diff --git a/benches/request_processing.rs b/benches/request_processing.rs
index 9c1ed893..af8c7463 100644
--- a/benches/request_processing.rs
+++ b/benches/request_processing.rs
@@ -210,6 +210,10 @@ fn create_large_chat_completion_request() -> ChatCompletionRequest {
             tool_calls: None,
             function_call: None,
             reasoning: None,
+            reasoning_content: None,
+            think: None,
+            think_fast: None,
+            think_faster: None,
         });
     }
 
diff --git a/src/protocols/spec.rs b/src/protocols/spec.rs
index 8b20bf9e..37d77761 100644
--- a/src/protocols/spec.rs
+++ b/src/protocols/spec.rs
@@ -87,6 +87,18 @@ pub enum ChatMessage {
         /// Reasoning content for reasoning models
         #[serde(skip_serializing_if = "Option::is_none")]
         reasoning: Option<String>,
+        /// Reasoning content for reasoning models using vLLM-compatible field names
+        #[serde(skip_serializing_if = "Option::is_none")]
+        reasoning_content: Option<String>,
+        /// Alternative reasoning field accepted by some chat templates
+        #[serde(skip_serializing_if = "Option::is_none")]
+        think: Option<String>,
+        /// Alternative reasoning field accepted by some chat templates
+        #[serde(skip_serializing_if = "Option::is_none")]
+        think_fast: Option<String>,
+        /// Alternative reasoning field accepted by some chat templates
+        #[serde(skip_serializing_if = "Option::is_none")]
+        think_faster: Option<String>,
     },
     Tool {
         role: String, // "tool"
@@ -151,6 +163,34 @@ impl<'de> Deserialize<'de> for ChatMessage {
                         r.as_str().map(String::from)
                     }
                 }),
+                reasoning_content: value.get("reasoning_content").and_then(|r| {
+                    if r.is_null() {
+                        None
+                    } else {
+                        r.as_str().map(String::from)
+                    }
+                }),
+                think: value.get("think").and_then(|r| {
+                    if r.is_null() {
+                        None
+                    } else {
+                        r.as_str().map(String::from)
+                    }
+                }),
+                think_fast: value.get("think_fast").and_then(|r| {
+                    if r.is_null() {
+                        None
+                    } else {
+                        r.as_str().map(String::from)
+                    }
+                }),
+                think_faster: value.get("think_faster").and_then(|r| {
+                    if r.is_null() {
+                        None
+                    } else {
+                        r.as_str().map(String::from)
+                    }
+                }),
             }),
             "system" => Ok(ChatMessage::System {
                 role: role.to_string(),
@@ -3672,6 +3712,10 @@ mod tests {
             tool_calls: None,
             function_call: None,
             reasoning: Some("Thinking...".to_string()),
+            reasoning_content: None,
+            think: None,
+            think_fast: None,
+            think_faster: None,
         };
 
         let serialized = serde_json::to_string(&original).unwrap();
diff --git a/tests/test_extra_args_chat.rs b/tests/test_extra_args_chat.rs
index 5599df2e..7a6b54d4 100644
--- a/tests/test_extra_args_chat.rs
+++ b/tests/test_extra_args_chat.rs
@@ -1,5 +1,20 @@
 //! Tests that unknown/extra fields in ChatCompletionRequest are preserved through serde roundtrip.
-use vllm_router_rs::protocols::spec::ChatCompletionRequest;
+use vllm_router_rs::protocols::spec::{ChatCompletionRequest, ChatMessage};
+
+fn assert_assistant_field_roundtrip(field: &str, expected: &str) {
+    let request_json = serde_json::json!({
+        "model": "test-model",
+        "messages": [
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": "Hi", field: expected},
+        ],
+    });
+
+    let req: ChatCompletionRequest = serde_json::from_value(request_json).unwrap();
+    let value = serde_json::to_value(&req).unwrap();
+
+    assert_eq!(value["messages"][1][field], serde_json::json!(expected));
+}
 
 #[test]
 fn test_extra_fields_preserved_on_deserialize() {
@@ -71,3 +86,107 @@ fn test_no_other_fields_gives_empty_map() {
     let req: ChatCompletionRequest = serde_json::from_str(json).unwrap();
     assert!(req.other.is_empty());
 }
+
+#[test]
+fn test_assistant_reasoning_content_empty_survives_roundtrip() {
+    let json = r#"{
+        "model": "test-model",
+        "messages": [
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "reasoning_content": "", "content": "Hi"}
+        ]
+    }"#;
+
+    let req: ChatCompletionRequest = serde_json::from_str(json).unwrap();
+    match &req.messages[1] {
+        ChatMessage::Assistant {
+            reasoning_content, ..
+        } => {
+            assert_eq!(reasoning_content.as_deref(), Some(""));
+        }
+        other => panic!("expected assistant message, got {other:?}"),
+    }
+
+    let value = serde_json::to_value(&req).unwrap();
+    assert_eq!(
+        value["messages"][1]["reasoning_content"],
+        serde_json::json!("")
+    );
+}
+
+#[test]
+fn test_assistant_reasoning_content_nonempty_survives_roundtrip() {
+    let json = r#"{
+        "model": "test-model",
+        "messages": [
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "reasoning_content": "scratchpad", "content": "Hi"}
+        ]
+    }"#;
+
+    let req: ChatCompletionRequest = serde_json::from_str(json).unwrap();
+    match &req.messages[1] {
+        ChatMessage::Assistant {
+            reasoning_content, ..
+        } => {
+            assert_eq!(reasoning_content.as_deref(), Some("scratchpad"));
+        }
+        other => panic!("expected assistant message, got {other:?}"),
+    }
+
+    let value = serde_json::to_value(&req).unwrap();
+    assert_eq!(
+        value["messages"][1]["reasoning_content"],
+        serde_json::json!("scratchpad")
+    );
+}
+
+#[test]
+fn test_assistant_reasoning_and_reasoning_content_survive_roundtrip() {
+    let json = r#"{
+        "model": "test-model",
+        "messages": [
+            {"role": "user", "content": "Hello"},
+            {
+                "role": "assistant",
+                "reasoning": "",
+                "reasoning_content": "",
+                "content": "Hi"
+            }
+        ]
+    }"#;
+
+    let req: ChatCompletionRequest = serde_json::from_str(json).unwrap();
+    match &req.messages[1] {
+        ChatMessage::Assistant {
+            reasoning,
+            reasoning_content,
+            ..
+        } => {
+            assert_eq!(reasoning.as_deref(), Some(""));
+            assert_eq!(reasoning_content.as_deref(), Some(""));
+        }
+        other => panic!("expected assistant message, got {other:?}"),
+    }
+
+    let value = serde_json::to_value(&req).unwrap();
+    assert_eq!(value["messages"][1]["reasoning"], serde_json::json!(""));
+    assert_eq!(
+        value["messages"][1]["reasoning_content"],
+        serde_json::json!("")
+    );
+}
+
+#[test]
+fn test_assistant_thinking_aliases_empty_survive_roundtrip() {
+    for field in ["think", "think_fast", "think_faster"] {
+        assert_assistant_field_roundtrip(field, "");
+    }
+}
+
+#[test]
+fn test_assistant_thinking_aliases_nonempty_survive_roundtrip() {
+    for field in ["think", "think_fast", "think_faster"] {
+        assert_assistant_field_roundtrip(field, "scratchpad");
+    }
+}