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Add ValidationOptions for lenient output schema validation - #1260
Add ValidationOptions for lenient output schema validation#1260lorenss-m wants to merge 3 commits into
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ochafik
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Hi @lorenss-m, thanks for sending this!
Seems reasonable to add this option given the spec only says clients "should" validate structured results against the output schema.
I'm curious tho, which examples of non-conforming outputs did you find / how bad were they?
cc/ @bhosmer-ant since authored initial support in #993
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What error would None produce here, do we need the two branches?
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Let's not introduce parameters that are BaseModel's please. The best experience is usually to have flat parameters.
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9a8592eComparelorenss-m
commented
Aug 26, 2025
Added the validate_structured_outputs suggestion! |
dsp-ant
commented
Sep 1, 2025
We should make sure that this matches how other SDKs behave before we merge this. @modelcontextprotocol/sdk-maintainers . |
jba
commented
Sep 1, 2025
In the Go SDK, currently all schema validation is done on the server side. |
mikekistler
commented
Sep 1, 2025
The C# SDK delegates the validation of the structured content to the MCP Host. This is mainly because there is no standard, built-in JSON Schema validator in .NET, so doing this in the client would require pulling in an external package, one of potentially several, which adds bulk to the SDK and usurps the MCP Host's choce of validation package. |
lorenss-m
commented
Sep 19, 2025
Anything else we should change here? Would love to get this rolled out, our library is forced to depend on our fork otherwise! |
felixweinberger
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Looks like there isn't necessarily a single approach across SDKs. Go does server-side validation, C# delegates it to the host, with neither currently doing any validation within the client.
This PR allows optionally disabling client side validation specifically in Python, which thus seems like it would enable behaviors like in Go & C# at least (no client side validation).
Also matches the spec as pointed out by @ochafik above, so this seems OK to me to merge.
However, left some comments - especially the change to auth seems unintentional.
| def construct_redirect_uri(redirect_uri_base: str, **params: str | None) -> str: | ||
| parsed_uri = urlparse(redirect_uri_base) | ||
| query_params = [(k, v) for k, vs in parse_qs(parsed_uri.query) for v in vs] | ||
| query_params = [(k, v) for k, vs in parse_qs(parsed_uri.query).items() for v in vs] |
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intentional? seems unrelated
| f"Invalid structured content returned by tool {name}: {e}. Continuing without validation." | ||
| ) | ||
| except SchemaError as e: | ||
| # Schema errors are always raised - they indicate a problem with the schema itself |
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nit: remove redundant comment
| if output_schema is not None: | ||
| if result.structuredContent is None: | ||
| raise RuntimeError(f"Tool {name} has an output schema but did not return structured content") | ||
| try: | ||
| validate(result.structuredContent, output_schema) | ||
| except ValidationError as e: | ||
| raise RuntimeError(f"Invalid structured content returned by tool {name}: {e}") | ||
| except SchemaError as e: | ||
| raise RuntimeError(f"Invalid schema for tool {name}: {e}") | ||
| if self._validate_structured_outputs: |
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nit: getting quite heavily nested here, could at least partially simplify with
if output_schema is None:
return
if result.structuredContent is None:
# raise / log depending
# handle the base case of try / except with raise / log depending
felixweinberger
commented
Oct 13, 2025
Hi @lorenss-m are you planning to come back to this one? |
felixweinberger
commented
Oct 23, 2025
Closing this for now due to inactivity. |
Motivation and Context
Currently, MCP clients raise
RuntimeErrorwhen tools don't return structured content matching theiroutputSchema. This breaks integrations with servers that have schema/implementation mismatches.This PR adds optional lenient validation that converts these errors to warnings, allowing clients to continue operating with imperfect servers.
How Has This Been Tested?
tests/client/test_validation_options.pyBreaking Changes
None. Default behavior unchanged. Lenient validation is opt-in:
Types of changes
Checklist
Additional context
ValidationOptionsclass following existing patterns (e.g.,TransportSecuritySettings)ClientSession.call_tool()to check options before raising validation errorsstructured_output_lowlevel.pyby creating proper package structure