));
+  ctx.logger.error('CART SAFETY CHECK FAILED: other cart lines disappeared or changed quantity.');
+  for (const line of unexpected) {
+    const command = remaining.has(line.skuId)
+      ? `takealot cart set-qty ${line.skuId} ${line.quantity} --confirm --yes`
+      : `takealot cart add --sku ${line.skuId} --qty ${line.quantity} --confirm --yes`;
+    ctx.logger.warn(`Restore ${line.title}: ${command}`);
+  }
+}
+
 /** Parse an optional leading quantity, e.g. "3 pencils" → { qty: 3, query: "pencils" }. */
 function parseQuantity(raw: string): { qty: number; query: string } {
   const m = raw.trim().match(/^(\d+)\s+(.+)$/);
@@ -154,10 +204,25 @@ export async function cartClear(ctx: Context, flags: CartWriteFlags = {}): Promi
 export async function cartSetQty(ctx: Context, skuId: number, quantity: number, flags: CartWriteFlags = {}): Promise<void> {
   await ctx.ensureCredentials();
   if (!Number.isFinite(quantity) || quantity < 1) throw new Error('quantity must be a positive integer');
-  if (!(await gate(ctx, flags, { action: `set SKU ${skuId} quantity to ${quantity}` }))) return;
+  const before = await ctx.client.getCart();
+  if (!before.items.some((item) => skuOf(item) === skuId)) throw new UsageError(`SKU ${skuId} is not in the current cart`);
+  const expected = cartLines(before).map((line) => line.skuId === skuId ? { ...line, quantity } : line);
+  const body = { products: [{ id: skuId, quantity }] };
+  const request = { ...ctx.client.describeCall('cart.update', { body }), remaining: expected };
+  if (!(await gate(ctx, flags, { action: `set SKU ${skuId} quantity to ${quantity}; lines after: ${previewLines(expected)}`, request }))) return;
   ctx.logger.info(`✏️  Setting SKU ${skuId} → qty ${quantity}…`);
   await ctx.client.setCartItemQuantity(skuId, quantity);
   const cart = await ctx.client.getCart();
+  const unexpectedRemovals = changedLines(before, cart, skuId, quantity);
+  if (unexpectedRemovals.length) {
+    warnRestoration(ctx, unexpectedRemovals, cart);
+    ctx.logger.result(
+      () => process.stdout.write(`${c.red('✗')} Cart update changed other lines; restore them with the commands above.\n`),
+      { updated: false, skuId, quantity, remaining: cart.items.length, total: cart.total, unexpectedRemovals },
+    );
+    process.exitCode = 1;
+    return;
+  }
   ctx.logger.result(
     () => process.stdout.write(`${c.green('✓')} Updated SKU ${skuId} to ${quantity}. Total ${rand(cart.total)}\n`),
     { updated: true, skuId, quantity, total: cart.total },
@@ -167,10 +232,25 @@ export async function cartSetQty(ctx: Context, skuId: number, quantity: number,
 /** `cart remove <sku>` — remove one cart line by its buyable SKU id. */
 export async function cartRemove(ctx: Context, skuId: number, flags: CartWriteFlags = {}): Promise<void> {
   await ctx.ensureCredentials();
-  if (!(await gate(ctx, flags, { action: `remove SKU ${skuId} from the cart` }))) return;
+  const before = await ctx.client.getCart();
+  if (!before.items.some((item) => skuOf(item) === skuId)) throw new UsageError(`SKU ${skuId} is not in the current cart`);
+  const remaining = cartLines(before).filter((line) => line.skuId !== skuId);
+  const body = { products: [{ id: skuId }] };
+  const request = { ...ctx.client.describeCall('cart.remove', { body }), remaining };
+  if (!(await gate(ctx, flags, { action: `remove SKU ${skuId} from the cart; lines remaining: ${previewLines(remaining)}`, request }))) return;
   ctx.logger.info(`➖ Removing SKU ${skuId}…`);
   await ctx.client.removeCartItem(skuId);
   const cart = await ctx.client.getCart();
+  const unexpectedRemovals = changedLines(before, cart, skuId, null);
+  if (unexpectedRemovals.length) {
+    warnRestoration(ctx, unexpectedRemovals, cart);
+    ctx.logger.result(
+      () => process.stdout.write(`${c.red('✗')} Cart removal changed other lines; restore them with the commands above.\n`),
+      { removed: !cart.items.some((item) => skuOf(item) === skuId), skuId, remaining: cart.items.length, total: cart.total, unexpectedRemovals },
+    );
+    process.exitCode = 1;
+    return;
+  }
   ctx.logger.result(
     () => process.stdout.write(`${c.green('✓')} Removed SKU ${skuId}. ${cart.items.length} item(s) left.\n`),
     { removed: true, skuId, remaining: cart.items.length, total: cart.total },
diff --git a/src/lib/api-client.ts b/src/lib/api-client.ts
index 311d7b2..a1a9c73 100644
--- a/src/lib/api-client.ts
+++ b/src/lib/api-client.ts
@@ -254,6 +254,7 @@ export class TakealotClient {
 
   private async rawAuthedFetch(path: string, init: RequestInit): Promise<Response> {
     const url = path.startsWith('http') ? path : this.mobileApiBase + path;
+    const method = init.method ?? 'GET';
     const headers: Record<string, string> = {
       accept: 'application/json, */*',
       'content-type': 'application/json',
@@ -261,7 +262,10 @@ export class TakealotClient {
       ...this.auth.authHeaders(),
       ...((init.headers as Record<string, string>) ?? {}),
     };
-    this.logger.debug(`${init.method ?? 'GET'} ${url}`);
+    this.logger.debug(`${method} ${url} customerId=${this.auth.customerId ?? 'none'} authGeneration=${this.auth.currentAuthGeneration}`);
+    if (method === 'DELETE' && rawPathname(url).endsWith('/cart/items')) {
+      this.logger.debug(`DELETE body ${String(init.body ?? '')}`);
+    }
     return this.send(url, { ...init, headers });
   }