Proposal: Add AddToolForType for Runtime Type Support

[rebaz94]

Currently, AddTool[In, Out any] only works with generics, requiring compile-time types. This blocks use cases where types are generated or resolved dynamically at runtime.

This is similar to the issue for jsonschema.ForType, where passing a reflect.Type enabled runtime flexibility.

1. New Function: AddToolForType

Introduce new function like:

func AddToolForType(
    s *Server,
    t *Tool,
    in, out reflect.Type,
    h ToolHandlerFor[map[string]any, any],
)

• Input type: Always handled as an object.
• Output type: Explicitly marked as structured or not, ensuring schemas are validated even when the handler returns any.

---

2. Option to Use Output Schema for any Types

Alternatively, we can extend the existing API by adding a configuration option:

type AddToolOpt struct {
    UseOutputSchemaForAnyType bool
}

Usage in ToolFor:

func ToolFor[In, Out any](
    t *Tool,
    h ToolHandlerFor[In, Out],
    opt ...AddToolOpt,
)

Then update schema resolution:

var (
    elemZero       any // only non-nil if Out is a pointer type
    outputResolved *jsonschema.Resolved
)
if reflect.TypeFor[Out]() != reflect.TypeFor[any]() || (tt.OutputSchema != nil && opts.UseOutputSchemaForAnyType) {
    var err error
    elemZero, err = setSchema[Out](&tt.OutputSchema, &outputResolved)
    if err != nil {
        return nil, nil, fmt.Errorf("output schema: %v", err)
    }
}

This addition would provide flexibility for dynamic type handling while keeping the validation and schema generation consistent with the generic approach.

[jba]

Couldn't you do this like so?

t := &Tool{...}
t.InputSchema, err = jsonschema.ForType(...)
// ditto for output schema
mcp.AddTool[map[string]any, any](...)

[rebaz94]

The issue is that when the output type is any, the handler ignores the OutputSchema. I believe the problem is this line:

go-sdk/mcp/server.go

Line 227 in 03c5113

if reflect.TypeFor[Out]() != reflect.TypeFor[any]() {

if the type is any, outputResolved is never set, so the output isn’t validated against the schema. We need a way to return results as any for dynamic types created via reflect, while still enforcing the schema if defined.

[jba]

I have a fix for this, dependent on #352.

[rebaz94]

@jba does that handle an output value with a type of any type but having a json schema?
I will check later, but it looks like not handling that case

[jba]

It says if t.OutputSchema != nil || ... so it should work if there is an output schema.

[rebaz94]

My bad, sorry. I missed the or when checking on my phone.