BoundaryML/baml · error

failed to encode kwargs

Error message

failed to encode kwargs: %w

What it means

Wraps failures from serde.EncodeMapEntries when BamlRuntime.NewCollector encodes the 'name' kwarg (rawobjects_constructors.go:17). The collector constructor arguments could not be serialized into the CFFI form required by the native runtime.

Solutions

1. Pass a non-empty, plain-string collector name.
2. Verify Go client and native runtime versions match.
3. Check the wrapped serde error for the failing map entry.

Example fix

// before
if name == "" { name = "collector" }
collector, err := rt.NewCollector(name)
// after
if strings.TrimSpace(name) == "" {
	return nil, errors.New("collector name must be non-empty")
}
collector, err := rt.NewCollector(name)

if strings.TrimSpace(name) == "" { return errors.New("collector name must be a non-empty string") }

collector, err := rt.NewCollector(name)
if err != nil {
	return fmt.Errorf("failed to create collector: %w", err)
}

When it happens

Trigger: BamlRuntime.NewCollector(name) where serde.EncodeMapEntries fails on the kwargs map — e.g. empty/invalid name value or an encoder version mismatch.

Common situations: Passing an empty string name where the runtime requires a value; skewed serde/CFFI encoding between Go client and native runtime.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• encoding client options
• encoding client registry
• encoding collector
• encoding env vars
• encoding function arguments

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/5b219fbf67944d02. Report an issue: GitHub.

Appendix: source

Thrown at engine/language_client_go/pkg/rawobjects_constructors.go:17

package baml

import (
	"fmt"

	"github.com/boundaryml/baml/engine/language_client_go/baml_go/raw_objects"
	"github.com/boundaryml/baml/engine/language_client_go/baml_go/serde"
	"github.com/boundaryml/baml/engine/language_client_go/pkg/cffi"
)

// / 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,

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