chat : tighten bare function parsing for Qwen models (#26793)

common/chat.cpp

@@ -1166,6 +1166,16 @@ static common_chat_params common_chat_params_init_qwen3_coder(const common_chat_
         data.prompt += data.generation_prompt;
     }
 
+    std::vector<std::string> tool_call_starts = { "<tool_call>" };
+
+    // Match complete <function=name> opener for Qwen3-Coder models that occasionally omit the
+    // starting <tool_call>. The model may hallucinate a tool name, but it is preferable over
+    // constraining on <function which may occur in valid content generation, e.g. #include <functional>
+    foreach_function(inputs.tools, [&](const json & tool) {
+        const std::string name = tool.at("function").at("name");
+        tool_call_starts.push_back("<function=" + name + ">");
+    });
+
     auto parser = build_chat_peg_parser([&](common_chat_peg_builder & p) {
         auto generation_prompt = p.literal(GEN_PREFIX);
 
@@ -1238,7 +1248,7 @@ static common_chat_params common_chat_params_init_qwen3_coder(const common_chat_
             auto tool_calls = p.trigger_rule("tool-call-root", p.repeat(calls, min_calls, 1));
 
             return generation_prompt +
-                   (reasoning << p.content(p.until_one_of({ "<tool_call>", "<function=" })) << tool_calls);
+                   (reasoning << p.content(p.until_one_of(tool_call_starts)) << tool_calls);
         }
 
         // Content only parser
@@ -1264,12 +1274,9 @@ static common_chat_params common_chat_params_init_qwen3_coder(const common_chat_
         });
 
         if (data.grammar_lazy) {
-            data.grammar_triggers = {
-                { COMMON_GRAMMAR_TRIGGER_TYPE_WORD, "<tool_call>" },
-                // Trigger on "<function" and not "<function=" because the trailing "=" is part of
-                // the token with the function name e.g. "=read"
-                { COMMON_GRAMMAR_TRIGGER_TYPE_WORD, "<function"   },
-            };
+            for (const auto & start : tool_call_starts) {
+                data.grammar_triggers.push_back({ COMMON_GRAMMAR_TRIGGER_TYPE_WORD, start });
+            }
         }
     }