common : delegate assistant continuation to underlying template handlers (#23089)

* common : delegate assistant continuation to template handler

* server : implement echo parameter to exclude assistant prefill in the response

* server : fix tests for prefill

* server : use existing llama template

* cont : clean up

Author: aldehir

common/chat-auto-parser-generator.cpp

@@ -43,11 +43,33 @@ common_chat_params peg_generator::generate_parser(const common_chat_template &
                                                   const autoparser &              autoparser) {
     // Create the result structure
     common_chat_params data;
-    data.prompt           = common_chat_template_direct_apply(tmpl, inputs);
-    data.format           = COMMON_CHAT_FORMAT_PEG_NATIVE;
-    data.preserved_tokens = autoparser.preserved_tokens;
+    data.prompt            = common_chat_template_direct_apply(tmpl, inputs);
+    data.generation_prompt = common_chat_template_generation_prompt(tmpl, inputs);
+    data.format            = COMMON_CHAT_FORMAT_PEG_NATIVE;
+    data.preserved_tokens  = autoparser.preserved_tokens;
+
+    std::string parser_generation_prompt = data.generation_prompt;
+
+    if (inputs.continue_final_message != COMMON_CHAT_CONTINUATION_NONE && !inputs.continue_msg.empty()) {
+        // Build up generation prompt manually
+        const auto & msg = inputs.continue_msg;
+
+        if (!autoparser.reasoning.start.empty()) {
+            data.generation_prompt = data.generation_prompt.substr(0, data.generation_prompt.find(autoparser.reasoning.start));
+            data.generation_prompt += autoparser.reasoning.start + msg.reasoning_content;
+            if (inputs.continue_final_message == COMMON_CHAT_CONTINUATION_CONTENT) {
+                data.generation_prompt += autoparser.reasoning.end;
+            }
+        }
+
+        if (inputs.continue_final_message == COMMON_CHAT_CONTINUATION_CONTENT) {
+            data.generation_prompt += msg.render_content();
+        }
+
+        data.prompt += data.generation_prompt;
+    }
 
-    auto parser = autoparser.build_parser(inputs);
+    auto parser = autoparser.build_parser(inputs, parser_generation_prompt);
     data.parser = parser.save();
 
     // Build grammar if tools are present
@@ -87,7 +109,7 @@ common_chat_params peg_generator::generate_parser(const common_chat_template &
     return data;
 }
 
-common_peg_arena autoparser::build_parser(const generation_params & inputs) const {
+common_peg_arena autoparser::build_parser(const generation_params & inputs, const std::string & generation_prompt) const {
     if (!analysis_complete) {
         throw std::invalid_argument("Cannot call build_parser on autoparser without performing analysis first, call analyze_template(...)");
     }
@@ -121,7 +143,7 @@ common_peg_arena autoparser::build_parser(const generation_params & inputs) cons
         } else {
             parser = content.build_parser(ctx);
         }
-        return pure_content ? p.prefix(inputs.generation_prompt, reasoning.start) + parser : p.prefix(inputs.generation_prompt, reasoning.start) << parser;
+        return pure_content ? p.prefix(generation_prompt, reasoning.start) + parser : p.prefix(generation_prompt, reasoning.start) << parser;
     });
 }
 

common/chat-auto-parser.h

@@ -60,16 +60,21 @@ struct generation_params {
     common_reasoning_format               reasoning_format    = COMMON_REASONING_FORMAT_AUTO;
     bool                                  stream              = true;
     std::string                           grammar;
-    bool                                  add_generation_prompt = false;
-    bool                                  enable_thinking       = true;
-    std::chrono::system_clock::time_point now                   = std::chrono::system_clock::now();
-    std::string                           generation_prompt;
+    bool                                  add_generation_prompt  = false;
+    common_chat_continuation              continue_final_message = COMMON_CHAT_CONTINUATION_NONE;
+    common_chat_msg                       continue_msg;
+    bool                                  enable_thinking        = true;
+    std::chrono::system_clock::time_point now                    = std::chrono::system_clock::now();
     json                                  extra_context;
     bool                                  add_bos       = false;
     bool                                  add_eos       = false;
     bool                                  is_inference  = true;
     bool                                  add_inference = false;
     bool                                  mark_input    = true;  // whether to mark input strings in the jinja context
+
+    bool has_continuation() const {
+        return continue_final_message != COMMON_CHAT_CONTINUATION_NONE && !continue_msg.empty();
+    }
 };
 
 // ============================================================================
@@ -386,7 +391,7 @@ struct autoparser {
     void analyze_template(const common_chat_template & tmpl);
 
     // Build the PEG parser for this template
-    common_peg_arena build_parser(const generation_params & inputs) const;
+    common_peg_arena build_parser(const generation_params & inputs, const std::string & generation_prompt) const;
 
   private:
     // Collect tokens from entire analysis to preserve

common/chat-peg-parser.cpp

@@ -785,7 +785,7 @@ common_peg_parser common_chat_peg_builder::prefix(const std::string & s, const s
     if (delimiter.empty()) {
         return literal(s);
     }
-    return literal(s.substr(0, s.rfind(delimiter)));
+    return literal(s.substr(0, s.find(delimiter)));
 }
 
 common_peg_parser common_chat_peg_builder::optspace(const std::string & tag) {