unsupported type: StreamState[any] cannot be passed as…

Error message

unsupported type: StreamState[any] cannot be passed as inputs to baml functions

What it means

shared.StreamState[any] is an internal streaming bookkeeping type produced by baml streaming calls; it is not a serializable input. encodeValue explicitly rejects it when it appears among baml function arguments.

Solutions

Extract the actual streamed value from the StreamState before passing it on
Pass the concrete data (string/struct/slice) rather than the stream container
Keep streaming state out of structs used as baml inputs

Example fix

// before
state := stream.State // shared.StreamState[any]
b.Next(ctx, state)
// after
b.Next(ctx, state.Value)

Defensive patterns

Strategy: type-guard

Validate before calling

func notStreamState(v any) bool { return !strings.Contains(reflect.TypeOf(v).String(), "StreamState") }

Type guard

func isStreamState(v any) bool { return strings.Contains(reflect.TypeOf(v).String(), "StreamState") }

Try / catch

if err := b.Fn(ctx, in); err != nil {
  if strings.Contains(err.Error(), "StreamState[any] cannot be passed") { /* extract .Value and retry */ }
}

Prevention

Extract the current value from StreamState before forwarding data between calls
Never store StreamState inside structs used as baml inputs
Keep streaming bookkeeping separate from payload data

When it happens

Trigger: Passing the StreamState returned by a streaming baml call (or an object/field containing it) as an argument to another baml function; storing a streaming state inside a class field passed to baml.

Common situations: Chaining streaming calls; accidentally using the StreamState container instead of its current value; holding stream state in a shared struct that is later reused as input.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/a632816b31d5f3cc. Report an issue: GitHub.

Appendix: source

Thrown at engine/language_client_go/baml_go/serde/encode.go:129

				Handle: handle,
			},
		}, nil
	}

	// Check for custom serializers first using the original value (could be pointer or value)
	if serializer, ok := originalValue.(BamlSerializer); ok {
		encoded, err := serializer.Encode()
		if err != nil {
			return nil, err
		}
		return encoded, nil
	}

	switch concreteValue.(type) {
	case shared.Checked[any]:
		return nil, fmt.Errorf("unsupported type: Checked[any] cannot be passed as inputs to baml functions")
	case shared.StreamState[any]:
		return nil, fmt.Errorf("unsupported type: StreamState[any] cannot be passed as inputs to baml functions")
	}

	// Handle primitive kinds and collections using reflection value rv (points to underlying value)
	switch rv.Kind() {
	case reflect.String:
		return &cffi.HostValue{
			Value: &cffi.HostValue_StringValue{
				StringValue: rv.String(),
			},
		}, nil

	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		return &cffi.HostValue{
			Value: &cffi.HostValue_IntValue{
				IntValue: rv.Int(),
			},
		}, nil

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