diff --git a/docs/source/features/reasoning_outputs.md b/docs/source/features/reasoning_outputs.md
index 323bf849a082..a079eb8b77e7 100644
--- a/docs/source/features/reasoning_outputs.md
+++ b/docs/source/features/reasoning_outputs.md
@@ -21,11 +21,10 @@ vLLM currently supports the following reasoning models:
 
 ## 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.
+To use reasoning models, you need to specify the `--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
+vllm serve deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B --reasoning-parser deepseek_r1
 ```
 
 Next, make a request to the model that should return the reasoning content in the response.
@@ -140,8 +139,7 @@ Remember to check whether the `reasoning_content` exists in the response before
 The reasoning content is also available in the structured output. The structured output engine like `xgrammar` will use the reasoning content to generate structured output. It is only supported in v0 engine now.
 
 ```bash
-VLLM_USE_V1=0 vllm serve deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B \
-    --enable-reasoning --reasoning-parser deepseek_r1
+VLLM_USE_V1=0 vllm serve deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B --reasoning-parser deepseek_r1
 ```
 
 Please note that the `VLLM_USE_V1` environment variable must be set to `0` to use the v0 engine.
@@ -316,9 +314,8 @@ class DeepSeekReasoner(Reasoner):
 
 The structured output engine like `xgrammar` will use `end_token_id` to check if the reasoning content is present in the model output and skip the structured output if it is the case.
 
-Finally, you can enable reasoning for the model by using the `--enable-reasoning` and `--reasoning-parser` flags.
+Finally, you can enable reasoning for the model by using the `--reasoning-parser` flags.
 
 ```bash
-vllm serve <model_tag> \
-    --enable-reasoning --reasoning-parser example
+vllm serve <model_tag> --reasoning-parser example
 ```
diff --git a/examples/online_serving/openai_chat_completion_structured_outputs_with_reasoning.py b/examples/online_serving/openai_chat_completion_structured_outputs_with_reasoning.py
index cb7f30d93255..5da9236c5306 100644
--- a/examples/online_serving/openai_chat_completion_structured_outputs_with_reasoning.py
+++ b/examples/online_serving/openai_chat_completion_structured_outputs_with_reasoning.py
@@ -9,7 +9,7 @@
 
 ```bash
 vllm serve deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B \
-     --enable-reasoning --reasoning-parser deepseek_r1
+    --reasoning-parser deepseek_r1
 ```
 
 This example demonstrates how to generate chat completions from reasoning models
diff --git a/examples/online_serving/openai_chat_completion_tool_calls_with_reasoning.py b/examples/online_serving/openai_chat_completion_tool_calls_with_reasoning.py
index 8c6470aa3dd4..9417abd3989a 100644
--- a/examples/online_serving/openai_chat_completion_tool_calls_with_reasoning.py
+++ b/examples/online_serving/openai_chat_completion_tool_calls_with_reasoning.py
@@ -9,7 +9,7 @@
 
 ```bash
 vllm serve Qwen/QwQ-32B \
-     --enable-reasoning --reasoning-parser deepseek_r1 \
+     --reasoning-parser deepseek_r1 \
      --enable-auto-tool-choice --tool-call-parser hermes
      
 ```
diff --git a/examples/online_serving/openai_chat_completion_with_reasoning.py b/examples/online_serving/openai_chat_completion_with_reasoning.py
index 6f5f7b5fa20b..4bf7731cb41e 100644
--- a/examples/online_serving/openai_chat_completion_with_reasoning.py
+++ b/examples/online_serving/openai_chat_completion_with_reasoning.py
@@ -8,7 +8,7 @@
 
 ```bash
 vllm serve deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B \
-     --enable-reasoning --reasoning-parser deepseek_r1
+    --reasoning-parser deepseek_r1
 ```
 
 This example demonstrates how to generate chat completions from reasoning models
diff --git a/examples/online_serving/openai_chat_completion_with_reasoning_streaming.py b/examples/online_serving/openai_chat_completion_with_reasoning_streaming.py
index 90481cdc0fb7..9cc0a5f2476b 100644
--- a/examples/online_serving/openai_chat_completion_with_reasoning_streaming.py
+++ b/examples/online_serving/openai_chat_completion_with_reasoning_streaming.py
@@ -8,7 +8,7 @@
 
 ```bash
 vllm serve deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B \
-     --enable-reasoning --reasoning-parser deepseek_r1
+     --reasoning-parser deepseek_r1
 ```
 
 Unlike openai_chat_completion_with_reasoning.py, this example demonstrates the
diff --git a/tests/entrypoints/openai/test_chat_with_tool_reasoning.py b/tests/entrypoints/openai/test_chat_with_tool_reasoning.py
index 53df1d9241b7..e00f001ef730 100644
--- a/tests/entrypoints/openai/test_chat_with_tool_reasoning.py
+++ b/tests/entrypoints/openai/test_chat_with_tool_reasoning.py
@@ -13,9 +13,9 @@
 @pytest.fixture(scope="module")
 def server():  # noqa: F811
     args = [
-        "--max-model-len", "8192", "--enforce-eager", "--enable-reasoning",
-        "--reasoning-parser", "deepseek_r1", "--enable-auto-tool-choice",
-        "--tool-call-parser", "hermes"
+        "--max-model-len", "8192", "--enforce-eager", "--reasoning-parser",
+        "deepseek_r1", "--enable-auto-tool-choice", "--tool-call-parser",
+        "hermes"
     ]
 
     with RemoteOpenAIServer(MODEL_NAME, args) as remote_server:
diff --git a/tests/entrypoints/openai/test_cli_args.py b/tests/entrypoints/openai/test_cli_args.py
index e0285b5e5566..8d1abe28a027 100644
--- a/tests/entrypoints/openai/test_cli_args.py
+++ b/tests/entrypoints/openai/test_cli_args.py
@@ -122,31 +122,23 @@ def test_enable_auto_choice_fails_with_enable_reasoning(serve_parser):
     """Ensure validation fails if reasoning is enabled with auto tool choice"""
     args = serve_parser.parse_args(args=[
         "--enable-auto-tool-choice",
-        "--enable-reasoning",
+        "--reasoning-parser",
+        "deepseek_r1",
     ])
     with pytest.raises(TypeError):
         validate_parsed_serve_args(args)
 
 
-def test_enable_reasoning_passes_with_reasoning_parser(serve_parser):
+def test_passes_with_reasoning_parser(serve_parser):
     """Ensure validation passes if reasoning is enabled 
     with a reasoning parser"""
     args = serve_parser.parse_args(args=[
-        "--enable-reasoning",
         "--reasoning-parser",
         "deepseek_r1",
     ])
     validate_parsed_serve_args(args)
 
 
-def test_enable_reasoning_fails_without_reasoning_parser(serve_parser):
-    """Ensure validation fails if reasoning is enabled 
-    without a reasoning parser"""
-    args = serve_parser.parse_args(args=["--enable-reasoning"])
-    with pytest.raises(TypeError):
-        validate_parsed_serve_args(args)
-
-
 def test_chat_template_validation_for_happy_paths(serve_parser):
     """Ensure validation passes if the chat template exists"""
     args = serve_parser.parse_args(
diff --git a/vllm/config.py b/vllm/config.py
index 43038da37302..41261ab578c4 100644
--- a/vllm/config.py
+++ b/vllm/config.py
@@ -3223,10 +3223,9 @@ def guided_decoding_backend(self, value: GuidedDecodingBackend):
     in the JSON schema. This is only supported for the `guidance` backend and
     is used to better align its behaviour with `outlines` and `xgrammar`."""
 
-    reasoning_backend: Optional[str] = None
+    reasoning_backend: str = ""
     """Select the reasoning parser depending on the model that you're using.
-    This is used to parse the reasoning content into OpenAI API format.
-    Required for `--enable-reasoning`."""
+    This is used to parse the reasoning content into OpenAI API format."""
 
     def compute_hash(self) -> str:
         """
diff --git a/vllm/engine/arg_utils.py b/vllm/engine/arg_utils.py
index c7a580cf1051..d4902bfc2b7c 100644
--- a/vllm/engine/arg_utils.py
+++ b/vllm/engine/arg_utils.py
@@ -364,8 +364,9 @@ class EngineArgs:
     calculate_kv_scales: bool = CacheConfig.calculate_kv_scales
 
     additional_config: Optional[Dict[str, Any]] = None
-    enable_reasoning: Optional[bool] = None
-    reasoning_parser: Optional[str] = DecodingConfig.reasoning_backend
+    enable_reasoning: Optional[bool] = None  # DEPRECATED
+    reasoning_parser: str = DecodingConfig.reasoning_backend
+
     use_tqdm_on_load: bool = LoadConfig.use_tqdm_on_load
 
     def __post_init__(self):
@@ -801,8 +802,15 @@ def add_cli_args(parser: FlexibleArgumentParser) -> FlexibleArgumentParser:
             "--enable-reasoning",
             action="store_true",
             default=False,
-            help="Whether to enable reasoning_content for the model. "
-            "If enabled, the model will be able to generate reasoning content."
+            help=
+            "[DEPRECATED] " \
+            "The --enable-reasoning flag is deprecated as of v0.8.6. "
+            "Use --reasoning-parser to specify " \
+            "the reasoning parser backend instead. "
+            "This flag (--enable-reasoning) will be " \
+            "removed in v0.10.0. "
+            "When --reasoning-parser is specified, " \
+            "reasoning mode is automatically enabled."
         )
 
         return parser
@@ -1091,7 +1099,6 @@ def create_engine_config(
             disable_additional_properties=\
                 self.guided_decoding_disable_additional_properties,
             reasoning_backend=self.reasoning_parser
-            if self.enable_reasoning else None,
         )
 
         show_hidden_metrics = False
diff --git a/vllm/engine/llm_engine.py b/vllm/engine/llm_engine.py
index 8481181eb8e8..0930bae02e41 100644
--- a/vllm/engine/llm_engine.py
+++ b/vllm/engine/llm_engine.py
@@ -2096,7 +2096,7 @@ def _build_logits_processors(
             guided_decoding.backend = guided_decoding.backend or \
                 self.decoding_config.backend
 
-            if self.decoding_config.reasoning_backend is not None:
+            if self.decoding_config.reasoning_backend:
                 logger.debug("Building with reasoning backend %s",
                              self.decoding_config.reasoning_backend)
 
diff --git a/vllm/entrypoints/openai/api_server.py b/vllm/entrypoints/openai/api_server.py
index 136819580897..9746d9697a66 100644
--- a/vllm/entrypoints/openai/api_server.py
+++ b/vllm/entrypoints/openai/api_server.py
@@ -967,7 +967,6 @@ async def init_app_state(
         return_tokens_as_token_ids=args.return_tokens_as_token_ids,
         enable_auto_tools=args.enable_auto_tool_choice,
         tool_parser=args.tool_call_parser,
-        enable_reasoning=args.enable_reasoning,
         reasoning_parser=args.reasoning_parser,
         enable_prompt_tokens_details=args.enable_prompt_tokens_details,
     ) if model_config.runner_type == "generate" else None
@@ -1053,7 +1052,7 @@ async def run_server(args, **uvicorn_kwargs) -> None:
                        f"(chose from {{ {','.join(valid_tool_parses)} }})")
 
     valid_reasoning_parses = ReasoningParserManager.reasoning_parsers.keys()
-    if args.enable_reasoning \
+    if args.reasoning_parser \
         and args.reasoning_parser not in valid_reasoning_parses:
         raise KeyError(
             f"invalid reasoning parser: {args.reasoning_parser} "
diff --git a/vllm/entrypoints/openai/cli_args.py b/vllm/entrypoints/openai/cli_args.py
index b3824013f055..a2639d374791 100644
--- a/vllm/entrypoints/openai/cli_args.py
+++ b/vllm/entrypoints/openai/cli_args.py
@@ -284,11 +284,6 @@ def validate_parsed_serve_args(args: argparse.Namespace):
         raise TypeError("Error: --enable-auto-tool-choice requires "
                         "--tool-call-parser")
 
-    # Enable reasoning needs a reasoning parser to be valid
-    if args.enable_reasoning and not args.reasoning_parser:
-        raise TypeError("Error: --enable-reasoning requires "
-                        "--reasoning-parser")
-
 
 def create_parser_for_docs() -> FlexibleArgumentParser:
     parser_for_docs = FlexibleArgumentParser(
diff --git a/vllm/entrypoints/openai/serving_chat.py b/vllm/entrypoints/openai/serving_chat.py
index dd0b67df4f15..83a92a98026e 100644
--- a/vllm/entrypoints/openai/serving_chat.py
+++ b/vllm/entrypoints/openai/serving_chat.py
@@ -58,8 +58,7 @@ def __init__(
         chat_template: Optional[str],
         chat_template_content_format: ChatTemplateContentFormatOption,
         return_tokens_as_token_ids: bool = False,
-        enable_reasoning: bool = False,
-        reasoning_parser: Optional[str] = None,
+        reasoning_parser: str = "",
         enable_auto_tools: bool = False,
         tool_parser: Optional[str] = None,
         enable_prompt_tokens_details: bool = False,
@@ -82,18 +81,17 @@ def __init__(
                 " the parallel_tool_calls client option is preset for "
                 "compatibility reasons, it will be ignored.")
 
-        self.enable_reasoning: bool = enable_reasoning
         self.reasoning_parser: Optional[Callable[[AnyTokenizer],
                                                  ReasoningParser]] = None
-        if self.enable_reasoning:
+        if reasoning_parser:
             try:
                 self.reasoning_parser = (
                     ReasoningParserManager.get_reasoning_parser(
                         reasoning_parser))
+                assert self.reasoning_parser is not None
             except Exception as e:
-                raise TypeError("Error: --enable-reasoning requires "
-                                f"reasoning_parser:'{reasoning_parser}' "
-                                "which has not been registered") from e
+                raise TypeError(
+                    f"{reasoning_parser=} has not been registered") from e
         self.tool_parser: Optional[Callable[[AnyTokenizer], ToolParser]] = None
         if self.enable_auto_tools:
             try:
@@ -423,15 +421,12 @@ async def chat_completion_stream_generator(
             not tool_choice_function_name
             and self._should_stream_with_auto_tool_parsing(request))
 
-        should_stream_with_reasoning_parsing = (
-            self._should_stream_with_reasoning_parsing(request))
-
         all_previous_token_ids: Optional[list[list[int]]]
         function_name_returned: Optional[list[bool]] = None
 
         # Only one of these will be used, thus previous_texts and
         # all_previous_token_ids will not be used twice in the same iteration.
-        if tool_choice_auto or should_stream_with_reasoning_parsing:
+        if tool_choice_auto or self.reasoning_parser:
             # These are only required in "auto" tool choice case
             previous_texts = [""] * num_choices
             all_previous_token_ids = [[]] * num_choices
@@ -446,12 +441,7 @@ async def chat_completion_stream_generator(
             previous_texts, all_previous_token_ids = None, None
 
         try:
-            # There is no need to check if the reasoning_parser is None
-            # because the should_stream_with_reasoning_parsing check
-            # already ensures that the reasoning_parser is not None.
-            # but the pre-commit hook requires it.
-            if should_stream_with_reasoning_parsing and \
-                self.reasoning_parser is not None:
+            if self.reasoning_parser:
                 reasoning_parser = self.reasoning_parser(tokenizer)
         except RuntimeError as e:
             logger.exception("Error in reasoning parser creation.")
@@ -459,7 +449,6 @@ async def chat_completion_stream_generator(
             yield f"data: {data}\n\n"
             yield "data: [DONE]\n\n"
             return
-
         # Prepare the tool parser if it's needed
         try:
             if tool_choice_auto and self.tool_parser:
@@ -592,7 +581,7 @@ async def chat_completion_stream_generator(
                     delta_message: Optional[DeltaMessage]
 
                     # just update previous_texts and previous_token_ids
-                    if tool_choice_auto or should_stream_with_reasoning_parsing:
+                    if tool_choice_auto or self.reasoning_parser:
                         assert previous_texts is not None
                         assert all_previous_token_ids is not None
                         previous_text = previous_texts[i]
@@ -603,7 +592,7 @@ async def chat_completion_stream_generator(
 
                     # handle streaming deltas for tools with named tool_choice
                     if tool_choice_function_name:
-                        if (self.enable_reasoning
+                        if (self.reasoning_parser
                                 and not reasoning_parser.is_reasoning_end(
                                     previous_token_ids)):
                             assert reasoning_parser is not None
@@ -630,7 +619,7 @@ async def chat_completion_stream_generator(
                                     current_text = ""
                         else:
                             # Just to add remaining `content`
-                            if self.enable_reasoning:
+                            if self.reasoning_parser:
                                 delta_text = previous_text + delta_text
                                 current_text = ""
 
@@ -660,7 +649,7 @@ async def chat_completion_stream_generator(
 
                     # handle streaming deltas for tools with "auto" tool choice
                     # and reasoning parser
-                    elif tool_choice_auto and self.enable_reasoning:
+                    elif tool_choice_auto and self.reasoning_parser:
                         assert tool_parser is not None
                         assert reasoning_parser is not None
                         assert added_content_delta_arr is not None
@@ -728,8 +717,7 @@ async def chat_completion_stream_generator(
                                 delta_token_ids=output.token_ids,
                                 request=request))
                     # when only reasoning
-                    elif self.enable_reasoning:
-                        assert reasoning_parser is not None
+                    elif self.reasoning_parser:
                         delta_message = (reasoning_parser.
                                          extract_reasoning_content_streaming(
                                              previous_text,
@@ -744,7 +732,7 @@ async def chat_completion_stream_generator(
                         delta_message = DeltaMessage(content=delta_text)
 
                     # update the previous values for the next iteration
-                    if tool_choice_auto or should_stream_with_reasoning_parsing:
+                    if tool_choice_auto or self.reasoning_parser:
                         assert previous_texts is not None
                         assert all_previous_token_ids is not None
                         previous_texts[i] = current_text
@@ -931,17 +919,9 @@ async def chat_completion_full_generator(
                 )
             else:
                 logprobs = None
-
-            should_stream_with_reasoning_parsing = (
-                self._should_stream_with_reasoning_parsing(request))
-
-            # In the OpenAI API the finish_reason is "tools_called"
-            # if the tool choice is auto and the model produced a tool
-            # call. The same is not true for named function calls
             auto_tools_called = False
 
-            if should_stream_with_reasoning_parsing and \
-                self.reasoning_parser is not None:
+            if self.reasoning_parser:
                 try:
                     reasoning_parser = self.reasoning_parser(tokenizer)
                 except RuntimeError as e:
@@ -1176,17 +1156,6 @@ def _should_stream_with_auto_tool_parsing(self,
         return (request.tools and self.tool_parser and self.enable_auto_tools
                 and request.tool_choice in ['auto', None])
 
-    def _should_stream_with_reasoning_parsing(self,
-                                              request: ChatCompletionRequest):
-        """
-            Utility function to check if streamed tokens should go through the
-            reasoning parser that was configured.
-    
-            We only want to do this IF reasoning is enabled and a reasoning 
-            parser is configured.
-            """
-        return self.enable_reasoning and self.reasoning_parser is not None
-
     def _should_check_for_unstreamed_tool_arg_tokens(
         self,
         delta_message: Optional[DeltaMessage],
diff --git a/vllm/model_executor/guided_decoding/__init__.py b/vllm/model_executor/guided_decoding/__init__.py
index 4e4d697f49a9..a2b61a1b19e4 100644
--- a/vllm/model_executor/guided_decoding/__init__.py
+++ b/vllm/model_executor/guided_decoding/__init__.py
@@ -103,7 +103,7 @@ async def get_guided_decoding_logits_processor(
         reasoning_backend: str | None = None) -> LogitsProcessor | None:
 
     reasoner = None
-    if reasoning_backend is not None:
+    if reasoning_backend:
         reasoner_class = ReasoningParserManager.get_reasoning_parser(
             reasoning_backend)
         reasoner = reasoner_class(tokenizer)
@@ -146,7 +146,7 @@ def get_local_guided_decoding_logits_processor(
     guided_params = maybe_backend_fallback(guided_params)
 
     reasoner = None
-    if reasoning_backend is not None:
+    if reasoning_backend:
         reasoner_class = ReasoningParserManager.get_reasoning_parser(
             reasoning_backend)
         reasoner = reasoner_class(tokenizer)
diff --git a/vllm/model_executor/guided_decoding/outlines_logits_processors.py b/vllm/model_executor/guided_decoding/outlines_logits_processors.py
index 31af4593f112..936fd0f06867 100644
--- a/vllm/model_executor/guided_decoding/outlines_logits_processors.py
+++ b/vllm/model_executor/guided_decoding/outlines_logits_processors.py
@@ -61,7 +61,7 @@ def __call__(self, input_ids: List[int],
         """Use the FSM to bias the logits before sampling the next token."""
 
         # Skip the structured logits processing if reasoning is not finished.
-        # reasoner is not None only when `--enable-reasoning` is set.
+        # reasoner is not None only when `--reasoning-parser` is set.
         if self._reasoner is not None:
             if not self._reasoner.is_reasoning_end(input_ids):
                 return scores
diff --git a/vllm/model_executor/guided_decoding/xgrammar_decoding.py b/vllm/model_executor/guided_decoding/xgrammar_decoding.py
index eb7b2b74ffbe..ac2d73626d78 100644
--- a/vllm/model_executor/guided_decoding/xgrammar_decoding.py
+++ b/vllm/model_executor/guided_decoding/xgrammar_decoding.py
@@ -346,7 +346,7 @@ def __call__(self, input_ids: list[int],
                  scores: torch.Tensor) -> torch.Tensor:
 
         # Skip the structured logits processing if reasoning is not finished.
-        # reasoner is not None only when `--enable-reasoning` is set.
+        # reasoner is not None only when `--reasoning-parser` is set.
         if self.reasoner is not None and \
         not self.reasoner.is_reasoning_end(
                 input_ids):