fix(mcp-server): create dataclass wrapper for VariableValue to resolve Pydantic compatibility #6265
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mscolnick
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Sep 5, 2025
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@mscolnick , when we make a msgspec class BaseStruct(msgspec.Struct):
def __get_pydantic_core_schema__(self): ...
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mscolnick
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Sep 26, 2025
…ma__ class method and refactor MCP msgspec.Struct types to use it (#6540) ## 📝 Summary <!-- Provide a concise summary of what this pull request is addressing. If this PR fixes any issues, list them here by number (e.g., Fixes #123). --> Create BaseStruct to make msgspec.Struct types compatible with pydantic typechecking (for MCP Server tools). Update old code to now use this and remove redundant types. This PR reverses the temporary fix implemented in #6265 ## 🔍 Description of Changes <!-- Detail the specific changes made in this pull request. Explain the problem addressed and how it was resolved. If applicable, provide before and after comparisons, screenshots, or any relevant details to help reviewers understand the changes easily. --> - Create BaseStruct with __get_pydantic_core_schema__ in marimo/_utils/msgspec_basestruct.py - Update datatypes in marimo/_data/models.py to use BaseStruct for marimo/_ai/_tools/tools/tables_and_variables.py tool - Update CellVariableValue -> VariableValue(BaseStruct) in tables_and_variables.py and cells.py - Update tests to use VariableValue, inherit Session for type saftey, and use correct DataType enums - Add pydantic >2 as mcp dependency in pyproject.toml - Add test that checks if all exposed msgspec.Struct classes exposed to tools Args or Output use BaseStruct ## 📋 Checklist - [x] I have read the [contributor guidelines](https://github.com/marimo-team/marimo/blob/main/CONTRIBUTING.md). - [ ] For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on [Discord](https://marimo.io/discord?ref=pr), or the community [discussions](https://github.com/marimo-team/marimo/discussions) (Please provide a link if applicable). - [ ] I have added tests for the changes made. - [x] I have run the code and verified that it works as expected.
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📝 Summary
During migration of the mcp-server from TypedDict to dataclass
VariableValuecaused a pydantic parsing error due tomgspec.Structnot being a pydantic-compatible type. I resolved this by creating a type calledCellVariableValuethat mirrorsVariableValuebut is pydantic-compatible.🔍 Description of Changes
📋 Checklist