fix(cohere): map string stop parameter to list of strings for stop_sequences

Fixes #46977: Cohere chat API reference expects stop_sequences to be a list of strings. Convert string stop parameter to a single-element list for CohereChatConfig and CohereV2ChatConfig while preserving lists.

litellm/llms/cohere/chat/transformation.py

@@ -205,7 +205,7 @@ class CohereChatConfig(BaseConfig):
             if param == "presence_penalty":
                 optional_params["presence_penalty"] = value
             if param == "stop":
-                optional_params["stop_sequences"] = value
+                optional_params["stop_sequences"] = [value] if isinstance(value, str) else value
             if param == "tools":
                 optional_params["tools"] = value
             if param == "seed":

litellm/llms/cohere/chat/v2_transformation.py

@@ -159,7 +159,7 @@ class CohereV2ChatConfig(OpenAIGPTConfig):
             if param == "presence_penalty":
                 optional_params["presence_penalty"] = value
             if param == "stop":
-                optional_params["stop_sequences"] = value
+                optional_params["stop_sequences"] = [value] if isinstance(value, str) else value
             if param == "tools":
                 optional_params["tools"] = value
             if param == "seed":

tests/unit/llms/cohere/chat/test_cohere_transformation.py

@@ -77,6 +77,26 @@ class TestCohereTransform:
         # The function should properly map max_tokens if max_completion_tokens is not provided
         assert result == {"temperature": 0.7, "max_tokens": 200}
 
+    def test_cohere_stop_string_to_list(self):
+        """Test that a string stop parameter is converted to a list of strings for stop_sequences"""
+        result = self.config.map_openai_params(
+            non_default_params={"stop": "END"},
+            optional_params={},
+            model=self.model,
+            drop_params=False,
+        )
+        assert result == {"stop_sequences": ["END"]}
+
+    def test_cohere_stop_list_preserved(self):
+        """Test that a list stop parameter is preserved as a list for stop_sequences"""
+        result = self.config.map_openai_params(
+            non_default_params={"stop": ["END", "STOP"]},
+            optional_params={},
+            model=self.model,
+            drop_params=False,
+        )
+        assert result == {"stop_sequences": ["END", "STOP"]}
+
 
 class TestCohereV2Transform:
     def setup_method(self):
@@ -141,6 +161,26 @@ class TestCohereV2Transform:
 
         assert optional_params["max_tokens"] == 256
 
+    def test_v2_stop_string_to_list(self):
+        """Test that a string stop parameter is converted to a list of strings for stop_sequences in v2"""
+        result = self.config.map_openai_params(
+            non_default_params={"stop": "END"},
+            optional_params={},
+            model=self.model,
+            drop_params=False,
+        )
+        assert result == {"stop_sequences": ["END"]}
+
+    def test_v2_stop_list_preserved(self):
+        """Test that a list stop parameter is preserved as a list for stop_sequences in v2"""
+        result = self.config.map_openai_params(
+            non_default_params={"stop": ["END", "STOP"]},
+            optional_params={},
+            model=self.model,
+            drop_params=False,
+        )
+        assert result == {"stop_sequences": ["END", "STOP"]}
+
 
 @pytest.mark.asyncio
 async def test_cohere_request_body_with_allowed_params():