test(together_ai): stop depending on a serverless model we do not control

Together moved openai/gpt-oss-20b off serverless, so three tests started
failing with a 400 model_not_available from the live API. None of them was
really testing Together: they cover provider-prefix parsing, prompt shaping
and streaming, all litellm side. Mock the transport and assert those, so the
tests answer to our code instead of a vendor catalog.

tests/local_testing/test_completion.py

@@ -11,6 +11,7 @@ import io
 
 from unittest.mock import AsyncMock, MagicMock, patch
 
+import httpx
 import pytest
 
 import litellm
@@ -57,21 +58,49 @@ def test_response_model_none():
     assert isinstance(x, litellm.ModelResponse)
 
 
+TOGETHER_AI_CHAT_URL = "https://api.together.ai/v1/chat/completions"
+
+
+def _together_ai_chat_response(content="Hello!"):
+    return httpx.Response(
+        200,
+        json={
+            "id": "chatcmpl-together",
+            "object": "chat.completion",
+            "created": 1,
+            "model": "openai/gpt-oss-20b",
+            "choices": [
+                {
+                    "index": 0,
+                    "message": {"role": "assistant", "content": content},
+                    "finish_reason": "stop",
+                }
+            ],
+            "usage": {
+                "prompt_tokens": 1,
+                "completion_tokens": 1,
+                "total_tokens": 2,
+            },
+        },
+        request=httpx.Request("POST", TOGETHER_AI_CHAT_URL),
+    )
+
+
 def test_completion_custom_provider_model_name():
-    try:
-        litellm.cache = None
+    litellm.cache = None
+    with patch.object(
+        HTTPHandler, "post", return_value=_together_ai_chat_response()
+    ) as mock_post:
         response = completion(
             model="together_ai/openai/gpt-oss-20b",
             messages=messages,
             logger_fn=logger_fn,
+            api_key="fake-key",
         )
-        # Add assertions here to check the-response
-        print(response)
-        print(response["choices"][0]["finish_reason"])
-    except litellm.Timeout as e:
-        pass
-    except Exception as e:
-        pytest.fail(f"Error occurred: {e}")
+
+    assert mock_post.call_args.kwargs["url"] == TOGETHER_AI_CHAT_URL
+    assert json.loads(mock_post.call_args.kwargs["data"])["model"] == "openai/gpt-oss-20b"
+    assert response.choices[0].finish_reason == "stop"
 
 
 def _openai_mock_response(*args, **kwargs) -> litellm.ModelResponse:
@@ -2804,12 +2833,11 @@ def test_completion_together_ai_llama():
 
 # test_completion_together_ai()
 def test_customprompt_together_ai():
-    try:
-        litellm.set_verbose = False
-        litellm.num_retries = 0
-        print("in test_customprompt_together_ai")
-        print(litellm.success_callback)
-        print(litellm._async_success_callback)
+    litellm.set_verbose = False
+    litellm.num_retries = 0
+    with patch.object(
+        HTTPHandler, "post", return_value=_together_ai_chat_response()
+    ) as mock_post:
         response = completion(
             model="together_ai/openai/gpt-oss-20b",
             messages=messages,
@@ -2827,14 +2855,14 @@ def test_customprompt_together_ai():
                     "post_message": "<|im_end|>",
                 },
             },
+            api_key="fake-key",
         )
-        print(response)
-    except litellm.exceptions.Timeout as e:
-        print(f"Timeout Error")
-        pass
-    except Exception as e:
-        print(f"ERROR TYPE {type(e)}")
-        pytest.fail(f"Error occurred: {e}")
+
+    body = json.loads(mock_post.call_args.kwargs["data"])
+    assert body["messages"] == messages
+    assert "prompt" not in body
+    assert "roles" not in body
+    assert response.choices[0].finish_reason == "stop"
 
 
 # test_customprompt_together_ai()
@@ -3648,19 +3676,42 @@ def test_completion_together_ai_stream():
     litellm.set_verbose = True
     user_message = "Write 1pg about YC & litellm"
     messages = [{"content": user_message, "role": "user"}]
-    try:
+    sse_body = (
+        'data: {"id":"chatcmpl-together","object":"chat.completion.chunk","created":1,'
+        '"model":"openai/gpt-oss-20b","choices":[{"index":0,"delta":{"role":"assistant",'
+        '"content":"YC"},"finish_reason":null}]}\n\n'
+        'data: {"id":"chatcmpl-together","object":"chat.completion.chunk","created":1,'
+        '"model":"openai/gpt-oss-20b","choices":[{"index":0,"delta":{"content":" and '
+        'litellm"},"finish_reason":null}]}\n\n'
+        'data: {"id":"chatcmpl-together","object":"chat.completion.chunk","created":1,'
+        '"model":"openai/gpt-oss-20b","choices":[{"index":0,"delta":{},'
+        '"finish_reason":"stop"}]}\n\n'
+        "data: [DONE]\n\n"
+    )
+    stream_response = httpx.Response(
+        200,
+        content=sse_body.encode(),
+        headers={"content-type": "text/event-stream"},
+        request=httpx.Request("POST", TOGETHER_AI_CHAT_URL),
+    )
+
+    with patch.object(
+        HTTPHandler, "post", return_value=stream_response
+    ) as mock_post:
         response = completion(
             model="together_ai/openai/gpt-oss-20b",
             messages=messages,
             stream=True,
             max_tokens=5,
+            api_key="fake-key",
         )
-        print(response)
-        for chunk in response:
-            print(chunk)
-        # print(string_response)
-    except Exception as e:
-        pytest.fail(f"Error occurred: {e}")
+        chunks = list(response)
+
+    assert json.loads(mock_post.call_args.kwargs["data"])["stream"] is True
+    assert "".join(
+        chunk.choices[0].delta.content or "" for chunk in chunks
+    ) == "YC and litellm"
+    assert chunks[-1].choices[0].finish_reason == "stop"
 
 
 # test_completion_together_ai_stream()