[Bug][Frontend] Fix structure of transcription's decoder_prompt (#18809)

Signed-off-by: sangbumlikeagod <oironese@naver.com>

tests/entrypoints/openai/test_transcription_validation.py

@@ -37,7 +37,6 @@ async def test_basic_audio(mary_had_lamb):
     model_name = "openai/whisper-large-v3-turbo"
     server_args = ["--enforce-eager"]
     # Based on https://github.com/openai/openai-cookbook/blob/main/examples/Whisper_prompting_guide.ipynb.
-    prompt = "THE FIRST WORDS I SPOKE"
     with RemoteOpenAIServer(model_name, server_args) as remote_server:
         client = remote_server.get_async_client()
         transcription = await client.audio.transcriptions.create(
@@ -48,16 +47,6 @@ async def test_basic_audio(mary_had_lamb):
             temperature=0.0)
         out = json.loads(transcription)['text']
         assert "Mary had a little lamb," in out
-        # This should "force" whisper to continue prompt in all caps
-        transcription_wprompt = await client.audio.transcriptions.create(
-            model=model_name,
-            file=mary_had_lamb,
-            language="en",
-            response_format="text",
-            prompt=prompt,
-            temperature=0.0)
-        out_capital = json.loads(transcription_wprompt)['text']
-        assert prompt not in out_capital
 
 
 @pytest.mark.asyncio
@@ -238,3 +227,31 @@ async def test_sampling_params(mary_had_lamb):
             extra_body=dict(seed=42))
 
         assert greedy_transcription.text != transcription.text
+
+
+@pytest.mark.asyncio
+async def test_audio_prompt(mary_had_lamb):
+    model_name = "openai/whisper-large-v3-turbo"
+    server_args = ["--enforce-eager"]
+    prompt = "This is a speech, recorded in a phonograph."
+    with RemoteOpenAIServer(model_name, server_args) as remote_server:
+        #Prompts should not omit the part of original prompt while transcribing.
+        prefix = "The first words I spoke in the original phonograph"
+        client = remote_server.get_async_client()
+        transcription = await client.audio.transcriptions.create(
+            model=model_name,
+            file=mary_had_lamb,
+            language="en",
+            response_format="text",
+            temperature=0.0)
+        out = json.loads(transcription)['text']
+        assert prefix in out
+        transcription_wprompt = await client.audio.transcriptions.create(
+            model=model_name,
+            file=mary_had_lamb,
+            language="en",
+            response_format="text",
+            prompt=prompt,
+            temperature=0.0)
+        out_prompt = json.loads(transcription_wprompt)['text']
+        assert prefix in out_prompt

vllm/model_executor/models/whisper.py

@@ -780,8 +780,9 @@ class WhisperForConditionalGeneration(nn.Module, SupportsTranscription,
     @classmethod
     def get_decoder_prompt(cls, language: str, task_type: str,
                            prompt: str) -> str:
-        return (f"<|startoftranscript|><|{language}|><|{task_type}|>"
-                f"<|notimestamps|>{prompt}")
+        return ((f"<|prev|>{prompt}" if prompt else "") +
+                f"<|startoftranscript|><|{language}|>" +
+                f"<|{task_type}|><|notimestamps|>")
 
     @classmethod
     def get_placeholder_str(cls, modality: str, i: int) -> Optional[str]: