Preserve omitted assistant content during OpenAI-compatible message round-tripping

common/chat.cpp

@@ -226,6 +226,11 @@ json common_chat_msg::to_json_oaicompat(bool concat_typed_text) const {
                 });
             }
         }
+    } else if (!content_present) {
+        // Preserve an omitted OpenAI-compatible content field. Some templates
+        // distinguish this from explicit null and an empty string.
+    } else if (content_is_null) {
+        jmsg["content"] = nullptr;
     } else {
         jmsg["content"] = "";
     }
@@ -390,6 +395,7 @@ std::vector<common_chat_msg> common_chat_msgs_parse_oaicompat(const json & messa
 
             auto has_content    = message.contains("content");
             auto has_tool_calls = message.contains("tool_calls");
+            msg.content_present = has_content;
             if (has_content) {
                 const auto & content = message.at("content");
                 if (content.is_string()) {
@@ -408,7 +414,9 @@ std::vector<common_chat_msg> common_chat_msgs_parse_oaicompat(const json & messa
                         msg_part.text = part.at("text");
                         msg.content_parts.push_back(msg_part);
                     }
-                } else if (!content.is_null()) {
+                } else if (content.is_null()) {
+                    msg.content_is_null = true;
+                } else {
                     throw std::invalid_argument("Invalid 'content' type: expected string or array, got " +
                                                 content.dump() +
                                                 " (ref: https://github.com/ggml-org/llama.cpp/issues/8367)");

common/chat.h

@@ -86,6 +86,8 @@ struct common_chat_msg {
     std::string                               reasoning_content;
     std::string                               tool_name;
     std::string                               tool_call_id;
+    bool                                      content_present = true;
+    bool                                      content_is_null = false;
 
     nlohmann::ordered_json to_json_oaicompat(bool concat_typed_text = false) const;
 
@@ -122,7 +124,8 @@ struct common_chat_msg {
     bool operator==(const common_chat_msg & other) const {
         return role == other.role && content == other.content && content_parts == other.content_parts &&
                tool_calls == other.tool_calls && reasoning_content == other.reasoning_content &&
-               tool_name == other.tool_name && tool_call_id == other.tool_call_id;
+               tool_name == other.tool_name && tool_call_id == other.tool_call_id &&
+               content_present == other.content_present && content_is_null == other.content_is_null;
     }
 
     bool operator!=(const common_chat_msg & other) const { return !(*this == other); }

tests/test-chat.cpp

@@ -1627,6 +1627,28 @@ static void test_msgs_oaicompat_json_conversion() {
     assert_equals<std::string>(res[0].role, "assistant");
     assert_equals(true, res[0].content.empty());
     assert_equals(true, res[0].tool_calls.empty());
+    assert_equals(false, res[0].content_present);
+    assert_equals(
+        std::string("[{\"role\":\"assistant\"}]"),
+        common_chat_msgs_to_json_oaicompat(res).dump());
+
+    auto null_content =
+        common_chat_msgs_parse_oaicompat(json::parse("[{\"role\":\"assistant\",\"content\":null,\"tool_calls\":[]}]"));
+    assert_equals<size_t>(1, null_content.size());
+    assert_equals(true, null_content[0].content_present);
+    assert_equals(true, null_content[0].content_is_null);
+    assert_equals(
+        std::string("[{\"role\":\"assistant\",\"content\":null}]"),
+        common_chat_msgs_to_json_oaicompat(null_content).dump());
+
+    auto empty_content =
+        common_chat_msgs_parse_oaicompat(json::parse("[{\"role\":\"assistant\",\"content\":\"\",\"tool_calls\":[]}]"));
+    assert_equals<size_t>(1, empty_content.size());
+    assert_equals(true, empty_content[0].content_present);
+    assert_equals(false, empty_content[0].content_is_null);
+    assert_equals(
+        std::string("[{\"role\":\"assistant\",\"content\":\"\"}]"),
+        common_chat_msgs_to_json_oaicompat(empty_content).dump());
 
     try {
         common_chat_msgs_parse_oaicompat(json::parse("[{\"role\": \"assistant\"}]"));