BoundaryML/baml · error

unexpected type for method: %T

Error message

unexpected type for method: %T

What it means

Go type-assertion failure on an HTTP request proxy object: CallMethod('method') succeeded but returned a value that is not a Go string (the %T shows what came back). Like the sibling URL/headers accessors, this signals the bridge protocol returning an unexpected shape — usually a runtime/bindings version mismatch.

Source

Thrown at engine/language_client_go/pkg/rawobjects_http_request.go:63

	}

	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
}

func (h *httpRequest) 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
}

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Fix the runtime/handler so 'method' returns a string
2. Read %T to identify the actual type
3. Set an explicit method in the script/handler (e.g. 'GET')
4. Verify client/runtime version compatibility

m, err := req.Method()
if err == nil && !validMethods[m] { log.Printf("unexpected method value: %q", m) }

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

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

When it happens

Trigger: The runtime's 'method' returns a non-string — for example nil when no method was set, or an enum/number in a custom implementation.

Common situations: Custom dispatchers storing the method as a number; scripts leaving method undefined; version drift in the request schema.

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/ce242f1d0ad8d395. Report an issue: GitHub.