fix: prevent tool exceptions from leaking internal details to client

src/mcp/server/mcpserver/server.py

@@ -31,7 +31,7 @@
 from mcp.server.lowlevel.server import LifespanResultT, Server
 from mcp.server.lowlevel.server import lifespan as default_lifespan
 from mcp.server.mcpserver.context import Context
-from mcp.server.mcpserver.exceptions import ResourceError
+from mcp.server.mcpserver.exceptions import ResourceError, ToolError
 from mcp.server.mcpserver.prompts import Prompt, PromptManager
 from mcp.server.mcpserver.resources import FunctionResource, Resource, ResourceManager
 from mcp.server.mcpserver.tools import Tool, ToolManager
@@ -303,8 +303,14 @@ async def _handle_call_tool(
             result = await self.call_tool(params.name, params.arguments or {}, context)
         except MCPError:
             raise
-        except Exception as e:
+        except ToolError as e:
             return CallToolResult(content=[TextContent(type="text", text=str(e))], is_error=True)
+        except Exception:
+            logger.exception(f"Error calling tool {params.name}")
+            return CallToolResult(
+                content=[TextContent(type="text", text=f"An unexpected error occurred calling tool {params.name}")],
+                is_error=True,
+            )
         if isinstance(result, CallToolResult):
             return result
         if isinstance(result, tuple) and len(result) == 2:

src/mcp/server/mcpserver/tools/base.py

@@ -116,8 +116,10 @@ async def run(
             # Re-raise UrlElicitationRequiredError so it can be properly handled
             # as an MCP error response with code -32042
             raise
+        except ToolError:
+            raise
         except Exception as e:
-            raise ToolError(f"Error executing tool {self.name}: {e}") from e
+            raise ToolError(f"An unexpected error occurred executing tool {self.name}") from e
 
 
 def _is_async_callable(obj: Any) -> bool:

tests/client/test_list_roots_callback.py

@@ -44,4 +44,4 @@ async def test_list_roots(context: Context[None], message: str):
         result = await client.call_tool("test_list_roots", {"message": "test message"})
         assert result.is_error is True
         assert isinstance(result.content[0], TextContent)
-        assert result.content[0].text == "Error executing tool test_list_roots: List roots not supported"
+        assert result.content[0].text == "An unexpected error occurred executing tool test_list_roots"

tests/client/test_sampling_callback.py

@@ -54,7 +54,7 @@ async def test_sampling_tool(message: str, ctx: Context) -> bool:
         result = await client.call_tool("test_sampling", {"message": "Test message for sampling"})
         assert result.is_error is True
         assert isinstance(result.content[0], TextContent)
-        assert result.content[0].text == "Error executing tool test_sampling: Sampling not supported"
+        assert result.content[0].text == "An unexpected error occurred executing tool test_sampling"
 
 
 @pytest.mark.anyio

tests/server/mcpserver/test_server.py

@@ -212,6 +212,10 @@ def error_tool_fn() -> None:
     raise ValueError("Test error")
 
 
+def tool_error_fn() -> None:
+    raise ToolError("Intentional tool error")
+
+
 def image_tool_fn(path: str) -> Image:
     return Image(path)
 
@@ -258,7 +262,8 @@ async def test_tool_exception_handling(self):
             assert len(result.content) == 1
             content = result.content[0]
             assert isinstance(content, TextContent)
-            assert "Test error" in content.text
+            assert "unexpected error" in content.text.lower()
+            assert "Test error" not in content.text
             assert result.is_error is True
 
     async def test_tool_error_handling(self):
@@ -269,20 +274,47 @@ async def test_tool_error_handling(self):
             assert len(result.content) == 1
             content = result.content[0]
             assert isinstance(content, TextContent)
-            assert "Test error" in content.text
+            assert "unexpected error" in content.text.lower()
+            assert "Test error" not in content.text
             assert result.is_error is True
 
     async def test_tool_error_details(self):
-        """Test that exception details are properly formatted in the response"""
+        """Test that exception details are NOT exposed to the client"""
         mcp = MCPServer()
         mcp.add_tool(error_tool_fn)
         async with Client(mcp) as client:
             result = await client.call_tool("error_tool_fn", {})
             content = result.content[0]
             assert isinstance(content, TextContent)
             assert isinstance(content.text, str)
-            assert "Test error" in content.text
+            assert "error_tool_fn" in content.text
+            assert "Test error" not in content.text
+            assert result.is_error is True
+
+    async def test_tool_error_passthrough(self):
+        """Test that ToolError raised in tool is passed through with its message."""
+        mcp = MCPServer()
+        mcp.add_tool(tool_error_fn)
+        async with Client(mcp) as client:
+            result = await client.call_tool("tool_error_fn", {})
             assert result.is_error is True
+            content = result.content[0]
+            assert isinstance(content, TextContent)
+            assert "Intentional tool error" in content.text
+
+    async def test_handle_call_tool_defensive_exception_handler(self):
+        """Test that _handle_call_tool returns generic error when call_tool raises unexpected Exception."""
+        mcp = MCPServer()
+        mcp.add_tool(tool_fn)
+
+        async with Client(mcp) as client:
+            with patch.object(mcp, "call_tool", new_callable=AsyncMock, side_effect=RuntimeError("internal db leak")):
+                result = await client.call_tool("tool_fn", {"x": 1, "y": 2})
+                assert result.is_error is True
+                content = result.content[0]
+                assert isinstance(content, TextContent)
+                assert "unexpected error" in content.text.lower()
+                assert "internal db leak" not in content.text
 
     async def test_tool_return_value_conversion(self):
         mcp = MCPServer()

tests/server/mcpserver/test_tool_manager.py

@@ -393,7 +393,7 @@ def tool_with_context(x: int, ctx: Context[ServerSessionT, None]) -> str:
         manager = ToolManager()
         manager.add_tool(tool_with_context)
 
-        with pytest.raises(ToolError, match="Error executing tool tool_with_context"):
+        with pytest.raises(ToolError, match="unexpected error occurred executing tool tool_with_context"):
             await manager.call_tool("tool_with_context", {"x": 42}, context=Context())
 
 

tests/server/mcpserver/test_url_elicitation_error_throw.py

@@ -106,4 +106,6 @@ async def failing_tool(ctx: Context[ServerSession, None]) -> str:
         assert result.is_error is True
         assert len(result.content) == 1
         assert isinstance(result.content[0], types.TextContent)
-        assert "Something went wrong" in result.content[0].text
+        assert "An unexpected error occurred executing tool failing_tool" in result.content[0].text
+        # Verify the original exception details are NOT leaked to the client
+        assert "Something went wrong" not in result.content[0].text