[Frontend] Support reasoning content for deepseek r1

docs/source/features/reasoning_outputs.md

@@ -0,0 +1,151 @@
+(reasoning-outputs)=
+
+# Reasoning Outputs
+
+vLLM offers support for reasoning models like [DeepSeek R1](https://huggingface.co/deepseek-ai/DeepSeek-R1), which are designed to generate outputs containing both reasoning steps and final conclusions.
+
+Reasoning models return a additional `reasoning_content` field in their outputs, which contains the reasoning steps that led to the final conclusion. This field is not present in the outputs of other models.
+
+## Supported Models
+
+vLLM currently supports the following reasoning models:
+
+- [DeepSeek R1 series](https://huggingface.co/collections/deepseek-ai/deepseek-r1-678e1e131c0169c0bc89728d) (`deepseek_r1`, which looks for `<think> ... </think>`)
+
+## Quickstart
+
+To use reasoning models, you need to specify the `--enable-reasoning` and `--reasoning-parser` flags when making a request to the chat completion endpoint. The `--reasoning-parser` flag specifies the reasoning parser to use for extracting reasoning content from the model output.
+
+```bash
+vllm serve deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B \
+    --enable-reasoning --reasoning-parser deepseek_r1
+```
+
+Next, make a request to the model that should return the reasoning content in the response.
+
+```python
+from openai import OpenAI
+
+# Modify OpenAI's API key and API base to use vLLM's API server.
+openai_api_key = "EMPTY"
+openai_api_base = "http://localhost:8000/v1"
+
+client = OpenAI(
+    api_key=openai_api_key,
+    base_url=openai_api_base,
+)
+
+models = client.models.list()
+model = models.data[0].id
+
+# Round 1
+messages = [{"role": "user", "content": "9.11 and 9.8, which is greater?"}]
+response = client.chat.completions.create(model=model, messages=messages)
+
+reasoning_content = response.choices[0].message.reasoning_content
+content = response.choices[0].message.content
+
+print("reasoning_content:", reasoning_content)
+print("content:", content)
+```
+
+The `reasoning_content` field contains the reasoning steps that led to the final conclusion, while the `content` field contains the final conclusion.
+
+## Streaming chat completions
+
+Streaming chat completions are also supported for reasoning models. The `reasoning_content` field is available in the `delta` field in [chat completion response chunks](https://platform.openai.com/docs/api-reference/chat/streaming).
+
+```json
+{
+    "id": "chatcmpl-123",
+    "object": "chat.completion.chunk",
+    "created": 1694268190,
+    "model": "deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B",
+    "system_fingerprint": "fp_44709d6fcb",
+    "choices": [
+        {
+            "index": 0,
+            "delta": {
+                "role": "assistant",
+                "reasoning_content": "is",
+            },
+            "logprobs": null,
+            "finish_reason": null
+        }
+    ]
+}
+```
+
+Please note that it is not compatible with the OpenAI Python client library. You can use the `requests` library to make streaming requests.
+
+## How to support a new reasoning model
+
+You can add a new `ReasoningParser` similar to `vllm/entrypoints/openai/reasoning_parsers/deepseek_r1_reasoning_parser.py`.
+
+```python
+# import the required packages
+
+from vllm.entrypoints.openai.reasoning_parsers.abs_reasoning_parsers import (
+    ReasoningParser, ReasoningParserManager)
+from vllm.entrypoints.openai.protocol import (ChatCompletionRequest,
+                                              DeltaMessage)
+
+# define a reasoning parser and register it to vllm
+# the name list in register_module can be used
+# in --reasoning-parser.
+@ReasoningParserManager.register_module(["example"])
+class ExampleParser(ReasoningParser):
+    def __init__(self, tokenizer: AnyTokenizer):
+        super().__init__(tokenizer)
+
+    def extract_reasoning_content_streaming(
+        self,
+        previous_text: str,
+        current_text: str,
+        delta_text: str,
+        previous_token_ids: Sequence[int],
+        current_token_ids: Sequence[int],
+        delta_token_ids: Sequence[int],
+    ) -> Union[DeltaMessage, None]:
+        """
+        Instance method that should be implemented for extracting reasoning
+        from an incomplete response; for use when handling reasoning calls and
+        streaming. Has to be an instance method because  it requires state -
+        the current tokens/diffs, but also the information about what has
+        previously been parsed and extracted (see constructor)
+        """
+
+    def extract_reasoning_content(
+            self, model_output: str, request: ChatCompletionRequest
+    ) -> Tuple[Optional[str], Optional[str]]:
+        """
+        Extract reasoning content from a complete model-generated string.
+
+        Used for non-streaming responses where we have the entire model response
+        available before sending to the client.
+
+        Parameters:
+        model_output: str
+            The model-generated string to extract reasoning content from.
+
+        request: ChatCompletionRequest
+            The request object that was used to generate the model_output.
+
+        Returns:
+        Tuple[Optional[str], Optional[str]]
+            A tuple containing the reasoning content and the content.
+        """
+```
+
+After defining the reasoning parser, you can use it by specifying the `--reasoning-parser` flag when making a request to the chat completion endpoint.
+
+```bash
+vllm serve <model_tag> \
+    --enable-reasoning --reasoning-parser example
+```
+
+## Limitations
+
+- The reasoning content is only available for online serving's chat completion endpoint (`/v1/chat/completions`).
+- It is not compatible with the [`structured_outputs`](#structured_outputs) and [`tool_calling`](#tool_calling) features.
+- The reasoning content is not available for all models. Check the model's documentation to see if it supports reasoning.

docs/source/index.md

@@ -90,6 +90,7 @@ models/extensions/index
 features/quantization/index
 features/lora
 features/tool_calling
+features/reasoning_outputs
 features/structured_outputs
 features/automatic_prefix_caching
 features/disagg_prefill

examples/online_serving/openai_chat_completion_with_reasoning.py

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+"""
+An example shows how to generate chat completions from reasoning models
+like DeepSeekR1.
+
+To run this example, you need to start the vLLM server with the reasoning 
+parser:
+
+```bash
+vllm serve deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B \
+     --enable-reasoning --reasoning-parser deepseek_r1
+```
+
+This example demonstrates how to generate chat completions from reasoning models
+using the OpenAI Python client library.
+"""
+
+from openai import OpenAI
+
+# Modify OpenAI's API key and API base to use vLLM's API server.
+openai_api_key = "EMPTY"
+openai_api_base = "http://localhost:8000/v1"
+
+client = OpenAI(
+    api_key=openai_api_key,
+    base_url=openai_api_base,
+)
+
+models = client.models.list()
+model = models.data[0].id
+
+# Round 1
+messages = [{"role": "user", "content": "9.11 and 9.8, which is greater?"}]
+response = client.chat.completions.create(model=model, messages=messages)
+
+reasoning_content = response.choices[0].message.reasoning_content
+content = response.choices[0].message.content
+
+print("reasoning_content:", reasoning_content)
+print("content:", content)
+
+# Round 2
+messages.append({"role": "assistant", "content": content})
+messages.append({
+    "role": "user",
+    "content": "How many Rs are there in the word 'strawberry'?",
+})
+response = client.chat.completions.create(model=model, messages=messages)
+
+reasoning_content = response.choices[0].message.reasoning_content
+content = response.choices[0].message.content
+
+print("reasoning_content:", reasoning_content)
+print("content:", content)

examples/online_serving/openai_chat_completion_with_reasoning_streaming.py

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+"""
+An example shows how to generate chat completions from reasoning models
+like DeepSeekR1.
+
+To run this example, you need to start the vLLM server with the reasoning 
+parser:
+
+```bash
+vllm serve deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B \
+     --enable-reasoning --reasoning-parser deepseek_r1
+```
+
+Unlike openai_chat_completion_with_reasoning.py, this example demonstrates the
+streaming chat completions feature.
+
+The streaming chat completions feature allows you to receive chat completions
+in real-time as they are generated by the model. This is useful for scenarios
+where you want to display chat completions to the user as they are generated
+by the model.
+
+Here we do not use the OpenAI Python client library, because it does not support
+`reasoning_content` fields in the response.
+"""
+
+import json
+
+import requests
+
+# Modify OpenAI's API key and API base to use vLLM's API server.
+openai_api_key = "EMPTY"
+openai_api_base = "http://localhost:8000/v1"
+
+models = requests.get(
+    f"{openai_api_base}/models",
+    headers={
+        "Authorization": f"Bearer {openai_api_key}"
+    },
+).json()
+model = models["data"][0]["id"]
+
+# Streaming chat completions
+messages = [{"role": "user", "content": "9.11 and 9.8, which is greater?"}]
+
+response = requests.post(
+    f"{openai_api_base}/chat/completions",
+    headers={"Authorization": f"Bearer {openai_api_key}"},
+    json={
+        "model": model,
+        "messages": messages,
+        "stream": True
+    },
+)
+
+print("client: Start streaming chat completions...")
+printed_reasoning_content = False
+printed_content = False
+# Make the streaming request
+if response.status_code == 200:
+    # Process the streaming response
+    for line in response.iter_lines():
+        if line:  # Filter out keep-alive new lines
+            # Decode the line and parse the JSON
+            decoded_line = line.decode("utf-8")
+            if decoded_line.startswith("data:"):
+                data = decoded_line[5:].strip()  # Remove "data:" prefix
+                if data == "[DONE]":  # End of stream
+                    print("\nclient: Stream completed.")
+                    break
+                try:
+                    # Parse the JSON data
+                    chunk = json.loads(data)
+                    reasoning_content = chunk["choices"][0]["delta"].get(
+                        "reasoning_content", "")
+                    content = chunk["choices"][0]["delta"].get("content", "")
+
+                    if reasoning_content:
+                        if not printed_reasoning_content:
+                            printed_reasoning_content = True
+                            print("reasoning_content:", end="", flush=True)
+                        print(reasoning_content, end="", flush=True)
+                    elif content:
+                        if not printed_content:
+                            printed_content = True
+                            print("\ncontent:", end="", flush=True)
+                        # Extract and print the content
+                        print(content, end="", flush=True)
+                except json.JSONDecodeError:
+                    print("Error decoding JSON:", decoded_line)
+else:
+    print(f"Error: {response.status_code} - {response.text}")