fix(rest/python): resolve the shop id before the profile is cached

rest/python/server/config.py

@@ -26,6 +26,12 @@
 
 _PROFILE_CACHE = None
 
+# The shop id this server instance advertises. discovery_profile.json is a
+# template, and this is the one value substituted into it that does not depend
+# on the request, so it is resolved here where every reader of the profile sees
+# the same value. routes/discovery.py substitutes {{ENDPOINT}} per request.
+SHOP_ID = str(uuid.uuid4())
+
 # checkout.json annotates `currency` with `ucp_request: omit` and describes
 # it as "reflecting the merchant's market determination ... buyers provide
 # signals, merchants determine currency". A conformant platform therefore does
@@ -48,7 +54,11 @@ def _get_profile() -> dict:
   profile_path = current_dir / "routes" / "discovery_profile.json"
 
   with profile_path.open(encoding="utf-8") as f:
-    _PROFILE_CACHE = json.load(f)
+    template = f.read()
+
+  # Resolve the request independent placeholder before parsing, so callers
+  # such as get_payment_handlers never hand a template value to a platform.
+  _PROFILE_CACHE = json.loads(template.replace("{{SHOP_ID}}", SHOP_ID))
   return _PROFILE_CACHE
 
 

rest/python/server/integration_test.py

@@ -2224,6 +2224,57 @@ def test_checkout_readiness_lifecycle(self) -> None:
       self.assertEqual(complete_res.status_code, 200, complete_res.text)
       self.assertEqual(complete_res.json()["status"], "completed")
 
+  def test_checkout_payment_handler_config_matches_discovery(self) -> None:
+    """The checkout envelope and discovery must advertise the same shop id.
+
+    discovery_profile.json is a template. routes/discovery.py reads it and
+    substitutes {{SHOP_ID}} before serving /.well-known/ucp, but
+    config._get_profile reads the same file separately and caches it raw, so
+    every checkout envelope carried the literal placeholder while discovery
+    carried a real value. A platform reading the handler config from the
+    checkout response therefore saw a different shop id than the one the
+    business advertises.
+    """
+    with self.client:
+      discovery = self.client.get("/.well-known/ucp")
+      self.assertEqual(discovery.status_code, 200, discovery.text)
+      advertised = discovery.json()["ucp"]["payment_handlers"]
+
+      payload = self._create_checkout_payload(
+        "shop_id_1", [("rose", "Red Rose", 1000, 1)]
+      )
+      response = self.client.post(
+        "/checkout-sessions",
+        headers=self._get_headers(idempotency_key="sid1", request_id="sid1"),
+        json=payload.model_dump(mode="json", exclude_none=True),
+      )
+      self.assertEqual(response.status_code, 201, response.text)
+      in_checkout = response.json()["ucp"]["payment_handlers"]
+
+    serialized = json.dumps(in_checkout)
+    self.assertNotIn(
+      "{{",
+      serialized,
+      msg="the checkout envelope must not carry template placeholders",
+    )
+    self.assertEqual(
+      in_checkout,
+      advertised,
+      msg="checkout payment_handlers must match the advertised profile",
+    )
+
+    # Equality alone would accept any shared constant, so pin the shape of
+    # the substituted value rather than only its agreement.
+    shop_ids = [
+      handler["config"]["shop_id"]
+      for entries in in_checkout.values()
+      for handler in entries
+      if "shop_id" in handler.get("config", {})
+    ]
+    self.assertNotEmpty(shop_ids, msg="expected a handler carrying a shop_id")
+    for shop_id in shop_ids:
+      uuid.UUID(shop_id)
+
 
 if __name__ == "__main__":
   absltest.main()

rest/python/server/routes/discovery.py

@@ -16,7 +16,7 @@
 
 import json
 import pathlib
-import uuid
+import config
 from fastapi import APIRouter
 from fastapi import Request
 from fastapi import Response
@@ -31,8 +31,9 @@
 # of at least 60 seconds, and never `private`/`no-store`/`no-cache`.
 PROFILE_CACHE_CONTROL = "public, max-age=3600"
 
-# Generate a unique shop ID for this server instance
-SHOP_ID = str(uuid.uuid4())
+# The shop id is owned by config so that the discovery document and every
+# other reader of the profile advertise the same value.
+SHOP_ID = config.SHOP_ID
 
 
 @router.get(