diff --git a/litellm/responses/litellm_completion_transformation/transformation.py b/litellm/responses/litellm_completion_transformation/transformation.py
index debd9f77b7e..e699811b33b 100644
--- a/litellm/responses/litellm_completion_transformation/transformation.py
+++ b/litellm/responses/litellm_completion_transformation/transformation.py
@@ -186,8 +186,14 @@ class LiteLLMCompletionResponsesConfig:
         if reasoning_param:
             if isinstance(reasoning_param, dict):
                 # reasoning can be {"effort": "low|medium|high", "summary": "detailed"}
-                # Preserve the full dict structure for reasoning_effort
-                reasoning_effort = reasoning_param
+                # Keep the full dict when summary is set so the responses API bridge can
+                # forward it; otherwise use the effort string for chat completion (e.g. Gemini).
+                if "summary" in reasoning_param:
+                    reasoning_effort = reasoning_param
+                elif "effort" in reasoning_param:
+                    reasoning_effort = reasoning_param.get("effort")
+                else:
+                    reasoning_effort = reasoning_param
             elif isinstance(reasoning_param, str):
                 # reasoning could be a string directly
                 reasoning_effort = reasoning_param