[BUG] MCPToolResolver does not log warning when native MCP config returns no tools

[AmoyCherry]

Description

MCPToolResolver._resolve_native() silently returns an empty tool list when:

The MCP server returns no tools, or
The tool_filter removes all tools.

The empty tools_list can trigger the error at tool_resolver.py", line 412:

for tool_def in tools_list:
                  ^^^^^^^^^^
UnboundLocalError: cannot access local variable 'tools_list' where it is not associated with a value

There is no warning logged, making it very hard for users to understand why their MCP server (Stdio / HTTP / SSE) is not providing any tools.
In contrast, _resolve_external() (for HTTPS URLs) does log a clear warning:

No tools discovered from MCP server: {server_url}

Affected Code

File: crewai/mcp/tool_resolver.py
Method: MCPToolResolver._resolve_native() (around line 412, right after tool filtering)

Steps to Reproduce

1. Configure an agent with a native MCP server via MCPServerStdio, MCPServerHTTP, or MCPServerSSE that either has no tools or uses a tool_filter that returns nothing.
2. Add it to the agent’s mcps list.
3. Run the agent/task → zero tools are available and no warning appears in the logs.

Expected behavior

A clear warning should be logged when no tools are discovered after listing + filtering, exactly like the external resolver path. Instead of the current 500 Error.

Screenshots/Code snippets

            tools = []
            for tool_def in tools_list:
                tool_name = tool_def.get("name", "")
                original_tool_name = tool_def.get("original_name", tool_name)
                if not tool_name:
                    continue

Operating System

Ubuntu 20.04

Python Version

3.12

crewAI Version

latest

crewAI Tools Version

latest

Virtual Environment

Venv

Evidence

Traceback (most recent call last):
  File ".../python3.12/site-packages/crewai/mcp/tool_resolver.py", line 412, in _resolve_native
    for tool_def in tools_list:
                    ^^^^^^^^^^
UnboundLocalError: cannot access local variable 'tools_list' where it is not associated with a value

Possible Solution

            if not tools_list:
                self._logger.log(
                    "warning",
                    f"No tools discovered from MCP server: {server_name}",
                )
                return cast(list[BaseTool], []), []

Additional context

It's an apparent bug.

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.