diff --git a/serve/mlc_serve/engine/engine_common.py b/serve/mlc_serve/engine/engine_common.py
index 3dd52269b3..21fbb9d044 100644
--- a/serve/mlc_serve/engine/engine_common.py
+++ b/serve/mlc_serve/engine/engine_common.py
@@ -16,6 +16,7 @@
     RequestState,
     SequenceId,
     StoppingCriteria,
+    RawLogprobsInfo,
 )
 from .model_module import (
     DecodeRequest,
@@ -80,8 +81,8 @@ def detokenize_incrementally(
     prompt_tokens: list[int],
     generation_sequence: GenerationSequence,
     tokenizer: TokenizerP,
-    new_token_id=None,
-    skip_special_tokens=False,
+    new_token_id: Optional[int] = None,
+    skip_special_tokens: bool = False,
 ) -> str:
     # tokenizer.decode() is similar to doing convert_tokens_to_string(convert_ids_to_tokens(token_ids))
     # in this function, we separate these two steps.
@@ -89,11 +90,10 @@ def detokenize_incrementally(
     new_token_id = (
         generation_sequence.generated_token_ids[-1] if not is_logprob else new_token_id
     )
-
     # This is the first iteration for this sequence
     if generation_sequence.prev_tokens is None:
         # TODO(masahi): Figure out a way to remove this concat
-        new_tokens: Union[str, List[str]] = tokenizer.convert_ids_to_tokens(
+        new_tokens: List[str] = tokenizer.convert_ids_to_tokens(  # type: ignore
             prompt_tokens + generation_sequence.generated_token_ids
         )
         output_tokens = new_tokens
@@ -109,7 +109,7 @@ def detokenize_incrementally(
             prefix_end_offset = max(len(output_tokens) - 1, 0)
     else:
         # Put new_token_id in a list so skip_special_tokens is respected
-        new_tokens: Union[str, List[str]] = tokenizer.convert_ids_to_tokens(
+        new_tokens: List[str] = tokenizer.convert_ids_to_tokens(  # type: ignore
             [new_token_id]
         )
         output_tokens = generation_sequence.prev_tokens + new_tokens
@@ -170,12 +170,12 @@ def check_stopping_sequences(stopping_criteria, output_text, delta, is_ended):
 
 
 def prepare_logprob(
-    logprob_info,
-    delta,
+    logprob_info: Optional[List[RawLogprobsInfo]],
+    delta: str,
     gen_seq: GenerationSequence,
-    prompt_token_ids,
-    tokenizer,
-):
+    prompt_token_ids: List[int],
+    tokenizer: TokenizerP,
+) -> List[Optional[LogprobsContent]]:
     if logprob_info is None:
         return []
 
@@ -219,7 +219,7 @@ def prepare_output(
     logprob_info,
     tokenizer: TokenizerP,
     stopping_criteria: StoppingCriteria,
-) -> str:
+) -> Tuple[str, List[Optional[LogprobsContent]]]:
     gen_seq.next_start_position = len(prompt_token_ids) + len(
         gen_seq.generated_token_ids
     )
@@ -227,7 +227,7 @@ def prepare_output(
     delta = detokenize_incrementally(prompt_token_ids, gen_seq, tokenizer)
     gen_seq.output_text += delta
 
-    out_logprob_info = prepare_logprob(
+    out_logprob_info: List[Optional[LogprobsContent]] = prepare_logprob(
         logprob_info, delta, gen_seq, prompt_token_ids, tokenizer
     )