fix(validators): reject valueHint on named arguments#1339
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valueHint identifies a positional slot for transport URL variable substitution, so it is only valid on positional arguments. The model carries ValueHint flat on Argument and validateArgument never checked it for named arguments, and the JSON schema does not catch it (the argument union branches do not set additionalProperties: false), so a named argument with a valueHint validated. Enforce it in the semantic validator with ErrValueHintOnNamedArgument (code valuehint-on-named-argument), with tests both directions. Closes modelcontextprotocol#662.
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Summary
Closes #662.
valueHintidentifies a positional slot for transport URL variable substitution, so it belongs only on positional arguments. Right now nothing stops a named argument from carrying one: the model holdsValueHintflat onArgument(pkg/model/types.go), andvalidateArgumentchecks the name and value fields for named arguments but not the hint.The JSON schema does not catch it either.
valueHintis declared onPositionalArgumentonly, butArgumentis ananyOfunion and neither branch setsadditionalProperties: false, so a named argument with avalueHintstill validates against theNamedArgumentbranch. That leaves the semantic validator as the enforcement point.Change
ErrValueHintOnNamedArgument.validateArgument, reject a non-emptyValueHinton a named argument (codevaluehint-on-named-argument), beside the existing named-argument checks.valueHintis rejected; a positional argument with avalueHintstays valid.The check is additive and matches the schema's positional-only declaration, so existing valid data is unaffected. I kept this at the validator level rather than tightening the generated schema, which felt like the smaller change; happy to put
additionalProperties: falseon the argument branches instead if you'd rather the schema carry it.Test
go test ./internal/validators/...passes.