match_message ambiguity with custom error objects in RSpec request specs

RSpec's match_message matcher is frequently used to validate error output in request specs, particularly for authentication and authorization failures. When custom error objects are raised by controllers relying on Devise or CanCanCan, the matcher's string-comparison logic may produce inconsistent results depending on error-message formatting, localization, or dynamic content. This ambiguity can lead to false positives where unrelated error messages pass validation, or false negatives where valid authentication-rejection messages are rejected. The precise matching strategy exact equality substring inclusion or pattern matching is not explicitly documented for all error-object types creating uncertainty in test reliability across diverse Rails configurations.

Does match_message require an exact string match, or does it support substring and pattern matching? How does the matcher handle error objects whose #message method returns different representations across test executions? Can the matcher reliably differentiate between credential-expired and permission-denied error signals in integrated authentication flows?