Substitute zero value for typed-nil pointer functool output

[PratikDhanave]

A functool created with a pointer output type — HandlerFor[In, *Out] — can never return (nil, nil), even though that is the idiomatic Go way to signal "no result".

The output schema is derived from the dereferenced type (jsonformat.ForType dereferences, and the root object schema is deliberately non-nullable). A typed-nil *Out marshals to JSON null, and output normalization then validates null against a type: "object" root, which always fails:

normalizing output: normalizing: validation error: validating root:
type: <invalid reflect.Value> has type "null", want "object"

So the handler’s return value is unreachable and every call to a pointer-output tool errors.

The upstream modelcontextprotocol/go-sdk — which this package’s HandlerFor is explicitly modeled on — handles exactly this case: it computes an elemZero (non-nil only when Out is a pointer) and substitutes it for a typed-nil output "to avoid typed nil, which will serialize as JSON null," so validation against the object schema succeeds. functool did no such substitution — a parity gap.

Fix

In New, when Out is a pointer type, precompute the zero element value and substitute it when the handler returns a typed nil:

var elemZero Out
outType := reflect.TypeFor[Out]()
hasPointerOut := outType != nil && outType.Kind() == reflect.Pointer
if hasPointerOut {
    elemZero = reflect.New(outType.Elem()).Interface().(Out)
}
...
if hasPointerOut && reflect.ValueOf(out).IsNil() {
    out = elemZero
}

Value-typed outputs are unaffected.

Test

TestFuncTool_PointerOutputNilReturnsZeroValue builds a tool whose handler returns (*Out)(nil) and asserts Call succeeds with a zero-value *Out. It fails on the old code (the null/object validation error) and passes with the fix.

[Copilot]

Pull request overview

This PR fixes a functool.HandlerFor parity gap for pointer-typed outputs (Out = *T) where a handler returning a typed-nil pointer ((nil, nil)) previously serialized to JSON null and then failed validation against the non-nullable, object-rooted output schema.

Changes:

• Precomputes a zero element pointer for pointer Out types and substitutes it when the handler returns a typed-nil pointer, avoiding null output normalization failures.
• Adds a regression test ensuring (*Out)(nil) output is accepted and becomes a non-nil zero-value pointer.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tool/functool/func.go	Adds typed-nil pointer output substitution logic during handler execution.
tool/functool/func_test.go	Adds regression test covering pointer output typed-nil behavior.

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[PratikDhanave]

Fixed in a follow-up commit — converts to outType before the assertion (reflect.New(outType.Elem()).Convert(outType).Interface().(Out)), with a named-pointer regression test.