Repository: vllm-project/vllm
Issue category: Bug Report

[Bug]: Internal Server Error when echo'ing logprobs with sampling
### Your current environment

<details>
<summary>The output of `python collect_env.py`</summary>

```text
Your output of `python collect_env.py` here
```

</details>


### Model Input Dumps

_No response_

### 🐛 Describe the bug

Sending an request that asks for prompt logprobs while also specifying certain sampling parameters can result in an Internal Server Error.  For example:
```
curl -v http://localhost:8000/v1/completions \
  -H "Content-Type: application/json" \
  -d '{
      "model": "meta-llama/Llama-3.2-1B",
      "prompt": "correct horse battery staple",
      "echo": 1,
      "logprobs": 1,
      "temperature": 1,
      "top_k": 1
  }'
```

Results in this stack trace:
```
INFO:     ::1:47926 - "POST /v1/completions HTTP/1.1" 500 Internal Server Error
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/opt/vllm/lib64/python3.12/site-packages/uvicorn/protocols/http/httptools_impl.py", line 401, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/vllm/lib64/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 60, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/vllm/lib64/python3.12/site-packages/fastapi/applications.py", line 1054, in __call__
    await super().__call__(scope, receive, send)
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/applications.py", line 113, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/middleware/errors.py", line 187, in __call__
    raise exc
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/middleware/errors.py", line 165, in __call__
    await self.app(scope, receive, _send)
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/middleware/cors.py", line 85, in __call__
    await self.app(scope, receive, send)
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/middleware/exceptions.py", line 62, in __call__
    await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    raise exc
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
    await app(scope, receive, sender)
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/routing.py", line 715, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/routing.py", line 735, in app
    await route.handle(scope, receive, send)
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/routing.py", line 288, in handle
    await self.app(scope, receive, send)
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/routing.py", line 76, in app
    await wrap_app_handling_exceptions(app, request)(scope, receive, send)
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    raise exc
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/_exception_handler.py", line 42, in wrapped_app
    await app(scope, receive, sender)
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/routing.py", line 73, in app
    response = await f(request)
               ^^^^^^^^^^^^^^^^
  File "/opt/vllm/lib64/python3.12/site-packages/fastapi/routing.py", line 301, in app
    raw_response = await run_endpoint_function(
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/vllm/lib64/python3.12/site-packages/fastapi/routing.py", line 212, in run_endpoint_function
    return await dependant.call(**values)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/vllm/lib64/python3.12/site-packages/vllm/entrypoints/openai/api_server.py", line 337, in create_completion
    return JSONResponse(content=generator.model_dump())
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/responses.py", line 180, in __init__
    super().__init__(content, status_code, headers, media_type, background)
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/responses.py", line 43, in __init__
    self.body = self.render(content)
                ^^^^^^^^^^^^^^^^^^^^
  File "/opt/vllm/lib64/python3.12/site-packages/starlette/responses.py", line 183, in render
    return json.dumps(
           ^^^^^^^^^^^
  File "/usr/lib64/python3.12/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
          ^^^^^^^^^^^
  File "/usr/lib64/python3.12/json/encoder.py", line 200, in encode
    chunks = self.iterencode(o, _one_shot=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.12/json/encoder.py", line 258, in iterencode
    return _iterencode(o, 0)
           ^^^^^^^^^^^^^^^^^
ValueError: Out of range float values are not JSON compliant: -inf
```

From what I can see this happens because the logprobs on at least one of the prompt tokens is -inf. The `top_k: 1` sampling parameter that makes the probablity of any token other than the top-1 0.

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Functions to edit:
vllm/entrypoints/openai/serving_completion.py:OpenAIServingCompletion.request_output_to_completion_response

Patch:
diff --git a/vllm/entrypoints/openai/serving_completion.py b/vllm/entrypoints/openai/serving_completion.py
index fc1c4908d6650..03bcf26ae7e91 100644
--- a/vllm/entrypoints/openai/serving_completion.py
+++ b/vllm/entrypoints/openai/serving_completion.py
@@ -392,6 +392,12 @@ def request_output_to_completion_response(
             prompt_token_ids = final_res.prompt_token_ids
             assert prompt_token_ids is not None
             prompt_logprobs = final_res.prompt_logprobs
+            if prompt_logprobs:
+                for logprob_dict in prompt_logprobs:
+                    if logprob_dict:
+                        for logprob_values in logprob_dict.values():
+                            if logprob_values.logprob == float('-inf'):
+                                logprob_values.logprob = -9999.0
             prompt_text = final_res.prompt
 
             token_ids: GenericSequence[int]