fix: prepare MCP tools during execute_task for delegated sub-agents

[Anandesh-Sharma]

Summary

• MCP tools were only loaded during kickoff() and the crew's _prepare_tools() path, but NOT during execute_task() which is the code path used when a main agent delegates work to a sub-agent via DelegateWorkTool
• Added _ensure_mcp_tools_loaded() method to Agent that idempotently loads MCP tools into the agent's tools list, gated by a _mcps_prepared flag
• Called at the start of both execute_task() and aexecute_task() so sub-agents with MCP servers receive their tools when delegated to

Root Cause

When DelegateWorkTool._execute() delegates to a sub-agent, it calls selected_agent.execute_task(task, context) directly. The execute_task method never prepared MCP tools -- it relied on either:

1. kickoff() -> _prepare_kickoff() (standalone agent path)
2. Crew._prepare_tools() -> _inject_mcp_tools() (crew task execution path)

Neither of these paths run when a sub-agent receives delegated work, so self.mcps was configured but the tools were never resolved and added to self.tools.

Fix Details

• New _ensure_mcp_tools_loaded() method checks self.mcps and the _mcps_prepared flag
• If MCP tools haven't been loaded yet, calls get_mcp_tools() and appends to self.tools (with duplicate-name protection)
• Sets _mcps_prepared = True after loading, making subsequent calls no-ops
• Also sets the flag in _prepare_kickoff() to prevent double-loading when kickoff is the entry point

Test plan

• Added test_execute_task_loads_mcp_tools_for_sub_agent -- verifies MCP tools are loaded when execute_task is called directly (the delegation path)
• Added test_ensure_mcp_tools_loaded_is_idempotent -- verifies no duplicate tools on repeat calls
• Added test_ensure_mcp_tools_loaded_no_mcps -- verifies no-op when agent has no MCP config
• All 3 new tests pass

Closes #4571

🤖 Generated with Claude Code

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Note

Medium Risk
Changes the agent execution path to dynamically mutate Agent.tools based on MCP configuration, which can affect tool availability and ordering during runtime. Scope is limited and covered by new regression/idempotency tests.

Overview
Fixes delegated sub-agent execution by loading MCP-provided tools at the start of Agent.execute_task and Agent.aexecute_task, via a new idempotent _ensure_mcp_tools_loaded() guarded by a private _mcps_prepared flag.

Also marks MCPs as prepared during _prepare_kickoff() to avoid double-loading, and adds targeted tests covering the direct execute_task delegation path, idempotency (no duplicate tools), and the no-MCP no-op case.

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Crew path causes duplicate MCP server connections

Medium Severity

In the standard crew execution path, crew._prepare_tools() → _inject_mcp_tools() already calls agent.get_mcp_tools(), but it never sets _mcps_prepared on the agent. So when execute_task subsequently runs _ensure_mcp_tools_loaded(), it sees _mcps_prepared is still False and calls get_mcp_tools() a second time. For native MCP configs (MCPServerStdio, MCPServerHTTP, MCPServerSSE), this creates duplicate server connections and appends duplicate clients to _mcp_clients. The redundant tools added to self.tools aren't even used because prepare_tools() prioritizes the passed tools parameter.

Additional Locations (1)

• lib/crewai/src/crewai/agent/core.py#L357-L360