fix xgrammar_backend crash with malformed inputs (#13752)

python/sglang/srt/constrained/xgrammar_backend.py

@@ -16,7 +16,7 @@
 import dataclasses
 import json
 import logging
-from typing import List, Optional, Tuple, Union
+from typing import Dict, List, Optional, Tuple, Union
 
 import torch
 from xgrammar import (
@@ -196,6 +196,34 @@ class XGrammarGrammarBackend(BaseGrammarBackend):
         self.override_stop_tokens = override_stop_tokens
         self.any_whitespace = any_whitespace
 
+    @staticmethod
+    def _sanitize_structural_format(structural_format):
+        """Recursively replace missing json_schema fields with an empty schema."""
+        if not isinstance(structural_format, dict):
+            return
+
+        fmt_type = structural_format.get("type")
+        if fmt_type in {"json_schema", "qwen_xml_parameter"}:
+            if structural_format.get("json_schema") is None:
+                structural_format["json_schema"] = {}
+
+        if fmt_type == "tag":
+            XGrammarGrammarBackend._sanitize_structural_format(
+                structural_format.get("content")
+            )
+        elif fmt_type in {"sequence", "or"}:
+            for element in structural_format.get("elements", []):
+                XGrammarGrammarBackend._sanitize_structural_format(element)
+        elif fmt_type in {"triggered_tags", "tags_with_separator"}:
+            for tag in structural_format.get("tags", []):
+                XGrammarGrammarBackend._sanitize_structural_format(tag)
+
+    @staticmethod
+    def _sanitize_structural_tag_structures(structural_tag: Dict) -> None:
+        for structure in structural_tag.get("structures", []):
+            if structure.get("schema") is None:
+                structure["schema"] = {}
+
     def _from_context(
         self, ctx: CompiledGrammar, key_string: str, grammar_stats: GrammarStats
     ) -> XGrammarGrammar:
@@ -249,6 +277,7 @@ class XGrammarGrammarBackend(BaseGrammarBackend):
             # TODO(dark): it's REALLY stupid to construct object from string and decode it again
             structural_tag = json.loads(key_string)
             if is_legacy_structural_tag(structural_tag):
+                self._sanitize_structural_tag_structures(structural_tag)
                 tags = [
                     StructuralTagItem(
                         begin=structure["begin"],
@@ -261,6 +290,11 @@ class XGrammarGrammarBackend(BaseGrammarBackend):
                     tags, structural_tag["triggers"]
                 )
             else:
+                format_dict = structural_tag.get("format")
+                if isinstance(format_dict, dict):
+                    self._sanitize_structural_format(format_dict)
+                    structural_tag["format"] = format_dict
+                    key_string = json.dumps(structural_tag)
                 ctx = self.grammar_compiler.compile_structural_tag(key_string)
         except (RuntimeError, json.decoder.JSONDecodeError) as e:
             logging.error(f"Hit invalid structural_tag: {key_string=}, {e=}")