Fix: raise on MCP tool errors instead of returning error text as a string

Root cause of the recurring "'str' object has no attribute 'items'" noise:
when a decoupleR tool raised server-side, MCP returns isError=True with the
message as plain text. _parse_mcp_content ignored isError, tried json.loads
(which fails on a plain message), and handed the agent the error TEXT as a
string. The agent's dict-expecting code then did result.items()/result['k']
on that string and failed with a misleading secondary error that hid the
real cause.

Now _parse_mcp_content checks result.isError and raises RuntimeError with the
message, so the executor reports the actual tool error (e.g. "dataset_id 'foo'
not found") and the agent self-corrects on the real problem. Verified
end-to-end against a raising FastMCP tool and a normal dict tool.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/managers/tools/mcp_manager.py

@@ -119,6 +119,13 @@ class MCPManager:
                             await session.initialize()
                             result = await session.call_tool(tool_name, kwargs)
                             content = result.content[0] if result.content else None
+                            text = content.text if (content is not None and hasattr(content, "text")) else (
+                                str(content) if content is not None else "")
+                            # Surface server-side tool errors as raised exceptions
+                            # instead of handing the agent the error text as a string
+                            # (which then breaks dict-expecting code misleadingly).
+                            if getattr(result, "isError", False):
+                                raise RuntimeError(text or "tool reported an error")
                             if content is None:
                                 return {}
                             if hasattr(content, "text"):
@@ -286,15 +293,31 @@ class MCPManager:
                 print(f"Failed to discover remote tools from {url}: {e}")
                 return []
 
-        def _parse_mcp_content(content) -> Any:
-            """Extract and JSON-parse the tool result from an MCP content object.
+        def _parse_mcp_content(result) -> Any:
+            """Extract the tool result from an MCP CallToolResult.
 
-            MCP tools return dicts serialized as JSON in content.text.
-            Returning a parsed Python dict lets agent code do result["key"]
+            MCP tools return dicts serialized as JSON in content[0].text;
+            returning a parsed Python dict lets agent code do result["key"]
             without hitting 'string indices must be integers, not str'.
+
+            If the tool raised server-side, MCP sets result.isError and puts the
+            error message in content[0].text. We must NOT hand that text back as a
+            plain string: the agent's dict-expecting code then does result.items()
+            on it and fails with a misleading "'str' object has no attribute
+            'items'" that hides the real cause. Instead we raise, so the executor
+            reports the actual tool error and the agent can self-correct on it.
             """
             import json
-            text = content.text if hasattr(content, "text") else str(content)
+
+            content = result.content[0] if getattr(result, "content", None) else None
+            text = content.text if (content is not None and hasattr(content, "text")) else (
+                str(content) if content is not None else "")
+
+            if getattr(result, "isError", False):
+                raise RuntimeError(text or "tool reported an error")
+
+            if content is None:
+                return {}
             try:
                 return json.loads(text)
             except (json.JSONDecodeError, TypeError):
@@ -313,7 +336,7 @@ class MCPManager:
                             async with ClientSession(reader, writer) as session:
                                 await session.initialize()
                                 result = await session.call_tool(tool_name, kwargs)
-                                return _parse_mcp_content(result.content[0])
+                                return _parse_mcp_content(result)
 
                     try:
                         loop = asyncio.get_running_loop()
@@ -339,7 +362,7 @@ class MCPManager:
                             async with ClientSession(read, write) as session:
                                 await session.initialize()
                                 result = await session.call_tool(tool_name, kwargs)
-                                return _parse_mcp_content(result.content[0])
+                                return _parse_mcp_content(result)
 
                     try:
                         loop = asyncio.get_running_loop()