diff --git a/.cspell/custom-words.txt b/.cspell/custom-words.txt
index c09cb03..9525e8c 100644
--- a/.cspell/custom-words.txt
+++ b/.cspell/custom-words.txt
@@ -38,6 +38,7 @@ Google
 gpay
 Gpay
 healthz
+idless
 ingestions
 inlinehilite
 Kroger
diff --git a/integration_test_utils.py b/integration_test_utils.py
index 461cd0d..40e714d 100644
--- a/integration_test_utils.py
+++ b/integration_test_utils.py
@@ -318,13 +318,17 @@ def _setup_routes(self) -> None:
     @self.app.post("/webhooks/partners/{partner_id}/events/order")
     async def order_event(partner_id: str, request: Request) -> dict[str, str]:
       """Record an incoming order event."""
-      payload = await request.json()
+      # Keep the raw bytes: Content-Digest (RFC 9530) is computed over the
+      # body as transmitted, and verifiers must not re-serialize JSON.
+      raw_body = await request.body()
+      payload = json.loads(raw_body)
       headers = dict(request.headers)
       self.events.append(
         {
           "partner_id": partner_id,
           "payload": payload,
           "headers": headers,
+          "raw_body": raw_body,
         }
       )
       return {"status": "ok"}
diff --git a/webhook_structure_test.py b/webhook_structure_test.py
new file mode 100644
index 0000000..27b0624
--- /dev/null
+++ b/webhook_structure_test.py
@@ -0,0 +1,357 @@
+#   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.
+
+"""Structural conformance for the order-event webhook contract (order.md).
+
+The 2026-04-08 order specification pins a delivery contract for order-event
+webhooks beyond "an event arrives" (which webhook_test.py already covers):
+
+  * Required headers follow Standard Webhooks: ``Webhook-Id`` (unique event
+    identifier) and ``Webhook-Timestamp`` (unix event timestamp) — order.md
+    "Order Event Webhook", Required Headers.
+  * The body is the FULL order entity: the same current-state snapshot as Get
+    Order. Businesses MUST send the "Order created" event with a fully
+    populated order entity and MUST send the full entity on updates, never an
+    incremental delta — order.md "Events" and its Business guidelines. The
+    schema's required properties (order.json) are the floor for "fully
+    populated".
+  * Deliveries MUST be signed. What is checked here is the signing ENVELOPE
+    (headers, digest, signed components), not cryptographic verification of
+    the signature value against the business's published keys: ``UCP-Agent``
+    (business profile URL), ``Signature``, ``Signature-Input`` and
+    ``Content-Digest`` headers, with
+    the SHA-256 ``Content-Digest`` matching the raw body — order.md "Webhook
+    Signature Verification". Because the ``UCP-Agent`` header is required on
+    webhook deliveries, ``ucp-agent`` is a required signed component
+    (signatures.md, REST Request Signing component table, footnote **).
+
+The signature tests skip loudly when the server sends completely unsigned
+deliveries (the current reference-server behavior), mirroring how other
+structural tests skip on shapes the server does not emit; the update-event
+test skips when the server exposes no /testing/simulate-shipping path.
+"""
+
+import base64
+import hashlib
+import re
+import time
+from typing import Any
+
+from absl.testing import absltest
+import integration_test_utils
+
+_ORDER_CAPABILITY = "dev.ucp.shopping.order"
+
+# order.json "required" — the floor for a "fully populated order entity"
+_ORDER_REQUIRED = (
+  "ucp",
+  "id",
+  "checkout_id",
+  "permalink_url",
+  "line_items",
+  "fulfillment",
+  "currency",
+  "totals",
+)
+
+# order.md "Webhook Signature Verification" Required Headers (lowercase)
+_SIGNATURE_HEADERS = (
+  "ucp-agent",
+  "signature",
+  "signature-input",
+  "content-digest",
+)
+
+
+class WebhookStructureTest(integration_test_utils.IntegrationTestBase):
+  """Order-event webhook contract: headers, full entity, signing."""
+
+  def setUp(self) -> None:
+    """Start the receiver; skip unless the order capability is advertised."""
+    super().setUp()
+    if not self._advertises_order():
+      self.skipTest(
+        f"business does not advertise {_ORDER_CAPABILITY}; skipping"
+      )
+    port = integration_test_utils.FLAGS.mock_webhook_port
+    self.webhook_server = integration_test_utils.MockWebhookServer(port=port)
+    self.webhook_server.start()
+
+  def tearDown(self) -> None:
+    """Stop the receiver."""
+    if hasattr(self, "webhook_server"):
+      self.webhook_server.stop()
+    super().tearDown()
+
+  def _advertises_order(self) -> bool:
+    """Return True if discovery advertises the order capability."""
+    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 {}
+    names = (
+      list(caps.keys())
+      if isinstance(caps, dict)
+      else [c.get("name") for c in caps if isinstance(c, dict)]
+    )
+    return _ORDER_CAPABILITY in names
+
+  # ── shared drivers ──────────────────────────────────────────────────────
+  def _events_for(self, order_id: str) -> list[dict[str, Any]]:
+    """All captured deliveries whose payload is the given order.
+
+    A delivery whose body has no ``id`` at all (a delta rather than the full
+    entity) cannot be matched here; callers surface that case explicitly so
+    the deviation reads "not a full entity", not "nothing delivered".
+    """
+    return [
+      e
+      for e in self.webhook_server.events
+      if isinstance(e.get("payload"), dict)
+      and e["payload"].get("id") == order_id
+    ]
+
+  def _idless_deliveries(self) -> int:
+    """Deliveries whose dict body carries no id (delta-shaped)."""
+    return sum(
+      1
+      for e in self.webhook_server.events
+      if isinstance(e.get("payload"), dict) and "id" not in e["payload"]
+    )
+
+  def _wait_for_events(self, order_id: str, count: int) -> list[dict[str, Any]]:
+    """Poll (up to 5s) until `count` deliveries for the order arrived."""
+    for _ in range(50):
+      events = self._events_for(order_id)
+      if len(events) >= count:
+        return events
+      time.sleep(0.1)
+    return self._events_for(order_id)
+
+  def _place_order(self) -> tuple[str, str, dict[str, Any]]:
+    """Create + complete a checkout; return (checkout_id, order_id, event).
+
+    The returned event is the first delivery for the order — the "Order
+    created" event the business MUST send (order.md Business guidelines).
+    """
+    checkout_data = self.create_checkout_session(headers=self.get_headers())
+    checkout_id = checkout_data["id"]
+    complete_response = self.complete_checkout_session(checkout_id)
+    order_id = complete_response["order"]["id"]
+    events = self._wait_for_events(order_id, 1)
+    if not events and self._idless_deliveries():
+      self.fail(
+        "webhook delivery arrived without an order id in the body - a delta "
+        "rather than the full order entity (order.md Events: deliveries are "
+        "fully populated order objects, never deltas)"
+      )
+    self.assertTrue(
+      events,
+      "no order-event webhook delivered for the completed order "
+      "(order.md: MUST send 'Order created' event)",
+    )
+    return checkout_id, order_id, events[0]
+
+  def _assert_full_order_entity(
+    self, payload: dict[str, Any], checkout_id: str, order_id: str
+  ) -> None:
+    """Assert the payload is the fully populated order entity for our order."""
+    missing = [k for k in _ORDER_REQUIRED if payload.get(k) is None]
+    self.assertFalse(
+      missing,
+      "webhook body is not a fully populated order entity; missing "
+      f"required order properties {missing} (order.md: the payload is the "
+      "same current-state snapshot as Get Order)",
+    )
+    self.assertTrue(
+      payload["line_items"],
+      "webhook order entity carries no line_items",
+    )
+    self.assertEqual(payload["id"], order_id)
+    self.assertEqual(
+      payload["checkout_id"],
+      checkout_id,
+      "webhook order entity does not reference the originating checkout",
+    )
+
+  # ── order.md "Order Event Webhook": Standard Webhooks headers ───────────
+  def test_delivery_carries_standard_webhook_headers(self) -> None:
+    """Every delivery carries Webhook-Id and a unix Webhook-Timestamp."""
+    _, _, event = self._place_order()
+    headers = event["headers"]
+    self.assertTrue(
+      headers.get("webhook-id"),
+      "delivery is missing the required Webhook-Id header",
+    )
+    timestamp = headers.get("webhook-timestamp", "")
+    self.assertRegex(
+      timestamp,
+      r"^\d+$",
+      "Webhook-Timestamp must be a unix timestamp",
+    )
+    # The spec requires a unix timestamp but sets no skew bound; assert the
+    # value parses into the unix-seconds era rather than inventing a window
+    # (a millisecond value or a date string fails, clock skew does not).
+    self.assertTrue(
+      1_000_000_000 <= int(timestamp) <= 4_000_000_000,
+      f"Webhook-Timestamp {timestamp} does not parse as unix seconds",
+    )
+
+  # ── order.md "Events": fully populated order entity ─────────────────────
+  def test_order_created_event_is_full_order_entity(self) -> None:
+    """The 'Order created' delivery body is the full order entity."""
+    checkout_id, order_id, event = self._place_order()
+    self._assert_full_order_entity(event["payload"], checkout_id, order_id)
+
+  def test_update_event_is_full_order_entity(self) -> None:
+    """An update delivery is the full current-state snapshot, not a delta."""
+    checkout_id, order_id, _ = self._place_order()
+    headers = self.get_headers()
+    headers["Simulation-Secret"] = (
+      integration_test_utils.FLAGS.simulation_secret
+    )
+    ship_response = self.client.post(
+      f"/testing/simulate-shipping/{order_id}",
+      headers=headers,
+    )
+    if ship_response.status_code == 403:
+      self.skipTest(
+        "simulation secret rejected (/testing/simulate-shipping -> 403); "
+        "configure --simulation_secret to match the server under test"
+      )
+    if ship_response.status_code in (404, 405):
+      self.skipTest(
+        "server exposes no webhook simulation path "
+        f"(/testing/simulate-shipping -> {ship_response.status_code}); "
+        "cannot trigger an order update"
+      )
+    self.assert_response_status(ship_response, 200)
+    events = self._wait_for_events(order_id, 2)
+    self.assertGreaterEqual(
+      len(events),
+      2,
+      "no update webhook delivered after the order shipped "
+      "(order.md: MUST send full order entity on updates)",
+    )
+    for event in events:
+      self._assert_full_order_entity(event["payload"], checkout_id, order_id)
+    fulfillment_events = [
+      fe
+      for event in events
+      for fe in (event["payload"]["fulfillment"].get("events") or [])
+    ]
+    self.assertTrue(
+      any(fe.get("type") == "shipped" for fe in fulfillment_events),
+      "no update delivery reflected the shipped state in the snapshot",
+    )
+
+  # ── order.md "Webhook Signature Verification" ───────────────────────────
+  def _signed_event(self) -> dict[str, Any]:
+    """Place an order and return its delivery, skipping if unsigned.
+
+    A delivery with NONE of the signature headers means the server does not
+    implement webhook signing at all; the structural assertions would all
+    fail on the same root cause, so skip with one loud message instead
+    (order.md: payloads MUST be signed — this skip is a finding, not a pass).
+    """
+    _, _, event = self._place_order()
+    if not any(event["headers"].get(h) for h in _SIGNATURE_HEADERS):
+      self.skipTest(
+        "server sends unsigned webhook deliveries (no UCP-Agent, Signature, "
+        "Signature-Input or Content-Digest header); order.md 'Webhook "
+        "Signature Verification' MUSTs are not exercised"
+      )
+    return event
+
+  def test_signed_delivery_carries_required_headers_and_digest(self) -> None:
+    """A signed delivery carries all four headers; the digest matches."""
+    event = self._signed_event()
+    headers = event["headers"]
+    missing = [h for h in _SIGNATURE_HEADERS if not headers.get(h)]
+    self.assertFalse(
+      missing,
+      f"signed delivery is missing required headers: {missing} "
+      "(order.md Webhook Signature Verification)",
+    )
+    agent = headers["ucp-agent"]
+    match = re.search(r'profile="([^"]+)"', agent)
+    self.assertIsNotNone(
+      match,
+      f"UCP-Agent is not an RFC 8941 dictionary with profile=: {agent!r}",
+    )
+    profile_url = match.group(1).split("?")[0].rstrip("/")
+    self.assertTrue(
+      profile_url.endswith("/.well-known/ucp"),
+      "UCP-Agent profile URL must point at the business's /.well-known/ucp "
+      f"(signatures.md UCP-Agent parsing): {agent!r}",
+    )
+    digest_match = re.search(
+      r"sha-256=:([A-Za-z0-9+/=]+):", headers["content-digest"]
+    )
+    self.assertIsNotNone(
+      digest_match,
+      f"Content-Digest is not sha-256 (RFC 9530): "
+      f"{headers['content-digest']!r}",
+    )
+    expected = hashlib.sha256(event["raw_body"]).digest()
+    self.assertEqual(
+      base64.b64decode(digest_match.group(1)),
+      expected,
+      "Content-Digest does not match the SHA-256 of the raw delivery body",
+    )
+
+  def test_signature_covers_ucp_agent(self) -> None:
+    """The signed components include ucp-agent (binds signer identity).
+
+    signatures.md's REST signing component table requires ``ucp-agent``
+    whenever the UCP-Agent header is present, and order.md requires that
+    header on every webhook delivery — so a conformant webhook signature
+    always covers it (along with the always-required derived components and
+    the body components).
+    """
+    event = self._signed_event()
+    signature_input = event["headers"].get("signature-input", "")
+    match = re.search(r"\(([^)]*)\)", signature_input)
+    self.assertIsNotNone(
+      match,
+      f"Signature-Input has no component list: {signature_input!r}",
+    )
+    components = re.findall(r'"([^"]+)"', match.group(1))
+    for required in (
+      "@method",
+      "@authority",
+      "@path",
+      "content-digest",
+      "content-type",
+    ):
+      self.assertIn(
+        required,
+        components,
+        f"webhook signature does not cover {required!r} "
+        f"(signatures.md signed-component table): {signature_input!r}",
+      )
+    # signatures.md's component table requires ucp-agent whenever the header
+    # is present, but order.md's own example webhook signature omits it; that
+    # inconsistency has a fix in flight (ucp#659). Until it lands, a merchant
+    # following the example verbatim must not be failed here.
+    if "ucp-agent" not in components:
+      self.skipTest(
+        "webhook signature does not cover ucp-agent; normative text requires "
+        "it but the spec example omits it (fix pending in ucp#659) - not "
+        "graded until the spec is self-consistent"
+      )
+
+
+if __name__ == "__main__":
+  absltest.main()