fix(server): wrap pydantic ValidationError from tool arg validation as fastmcp ValidationError by SarthakB11 · Pull Request #4172 · PrefectHQ/fastmcp

SarthakB11 · 2026-05-17T18:40:38Z

Description

When invalid arguments are passed to a tool, pydantic-core's call validator raises a raw pydantic.ValidationError which FastMCP.call_tool re-raises unchanged. The error then propagates through the middleware chain as a non-fastmcp type, so consumers filtering on the framework's own error taxonomy (e.g. a Sentry ignore_errors=[fastmcp.exceptions.ValidationError] filter) can't match it. The call_tool docstring already advertises ValidationError: If arguments fail validation, so the implementation had drifted from the contract.

This wraps the caught PydanticValidationError in fastmcp.exceptions.ValidationError with from e, preserving the structured pydantic error as __cause__ so consumers that still want .errors() can walk the chain. The warning log just above the re-raise is unchanged, and the client-side wire contract (a ToolError reaches the client) is preserved.

The 10 existing pytest.raises(ValidationError) blocks in tests/server/providers/local_provider_tools/test_parameters.py that target mcp.call_tool(...) are migrated from pydantic.ValidationError to fastmcp.exceptions.ValidationError in the same PR, since they assert against the new error type. No TypeAdapter-direct tests exist in that file, so no pydantic.ValidationError raises are left intact.

The reporter notes in the issue thread that tool bodies can also raise pydantic.ValidationError from their own bugs, and those should arguably stay distinguishable from bad-call errors. Distinguishing the two requires splitting argument validation from invocation inside FunctionTool.run (today type_adapter.validate_python(arguments) does both), which is a deeper refactor than this bug warrants.

Closes #4128.

Contribution type

Bug fix (simple, well-scoped fix for a clearly broken behavior)
Documentation improvement
Enhancement (maintainers typically implement enhancements — see CONTRIBUTING.md)

Checklist

This PR addresses an existing issue (or fixes a self-evident bug)
I have read CONTRIBUTING.md
I have added tests that cover my changes
I have run uv run prek run --all-files and all checks pass
I have self-reviewed my changes
If I used an LLM, it followed the repo's contributing conventions (not generic output)

Closes PrefectHQ#4128

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marvin-context-protocol · 2026-05-17T18:42:48Z

Edited to reflect the latest CI run — the original ruff issue is fixed; new failures are test expectations that weren't migrated alongside the behavior change.

tl;dr: Three tests still expect pydantic.ValidationError from mcp.call_tool(...), but this PR now wraps that as fastmcp.exceptions.ValidationError. Update the three pytest.raises(pydantic.ValidationError) blocks listed below to match.

Root Cause: The PR's whole point is to wrap pydantic's ValidationError from call_tool arg validation in fastmcp.exceptions.ValidationError (see fastmcp_slim/fastmcp/server/server.py:1288 — raise ValidationError(str(e)) from e). The PR description says the 10 raises in tests/server/providers/local_provider_tools/test_parameters.py were migrated, but three other call sites that also call mcp.call_tool(...) inside pytest.raises(pydantic.ValidationError) were missed and now fail because the exception they see is the wrapped fastmcp.exceptions.ValidationError, not the bare pydantic one.

Fix: In each of the three locations, replace pydantic.ValidationError with fastmcp.exceptions.ValidationError (the test file already imports pytest/pydantic; either import fastmcp.exceptions.ValidationError or assert on the wrapped error). The original pydantic error is preserved via __cause__, so existing message-matching like match="title" should still work via the wrapped str(e).

tests/apps/test_form.py:129 — pytest.raises(pydantic.ValidationError, match="title") around await server.call_tool(...).
tests/server/test_dependencies.py:456 — pytest.raises(pydantic.ValidationError) around await mcp.call_tool("validated_tool", {"age": "not a number"}).
tests/server/test_dependencies.py:604 — pytest.raises(pydantic.ValidationError) around await mcp.call_tool("check_permission", {"action": "read", "admin": "hacker"}).

All three failing jobs (Tests with lowest-direct dependencies, Tests: Python 3.10 on ubuntu-latest, Tests: Python 3.10 on windows-latest, Tests: Python 3.13 on ubuntu-latest) hit the same three tests.

Log excerpts

From Tests with lowest-direct dependencies:

FAILED tests/apps/test_form.py::TestFormInputProvider::test_submit_no_data_does_not_crash
  - fastmcp.exceptions.ValidationError: 2 validation errors for NoteForm
    title  Field required [type=missing, input_value={'archived': False}, input_type=dict]
    content  Field required [type=missing, input_value={'archived': False}, input_type=dict]

FAILED tests/server/test_dependencies.py::test_argument_validation_with_dependencies
  - fastmcp.exceptions.ValidationError: 1 validation error for call[validated_tool]
    age  Input should be a valid integer, unable to parse string as an integer
         [type=int_parsing, input_value='not a number', input_type=str]

FAILED tests/server/test_dependencies.py::test_external_user_cannot_override_dependency
  - fastmcp.exceptions.ValidationError: 1 validation error for call[check_permission]
    admin  Unexpected keyword argument [type=unexpected_keyword_argument, input_value='hacker', input_type=str]

======= 3 failed, 5673 passed, 2 skipped, 1 xfailed in 153.46s =======

Why this happened

The PR description correctly identifies tests/server/providers/local_provider_tools/test_parameters.py as the file with pytest.raises(ValidationError) blocks against mcp.call_tool(...) — but there are other such call sites in the repo. A repo-wide grep for pytest.raises(pydantic.ValidationError) near await ... call_tool would have surfaced these three.

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chatgpt-codex-connector · 2026-05-17T18:43:02Z

                        e.errors(include_url=False),
                    )
-                    raise
+                    raise ValidationError(str(e)) from e


Don't classify tool-body validation failures as bad input

When a tool itself raises a pydantic.ValidationError (for example by constructing a Pydantic model from bad upstream data), that exception also exits tool._run() through the same PydanticValidationError handler because FunctionTool.run() performs argument validation and invokes the function inside one TypeAdapter.validate_python(...) call. Wrapping every such error as fastmcp.exceptions.ValidationError means middleware or Sentry filters intended to ignore caller argument mistakes will now also suppress real tool-body bugs. Please only wrap validation errors known to come from argument binding, or leave body-raised Pydantic errors on the normal tool-error path.

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strawgate · 2026-05-17T18:48:21Z

Please follow the contributing guide and do not raise PRs without discussion on the issue first.

fix(server): wrap pydantic ValidationError as fastmcp ValidationError

0c9b729

Closes PrefectHQ#4128

claude Bot reviewed May 17, 2026

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marvin-context-protocol Bot added bug Something isn't working. Reports of errors, unexpected behavior, or broken functionality. server Related to FastMCP server implementation or server-side functionality. labels May 17, 2026

chatgpt-codex-connector Bot reviewed May 17, 2026

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strawgate closed this May 17, 2026

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fix(server): wrap pydantic ValidationError from tool arg validation as fastmcp ValidationError - #4172

fix(server): wrap pydantic ValidationError from tool arg validation as fastmcp ValidationError#4172
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