ADR-080: Typed provider HTTP exceptions (authentication 401, rate-limit 429) 

Context 

A provider's 4xx responses were all flattened into one class. The standard adapter path (AbstractProvider::handleResponse() and assertStreamingResponseOk()) mapped every 4xx — including 401 (authentication) and 429 (rate limit) — to a generic ProviderResponseException carrying the numeric httpStatus. A consumer that wanted to react differently to "the API key is wrong" versus "we are being throttled" (a controller turning either into user-facing copy, a retry policy) had to inspect getCode()/$httpStatus or, worse, re-parse the message string — the exact fragile string-matching ADR-053's marker interface was meant to end.

The mapping was also inconsistent across adapters: OpenRouterProvider::handleOpenRouterError() routed 401 to ProviderConfigurationException and 429 to ProviderConnectionException, so the same HTTP status produced a different class on OpenRouter than on the other six providers.

Decision 

Introduce two status-specific subclasses of `ProviderResponseException`: ProviderAuthenticationException (401) and ProviderRateLimitException (429). They are empty final subclasses — they inherit the constructor and the typed httpStatus / responseBody / endpoint fields unchanged. ProviderResponseException drops its own final so it can be extended; it stays otherwise identical.

A private AbstractProvider::clientErrorException() factory picks the class from the status (401 → authentication, 429 → rate-limit, every other 4xx → the base ProviderResponseException) and is used by both the buffered and the streaming 4xx branches, so the two paths never drift. OpenRouterProvider is realigned to the same 401/429 classes (402 stays ProviderConfigurationException, 503 stays ProviderConnectionException).

Backward compatibility is preserved by inheritance:

• catch (ProviderResponseException) still catches a 401 or 429 — the new classes are ProviderResponseException.
• catch (ProviderException) and catch (\Netresearch\NrLlm\Exception\NrLlmExceptionInterface) are unaffected.
• getCode() still returns the HTTP status, so the retry/fallback (FallbackMiddleware) and circuit-breaker (CircuitBreakerMiddleware) checks that key off getCode() === 429 keep firing for the rate-limit class.

Consequences 

• Consumers branch on the exception class (catch (ProviderRateLimitException)) instead of the status code or the message text.
• Behaviour change on OpenRouter only: a 401 is now ProviderAuthenticationException (was ProviderConfigurationException) and a 429 is now ProviderRateLimitException (was ProviderConnectionException). Both remain ProviderResponseException/ProviderException, and 429 keeps getCode() === 429, so retry/circuit-breaker semantics are unchanged; only a consumer catching those two specific OpenRouter classes sees the more correct type. The realignment is a separate commit in the introducing PR.
• ProviderResponseException is no longer final; the two shipped subclasses are the only intended extensions.
• The exceptions carry no Retry-After value — none is parsed today; adding it is a separate change.