[Bugfix] Fix LLM priority normalization for single-string prompts (#40011)

Signed-off-by: daiyu1111 <2356690121@qq.com>

tests/entrypoints/llm/test_generate.py

@@ -91,6 +91,12 @@ def test_multiple_priority(llm: LLM):
         outputs = llm.generate(PROMPTS, sampling_params=None, priority=[])
 
 
+def test_single_prompt_priority(llm: LLM):
+    # Single string prompts should be normalized to one request.
+    outputs = llm.generate(PROMPTS[0], sampling_params=None, priority=[0])
+    assert len(outputs) == 1
+
+
 def test_max_model_len():
     max_model_len = 20
     llm = LLM(

vllm/entrypoints/llm.py

@@ -1575,7 +1575,7 @@ class LLM:
         seq_prompts = prompt_to_seq(prompts)
         seq_params = self._params_to_seq(params, len(seq_prompts))
         seq_lora_requests = self._lora_request_to_seq(lora_request, len(seq_prompts))
-        seq_priority = self._priority_to_seq(priority, len(prompts))
+        seq_priority = self._priority_to_seq(priority, len(seq_prompts))
 
         return self._render_and_add_requests(
             prompts=(