implement Structural Tag with Guidance backend (#17333)

Signed-off-by: Michal Moskal <michal@moskal.me>

tests/v1/entrypoints/llm/test_struct_output_generate.py

@@ -435,13 +435,10 @@ Given the previous instructions, what is the weather in New York City?
 """
 
     # Change this once other backends support structural_tag
-    if guided_decoding_backend.startswith("xgrammar"):
     outputs = llm.generate(prompts=prompt,
                            sampling_params=sampling_params,
                            use_tqdm=True)
     assert outputs is not None
-    else:
-        outputs = []
 
     for output in outputs:
         assert output is not None

vllm/v1/structured_output/backend_guidance.py

@@ -173,7 +173,8 @@ def serialize_guidance_grammar(
     disable_any_whitespace: bool = False,
     no_additional_properties: bool = False,
 ) -> str:
-    if request_type == StructuredOutputOptions.JSON:
+
+    def _process_schema(grammar_spec: Union[str, dict[str, Any]], ) -> str:
         if no_additional_properties:
             grammar_spec = process_for_additional_properties(grammar_spec)
         return llguidance.LLMatcher.grammar_from_json_schema(
@@ -181,6 +182,9 @@ def serialize_guidance_grammar(
             defaults={
                 "whitespace_flexible": not disable_any_whitespace,
             })
+
+    if request_type == StructuredOutputOptions.JSON:
+        return _process_schema(grammar_spec)
     elif request_type == StructuredOutputOptions.JSON_OBJECT:
         return llguidance.LLMatcher.grammar_from_json_schema(
             '{"type": "object"}',
@@ -195,8 +199,29 @@ def serialize_guidance_grammar(
         elif request_type == StructuredOutputOptions.CHOICE:
             tp = "choice"
         elif request_type == StructuredOutputOptions.STRUCTURAL_TAG:
-            raise ValueError("Structural tag is not supported "
-                             "for guidance backend yet")
+            if isinstance(grammar_spec, str):
+                s_tag = json.loads(grammar_spec)
+            else:
+                s_tag = grammar_spec
+            triggers: list[str] = s_tag["triggers"]
+            tags: list[llguidance.StructTag] = []
+            for s in s_tag["structures"]:
+                begin: str = s["begin"]
+                trig = next((t for t in triggers if begin.startswith(t)), None)
+                if trig is None:
+                    raise ValueError(
+                        f"Trigger {begin} not found in triggers {triggers}")
+                tags.append(
+                    llguidance.StructTag(
+                        trigger=trig,
+                        begin=s["begin"],
+                        grammar=_process_schema(s["schema"]),
+                        end=s["end"],
+                    ))
+            if not tags:
+                raise ValueError(
+                    "No structural tags found in the grammar spec.")
+            return llguidance.StructTag.to_grammar(tags)
         else:
             logger.error("Validation should have already occurred. "
                          "Please file an issue.")