diff --git a/cart_test.py b/cart_test.py
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+++ b/cart_test.py
@@ -0,0 +1,230 @@
+# Copyright 2026 UCP Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Conformance tests for the cart capability (cart.md).
+
+The suite has no cart coverage today, while the capability defines normative
+business obligations that are observable on the wire. This module asserts the
+ones that are stated as MUST and are not covered elsewhere:
+
+  cart.md:66       cart contents win; overlapping checkout payload fields are
+                   ignored on conversion
+  cart.md:72-74    a second conversion of the same cart returns the existing
+                   incomplete checkout rather than creating a new one
+  cart.md:185      cancel returns the cart state as it was before deletion
+  discount.md:143  discount codes applied to a cart carry forward to the
+                   checkout the cart converts into
+
+Server-agnostic by construction. Line items are compared as a bag of
+(item id, quantity) rather than by server-assigned identifiers; the conversion
+identity check compares checkout ids rather than totals arithmetic; both 200 and
+201 are accepted where the specification does not pin a status; and the discount
+assertion accepts any of the three wire signals a conformant business may use.
+
+Not asserted here, deliberately. cart.md:176 ("full replacement") binds the
+platform rather than the business, so it is not a business obligation this suite
+can hold a merchant to. cart.md:186 (subsequent operations return not_found) is
+a SHOULD, so the post-cancel status is not asserted.
+"""
+
+import uuid
+
+from absl.testing import absltest
+import integration_test_utils
+
+_CART_CAPABILITY = "dev.ucp.shopping.cart"
+_DISCOUNT_CAPABILITY = "dev.ucp.shopping.discount"
+
+
+class CartTest(integration_test_utils.IntegrationTestBase):
+  """Cart capability conformance (cart.md)."""
+
+  def setUp(self) -> None:
+    """Skip unless the business advertises the cart capability."""
+    super().setUp()
+    self._capabilities = self._advertised_capabilities()
+    if _CART_CAPABILITY not in self._capabilities:
+      self.skipTest(f"business does not advertise {_CART_CAPABILITY}; skipping")
+    self._sku = self.fixture_ctx.get_test_sku()
+
+  def _advertised_capabilities(self) -> dict:
+    """Return the advertised capability map, keyed by capability name."""
+    resp = self.client.get("/.well-known/ucp")
+    self.assert_response_status(resp, 200)
+    ucp = resp.json().get("ucp", resp.json())
+    caps = ucp.get("capabilities") or {}
+    if isinstance(caps, dict):
+      return caps
+    return {c.get("name"): c for c in caps if isinstance(c, dict)}
+
+  def _discount_extends_cart(self) -> bool:
+    """Return True if the discount capability declares it extends cart."""
+    entry = self._capabilities.get(_DISCOUNT_CAPABILITY)
+    if entry is None:
+      return False
+    entries = entry if isinstance(entry, list) else [entry]
+    for item in entries:
+      if not isinstance(item, dict):
+        continue
+      extends = item.get("extends")
+      if extends is None:
+        continue
+      if isinstance(extends, str):
+        extends = [extends]
+      if _CART_CAPABILITY in extends:
+        return True
+    return False
+
+  # ── drivers ────────────────────────────────────────────────────────────
+  def _create_cart(self, quantity: int = 2, **extra) -> dict:
+    """Create a cart holding the configured test item; return the raw dict."""
+    payload = {
+      "line_items": [{"item": {"id": self._sku}, "quantity": quantity}]
+    }
+    payload.update(extra)
+    resp = self.client.post(
+      self.get_shopping_url("/carts"),
+      json=payload,
+      headers=integration_test_utils.get_headers(),
+    )
+    self.assertIn(
+      resp.status_code,
+      (200, 201),
+      f"cart creation failed: {resp.status_code} {resp.text}",
+    )
+    return resp.json()
+
+  def _convert(self, cart_id: str, **extra) -> dict:
+    """Convert a cart into a checkout; return the raw checkout dict."""
+    payload = {"cart_id": cart_id}
+    payload.update(extra)
+    resp = self.client.post(
+      self.get_shopping_url("/checkout-sessions"),
+      json=payload,
+      headers=integration_test_utils.get_headers(
+        idempotency_key=str(uuid.uuid4())
+      ),
+    )
+    self.assertIn(
+      resp.status_code,
+      (200, 201),
+      f"cart conversion failed: {resp.status_code} {resp.text}",
+    )
+    return resp.json()
+
+  @staticmethod
+  def _item_bag(raw: dict) -> list:
+    """Return line items as a sorted bag of (item id, quantity) pairs."""
+    return sorted(
+      (li.get("item", {}).get("id"), li.get("quantity"))
+      for li in (raw.get("line_items") or [])
+    )
+
+  # ── cart.md:66 ─────────────────────────────────────────────────────────
+  def test_conversion_uses_cart_contents_over_payload(self) -> None:
+    """Cart contents win over overlapping fields in the checkout payload."""
+    cart = self._create_cart(quantity=2)
+    conflicting = [{"item": {"id": self._sku}, "quantity": 9}]
+
+    checkout = self._convert(cart["id"], line_items=conflicting)
+
+    self.assertEqual(
+      self._item_bag(checkout),
+      self._item_bag(cart),
+      "checkout line items must come from the cart, not the request payload",
+    )
+
+  # ── cart.md:72-74 ──────────────────────────────────────────────────────
+  def test_second_conversion_returns_the_existing_checkout(self) -> None:
+    """A cart with an incomplete checkout converts to that same checkout."""
+    cart = self._create_cart()
+
+    first = self._convert(cart["id"])
+    second = self._convert(cart["id"])
+
+    self.assertEqual(
+      second.get("id"),
+      first.get("id"),
+      "a second conversion must return the existing incomplete checkout",
+    )
+
+  # ── cart.md:185 ────────────────────────────────────────────────────────
+  def test_cancel_returns_the_cart_state_before_deletion(self) -> None:
+    """Cancelling a cart returns the cart as it was before deletion."""
+    cart = self._create_cart(quantity=3)
+
+    resp = self.client.post(
+      self.get_shopping_url(f"/carts/{cart['id']}/cancel"),
+      headers=integration_test_utils.get_headers(),
+    )
+    self.assertIn(
+      resp.status_code,
+      (200, 201),
+      f"cart cancel failed: {resp.status_code} {resp.text}",
+    )
+    cancelled = resp.json()
+
+    self.assertEqual(
+      cancelled.get("id"),
+      cart.get("id"),
+      "cancel must return the cancelled cart, identified by its own id",
+    )
+    self.assertEqual(
+      self._item_bag(cancelled),
+      self._item_bag(cart),
+      "cancel must return the cart state as it was before deletion",
+    )
+
+  # ── discount.md:143 ────────────────────────────────────────────────────
+  def test_cart_discount_codes_carry_forward_to_checkout(self) -> None:
+    """Discount codes applied to a cart survive conversion to a checkout."""
+    if not self._discount_extends_cart():
+      self.skipTest(
+        f"{_DISCOUNT_CAPABILITY} does not declare it extends "
+        f"{_CART_CAPABILITY}; skipping"
+      )
+    code = self.fixture_ctx.get_test_discount_code()
+    cart = self._create_cart(discounts={"codes": [code]})
+    if not self._discount_is_visible(cart, code):
+      self.skipTest(f"discount code {code} did not apply to the cart; skipping")
+
+    checkout = self._convert(cart["id"])
+
+    self.assertTrue(
+      self._discount_is_visible(checkout, code),
+      "a discount applied to the cart must carry forward to the checkout",
+    )
+
+  @staticmethod
+  def _discount_is_visible(raw: dict, code: str) -> bool:
+    """Return True if the response shows the code, or a discount it caused."""
+    discounts = raw.get("discounts") or {}
+    codes = [c.casefold() for c in (discounts.get("codes") or [])]
+    if code.casefold() in codes:
+      return True
+    applied = discounts.get("applied") or []
+    if any(
+      (a.get("code") or "").casefold() == code.casefold()
+      for a in applied
+      if isinstance(a, dict)
+    ):
+      return True
+    return any(
+      t.get("type") == "discount" and (t.get("amount") or 0) < 0
+      for t in (raw.get("totals") or [])
+    )
+
+
+if __name__ == "__main__":
+  absltest.main()