diff --git a/verl/experimental/fully_async_policy/vllm_rollout/vllm_async_server.py b/verl/experimental/fully_async_policy/vllm_rollout/vllm_async_server.py
index d45dbec890f..865019d0cec 100644
--- a/verl/experimental/fully_async_policy/vllm_rollout/vllm_async_server.py
+++ b/verl/experimental/fully_async_policy/vllm_rollout/vllm_async_server.py
@@ -73,8 +73,35 @@ async def _generate_step(
         image_data: Optional[list[Any]] = None,
         video_data: Optional[list[Any]] = None,
     ):
+        """Generate sequence with token-in-token-out."""
         prompt_ids = normalize_token_ids(prompt_ids)
-        max_tokens = self.config.max_model_len - len(prompt_ids)
+
+        # Calculate the maximum possible new tokens based on available context space
+        # This serves as a safety upper bound
+        max_possible_tokens = self.config.max_model_len - len(prompt_ids)
+        if max_possible_tokens < 0:
+            raise ValueError(
+                f"Prompt length ({len(prompt_ids)}) exceeds the model's maximum context length "
+                f"({self.config.max_model_len})."
+            )
+
+        # Determine max_tokens from sampling_params or use configured response_length as default
+        if "max_tokens" in sampling_params:
+            max_tokens = sampling_params.pop("max_tokens")
+        elif "max_new_tokens" in sampling_params:
+            # support sglang-style 'max_new_tokens' param
+            max_tokens = sampling_params.pop("max_new_tokens")
+        else:
+            # Normal case:          len(prompt_ids) <= prompt_length → max_tokens = response_length (no reduction)
+            # Multi-turn / partial: len(prompt_ids) >  prompt_length → previous responses are concatenated
+            #                       into prompt_ids, causing it to exceed the configured prompt_length
+            max_tokens = self.config.response_length - max(0, len(prompt_ids) - self.config.prompt_length)
+
+        # Clamp max_tokens to the valid range [0, max_possible_tokens]
+        max_tokens = max(0, min(max_tokens, max_possible_tokens))
+        assert max_tokens <= max_possible_tokens, (
+            f"max_tokens {max_tokens} exceeds available context space {max_possible_tokens}"
+        )
         sampling_params["logprobs"] = 1
         sampling_params.setdefault("repetition_penalty", self.config.get("repetition_penalty", 1.0))
         sampling_params = SamplingParams(max_tokens=max_tokens, **sampling_params)
diff --git a/verl/workers/rollout/vllm_rollout/vllm_async_server.py b/verl/workers/rollout/vllm_rollout/vllm_async_server.py
index 1e30a01beda..3411ae39be7 100644
--- a/verl/workers/rollout/vllm_rollout/vllm_async_server.py
+++ b/verl/workers/rollout/vllm_rollout/vllm_async_server.py
@@ -531,8 +531,10 @@ async def generate(
             # support sglang-style 'max_new_tokens' param
             max_tokens = sampling_params.pop("max_new_tokens")
         else:
-            # Default to a calculation that considers configured lengths
-            max_tokens = self.config.response_length + self.config.prompt_length - len(prompt_ids)
+            # Normal case:          len(prompt_ids) <= prompt_length → max_tokens = response_length (no reduction)
+            # Multi-turn / partial: len(prompt_ids) >  prompt_length → previous responses are concatenated
+            #                       into prompt_ids, causing it to exceed the configured prompt_length
+            max_tokens = self.config.response_length - max(0, len(prompt_ids) - self.config.prompt_length)
 
         # Clamp max_tokens to the valid range [0, max_possible_tokens]
         max_tokens = max(0, min(max_tokens, max_possible_tokens))