Add toolset() method to AgentBuilder for AbstractToolset integration

[acoliver]

Summary

Add a .toolset() method to AgentBuilder that allows registering an AbstractToolset (like McpToolset) with an agent.

Motivation

Currently, AgentBuilder supports registering individual tools via:

• .tool_fn() - register a closure as a tool
• .tool_fn_async() - register an async closure
• .tool_with_executor() - register with a custom executor

However, there's no way to register an entire AbstractToolset which would import all its tools. This is needed to integrate McpToolset (from serdes-ai-mcp) with agents.

Proposed API

let mcp_toolset = McpToolset::stdio("npx", &["-y", "@mcp/server"]).await?;

let agent = agent(model)
    .system_prompt("You have access to filesystem tools.")
    .toolset(mcp_toolset)  // <-- New method
    .build()
    .await?;  // Note: build() may need to become async

Implementation Considerations

1. Async in Builder: AbstractToolset::get_tools() is async, so either:

   • Make build() async
   • Discover tools lazily at first run
   • Store toolset and discover at agent creation

2. Multiple Toolsets: Support chaining .toolset(ts1).toolset(ts2) for multiple MCP servers

3. Tool Routing: Track which toolset owns which tool to route call_tool correctly

4. Lifecycle: Call toolset.enter() when agent starts and toolset.exit() when done

5. Name Conflicts: Handle tool name collisions between toolsets (error or prefix)

Related

• AbstractToolset trait in serdes-ai-toolsets
• McpToolset in serdes-ai-mcp
• AgentBuilder in serdes-ai-agent