diff --git a/lib/crewai/src/crewai/llm.py b/lib/crewai/src/crewai/llm.py
index 77053deeb..c0c0d0743 100644
--- a/lib/crewai/src/crewai/llm.py
+++ b/lib/crewai/src/crewai/llm.py
@@ -676,14 +676,13 @@ class LLM(BaseLLM):
         formatted_messages = self._format_messages_for_provider(messages)
 
         # --- 2) Prepare the parameters for the completion call
-        params = {
+        params: dict[str, Any] = {
             "model": self.model,
             "messages": formatted_messages,
             "timeout": self.timeout,
             "temperature": self.temperature,
             "top_p": self.top_p,
             "n": self.n,
-            "stop": self.stop,
             "max_tokens": self.max_tokens or self.max_completion_tokens,
             "presence_penalty": self.presence_penalty,
             "frequency_penalty": self.frequency_penalty,
@@ -702,6 +701,12 @@ class LLM(BaseLLM):
             **self.additional_params,
         }
 
+        # Only include stop if it has values and is not in additional_drop_params
+        # Some models (e.g., gpt-5.1) don't support the stop parameter at all
+        drop_params = self.additional_params.get("additional_drop_params", [])
+        if self.stop and "stop" not in drop_params:
+            params["stop"] = self.stop
+
         # Remove None values from params
         return {k: v for k, v in params.items() if v is not None}
 
diff --git a/lib/crewai/tests/test_llm.py b/lib/crewai/tests/test_llm.py
index 6f3bcd70a..be19bb715 100644
--- a/lib/crewai/tests/test_llm.py
+++ b/lib/crewai/tests/test_llm.py
@@ -877,3 +877,62 @@ def test_validate_model_in_constants():
         LLM._validate_model_in_constants("anthropic.claude-future-v1:0", "bedrock")
         is True
     )
+
+
+def test_prepare_completion_params_excludes_empty_stop():
+    """Test that _prepare_completion_params excludes stop when it's empty.
+
+    This is a regression test for issue #4149 where models like gpt-5.1
+    don't support the stop parameter at all, and passing an empty list
+    would cause an error.
+    """
+    llm = LLM(model="gpt-4o", is_litellm=True)
+    # By default, stop is initialized to an empty list
+    assert llm.stop == []
+
+    params = llm._prepare_completion_params("Hello")
+    # stop should not be in params when it's empty
+    assert "stop" not in params
+
+
+def test_prepare_completion_params_includes_stop_when_provided():
+    """Test that _prepare_completion_params includes stop when it has values."""
+    llm = LLM(model="gpt-4o", stop=["Observation:"], is_litellm=True)
+    assert llm.stop == ["Observation:"]
+
+    params = llm._prepare_completion_params("Hello")
+    # stop should be in params when it has values
+    assert "stop" in params
+    assert params["stop"] == ["Observation:"]
+
+
+def test_prepare_completion_params_excludes_stop_when_in_drop_params():
+    """Test that _prepare_completion_params excludes stop when it's in additional_drop_params.
+
+    This ensures the retry logic works correctly when a model doesn't support stop.
+    """
+    llm = LLM(
+        model="gpt-4o",
+        stop=["Observation:"],
+        additional_drop_params=["stop"],
+        is_litellm=True,
+    )
+    assert llm.stop == ["Observation:"]
+
+    params = llm._prepare_completion_params("Hello")
+    # stop should not be in params when it's in additional_drop_params
+    assert "stop" not in params
+
+
+def test_prepare_completion_params_excludes_stop_with_existing_drop_params():
+    """Test that stop is excluded when additional_drop_params already has other params."""
+    llm = LLM(
+        model="gpt-4o",
+        stop=["Observation:"],
+        additional_drop_params=["another_param", "stop"],
+        is_litellm=True,
+    )
+
+    params = llm._prepare_completion_params("Hello")
+    # stop should not be in params
+    assert "stop" not in params