[Wali05]

Fixes #221.

_toolset.py imported pydantic_ai.mcp at module level for RestateMCPToolset, and pydantic_ai.mcp raises ImportError when the MCP client packages aren't installed. Because restate/ext/pydantic/__init__.py imports _toolset, the whole extension failed to import unless pydantic-ai-slim[mcp] was installed, even for agents that only use function tools.

RestateAgent already imports RestateMCPToolset lazily inside a try/except ImportError, but the guard never got a chance to work because _toolset was imported unconditionally a few lines above it. So I moved RestateMCPToolset and its two serde wrappers into a new _mcp_toolset.py, and pointed the lazy import there. _toolset.py keeps RestateContextRunToolSet and no longer touches pydantic_ai.mcp. The MCP code itself is moved as-is, no behaviour change.

RestateMCPToolset was never exported from restate.ext.pydantic, so nothing public moves.

One thing I noticed while testing: the repo's own uv sync --all-extras environment has mcp but not fastmcp (pydantic-ai-slim 2.15 needs both for pydantic_ai.mcp), so on main import restate.ext.pydantic fails there as well.

Testing

Added tests/ext_pydantic.py:

• test_import_without_mcp imports restate.ext.pydantic in a subprocess with mcp/fastmcp blocked
• test_agent_with_function_tools_without_mcp builds a RestateAgent with a function tool in the same setup and checks the tool is still wrapped in RestateContextRunToolSet
• test_agent_wraps_mcp_toolsets checks an MCPToolset is still wrapped in RestateMCPToolset (skipped when pydantic_ai.mcp can't be imported)

The first two fail on main with the ImportError from the issue and pass with the change. I ran the third with pydantic-ai-slim[mcp]==2.15.0 installed, and it passes. Because the dev environment doesn't have fastmcp yet, test_agent_wraps_mcp_toolsets will be skipped in CI for now. Happy to add fastmcp to the dev deps in a follow-up so that path gets covered.

I also checked the exact scenario from the issue in a clean venv with only pydantic-ai-slim==2.15.0: import restate.ext.pydantic fails on main and works on this branch.

All the just verify steps pass locally (ruff format/check, pyright and mypy on python/, examples/, tests/, test-services/, plus pytest tests/*: 42 passed, 1 skipped).

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[Wali05]

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