That's what I get for cargo culting a newline.

Author: createthis

common/chat.cpp

@@ -1415,7 +1415,7 @@ static common_chat_params common_chat_params_init_deepseek_v3_1(const common_cha
                        additional_context);
     data.prompt = prompt;
     data.format = COMMON_CHAT_FORMAT_DEEPSEEK_V3_1;
-    if (string_ends_with(data.prompt, "<think>\n")) {
+    if (string_ends_with(data.prompt, "<think>")) {
         if (!inputs.enable_thinking) {
             data.prompt += "</think>";
         } else {

tests/test-chat-parser.cpp

@@ -282,6 +282,23 @@ static void test_deepseek_v3_1_tool_calls() {
         assert_equals(variant, std::string("REASONING<｜tool▁calls▁begin｜><｜tool▁call▁begin｜>get_time2<｜tool▁sep｜>{\"city\": \"Tokyo2\"}<｜tool▁call▁end｜><｜tool▁calls▁end｜>REASONING"), m.reasoning_content);
     }
 
+    // variant: thinking forced open + tool call in reasoning content + no closing think
+    {
+        common_chat_syntax syntax = {
+            /* .format = */ COMMON_CHAT_FORMAT_DEEPSEEK_V3_1,
+            /* .reasoning_format = */ COMMON_REASONING_FORMAT_DEEPSEEK,
+            /* .reasoning_in_content = */ false,
+            /* .thinking_forced_open = */ true,
+            /* .parse_tool_calls = */ true,
+        };
+        const char* variant = "thinking_forced_open_tool_call_in_reasoning_no_closing_think";
+        const std::string in = "REASONING<｜tool▁calls▁begin｜><｜tool▁call▁begin｜>get_time2<｜tool▁sep｜>{\"city\": \"Tokyo2\"}<｜tool▁call▁end｜><｜tool▁calls▁end｜>REASONING";
+        auto m = common_chat_parse(in, false, syntax);
+        assert_equals<std::size_t>(variant, 0, m.tool_calls.size());
+        assert_equals(variant, std::string(""), m.content);
+        assert_equals(variant, std::string("REASONING<｜tool▁calls▁begin｜><｜tool▁call▁begin｜>get_time2<｜tool▁sep｜>{\"city\": \"Tokyo2\"}<｜tool▁call▁end｜><｜tool▁calls▁end｜>REASONING"), m.reasoning_content);
+    }
+
     // variant: thinking not forced open + missing reasoning + no tool calls
     {
         common_chat_syntax syntax = {