[Bugfix] Validate logit biases to prevent out of vocab ids crashing engine (#16529)

Signed-off-by: Ryan McConville <ryan@ryanmcconville.com>

vllm/v1/engine/processor.py

@@ -77,6 +77,7 @@ class Processor:
         params: SamplingParams,
     ) -> None:
         self._validate_structured_output(params)
+        self._validate_logit_bias(params)
 
         if params.allowed_token_ids is None:
             return
@@ -87,6 +88,26 @@ class Processor:
             raise ValueError(
                 "allowed_token_ids contains out-of-vocab token id!")
 
+    def _validate_logit_bias(
+        self,
+        params: SamplingParams,
+    ) -> None:
+        """Validate logit_bias token IDs are within vocabulary range."""
+        if not params.logit_bias:
+            return
+
+        vocab_size = self.model_config.get_vocab_size()
+        invalid_token_ids = []
+
+        for token_id in params.logit_bias:
+            if token_id < 0 or token_id >= vocab_size:
+                invalid_token_ids.append(token_id)
+
+        if invalid_token_ids:
+            raise ValueError(
+                f"token_id(s) {invalid_token_ids} in logit_bias contain "
+                f"out-of-vocab token ids. Vocabulary size: {vocab_size}")
+
     def _validate_supported_sampling_params(
         self,
         params: SamplingParams,