[Rust Frontend] Cover different thinking modes in roundtrip tests (#44320)

Signed-off-by: Bugen Zhao <i@bugenzhao.com>

rust/src/chat/tests/roundtrip.rs

@@ -20,6 +20,7 @@ use vllm_chat::{
 use vllm_text::{DecodedTextEvent, Finished, Prompt};
 
 /// One model/parser configuration used to run the fixed roundtrip fixtures.
+#[derive(Clone)]
 struct RoundtripCase {
     /// Hugging Face model id resolved through the production backend loader.
     model_id: &'static str,
@@ -31,11 +32,45 @@ struct RoundtripCase {
     tool_call_parser: ParserSelection,
     /// Reasoning parser selection used by the output processor.
     reasoning_parser: ParserSelection,
+    /// How this model's chat template handles thinking mode.
+    thinking_behavior: ThinkingBehavior,
     /// JSON formatting expected after this model's template has materialized
     /// tool-call arguments.
     json_fmt: JsonFmt,
 }
 
+#[derive(Clone, Copy)]
+enum ThinkingBehavior {
+    /// The chat template accepts explicit thinking on/off kwargs, and uses
+    /// `default` when the request does not specify either kwarg.
+    Toggleable { default: bool },
+    /// The chat template always behaves as `value` for this fixture.
+    Always { value: bool },
+}
+
+impl ThinkingBehavior {
+    fn default(self) -> bool {
+        match self {
+            Self::Toggleable { default } => default,
+            Self::Always { value } => value,
+        }
+    }
+
+    fn fixtures(self) -> Vec<Option<bool>> {
+        match self {
+            Self::Toggleable { .. } => vec![
+                Some(true),  // explicitly enable thinking
+                Some(false), // explicitly disable thinking
+                None,        // use default template behavior
+            ],
+            Self::Always { value } => vec![
+                Some(value), // explicitly request the supported thinking behavior
+                None,        // use default template behavior
+            ],
+        }
+    }
+}
+
 impl RoundtripCase {
     /// Qwen3 XML tool-call format with `qwen3` reasoning tags.
     fn qwen3() -> Self {
@@ -44,6 +79,7 @@ impl RoundtripCase {
             assistant_stop_suffix: "<|im_end|>\n",
             tool_call_parser: ParserSelection::Auto,
             reasoning_parser: ParserSelection::Auto,
+            thinking_behavior: ThinkingBehavior::Toggleable { default: true },
             json_fmt: spaced_json_fmt(),
         }
     }
@@ -55,6 +91,7 @@ impl RoundtripCase {
             assistant_stop_suffix: "<|im_end|>\n",
             tool_call_parser: ParserSelection::Auto,
             reasoning_parser: ParserSelection::Auto,
+            thinking_behavior: ThinkingBehavior::Toggleable { default: true },
             json_fmt: compact_json_fmt(),
         }
     }
@@ -66,6 +103,7 @@ impl RoundtripCase {
             assistant_stop_suffix: "[e~[\n",
             tool_call_parser: ParserSelection::Auto,
             reasoning_parser: ParserSelection::Auto,
+            thinking_behavior: ThinkingBehavior::Always { value: true },
             json_fmt: compact_json_fmt(),
         }
     }
@@ -77,6 +115,7 @@ impl RoundtripCase {
             assistant_stop_suffix: "<｜end▁of▁sentence｜>",
             tool_call_parser: ParserSelection::Auto,
             reasoning_parser: ParserSelection::Auto,
+            thinking_behavior: ThinkingBehavior::Toggleable { default: false },
             json_fmt: compact_json_fmt(),
         }
     }
@@ -88,6 +127,7 @@ impl RoundtripCase {
             assistant_stop_suffix: "",
             tool_call_parser: ParserSelection::Auto,
             reasoning_parser: ParserSelection::Auto,
+            thinking_behavior: ThinkingBehavior::Toggleable { default: true },
             json_fmt: compact_json_fmt(),
         }
     }
@@ -100,6 +140,7 @@ impl RoundtripCase {
             assistant_stop_suffix: "<|im_end|>",
             tool_call_parser: ParserSelection::Auto,
             reasoning_parser: ParserSelection::Auto,
+            thinking_behavior: ThinkingBehavior::Toggleable { default: true },
             json_fmt: spaced_json_fmt(),
         }
     }
@@ -135,35 +176,44 @@ roundtrip_tests! {
 
 /// Run the fixed reasoning+content fixture for one model/parser case.
 async fn run_roundtrip_reasoning_and_content(case: RoundtripCase) -> Result<()> {
+    for thinking in case.thinking_behavior.fixtures() {
+        run_roundtrip_reasoning_and_content_inner(case.clone(), thinking).await?;
+    }
+    Ok(())
+}
+
+async fn run_roundtrip_reasoning_and_content_inner(
+    case: RoundtripCase,
+    thinking: Option<bool>,
+) -> Result<()> {
     let backends = load_roundtrip_backends(&case).await?;
     let request = roundtrip_request(
         "roundtrip-reasoning-content",
         vec![ChatMessage::text(ChatRole::User, "What is 2 + 2?")],
         Vec::new(),
+        thinking,
     );
     let expected_reasoning = "Need compute 2 + 2 directly.";
     let expected_text = "The answer is 4.";
+    let effective_thinking = thinking.unwrap_or(case.thinking_behavior.default());
 
-    let result = run_roundtrip(
-        &case,
-        &backends,
-        &request,
-        AssistantMessage {
-            content: vec![
-                AssistantContentBlock::Reasoning {
-                    text: expected_reasoning.to_string(),
-                },
-                AssistantContentBlock::Text {
-                    text: expected_text.to_string(),
-                },
-            ],
-        },
-    )
-    .await?;
+    let assistant = {
+        let mut content = Vec::new();
+        if effective_thinking {
+            content.push(AssistantContentBlock::Reasoning {
+                text: expected_reasoning.to_string(),
+            });
+        }
+        content.push(AssistantContentBlock::Text {
+            text: expected_text.to_string(),
+        });
+        AssistantMessage { content }
+    };
+    let result = run_roundtrip(&case, &backends, &request, assistant).await?;
 
     assert_eq!(
         result.parsed_message.reasoning().as_deref().map(str::trim),
-        Some(expected_reasoning)
+        effective_thinking.then_some(expected_reasoning)
     );
     assert_eq!(result.parsed_message.text().trim(), expected_text);
     assert_eq!(result.parsed_message.tool_calls().count(), 0);
@@ -186,6 +236,7 @@ async fn run_roundtrip_tool_call_mix(case: RoundtripCase) -> Result<()> {
             "Check Shanghai weather and add 1.00 plus 2.",
         )],
         test_tools(),
+        Some(true), // always enable thinking in this fixture
     );
     let expected_reasoning = "Need call the weather and add tools.";
     let expected_text = "I will call the tools.";
@@ -487,6 +538,7 @@ fn roundtrip_request(
     request_id: impl Into<String>,
     messages: Vec<ChatMessage>,
     tools: Vec<ChatTool>,
+    thinking: Option<bool>,
 ) -> ChatRequest {
     let mut request = ChatRequest {
         request_id: request_id.into(),
@@ -500,10 +552,12 @@ fn roundtrip_request(
         ..ChatRequest::for_test()
     };
 
-    // Enable thinking for some models so that rendering and parsing the reasoning block is
-    // exercised in the roundtrip.
-    for key in ["thinking", "enable_thinking"] {
-        request.chat_options.template_kwargs.insert(key.to_string(), true.into());
+    // Explicitly enable or disable thinking so that rendering and parsing the reasoning block is
+    // exercised or skipped in the roundtrip. If unspecified, use the default template behavior.
+    if let Some(thinking) = thinking {
+        for key in ["thinking", "enable_thinking"] {
+            request.chat_options.template_kwargs.insert(key.to_string(), thinking.into());
+        }
     }
 
     request