Why:

Separates pipeline validation from MCP server connection. Allows pipelines to validate successfully without requiring server connectivity, then connect during pipeline warm_up.

Keep in draft stage until deepset-ai/haystack#9856 is merge and Haystack 2.19 released.

What:

• Added eager_connect parameter to MCPTool and MCPToolset (defaults to True)
• Implemented _ensure_connected() with thread-safe lazy initialization
• Added warm_up() methods for connecting during pipeline warm_up phase

How can it be used:

# Validate pipeline without connecting to MCP server
tool = MCPTool(name="add", server_info=info, eager_connect=False)
pipeline.add_component("invoker", ToolInvoker(tools=[tool]))  # ✓ Validates

# Connect during pipeline warm_up
pipeline.run()  # triggers tool.warm_up() → connects to server

How did you test it:

• Test verifies permissive schema → strict schema transition on first use
• Confirmed backward compatibility (eager_connect=True by default)
• Tested with itinerary agent

Notes for the reviewer:

• Placeholder schema allows validation; strict schema loaded post-connection
• warm_up() is idempotent