BoundaryML/baml · error

unexpected type for collector creation: %T

Error message

unexpected type for collector creation: %T

What it means

NewCollector calls the Rust runtime's raw object factory and then asserts the returned generic pointer is a *collector. If the runtime returns any other concrete Go wrapper type, this error is thrown. It is an internal consistency check: it means the cffi layer and the Go wrapper types have drifted out of sync, not that caller input was wrong.

Source

Thrown at engine/language_client_go/pkg/rawobjects_constructors.go:27

)

// / Construct Collector
func (r *BamlRuntime) NewCollector(name string) (Collector, error) {
	kwargs, err := serde.EncodeMapEntries(map[string]any{
		"name": name,
	}, "collector constructor args")
	if err != nil {
		return nil, fmt.Errorf("failed to encode kwargs: %w", err)
	}

	ptr, err := raw_objects.NewRawObject(r.runtime, cffi.BamlObjectType_OBJECT_COLLECTOR, kwargs)
	if err != nil {
		return nil, fmt.Errorf("failed to create collector: %w", err)
	}

	as_collector, ok := ptr.(*collector)
	if !ok {
		return nil, fmt.Errorf("unexpected type for collector creation: %T", ptr)
	}

	return as_collector, nil
}

func (r *BamlRuntime) newMediaFromUrl(mediaType MediaType, url string, mimeType *string) (media, error) {
	kwargs, err := serde.EncodeMapEntries(map[string]any{
		"mime_type": mimeType,
		"url":       url,
	}, "media constructor args")
	if err != nil {
		return nil, fmt.Errorf("failed to encode kwargs: %w", err)
	}

	ptr, err := raw_objects.NewRawObject(r.runtime, mediaType.objectType(), kwargs)
	if err != nil {
		return nil, fmt.Errorf("failed to create media: %w", err)
	}

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Check that the baml Go module version and the bundled native runtime are from the same release; run `go mod tidy` and upgrade to a consistent version
2. Rebuild/re-download the native BAML runtime binary so it matches the Go bindings
3. If reproducible on a clean install, file a bug with the BAML maintainers including the %T value printed in the message

// Go: ensure module consistency before use
import (
    "runtime/debug"
    baml "github.com/boundaryml/baml/engine/language_client_go"
)
func checkBamlVersion() error {
    bi, ok := debug.ReadBuildInfo()
    if !ok { return nil }
    for _, d := range bi.Deps {
        if strings.HasPrefix(d.Path, "github.com/boundaryml/baml") {
            _ = d.Version // log and compare against native runtime version
        }
    }
    return nil
}

func asCollector(v any) (*collector, bool) {
    c, ok := v.(*collector)
    return c, ok
}

c, err := baml.NewCollector(ctx)
if err != nil {
    if strings.Contains(err.Error(), "unexpected type for collector creation") {
        return nil, fmt.Errorf("baml bindings/runtime version mismatch: %w", err)
    }
    return nil, err
}

When it happens

Trigger: Calling baml.NewCollector() when raw_objects.NewRawObject(OBJECT_TYPE_COLLECTOR) returns a non-*collector value — i.e. a version mismatch between the generated cffi bindings and the hand-written Go wrappers, or a runtime bug in NewRawObject dispatch.

Common situations: Mixing a BAML Go SDK version with a mismatched native library/binary (e.g. after a partial upgrade), custom builds of the engine, or an upstream cffi change that renamed or reordered object types.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• unexpected type for media creation: %T
• unexpected type for type builder creation: %T
• unexpected type for enum value builder: %T
• unexpected type for enum type: %T
• unexpected type for enum value builders: %T

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/bd199e7a533973d1. Report an issue: GitHub.