BoundaryML/baml · error

unexpected type for URL: %T

Error message

unexpected type for URL: %T

What it means

httpRequest.Url() expects the runtime 'url' method to return a Go string. 'unexpected type for URL: %T' means the bridge call succeeded but the returned value is not a string (nil, float64, struct, etc.).

Source

Thrown at engine/language_client_go/pkg/rawobjects_http_request.go:49

	}

	id, ok := result.(string)
	if !ok {
		return "", fmt.Errorf("unexpected type for ID: %T", result)
	}

	return id, nil
}

func (h *httpRequest) Url() (string, error) {
	result, err := raw_objects.CallMethod(h, "url", nil)
	if err != nil {
		return "", fmt.Errorf("failed to get URL: %w", err)
	}

	url, ok := result.(string)
	if !ok {
		return "", fmt.Errorf("unexpected type for URL: %T", result)
	}

	return url, nil
}

func (h *httpRequest) Method() (string, error) {
	result, err := raw_objects.CallMethod(h, "method", nil)
	if err != nil {
		return "", fmt.Errorf("failed to get method: %w", err)
	}

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

	return method, nil
}

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Fix the runtime/handler so 'url' returns a plain string
2. Check %T in the message to learn the actual type
3. If the value is a URL object, extract its string form on the runtime side
4. Align client and runtime versions

u, err := req.Url()
if err == nil && u == "" { log.Printf("warning: empty URL returned") }

func isString(v any) bool { _, ok := v.(string); return ok }

u, err := req.Url()
if err != nil {
    if strings.Contains(err.Error(), "unexpected type") {
        return fmt.Errorf("runtime URL is not a string: %w", err)
    }
    return err
}

When it happens

Trigger: The runtime 'url' implementation returns a non-string — e.g. a URL object, nil for malformed requests, or a numeric value.

Common situations: Handlers returning URL objects instead of strings; scripts producing nil URLs for unmatched routes; cross-version schema drift.

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

• unexpected type for logs: %T
• unexpected type in logs: %T
• unexpected type for last log: %T %v
• unexpected type for log by id: %T
• unexpected type for clear result: %T

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/cec6f0bb5eeec606. Report an issue: GitHub.