What causes "failed to get headers: %w" in BoundaryML/baml?

Unwrap the error to identify the core-side failure

failed to get headers

Error message

failed to get headers: %w

What it means

Thrown by httpResponse.Headers() in BAML's Go client when raw_objects.CallMethod(h, "headers") fails at the FFI boundary; the core-side error is wrapped with %w. This is a method-invocation failure on the raw object, not a map conversion problem.

Solutions

Unwrap the error to identify the core-side failure
Use only HTTPResponse objects obtained live from the current runtime
Align Go bindings and BAML core versions
Skip header processing gracefully when unavailable (log and continue)

Example fix

headers, err := resp.Headers()
if err != nil {
    log.Printf("headers unavailable: %v", err)
    headers = map[string]string{} // empty map fallback
}

Defensive patterns

Strategy: fallback

Validate before calling

if resp == nil { return errors.New("nil HTTPResponse") }

Try / catch

headers, err := resp.Headers()
if err != nil {
    log.Printf("headers lookup failed: %v", err)
    headers = map[string]string{}
}

Prevention

Use only live runtime objects for header reads
Provide an empty-map fallback in log-processing code
Match baml engine and Go module versions

When it happens

Trigger: Calling HTTPResponse.Headers() on an object whose raw pointer the core cannot resolve or that rejects the "headers" method — stale handles, runtime teardown, or bindings/core version drift.

Common situations: Header inspection in BAML event-log processors accessing objects after the runtime disposed of them; mismatched baml Go module vs installed engine versions.

Understand the failure class

Background: "API request failed": what wrapped HTTP errors from external APIs mean and how to find the real cause — this error's family across 29 libraries.

Related errors

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/53a55c8c64366b4e. Report an issue: GitHub.

Appendix: source

Thrown at engine/language_client_go/pkg/rawobjects_http_response.go:58

func (h *httpResponse) Status() (int64, error) {
	result, err := raw_objects.CallMethod(h, "status", nil)
	if err != nil {
		return 0, fmt.Errorf("failed to get status: %w", err)
	}

	status, ok := result.(int64)
	if !ok {
		return 0, fmt.Errorf("unexpected type for status: %T", result)
	}

	return status, nil
}

func (h *httpResponse) Headers() (map[string]string, error) {
	result, err := raw_objects.CallMethod(h, "headers", nil)
	if err != nil {
		return nil, fmt.Errorf("failed to get headers: %w", err)
	}

	headers, ok := result.(map[string]string)
	if !ok {
		return nil, fmt.Errorf("unexpected type for headers: %T", result)
	}

	return headers, nil
}

func (h *httpResponse) Body() (HTTPBody, error) {
	result, err := raw_objects.CallMethod(h, "body", nil)
	if err != nil {
		return nil, fmt.Errorf("failed to get body: %w", err)
	}

	body, ok := result.(HTTPBody)
	if !ok {

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