[router][bugfix] Fix input_logprobs handling with None value and logprob_start_len = -1 (#11113)

sgl-router/src/proto/sglang_scheduler.proto

@@ -175,13 +175,13 @@ message GenerateStreamChunk {
   int32 cached_tokens = 4;
 
   // Output logprobs (if requested) - incremental for streaming
-  LogProbs output_logprobs = 5;
+  OutputLogProbs output_logprobs = 5;
 
   // Hidden states (if requested)
   repeated float hidden_states = 6;
 
   // Input logprobs (if requested) - only in first chunk
-  LogProbs input_logprobs = 7;
+  InputLogProbs input_logprobs = 7;
 }
 
 message GenerateComplete {
@@ -197,7 +197,7 @@ message GenerateComplete {
   int32 cached_tokens = 5;
 
   // Output logprobs if requested (cumulative)
-  LogProbs output_logprobs = 6;
+  OutputLogProbs output_logprobs = 6;
 
   // All hidden states if requested
   repeated HiddenStates all_hidden_states = 7;
@@ -209,7 +209,7 @@ message GenerateComplete {
   }
 
   // Input logprobs if requested (for prompt tokens)
-  LogProbs input_logprobs = 10;
+  InputLogProbs input_logprobs = 10;
 }
 
 message GenerateError {
@@ -218,7 +218,8 @@ message GenerateError {
   string details = 3;
 }
 
-message LogProbs {
+// Output logprobs - all values are present (no None)
+message OutputLogProbs {
   repeated float token_logprobs = 1;
   repeated int32 token_ids = 2;
 
@@ -226,6 +227,20 @@ message LogProbs {
   repeated TopLogProbs top_logprobs = 3;
 }
 
+// Input logprobs - first token has no logprob (None)
+message InputLogProbs {
+  repeated InputTokenLogProb token_logprobs = 1;
+  repeated int32 token_ids = 2;
+
+  // Top logprobs at each position
+  repeated TopLogProbs top_logprobs = 3;
+}
+
+// Wrapper to represent optional logprob (first input token has no logprob)
+message InputTokenLogProb {
+  optional float value = 1;
+}
+
 message TopLogProbs {
   repeated float values = 1;
   repeated int32 token_ids = 2;

sgl-router/src/routers/grpc/router.rs

@@ -1239,7 +1239,7 @@ impl GrpcRouter {
     /// Note: Always decodes with skip_special_tokens=false to show actual tokens generated
     fn convert_proto_to_openai_logprobs(
         &self,
-        proto_logprobs: &proto::LogProbs,
+        proto_logprobs: &proto::OutputLogProbs,
     ) -> Result<crate::protocols::spec::ChatLogProbs, String> {
         let mut content_items = Vec::new();