fix(client): resolve REST endpoint from discovery profile

rest/python/client/flower_shop/endpoint_resolution.py

@@ -0,0 +1,48 @@
+#   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.
+
+"""UCP endpoint resolution utilities.
+
+Implements the Endpoint Resolution logic from the UCP specification:
+https://ucp.dev/2026-04-08/specification/overview#endpoint-resolution
+
+  "The endpoint field provides the base URL for API calls. OpenAPI paths
+   are appended to this endpoint to form the complete URL."
+"""
+
+
+def resolve_rest_endpoint(discovery_data: dict) -> str | None:
+  """Extract the REST endpoint from a UCP discovery response.
+
+  Looks up ``services["dev.ucp.shopping"]`` in the discovery payload,
+  finds the entry with ``transport == "rest"``, and returns its
+  ``endpoint`` value.  All subsequent API paths (e.g.
+  ``/checkout-sessions``) should be appended to this base URL.
+
+  Args:
+    discovery_data: Parsed JSON body from ``GET /.well-known/ucp``.
+
+  Returns:
+    The resolved endpoint URL, or ``None`` if no REST service was found.
+
+  """
+  shopping_services = discovery_data.get("services", {}).get(
+    "dev.ucp.shopping", []
+  )
+  rest_service = next(
+    (s for s in shopping_services if s.get("transport") == "rest"), None
+  )
+  if rest_service and rest_service.get("endpoint"):
+    return rest_service["endpoint"]
+  return None

rest/python/client/flower_shop/simple_happy_path_client.py

@@ -43,6 +43,8 @@
 from ucp_sdk.models.schemas.shopping.types import line_item_create_request
 from ucp_sdk.models.schemas.shopping.types import line_item_update_request
 
+from endpoint_resolution import resolve_rest_endpoint
+
 
 def get_headers() -> dict[str, str]:
   """Generate necessary headers for UCP requests."""
@@ -237,6 +239,13 @@ def main() -> None:
 
     discovery_data = response.json()
 
+    api_base_url = resolve_rest_endpoint(discovery_data) or args.server_url
+    logger.info("Resolved REST endpoint: %s", api_base_url)
+    if api_base_url != args.server_url:
+      client.close()
+      client = httpx.Client(base_url=api_base_url)
+      global_replacements[api_base_url] = "ENDPOINT"
+
     supported_handlers = []
     for handlers in discovery_data.get("payment_handlers", {}).values():
       supported_handlers.extend(handlers)
@@ -324,7 +333,7 @@ def main() -> None:
       log_interaction(
         args.export_requests_to,
         "POST",
-        f"{args.server_url}{url}",
+        f"{api_base_url}{url}",
         headers,
         json_body,
         response,
@@ -414,7 +423,7 @@ def main() -> None:
       log_interaction(
         args.export_requests_to,
         "PUT",
-        f"{args.server_url}{url}",
+        f"{api_base_url}{url}",
         headers,
         json_body,
         response,
@@ -505,7 +514,7 @@ def main() -> None:
       log_interaction(
         args.export_requests_to,
         "PUT",
-        f"{args.server_url}{url}",
+        f"{api_base_url}{url}",
         headers,
         json_body,
         response,
@@ -646,7 +655,7 @@ def main() -> None:
         log_interaction(
           args.export_requests_to,
           "PUT",
-          f"{args.server_url}{url}",
+          f"{api_base_url}{url}",
           headers,
           trigger_payload,
           response,
@@ -704,7 +713,7 @@ def main() -> None:
           log_interaction(
             args.export_requests_to,
             "PUT",
-            f"{args.server_url}{url}",
+            f"{api_base_url}{url}",
             headers,
             payload,
             response,
@@ -762,7 +771,7 @@ def main() -> None:
             log_interaction(
               args.export_requests_to,
               "PUT",
-              f"{args.server_url}{url}",
+              f"{api_base_url}{url}",
               headers,
               payload,
               response,
@@ -864,7 +873,7 @@ def main() -> None:
       log_interaction(
         args.export_requests_to,
         "POST",
-        f"{args.server_url}{url}",
+        f"{api_base_url}{url}",
         headers,
         final_payload,
         response,

rest/python/client/flower_shop/simple_happy_path_client_test.py

@@ -0,0 +1,176 @@
+#   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.
+
+"""Tests for endpoint resolution in the happy path client.
+
+Verifies that resolve_rest_endpoint correctly extracts the REST endpoint
+from a UCP discovery response, per the spec's Endpoint Resolution section:
+https://ucp.dev/2026-04-08/specification/overview#endpoint-resolution
+"""
+
+import unittest
+
+from endpoint_resolution import resolve_rest_endpoint
+
+
+class ResolveRestEndpointTest(unittest.TestCase):
+  """Tests for resolve_rest_endpoint."""
+
+  def test_returns_endpoint_with_path_prefix(self):
+    """Merchant mounts API under a path prefix (the bug scenario)."""
+    discovery = {
+      "services": {
+        "dev.ucp.shopping": [
+          {
+            "version": "2026-04-08",
+            "transport": "rest",
+            "endpoint": "https://merchant.example.com/buy/v1",
+            "schema": "https://ucp.dev/2026-04-08/services/shopping/rest.openapi.json",
+          }
+        ]
+      }
+    }
+    self.assertEqual(
+      resolve_rest_endpoint(discovery),
+      "https://merchant.example.com/buy/v1",
+    )
+
+  def test_returns_endpoint_at_root(self):
+    """Merchant serves API at domain root (sample server scenario)."""
+    discovery = {
+      "services": {
+        "dev.ucp.shopping": [
+          {
+            "version": "2026-04-08",
+            "transport": "rest",
+            "endpoint": "http://localhost:8182",
+            "schema": "https://ucp.dev/2026-04-08/services/shopping/rest.openapi.json",
+          }
+        ]
+      }
+    }
+    self.assertEqual(
+      resolve_rest_endpoint(discovery),
+      "http://localhost:8182",
+    )
+
+  def test_selects_rest_transport_among_multiple(self):
+    """Discovery lists multiple transports; only REST is selected."""
+    discovery = {
+      "services": {
+        "dev.ucp.shopping": [
+          {
+            "version": "2026-04-08",
+            "transport": "mcp",
+            "endpoint": "https://merchant.example.com/ucp/mcp",
+            "schema": "https://ucp.dev/2026-04-08/services/shopping/mcp.openrpc.json",
+          },
+          {
+            "version": "2026-04-08",
+            "transport": "rest",
+            "endpoint": "https://merchant.example.com/api/v2",
+            "schema": "https://ucp.dev/2026-04-08/services/shopping/rest.openapi.json",
+          },
+          {
+            "version": "2026-04-08",
+            "transport": "a2a",
+            "endpoint": "https://merchant.example.com/.well-known/agent-card.json",
+          },
+        ]
+      }
+    }
+    self.assertEqual(
+      resolve_rest_endpoint(discovery),
+      "https://merchant.example.com/api/v2",
+    )
+
+  def test_returns_none_when_no_services(self):
+    """Discovery response has no services key at all."""
+    discovery = {"payment_handlers": {}}
+    self.assertIsNone(resolve_rest_endpoint(discovery))
+
+  def test_returns_none_when_services_empty(self):
+    """Discovery response has empty services."""
+    discovery = {"services": {}}
+    self.assertIsNone(resolve_rest_endpoint(discovery))
+
+  def test_returns_none_when_no_rest_transport(self):
+    """Shopping service exists but only non-REST transports are listed."""
+    discovery = {
+      "services": {
+        "dev.ucp.shopping": [
+          {
+            "version": "2026-04-08",
+            "transport": "mcp",
+            "endpoint": "https://merchant.example.com/ucp/mcp",
+            "schema": "https://ucp.dev/2026-04-08/services/shopping/mcp.openrpc.json",
+          }
+        ]
+      }
+    }
+    self.assertIsNone(resolve_rest_endpoint(discovery))
+
+  def test_returns_none_when_rest_has_no_endpoint(self):
+    """REST transport exists but endpoint field is missing."""
+    discovery = {
+      "services": {
+        "dev.ucp.shopping": [
+          {
+            "version": "2026-04-08",
+            "transport": "rest",
+            "schema": "https://ucp.dev/2026-04-08/services/shopping/rest.openapi.json",
+          }
+        ]
+      }
+    }
+    self.assertIsNone(resolve_rest_endpoint(discovery))
+
+  def test_returns_none_when_endpoint_is_empty_string(self):
+    """REST transport exists but endpoint is an empty string."""
+    discovery = {
+      "services": {
+        "dev.ucp.shopping": [
+          {
+            "version": "2026-04-08",
+            "transport": "rest",
+            "endpoint": "",
+            "schema": "https://ucp.dev/2026-04-08/services/shopping/rest.openapi.json",
+          }
+        ]
+      }
+    }
+    self.assertIsNone(resolve_rest_endpoint(discovery))
+
+  def test_handles_deeply_nested_path_prefix(self):
+    """Endpoint has multiple path segments."""
+    discovery = {
+      "services": {
+        "dev.ucp.shopping": [
+          {
+            "version": "2026-04-08",
+            "transport": "rest",
+            "endpoint": "https://api.merchant.com/ucp/shopping/v1",
+            "schema": "https://ucp.dev/2026-04-08/services/shopping/rest.openapi.json",
+          }
+        ]
+      }
+    }
+    self.assertEqual(
+      resolve_rest_endpoint(discovery),
+      "https://api.merchant.com/ucp/shopping/v1",
+    )
+
+
+if __name__ == "__main__":
+  unittest.main()