fix(mcp): prevent UnboundLocalError and log warning when native MCP returns no tools

[r266-tech]

Closes #5116

Problem

MCPToolResolver._resolve_native() has two issues:

1. UnboundLocalError: When the MCP server connection raises a RuntimeError that doesn't contain "cancel scope" or "task" in its message, the exception is silently swallowed (not re-raised). This leaves tools_list unassigned, causing UnboundLocalError: cannot access local variable 'tools_list' where it is not associated with a value at the iteration loop (line 412).

2. No warning for empty tools: When the MCP server returns no tools (or tool_filter removes all tools), no warning is logged. In contrast, _resolve_external() logs a clear warning: "No tools discovered from MCP server: {server_url}".

Fix

1. Re-raise unrecognized RuntimeError: Added raise at the end of the RuntimeError handler so that non-cancel-scope errors propagate instead of being silently swallowed.

2. Log warning and return early: Added checks after getting tools_list and after tool filtering to log a warning and return ([], []) when no tools are available, matching _resolve_external() behavior.

Changes

• lib/crewai/src/crewai/mcp/tool_resolver.py: +14 lines

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Note

Low Risk
Low risk: small control-flow and logging changes in MCP native tool discovery; main impact is different error propagation and earlier returns when no tools are available.

Overview
Fixes native MCP tool resolution to avoid silently swallowing unexpected RuntimeErrors during discovery (preventing an UnboundLocalError later) by re-raising unrecognized runtime errors.

Adds explicit empty-results handling: logs a warning and returns ([], []) when the MCP server returns no tools, and when tool_filter removes all tools, aligning behavior with external MCP resolution.

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