Fix #348: Allow ToolError to carry custom content and is_error

Adds `content` and `is_error` parameters to ToolError so tool functions
can return arbitrary content blocks with isError=True (e.g., images,
structured data alongside error status).

- exceptions.py: ToolError gains content and is_error parameters
- tools/base.py: Re-raise ToolError instead of wrapping it
- server.py: Catch ToolError, construct CallToolResult with proper content
- tests: 2 new tests covering ToolError with custom content and default

Closes #348

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>

Author: Stoobyy

src/mcp/server/mcpserver/exceptions.py

@@ -1,4 +1,5 @@
 """Custom exceptions for MCPServer."""
+from __future__ import annotations
 
 
 class MCPServerError(Exception):
@@ -14,7 +15,27 @@ class ResourceError(MCPServerError):
 
 
 class ToolError(MCPServerError):
-    """Error in tool operations."""
+    """Error in tool operations.
+
+    Can be raised from tool functions to return a tool result with
+    is_error=True and arbitrary content (e.g., images, structured data).
+
+    Args:
+        message: Error message (used as text content if no content provided).
+        content: Optional list of ContentBlock items to return as tool result content.
+        is_error: Whether to set is_error on the CallToolResult (default True).
+    """
+
+    def __init__(
+        self,
+        message: str,
+        *,
+        content: list | None = None,
+        is_error: bool = True,
+    ) -> None:
+        super().__init__(message)
+        self.content = content
+        self.is_error = is_error
 
 
 class InvalidSignature(Exception):

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
@@ -310,6 +310,9 @@ async def _handle_call_tool(
         context = Context(request_context=ctx, mcp_server=self)
         try:
             result = await self.call_tool(params.name, params.arguments or {}, context)
+        except ToolError as e:
+            content = e.content if e.content is not None else [TextContent(type="text", text=str(e))]
+            return CallToolResult(content=content, is_error=e.is_error)
         except MCPError:
             raise
         except Exception as e:

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

@@ -111,6 +111,9 @@ async def run(
                 result = self.fn_metadata.convert_result(result)
 
             return result
+        except ToolError:
+            # Re-raise ToolError so custom content and is_error propagate
+            raise
         except UrlElicitationRequiredError:
             # Re-raise UrlElicitationRequiredError so it can be properly handled
             # as an MCP error response with code -32042

tests/server/mcpserver/test_server.py

@@ -292,6 +292,43 @@ async def test_tool_error_details(self):
             assert "Test error" in content.text
             assert result.is_error is True
 
+    async def test_tool_error_with_content(self):
+        """Test that ToolError with custom content returns is_error=True."""
+
+        def tool_fn() -> None:
+            raise ToolError(
+                "Something went wrong",
+                content=[
+                    TextContent(type="text", text="Custom error"),
+                    ImageContent(type="image", data="base64...", mimeType="image/png"),
+                ],
+            )
+
+        mcp = MCPServer()
+        mcp.add_tool(tool_fn)
+        async with Client(mcp) as client:
+            result = await client.call_tool("tool_fn", {})
+            assert result.is_error is True
+            assert len(result.content) == 2
+            assert isinstance(result.content[0], TextContent)
+            assert result.content[0].text == "Custom error"
+            assert isinstance(result.content[1], ImageContent)
+
+    async def test_tool_error_default_content(self):
+        """Test that ToolError without custom content falls back to the error message."""
+
+        def tool_fn() -> None:
+            raise ToolError("Default error message")
+
+        mcp = MCPServer()
+        mcp.add_tool(tool_fn)
+        async with Client(mcp) as client:
+            result = await client.call_tool("tool_fn", {})
+            assert result.is_error is True
+            assert len(result.content) == 1
+            assert isinstance(result.content[0], TextContent)
+            assert "Default error message" in result.content[0].text
+
     async def test_tool_return_value_conversion(self):
         mcp = MCPServer()
         mcp.add_tool(tool_fn)