[Rust Frontend] Use the MiMo structural-tag builder (#59148)

Signed-off-by: Bugen Zhao <i@bugenzhao.com>
Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

rust/Cargo.lock

@@ -6605,9 +6605,9 @@ checksum = "9edde0db4769d2dc68579893f2306b26c6ecfbe0ef499b013d731b7b9247e0b9"
 
 [[package]]
 name = "xgrammar-structural-tag"
-version = "0.3.0+xgrammar.0.2.7.82505d0"
+version = "0.4.1+xgrammar.0.2.8.9778737"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3864005575c2ab76057f789898f59fb9fa1378d07c5ceabc24764e012d0e90bb"
+checksum = "06637fe6ff753ef929b4cea2933b9432f78335255c6931dc748957122b9b492c"
 dependencies = [
  "auto_impl",
  "serde",

rust/Cargo.toml

@@ -153,7 +153,7 @@ vllm-text = { path = "src/text" }
 vllm-tokenizer = { path = "src/tokenizer" }
 vllm-tracing = { path = "src/tracing" }
 winnow = { version = "1.0.2", features = ["simd"] }
-xgrammar-structural-tag = "0.3.0"
+xgrammar-structural-tag = "0.4.1"
 zeromq = { version = "0.6.0", default-features = false, features = [
     "tokio-runtime",
     "all-transport",

rust/src/parser/src/tool/mimo.rs

@@ -26,7 +26,8 @@ impl ToolParser for MiMoToolParser {
     }
 
     fn structural_tag_builder(&self) -> Option<&dyn StructuralTagBuilder> {
-        self.inner.structural_tag_builder()
+        // MiMo's template emits compact tags without the newlines Qwen3-Coder uses.
+        Some(xgrammar_structural_tag::Model::Mimo.builder())
     }
 
     fn parse_into(&mut self, chunk: &str, output: &mut ToolParserOutput) -> Result<()> {
@@ -88,4 +89,32 @@ mod tests {
             );
         }
     }
+
+    #[test]
+    fn structural_tag_uses_compact_mimo_tags() {
+        use xgrammar_structural_tag::builders::StructuralTagOptions;
+        use xgrammar_structural_tag::{
+            FunctionDefinition, FunctionToolParam, ToolChoice, ToolParam, build_structural_tag,
+        };
+
+        let parser = MiMoToolParser::create(&[]).unwrap();
+        let builder = parser.structural_tag_builder().expect("MiMo exposes a builder");
+        let tools = [ToolParam::Function(FunctionToolParam::new(
+            FunctionDefinition::new("get_weather"),
+        ))];
+        let tag = build_structural_tag(
+            builder,
+            &tools,
+            ToolChoice::required(),
+            StructuralTagOptions::default().with_reasoning(false),
+        )
+        .unwrap();
+        let value = serde_json::to_value(tag).unwrap();
+        assert_eq!(value["format"]["triggers"], json!(["<tool_call>"]));
+        assert_eq!(
+            value["format"]["tags"][0]["begin"],
+            "<tool_call><function=get_weather>"
+        );
+        assert_eq!(value["format"]["tags"][0]["end"], "</function></tool_call>");
+    }
 }

rust/src/parser/src/unified/kimi_k3/structural_tag.rs

@@ -395,7 +395,7 @@ mod tests {
         )
         .unwrap();
 
-        expect![[r##"{"type":"structural_tag","format":{"type":"sequence","elements":[{"type":"optional","content":{"type":"const_string","value":"<|open|>response<|sep|>"}},{"type":"tag","begin":"","content":{"type":"any_text","excludes":["<|open|>","<|close|>","<|end_of_msg|>"],"max_tokens":null,"max_chars":null},"end":"<|close|>response<|sep|>"},{"type":"tag","begin":"<|open|>tools<|sep|>","content":{"type":"tags_with_separator","tags":[{"type":"tag","begin":"<|open|>call tool=\"get_weather\" index=\"","content":{"type":"sequence","elements":[{"type":"regex","pattern":"[1-9][0-9]*"},{"type":"const_string","value":"\"<|sep|>"},{"type":"or","elements":[{"type":"plus","content":{"type":"or","elements":[{"type":"tag","begin":"<|open|>argument key=\"unit\" type=\"string\"<|sep|>","content":{"type":"or","elements":[{"type":"const_string","value":"celsius"},{"type":"const_string","value":"fahrenheit"}]},"end":"<|close|>argument<|sep|>"},{"type":"tag","begin":"<|open|>argument key=\"place\" type=\"object\"<|sep|>","content":{"type":"json_schema","json_schema":{"$ref":"#/$defs/place","type":"object","$defs":{"place":{"type":"object","properties":{"city":{"type":"string"}}}}},"style":"json","any_order":false,"max_whitespace_cnt":null},"end":"<|close|>argument<|sep|>"}]}},{"type":"tag","begin":"<|open|>json type=\"object\"<|sep|>","content":{"type":"json_schema","json_schema":{"$defs":{"place":{"type":"object","properties":{"city":{"type":"string"}}}},"type":"object","properties":{"unit":{"type":"string","enum":["celsius","fahrenheit"]},"place":{"$ref":"#/$defs/place","type":"object"}},"required":["place"]},"style":"json","any_order":false,"max_whitespace_cnt":null},"end":"<|close|>json<|sep|>"}]}]},"end":"<|close|>call<|sep|>"}],"separator":"","at_least_one":true,"stop_after_first":false},"end":"<|close|>tools<|sep|>"},{"type":"optional","content":{"type":"const_string","value":"<|close|>message<|sep|>"}}]}}"##]].assert_eq(&tag.to_json_string().unwrap());
+        expect![[r##"{"type":"structural_tag","format":{"type":"sequence","elements":[{"type":"optional","content":{"type":"const_string","value":"<|open|>response<|sep|>"}},{"type":"tag","begin":"","content":{"type":"any_text","excludes":["<|open|>","<|close|>","<|end_of_msg|>"],"max_tokens":null,"max_chars":null},"end":"<|close|>response<|sep|>"},{"type":"tag","begin":"<|open|>tools<|sep|>","content":{"type":"tags_with_separator","tags":[{"type":"tag","begin":"<|open|>call tool=\"get_weather\" index=\"","content":{"type":"sequence","elements":[{"type":"regex","pattern":"[1-9][0-9]*"},{"type":"const_string","value":"\"<|sep|>"},{"type":"or","elements":[{"type":"plus","content":{"type":"or","elements":[{"type":"tag","begin":"<|open|>argument key=\"unit\" type=\"string\"<|sep|>","content":{"type":"or","elements":[{"type":"const_string","value":"celsius"},{"type":"const_string","value":"fahrenheit"}]},"end":"<|close|>argument<|sep|>"},{"type":"tag","begin":"<|open|>argument key=\"place\" type=\"object\"<|sep|>","content":{"type":"json_schema","json_schema":{"$ref":"#/$defs/place","type":"object","$defs":{"place":{"type":"object","properties":{"city":{"type":"string"}}}}},"style":"json","any_order":false,"max_whitespace_cnt":null,"excludes":[]},"end":"<|close|>argument<|sep|>"}]}},{"type":"tag","begin":"<|open|>json type=\"object\"<|sep|>","content":{"type":"json_schema","json_schema":{"$defs":{"place":{"type":"object","properties":{"city":{"type":"string"}}}},"type":"object","properties":{"unit":{"type":"string","enum":["celsius","fahrenheit"]},"place":{"$ref":"#/$defs/place","type":"object"}},"required":["place"]},"style":"json","any_order":false,"max_whitespace_cnt":null,"excludes":[]},"end":"<|close|>json<|sep|>"}]}]},"end":"<|close|>call<|sep|>"}],"separator":"","at_least_one":true,"stop_after_first":false},"end":"<|close|>tools<|sep|>"},{"type":"optional","content":{"type":"const_string","value":"<|close|>message<|sep|>"}}]}}"##]].assert_eq(&tag.to_json_string().unwrap());
     }
 
     #[test]