I fixed a smolagents bug that confused everyone who hit it (with Sentry watching the whole time)

Short summary

A detailed account of fixing a long-standing bug in HuggingFace's smolagents framework (issue #1108) where calling to_dict() on a CodeAgent with MCP tools crashed with a confusing ValueError. The root cause: smolagents serializes tools by reconstructing standalone Python source, but MCP tools are runtime-generated closures wrapping live server sessions that cannot be serialized. The fix extends an existing guard clause to detect MCP tools and raise a clear, actionable error message instead of the cryptic internal failure.

• •smolagents crashed when serializing agents with MCP tools due to runtime-generated closures that can't be reconstructed from source
• •Root cause: MCPAdaptTool is generated at runtime by mcpadapt with required __init__ params and closure-captured session state
• •Fix adds a guard clause that detects MCP tools and raises a clear error telling users to remove MCP tools before serialization

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