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

Unwrap the error to see the underlying FFI cause

failed to get body

Error message

failed to get body: %w

What it means

Thrown by httpResponse.Body() in BAML's Go client when raw_objects.CallMethod(h, "body") fails at the FFI boundary; the core-side error is wrapped with %w. It means the "body" method could not be invoked on the raw response object, distinct from a wrong body type.

Solutions

Unwrap the error to see the underlying FFI cause
Re-acquire the HTTPResponse from the active runtime instead of caching handles
Align Go bindings and BAML core versions
Handle body absence gracefully (return a typed error to callers instead of panicking)

Example fix

body, err := resp.Body()
if err != nil {
    return fmt.Errorf("cannot read response body: %w", err)
}

Defensive patterns

Strategy: try-catch

Validate before calling

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

Try / catch

body, err := resp.Body()
if err != nil {
    return fmt.Errorf("response body unavailable: %w", err)
}

Prevention

Re-acquire response objects from the active runtime
Handle body absence explicitly in log processors
Keep engine and Go bindings aligned

When it happens

Trigger: Calling HTTPResponse.Body() on a raw object handle the core cannot resolve or that rejects the "body" method — stale/expired pointer, object created by a different runtime, or bindings/core version drift.

Common situations: Body extraction in BAML log/event pipelines after runtime teardown; mismatched versions after upgrading the engine.

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/f884a11971b95d6f. Report an issue: GitHub.

Appendix: source

Thrown at engine/language_client_go/pkg/rawobjects_http_response.go:72

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 {
		return nil, fmt.Errorf("unexpected type for body: %T", result)
	}

	return body, nil
}

View on GitHub (pinned to bd85ce9dee)