common/parser: fix nasty bug causing subtle corruption of generation prompt (#20825)

common/chat-auto-parser-helpers.cpp

@@ -188,6 +188,21 @@ diff_split calculate_diff_split(const std::string & left, const std::string & ri
         result.suffix = "";
         // pick prefix = all as representation
     }
+
+    // When left has no unique content (result.left is empty), left is entirely
+    // shared with right. The simultaneous prefix/suffix segment matching can
+    // incorrectly consume trailing segments of left as suffix when those same
+    // segments also appear at the end of right (e.g. "\n" at the end of both
+    // the shared content and the generation prompt). This rotates the diff.
+    // Fix: if left is a prefix of right, enforce that directly.
+    if (result.left.empty() && !result.right.empty() &&
+            left.size() <= right.size() &&
+            right.substr(0, left.size()) == left) {
+        result.prefix = left;
+        result.suffix = "";
+        result.right  = right.substr(left.size());
+    }
+
     return result;
 }
 

tests/test-chat-auto-parser.cpp

@@ -22,6 +22,7 @@ static void test_calculate_diff_split_no_common(testing & t);
 static void test_calculate_diff_split_single_char(testing & t);
 static void test_calculate_diff_split_overlaps(testing & t);
 static void test_calculate_diff_split_tag_boundaries(testing & t);
+static void test_calculate_diff_split_generation_prompt(testing & t);
 static void test_calculate_diff_split(testing & t);
 
 static void test_until_common_prefix_basic(testing & t);
@@ -179,6 +180,7 @@ static void test_calculate_diff_split(testing & t) {
     t.test("calculate_diff_split single char", test_calculate_diff_split_single_char);
     t.test("calculate_diff_split overlaps", test_calculate_diff_split_overlaps);
     t.test("calculate_diff_split tag boundaries", test_calculate_diff_split_tag_boundaries);
+    t.test("calculate_diff_split generation prompt", test_calculate_diff_split_generation_prompt);
 }
 
 static void test_calculate_diff_split_basic(testing & t) {
@@ -502,6 +504,39 @@ static void test_calculate_diff_split_tag_boundaries(testing & t) {
     }
 }
 
+static void test_calculate_diff_split_generation_prompt(testing & t) {
+    // ChatML thinking template: left is a prefix of right, generation_prompt is the appended part.
+    // The trailing \n in left matches the trailing \n in the generation_prompt, causing
+    // the suffix matcher to steal it and rotate the diff result.
+    {
+        // Simplified reproduction: left ends with \n, right = left + "<|im_start|>assistant\n<think>\n"
+        std::string left  = "<|im_start|>user\nHello<|im_end|>\n";
+        std::string right = left + "<|im_start|>assistant\n<think>\n";
+        diff_split result = calculate_diff_split(left, right);
+        t.assert_equal("chatml prefix", left, result.prefix);
+        t.assert_equal("chatml left", "", result.left);
+        t.assert_equal("chatml right should be generation prompt",
+                       "<|im_start|>assistant\n<think>\n", result.right);
+        t.assert_equal("chatml suffix", "", result.suffix);
+    }
+
+    {
+        // More realistic: longer conversation ending with tool_response
+        std::string common =
+            "<|im_start|>system\nYou are a helpful assistant.<|im_end|>\n"
+            "<|im_start|>user\nSearch for files<|im_end|>\n"
+            "<|im_start|>assistant\n<think>\nLet me search.\n</think>\n\n"
+            "<tool_call>\n<function=search>\n</function>\n</tool_call><|im_end|>\n"
+            "<|im_start|>user\n<tool_response>\nNo files found\n</tool_response><|im_end|>\n";
+        std::string left  = common;
+        std::string right = common + "<|im_start|>assistant\n<think>\n";
+        diff_split result = calculate_diff_split(left, right);
+        t.assert_equal("tool_response left", "", result.left);
+        t.assert_equal("tool_response right should be generation prompt",
+                       "<|im_start|>assistant\n<think>\n", result.right);
+    }
+}
+
 static void test_until_common_prefix(testing & t) {
     t.test("until_common_prefix basic", test_until_common_prefix_basic);
 }

tests/test-chat.cpp

@@ -1337,7 +1337,7 @@ static void test_template_output_peg_parsers(bool detailed_debug) {
         tst.test("I'm\nthinking\n</think>\nHello, world!\nWhat's up?")
             .enable_thinking(true)
             .reasoning_format(COMMON_REASONING_FORMAT_NONE)
-            .expect_content("<think>I'm\nthinking\n</think>\nHello, world!\nWhat's up?")
+            .expect_content("<think>\nI'm\nthinking\n</think>\nHello, world!\nWhat's up?")
             .run();
 
         tst.test("I'm\nthinking\n</think>\nHello, world!\nWhat's up?")

tools/server/server-task.cpp

@@ -415,6 +415,7 @@ task_params server_task::params_from_json_cmpl(
         params.chat_parser_params.reasoning_in_content = params.stream && (reasoning_format == COMMON_REASONING_FORMAT_DEEPSEEK_LEGACY);
         params.chat_parser_params.generation_prompt = json_value(data, "generation_prompt", std::string());
         params.sampling.generation_prompt = params.chat_parser_params.generation_prompt;
+        SRV_DBG("Generation prompt: '%s'\n", params.chat_parser_params.generation_prompt.c_str());
         params.chat_parser_params.parse_tool_calls = json_value(data, "parse_tool_calls", false);
         if (data.contains("chat_parser")) {
             params.chat_parser_params.parser.load(data.at("chat_parser").get<std::string>());