Support min_tokens in sgl.gen (#1573)

examples/frontend_language/usage/sgl_gen_min_tokens.py

+"""
+This example demonstrates how to use `min_tokens` to enforce sgl.gen to generate a longer sequence
+
+Usage:
+python3 sgl_gen_min_tokens.py
+"""
+
+import sglang as sgl
+
+
+@sgl.function
+def long_answer(s):
+    s += sgl.user("What is the capital of the United States?")
+    s += sgl.assistant(sgl.gen("answer", min_tokens=64, max_tokens=128))
+
+
+@sgl.function
+def short_answer(s):
+    s += sgl.user("What is the capital of the United States?")
+    s += sgl.assistant(sgl.gen("answer"))
+
+
+if __name__ == "__main__":
+    runtime = sgl.Runtime(model_path="meta-llama/Meta-Llama-3.1-8B-Instruct")
+    sgl.set_default_backend(runtime)
+
+    state = long_answer.run()
+    print("=" * 20)
+    print("Longer Answer", state["answer"])
+
+    state = short_answer.run()
+    print("=" * 20)
+    print("Short Answer", state["answer"])
+
+    runtime.shutdown()

python/sglang/api.py

@@ -69,6 +69,7 @@ def get_server_args(backend: Optional[BaseBackend] = None):
 def gen(
     name: Optional[str] = None,
     max_tokens: Optional[int] = None,
+    min_tokens: Optional[int] = None,
     stop: Optional[Union[str, List[str]]] = None,
     stop_token_ids: Optional[List[int]] = None,
     temperature: Optional[float] = None,
@@ -108,6 +109,7 @@ def gen(
     return SglGen(
         name,
         max_tokens,
+        min_tokens,
         stop,
         stop_token_ids,
         temperature,
@@ -147,6 +149,7 @@ def gen_int(
     return SglGen(
         name,
         max_tokens,
+        None,
         stop,
         stop_token_ids,
         temperature,
@@ -185,6 +188,7 @@ def gen_string(
     return SglGen(
         name,
         max_tokens,
+        None,
         stop,
         stop_token_ids,
         temperature,

python/sglang/lang/interpreter.py

@@ -668,6 +668,7 @@ class StreamExecutor:
 
         for item in [
             "max_new_tokens",
+            "min_new_tokens",
             "stop",
             "stop_token_ids",
             "temperature",

python/sglang/lang/ir.py

@@ -17,6 +17,7 @@ REGEX_STR = r"\"[\w\d\s]*\""  # bugs with regex r"\".*\"" in interegular pkg
 @dataclasses.dataclass
 class SglSamplingParams:
     max_new_tokens: int = 128
+    min_new_tokens: int = 0
     stop: Union[str, List[str]] = ()
     stop_token_ids: Optional[List[int]] = ()
     temperature: float = 1.0
@@ -39,6 +40,7 @@ class SglSamplingParams:
     def clone(self):
         return SglSamplingParams(
             self.max_new_tokens,
+            self.min_new_tokens,
             self.stop,
             self.stop_token_ids,
             self.temperature,
@@ -113,6 +115,7 @@ class SglSamplingParams:
     def to_srt_kwargs(self):
         return {
             "max_new_tokens": self.max_new_tokens,
+            "min_new_tokens": self.min_new_tokens,
             "stop": self.stop,
             "stop_token_ids": self.stop_token_ids,
             "temperature": self.temperature,
@@ -424,6 +427,7 @@ class SglGen(SglExpr):
         self,
         name: Optional[str] = None,
         max_new_tokens: Optional[int] = None,
+        min_new_tokens: Optional[int] = None,
         stop: Optional[Union[str, List[str]]] = None,
         stop_token_ids: Optional[List[int]] = None,
         temperature: Optional[float] = None,
@@ -446,6 +450,7 @@ class SglGen(SglExpr):
         self.name = name
         self.sampling_params = SglSamplingParams(
             max_new_tokens=max_new_tokens,
+            min_new_tokens=min_new_tokens,
             stop=stop,
             stop_token_ids=stop_token_ids,
             temperature=temperature,

python/sglang/test/test_programs.py

@@ -517,3 +517,36 @@ def test_hellaswag_select():
     accuracy = np.mean(np.array(preds) == np.array(labels))
 
     return accuracy, latency
+
+
+def test_gen_min_new_tokens():
+    """
+    Validate sgl.gen(min_tokens) functionality.
+
+    The test asks a question where, without a min_tokens constraint, the generated answer is expected to be short.
+    By enforcing the min_tokens parameter, we ensure the generated answer has at least the specified number of tokens.
+    We verify that the number of tokens in the answer is >= the min_tokens threshold.
+    """
+    import sglang as sgl
+    from sglang.srt.hf_transformers_utils import get_tokenizer
+
+    model_path = sgl.global_config.default_backend.endpoint.get_model_name()
+    MIN_TOKENS, MAX_TOKENS = 64, 128
+
+    @sgl.function
+    def convo_1(s):
+        s += sgl.user("What is the capital of the United States?")
+        s += sgl.assistant(
+            sgl.gen("answer", min_tokens=MIN_TOKENS, max_tokens=MAX_TOKENS)
+        )
+
+    def assert_min_tokens(tokenizer, text):
+        token_ids = tokenizer.encode(text)
+        assert (
+            len(token_ids) >= MIN_TOKENS
+        ), f"Generated {len(token_ids)} tokens, min required: {MIN_TOKENS}. Text: {text}"
+
+    tokenizer = get_tokenizer(model_path)
+
+    state = convo_1.run()
+    assert_min_tokens(tokenizer, state["answer"])

test/lang/test_srt_backend.py

@@ -7,6 +7,7 @@ from sglang.test.test_programs import (
     test_dtype_gen,
     test_expert_answer,
     test_few_shot_qa,
+    test_gen_min_new_tokens,
     test_hellaswag_select,
     test_mt_bench,
     test_parallel_decoding,
@@ -69,6 +70,9 @@ class TestSRTBackend(unittest.TestCase):
             accuracy, latency = test_hellaswag_select()
             assert accuracy > 0.71, f"{accuracy=}"
 
+    def test_gen_min_new_tokens(self):
+        test_gen_min_new_tokens()
+
 
 if __name__ == "__main__":
     unittest.main()