fix(agentgateway): raise AgentGatewayServerError on isError tool results

src/sap_cloud_sdk/agentgateway/_customer.py

@@ -46,7 +46,10 @@
     MCPToolFilter,
 )
 from sap_cloud_sdk.agentgateway._token_cache import _TokenCache
-from sap_cloud_sdk.agentgateway.exceptions import AgentGatewaySDKError
+from sap_cloud_sdk.agentgateway.exceptions import (
+    AgentGatewaySDKError,
+    AgentGatewayServerError,
+)
 from sap_cloud_sdk.core.secret_resolver import resolve_base_mount
 
 logger = logging.getLogger(__name__)
@@ -841,11 +844,8 @@ async def call_mcp_tool_customer(
                 text = str(getattr(first, "text", ""))
 
                 if mcp_is_error(result):
-                    logger.error(
-                        "Tool '%s' on '%s' returned an error: %s",
-                        tool.name,
-                        tool.url,
-                        text,
+                    raise AgentGatewayServerError(
+                        f"Tool '{tool.name}' on '{tool.url}' returned an error: {text}"
                     )
 
                 return text

src/sap_cloud_sdk/agentgateway/_lob.py

@@ -50,6 +50,7 @@
 from sap_cloud_sdk.agentgateway._token_cache import _GatewayUrlCache, _TokenCache
 from sap_cloud_sdk.agentgateway.exceptions import (
     AgentGatewaySDKError,
+    AgentGatewayServerError,
     MCPServerNotFoundError,
 )
 
@@ -526,11 +527,8 @@ async def call_mcp_tool_lob(
                 text = str(getattr(first, "text", ""))
 
                 if mcp_is_error(result):
-                    logger.error(
-                        "Tool '%s' on '%s' returned an error: %s",
-                        tool.name,
-                        tool.url,
-                        text,
+                    raise AgentGatewayServerError(
+                        f"Tool '{tool.name}' on '{tool.url}' returned an error: {text}"
                     )
 
                 return text

tests/agentgateway/unit/test_customer.py

@@ -27,7 +27,10 @@
 )
 from sap_cloud_sdk.agentgateway._token_cache import _TokenCache
 from sap_cloud_sdk.agentgateway.config import ClientConfig
-from sap_cloud_sdk.agentgateway.exceptions import AgentGatewaySDKError
+from sap_cloud_sdk.agentgateway.exceptions import (
+    AgentGatewaySDKError,
+    AgentGatewayServerError,
+)
 
 
 # ============================================================
@@ -766,6 +769,8 @@ async def test_calls_tool_with_pre_fetched_token(self, credentials, mock_tool):
             mock_content = MagicMock()
             mock_content.text = "Order created successfully"
             mock_result.content = [mock_content]
+            mock_result.is_error = False
+            mock_result.isError = False
             mock_session.call_tool = AsyncMock(return_value=mock_result)
             mock_session_ctx = AsyncMock()
             mock_session_ctx.__aenter__ = AsyncMock(return_value=mock_session)
@@ -824,6 +829,58 @@ async def test_returns_empty_string_when_no_content(self, credentials, mock_tool
 
             assert result == ""
 
+    @pytest.mark.asyncio
+    async def test_raises_server_error_on_is_error(
+        self, credentials, mock_tool
+    ):
+        """Raise AgentGatewayServerError when tool returns isError."""
+        tool = MCPTool(
+            name="test-tool",
+            server_name="test-server",
+            description="Test tool",
+            input_schema={},
+            url="https://example.com/mcp",
+            fragment_name="test-fragment",
+        )
+
+        with (
+            patch(
+                "httpx.AsyncClient",
+            ),
+            patch(
+                "sap_cloud_sdk.agentgateway._customer.streamable_http_client",
+            ) as mock_stream,
+            patch(
+                "sap_cloud_sdk.agentgateway._customer.ClientSession",
+            ) as mock_session_class,
+        ):
+            mock_stream_ctx = AsyncMock()
+            mock_stream_ctx.__aenter__ = AsyncMock(
+                return_value=(AsyncMock(), AsyncMock(), None)
+            )
+            mock_stream_ctx.__aexit__ = AsyncMock(return_value=None)
+            mock_stream.return_value = mock_stream_ctx
+
+            mock_session = AsyncMock()
+            mock_session.initialize = AsyncMock()
+            mock_result = MagicMock()
+            mock_result.content = [MagicMock()]
+            mock_result.content[0].text = "backend exploded"
+            mock_result.is_error = True
+            mock_result.isError = True
+            mock_session.call_tool = AsyncMock(return_value=mock_result)
+            mock_session_ctx = AsyncMock()
+            mock_session_ctx.__aenter__ = AsyncMock(return_value=mock_session)
+            mock_session_ctx.__aexit__ = AsyncMock(return_value=None)
+            mock_session_class.return_value = mock_session_ctx
+
+            with pytest.raises(
+                AgentGatewayServerError, match="backend exploded"
+            ):
+                await call_mcp_tool_customer(tool, "auth-token", 60.0)
+
+            mock_session.call_tool.assert_called_once()
+
 
 # ============================================================
 # Test: detect_transparent_credentials

tests/agentgateway/unit/test_lob.py

@@ -41,6 +41,7 @@
 from sap_cloud_sdk.destination import ConsumptionOptions, ConsumptionLevel
 from sap_cloud_sdk.agentgateway.exceptions import (
     AgentGatewaySDKError,
+    AgentGatewayServerError,
     MCPServerNotFoundError,
 )
 from sap_cloud_sdk.destination import ConsumptionLevel
@@ -953,6 +954,8 @@ async def test_calls_tool_with_pre_fetched_token(self):
         mock_result = MagicMock()
         mock_result.content = [MagicMock()]
         mock_result.content[0].text = "Tool result"
+        mock_result.is_error = False
+        mock_result.isError = False
 
         with (
             patch("sap_cloud_sdk.agentgateway._lob.httpx.AsyncClient") as mock_http,
@@ -1030,6 +1033,53 @@ async def test_returns_empty_string_when_no_content(self):
 
             assert result == ""
 
+    @pytest.mark.asyncio
+    async def test_raises_server_error_on_is_error(self):
+        """Raise AgentGatewayServerError when tool returns isError."""
+        tool = MCPTool(
+            name="test-tool",
+            server_name="test-server",
+            description="Test tool",
+            input_schema={},
+            url="https://example.com/mcp",
+            fragment_name="test-fragment",
+        )
+
+        mock_result = MagicMock()
+        mock_result.content = [MagicMock()]
+        mock_result.content[0].text = "backend exploded"
+        mock_result.is_error = True
+        mock_result.isError = True
+
+        with (
+            patch("sap_cloud_sdk.agentgateway._lob.httpx.AsyncClient"),
+            patch(
+                "sap_cloud_sdk.agentgateway._lob.streamable_http_client"
+            ) as mock_stream,
+            patch("sap_cloud_sdk.agentgateway._lob.ClientSession") as mock_session,
+        ):
+            mock_stream.return_value.__aenter__.return_value = (
+                AsyncMock(),
+                AsyncMock(),
+                None,
+            )
+
+            mock_session_instance = AsyncMock()
+            mock_session_instance.initialize = AsyncMock()
+            mock_session_instance.call_tool = AsyncMock(
+                return_value=mock_result
+            )
+            mock_session.return_value.__aenter__.return_value = (
+                mock_session_instance
+            )
+
+            with pytest.raises(
+                AgentGatewayServerError, match="backend exploded"
+            ):
+                await call_mcp_tool_lob(tool, "user-auth-token", 60.0)
+
+            mock_session_instance.call_tool.assert_called_once()
+
 
 # ============================================================
 # Test: list_a2a_fragments