fix: preserve output logprobs without input logprobs (#34627)

Signed-off-by: jain-ria <riajain@NVIDIA.com>

python/sglang/srt/managers/tokenizer_manager.py

@@ -2655,11 +2655,9 @@ class TokenizerManager(TokenizerControlMixin, TokenizerManagerScoreMixin):
         recv_obj: BatchStrOutput,
         recv_obj_index: int,
     ):
-        if recv_obj.input_token_logprobs_val is None:
-            return
-
         if (
-            len(recv_obj.input_token_logprobs_val) > 0
+            recv_obj.input_token_logprobs_val is not None
+            and len(recv_obj.input_token_logprobs_val) > 0
             and recv_obj.input_token_logprobs_val[recv_obj_index] is not None
         ):
             state.input_token_logprobs_val.extend(
@@ -2668,12 +2666,16 @@ class TokenizerManager(TokenizerControlMixin, TokenizerManagerScoreMixin):
             state.input_token_logprobs_idx.extend(
                 recv_obj.input_token_logprobs_idx[recv_obj_index]
             )
-        state.output_token_logprobs_val.extend(
+        if (
-            recv_obj.output_token_logprobs_val[recv_obj_index]
+            recv_obj.output_token_logprobs_val is not None
-        )
+            and recv_obj.output_token_logprobs_val[recv_obj_index] is not None
-        state.output_token_logprobs_idx.extend(
+        ):
-            recv_obj.output_token_logprobs_idx[recv_obj_index]
+            state.output_token_logprobs_val.extend(
-        )
+                recv_obj.output_token_logprobs_val[recv_obj_index]
+            )
+            state.output_token_logprobs_idx.extend(
+                recv_obj.output_token_logprobs_idx[recv_obj_index]
+            )
 
         if top_logprobs_num > 0:
             if len(recv_obj.input_top_logprobs_val) > 0:

test/registered/unit/managers/test_flat_raw_top_logprobs.py

@@ -57,6 +57,7 @@ _EXACT_VAL_ROWS = [None, [-0.5, -2.5], [-0.25, -1.5], [-0.125, -4.0]]
 class _TokenizerManagerStub:
     """Borrow the real logprob meta_info methods without a full manager."""
 
+    convert_logprob_style = TokenizerManager.convert_logprob_style
     add_logprob_to_meta_info = TokenizerManager.add_logprob_to_meta_info
     detokenize_logprob_tokens = TokenizerManager.detokenize_logprob_tokens
     detokenize_top_logprobs_tokens = TokenizerManager.detokenize_top_logprobs_tokens
@@ -543,6 +544,32 @@ class TestMetaInfoFromSchedulerArrays(CustomTestCase):
         )
 
 
+class TestTokenizerManagerLogprobs(CustomTestCase):
+    def test_output_logprobs_without_input_logprobs(self):
+        state = _make_state(return_logprob=True, top_logprobs_num=0)
+        recv_obj = SimpleNamespace(
+            input_token_logprobs_val=None,
+            input_token_logprobs_idx=None,
+            output_token_logprobs_val=[[-0.25]],
+            output_token_logprobs_idx=[[42]],
+        )
+        meta_info = {}
+
+        _TokenizerManagerStub().convert_logprob_style(
+            meta_info,
+            state,
+            top_logprobs_num=0,
+            token_ids_logprob=None,
+            return_text_in_logprobs=False,
+            recv_obj=recv_obj,
+            recv_obj_index=0,
+        )
+
+        self.assertEqual(meta_info["input_token_logprobs"], [])
+        self.assertEqual(meta_info["output_token_logprobs"], [(-0.25, 42, None)])
+        self.assertEqual(meta_info["output_token_logprobs_length"], 1)
+
+
 def _make_batch_token_id_output(**overrides) -> BatchTokenIDOutput:
     """A two-request BatchTokenIDOutput with the required fields stubbed."""
     n = 2