Fix unstructured tool errors being masked as 'outputSchema defined but no structured output returned'

.changes/unreleased/Bug Fix-20260514-154649.yaml

@@ -0,0 +1,3 @@
+kind: Bug Fix
+body: Fix unstructured tool errors being masked as 'outputSchema defined but no structured output returned'
+time: 2026-05-14T15:46:49.572423-07:00

src/dbt_mcp/mcp/server.py

@@ -9,7 +9,7 @@
 from dbtlabs_vortex.producer import shutdown
 from mcp.server.fastmcp import FastMCP
 from mcp.server.lowlevel.server import LifespanResultT
-from mcp.types import ContentBlock, TextContent, Tool
+from mcp.types import ContentBlock, Tool
 
 from dbt_mcp.config.config import Config
 from dbt_mcp.dbt_admin.tools import register_admin_api_tools
@@ -108,12 +108,8 @@ async def call_tool(
                 )
             except Exception:
                 logger.debug("Usage tracking failed — skipping", exc_info=True)
-            return [
-                TextContent(
-                    type="text",
-                    text=str(e),
-                )
-            ]
+            raise
+
         end_time = int(time.time() * 1000)
         logger.info(f"Tool {name} called successfully in {end_time - start_time}ms")
         try:

tests/unit/mcp/test_dispatcher.py

@@ -191,7 +191,7 @@ async def test_routes_based_on_project_mode(
         mcps[called].call_tool.assert_awaited_once()
         mcps[not_called].call_tool.assert_not_awaited()
 
-    async def test_returns_text_content_on_tool_error(self):
+    async def test_raises_on_tool_error(self):
         multi = MagicMock(spec=FastMCP)
         single = MagicMock(spec=FastMCP)
         single.call_tool = AsyncMock(side_effect=RuntimeError("something broke"))
@@ -202,13 +202,10 @@ async def test_returns_text_content_on_tool_error(self):
         with patch.object(
             dispatcher, "_is_multi_project", AsyncMock(return_value=False)
         ):
-            result = await dispatcher.call_tool("bad_tool", {})
+            with pytest.raises(RuntimeError, match="something broke"):
+                await dispatcher.call_tool("bad_tool", {})
 
-        assert len(result) == 1
-        assert isinstance(result[0], TextContent)
-        assert "something broke" in result[0].text
-
-    async def test_tracking_failure_does_not_crash_call_tool(self):
+    async def test_tracking_failure_does_not_suppress_tool_error(self):
         single = MagicMock(spec=FastMCP)
         single.call_tool = AsyncMock(
             side_effect=MissingHostError("DBT_HOST is a required environment variable")
@@ -221,13 +218,10 @@ async def test_tracking_failure_does_not_crash_call_tool(self):
         with patch.object(
             dispatcher, "_is_multi_project", AsyncMock(return_value=False)
         ):
-            result = await dispatcher.call_tool("some_tool", {})
+            with pytest.raises(MissingHostError, match="DBT_HOST"):
+                await dispatcher.call_tool("some_tool", {})
 
-        # Tool error is returned cleanly despite tracking also failing
-        assert len(result) == 1
-        assert isinstance(result[0], TextContent)
-        assert "DBT_HOST" in result[0].text
-        # Tracking was attempted (and failed, but didn't crash)
+        # Tracking was attempted (and failed, but didn't suppress the tool error)
         dispatcher.usage_tracker.emit_tool_called_event.assert_awaited_once()