Tighten Responses bad request fallback

Keep fallback for endpoint-specific and Venice-style Responses payload rejections, but stop treating generic Bad Request validation text as proof that Responses is unsupported.

Add a regression that generic pre-output Responses 400s raise without trying Chat Completions.

helpers/litellm_transport.py

@@ -1574,10 +1574,9 @@ def _looks_like_responses_request_rejected(text: str) -> bool:
             "expected object, received string",
             "expected string, received array",
             "zod",
-            "invalid request",
-            "invalid type",
-            "failed to deserialize",
-            "validation error",
+            "failed to deserialize input",
+            "failed to deserialize response",
+            "failed to deserialize responses",
         )
     )
 

helpers/litellm_transport.py.dox.md

@@ -26,7 +26,7 @@
 - Strip Agent Zero internal kwargs before sending requests to LiteLLM.
 - Do not send orphan tool controls when no tools are present; strict OpenAI-compatible servers can reject empty `tools` arrays.
 - Prefer Responses API when configured, but fallback to Chat Completions when the provider does not support Responses.
-- Fall back to Chat Completions when a Responses request is rejected before any output by a Bad Request/validation error that indicates the provider cannot parse the Responses request shape.
+- Fall back to Chat Completions when a Responses request is rejected before any output by an endpoint-specific or shape-specific Bad Request indicating the provider cannot parse Responses payloads.
 - Preserve provider-state metadata when Responses API calls succeed, and fall back to local replay when provider state is unsupported.
 - Keep prompt-cache markers only for providers that accept them.