perf(qwen3.5): prefill the empty think block for single-turn models

src/common/AutoModel/automodel.cpp

@@ -232,7 +232,15 @@ void AutoModel::_shared_load_model(std::string model_path, json model_info, int
 }
 
 void AutoModel::_shared_initialize_model_state(
-    std::string model_path, json, int context_length) {
+    std::string model_path, json model_info, int context_length) {
+    // The catalog marks a model single-turn with a label, and some classes set
+    // the flag themselves. Only ever switched on here, so an entry without the
+    // label does not override a class that knows it is single-turn.
+    if (model_info.contains("label") &&
+        std::find(model_info["label"].begin(), model_info["label"].end(),
+                  "single-turn") != model_info["label"].end()) {
+        this->single_turn = true;
+    }
     this->model_path = std::move(model_path);
     header_print("FLM", "Loading model: " << this->model_path);
     this->lm_config = std::make_unique<LM_Config>();
@@ -420,7 +428,13 @@ bool AutoModel::_shared_insert(chat_meta_info_t& meta_info, std::vector<int>& to
             break;
         }
     }
-    if (skip_count != idx) {
+    // A request that matches the whole history leaves nothing to prefill, and
+    // then there are no logits to draw from: the sampler would read an empty
+    // buffer. It happens when a request repeats the previous one exactly and
+    // that answer added nothing to the history -- a single-turn Qwen3.5 model
+    // with the think block prefilled and max_tokens=1. Recurrent state cannot
+    // be stepped back by one token, so treat it like a divergence.
+    if (skip_count != idx || skip_count == tokens.size()) {
         if (is_server_mode) {
             header_print("FLM", "Conversation context diverged from cache, clearing context...");
         }

src/common/AutoModel/modeling_qwen3_5vl.cpp

@@ -287,8 +287,19 @@ bool Qwen3_5VL::insert(chat_meta_info_t& meta_info, lm_uniform_input_t& input, s
         this->token_history = checkpoint_his; // restore the token history to be consistent with the restored KV cache, which is crucial for correct functioning of _shared_insert's prefix-matching logic
     }
 
-    size_t n = tokens.size();
-    tokens.resize(n - (this->enable_think ? 2 : 4));
+    // The template ends with the opening of the assistant turn and, without
+    // thinking, an empty think block. Normally the prefill stops before that
+    // block, so the checkpoint below sits where the next turn of the
+    // conversation will diverge, and generate() feeds the block one forward()
+    // at a time. A single-turn model never comes back to that checkpoint, so
+    // the four extra decode steps buy nothing: prefill the block with the
+    // prompt, and the token drawn from the prefill logits is already the first
+    // token of the answer.
+    this->think_block_prefilled = this->single_turn && !this->enable_think;
+    if (!this->think_block_prefilled) {
+        size_t n = tokens.size();
+        tokens.resize(n - (this->enable_think ? 2 : 4));
+    }
 
     bool success = has_images
         ? this->_shared_insert(meta_info, tokens, is_cancelled, &image_payload, last_image_token_index)
@@ -316,7 +327,16 @@ std::string Qwen3_5VL::generate(chat_meta_info_t& meta_info, int length_limit, s
     this->profiler_list[TKOEN_DECODE_TIME].reset();
     std::string token_str;
     int sampled_token;
-    if(this->enable_think) {
+    if (this->think_block_prefilled) {
+        // insert() prefilled the empty think block; _shared_insert drew the
+        // answer's first token from those logits, under the constraint. That
+        // draw is kept, so nothing is rolled back: the constraint started
+        // behind the closed block (the prompt tail ends with it) and has
+        // already accepted the token.
+        sampled_token = this->last_token;
+        meta_info.insert_sample_pending = false;
+    }
+    else if(this->enable_think) {
         this->token_history.push_back(think_start_id);
         this->profiler_list[DECODING_TIME].start();
         this->_force_token(meta_info, think_start_id);

src/include/AutoModel/modeling_qwen3_5vl.hpp

@@ -28,6 +28,10 @@ class Qwen3_5VL : public AutoModel {
     bool enable_tool = false;
     int think_start_id = 248068;
     int think_end_id = 248069;
+    /// Set by insert() when it prefilled the empty think block together with
+    /// the prompt; generate() then takes the first token from that draw
+    /// instead of feeding the block one forward() at a time.
+    bool think_block_prefilled = false;
 
     void setup_tokenizer(std::string model_path);