DX-122122: propagate remote tool errors as ToolError (isError=True)

- Import ToolError from mcp.server.fastmcp.exceptions in mcp.py
- call_tool raises ToolError when remote tools are disabled or when
  InvokeToolResponse.error is set, so the MCP SDK sets isError=True
  on the CallToolResult instead of silently reporting success
- Fix E2E assertion in stremable_http_cli: check not result.isError
  and "error" not in structuredContent (was always-true before)
- Update test_remote_tools: error paths now expect pytest.raises(ToolError)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: aniket-s-kulkarni

src/dremioai/servers/mcp.py

@@ -43,6 +43,7 @@
 from mcp.server.auth.middleware.bearer_auth import BearerAuthBackend
 from mcp.server.auth.provider import AccessToken, TokenVerifier
 from mcp.server.fastmcp import FastMCP
+from mcp.server.fastmcp.exceptions import ToolError
 from mcp.server.fastmcp.prompts import Prompt
 from mcp.server.fastmcp.resources import FunctionResource
 from mcp.server.lowlevel.server import request_ctx
@@ -469,10 +470,12 @@ async def call_tool(
         if name in static_names:
             return await super().call_tool(name, arguments)
 
-        if not settings.instance().dremio.get("enable_remote_tools"):
-            return {"error": f"Tool '{name}' not found (remote tools disabled)"}
+        if not self.expose_remote_tools():
+            raise ToolError(f"Tool '{name}' not found (remote tools not enabled)")
 
         result = await self._invoke_remote_tool(name, arguments)
+        if result.error:
+            raise ToolError(result.error)
         return result.model_dump(exclude_none=True)
 
     def streamable_http_app(self):

tests/servers/test_remote_tools.py

@@ -23,6 +23,7 @@
 from dremioai.config import settings
 from dremioai.config.tools import ToolType
 from dremioai.servers.mcp import FastMCPServerWithAuthToken
+from mcp.server.fastmcp.exceptions import ToolError
 
 
 def _make_server() -> FastMCPServerWithAuthToken:
@@ -161,16 +162,14 @@ async def colliding_tool() -> str:
 
 @pytest.mark.asyncio
 async def test_call_unknown_tool_returns_error():
-    """Unknown remote tool → invoke_tool returns error dict."""
+    """Unknown remote tool → invoke_tool error propagates as ToolError (isError=True)."""
     server = _make_server()
 
     invoke_response = InvokeToolResponse(error="tool 'nonexistent_tool' not found")
 
     with patch.object(server, "_invoke_remote_tool", new=AsyncMock(return_value=invoke_response)):
-        result = await server.call_tool("nonexistent_tool", {})
-
-    assert isinstance(result, dict)
-    assert "error" in result
+        with pytest.raises(ToolError, match="nonexistent_tool"):
+            await server.call_tool("nonexistent_tool", {})
 
 
 @pytest.mark.asyncio
@@ -188,12 +187,10 @@ async def test_remote_tool_api_error_handled():
 
 
 @pytest.mark.asyncio
-async def test_remote_tools_disabled_call_returns_error_dict():
-    """call_tool for non-static tool when enable_remote_tools=False returns an error dict."""
+async def test_remote_tools_disabled_call_raises_tool_error():
+    """call_tool for non-static tool when remote tools disabled raises ToolError (isError=True)."""
     settings.set_base_settings(_make_settings(enable_remote_tools=False))
     server = _make_server()
 
-    result = await server.call_tool("any_remote_tool", {})
-
-    assert isinstance(result, dict)
-    assert "error" in result
+    with pytest.raises(ToolError):
+        await server.call_tool("any_remote_tool", {})

tests/stremable_http_cli.py

@@ -1219,7 +1219,8 @@ async def _run_smoketests(
             pp(f"Calling remote tool '{first_remote.name}'..", end=" ")
             result = await session.call_tool(first_remote.name, {})
             _assert(
-                result.structuredContent is not None or not result.isError,
+                not result.isError
+                and "error" not in (result.structuredContent or {}),
                 f"Remote tool call failed: {result.content}",
             )
             pp("[green]OK[/green]")