server: add full streaming compliance for Responses API events

- Add sequence_number to ALL streaming events (created, in_progress,
  output_item.added, content_part.added, all delta events)
- Add output_index to all events referencing output items
- Add content_index to content-related events
- Populate full response object in response.created and
  response.in_progress events (was only {id, object, status})
- Add id field to function_call output_item.added events
- Add status: completed to reasoning output_item.done events
- Counter state persisted across streaming chunks via task_result_state

Fixes: spec-compliant client libraries (async-openai) that require
these fields can now parse all streaming events without error.

Refs: ggml-org/llama.cpp#21174 (fumlig review comment)

tools/server/server-task.cpp

@@ -1053,6 +1053,7 @@ json server_task_result_cmpl_final::to_json_oaicompat_resp_stream() {
                 {"type", "reasoning_text"},
             }})},
             {"encrypted_content", ""},
+            {"status",            "completed"},
         };
 
         server_sent_events.push_back(json {
@@ -1428,20 +1429,42 @@ void server_task_result_cmpl_partial::update(task_result_state & state) {
     oai_resp_reasoning_id  = state.oai_resp_reasoning_id;
     oai_resp_message_id    = state.oai_resp_message_id;
     oai_resp_fc_id         = state.oai_resp_fc_id;
+    // seq_num/output_idx: read from state (may have been advanced by previous to_json call)
+    oai_resp_seq_num       = state.oai_resp_seq_num;
+    oai_resp_output_idx    = state.oai_resp_output_idx;
 
     // track if the accumulated message has any reasoning content
     anthropic_has_reasoning = !state.chat_msg.reasoning_content.empty();
 
     // Pre-compute state updates based on diffs (for next chunk)
+    // Also advance seq_num/output_idx to match events that to_json_oaicompat_resp() will emit
+    if (n_decoded == 1) {
+        state.oai_resp_seq_num += 2; // response.created + response.in_progress
+    }
     for (const common_chat_msg_diff & diff : oaicompat_msg_diffs) {
-        if (!diff.reasoning_content_delta.empty() && !state.thinking_block_started) {
+        if (!diff.reasoning_content_delta.empty()) {
-            state.thinking_block_started = true;
+            if (!state.thinking_block_started) {
+                state.thinking_block_started = true;
+                state.oai_resp_seq_num++;    // output_item.added
+                state.oai_resp_output_idx++;
+            }
+            state.oai_resp_seq_num++; // reasoning_text.delta
         }
-        if (!diff.content_delta.empty() && !state.text_block_started) {
+        if (!diff.content_delta.empty()) {
-            state.text_block_started = true;
+            if (!state.text_block_started) {
+                state.text_block_started = true;
+                state.oai_resp_seq_num += 2; // output_item.added + content_part.added
+                state.oai_resp_output_idx++;
+            }
+            state.oai_resp_seq_num++; // output_text.delta
         }
         if (!diff.tool_call_delta.name.empty()) {
             state.oai_resp_fc_id = diff.tool_call_delta.id;
+            state.oai_resp_seq_num++;    // output_item.added
+            state.oai_resp_output_idx++;
+        }
+        if (!diff.tool_call_delta.arguments.empty()) {
+            state.oai_resp_seq_num++; // function_call_arguments.delta
         }
     }
 }
@@ -1583,28 +1606,31 @@ json server_task_result_cmpl_partial::to_json_oaicompat_chat() {
 
 json server_task_result_cmpl_partial::to_json_oaicompat_resp() {
     std::vector<json> events;
+    int & seq_num    = oai_resp_seq_num;
+    int & output_idx = oai_resp_output_idx;
 
     if (n_decoded == 1) {
+        // Build initial response object with all required fields but empty output
+        json initial_resp = build_oai_resp_metadata(
+            oai_resp_id, oaicompat_model, {}, "",
+            n_prompt_tokens, 0, n_prompt_tokens_cache);
+        initial_resp["status"] = "in_progress";
+        initial_resp["completed_at"] = nullptr;
+
         events.push_back(json {
             {"event", "response.created"},
             {"data", json {
-                {"type", "response.created"},
+                {"type",            "response.created"},
-                {"response", json {
+                {"sequence_number", seq_num++},
-                    {"id",     oai_resp_id},
+                {"response",        initial_resp},
-                    {"object", "response"},
-                    {"status", "in_progress"},
-                }},
             }},
         });
         events.push_back(json {
             {"event", "response.in_progress"},
             {"data", json {
-                {"type", "response.in_progress"},
+                {"type",            "response.in_progress"},
-                {"response", json {
+                {"sequence_number", seq_num++},
-                    {"id",     oai_resp_id},
+                {"response",        initial_resp},
-                    {"object", "response"},
-                    {"status", "in_progress"},
-                }},
             }},
         });
     }
@@ -1615,7 +1641,9 @@ json server_task_result_cmpl_partial::to_json_oaicompat_resp() {
                 events.push_back(json {
                     {"event", "response.output_item.added"},
                     {"data", json {
-                        {"type", "response.output_item.added"},
+                        {"type",            "response.output_item.added"},
+                        {"sequence_number", seq_num++},
+                        {"output_index",    output_idx++},
                         {"item", json {
                             {"id",                oai_resp_reasoning_id},
                             {"summary",           json::array()},
@@ -1631,9 +1659,12 @@ json server_task_result_cmpl_partial::to_json_oaicompat_resp() {
             events.push_back(json {
                 {"event", "response.reasoning_text.delta"},
                 {"data", json {
-                    {"type",    "response.reasoning_text.delta"},
+                    {"type",            "response.reasoning_text.delta"},
-                    {"delta",   diff.reasoning_content_delta},
+                    {"sequence_number", seq_num++},
-                    {"item_id", oai_resp_reasoning_id},
+                    {"output_index",    output_idx - 1},
+                    {"content_index",   0},
+                    {"delta",           diff.reasoning_content_delta},
+                    {"item_id",         oai_resp_reasoning_id},
                 }},
             });
         }
@@ -1643,7 +1674,9 @@ json server_task_result_cmpl_partial::to_json_oaicompat_resp() {
                 events.push_back(json {
                     {"event", "response.output_item.added"},
                     {"data", json {
-                        {"type", "response.output_item.added"},
+                        {"type",            "response.output_item.added"},
+                        {"sequence_number", seq_num++},
+                        {"output_index",    output_idx++},
                         {"item", json {
                             {"content", json::array()},
                             {"id",      oai_resp_message_id},
@@ -1656,8 +1689,11 @@ json server_task_result_cmpl_partial::to_json_oaicompat_resp() {
                 events.push_back(json {
                     {"event", "response.content_part.added"},
                     {"data", json {
-                        {"type",    "response.content_part.added"},
+                        {"type",            "response.content_part.added"},
-                        {"item_id", oai_resp_message_id},
+                        {"sequence_number", seq_num++},
+                        {"output_index",    output_idx - 1},
+                        {"content_index",   0},
+                        {"item_id",         oai_resp_message_id},
                         {"part", json {
                             {"type", "output_text"},
                             {"text", ""},
@@ -1669,9 +1705,12 @@ json server_task_result_cmpl_partial::to_json_oaicompat_resp() {
             events.push_back(json {
                 {"event", "response.output_text.delta"},
                 {"data", json {
-                    {"type",    "response.output_text.delta"},
+                    {"type",            "response.output_text.delta"},
-                    {"item_id", oai_resp_message_id},
+                    {"sequence_number", seq_num++},
-                    {"delta",   diff.content_delta},
+                    {"output_index",    output_idx - 1},
+                    {"content_index",   0},
+                    {"item_id",         oai_resp_message_id},
+                    {"delta",           diff.content_delta},
                 }},
             });
         }
@@ -1680,10 +1719,13 @@ json server_task_result_cmpl_partial::to_json_oaicompat_resp() {
             events.push_back(json {
                 {"event", "response.output_item.added"},
                 {"data", json {
-                    {"type",  "response.output_item.added"},
+                    {"type",            "response.output_item.added"},
+                    {"sequence_number", seq_num++},
+                    {"output_index",    output_idx++},
                     {"item", json {
+                        {"id",        "fc_" + random_string()},
                         {"arguments", ""},
-                        {"call_id",   "fc_" + diff.tool_call_delta.id},
+                        {"call_id",   diff.tool_call_delta.id},
                         {"name",      diff.tool_call_delta.name},
                         {"type",      "function_call"},
                         {"status",    "in_progress"},
@@ -1697,9 +1739,11 @@ json server_task_result_cmpl_partial::to_json_oaicompat_resp() {
             events.push_back(json {
                 {"event", "response.function_call_arguments.delta"},
                 {"data", json {
-                    {"type",    "response.function_call_arguments.delta"},
+                    {"type",            "response.function_call_arguments.delta"},
-                    {"delta",   diff.tool_call_delta.arguments},
+                    {"sequence_number", seq_num++},
-                    {"item_id", "fc_" + oai_resp_fc_id},
+                    {"output_index",    output_idx - 1},
+                    {"delta",           diff.tool_call_delta.arguments},
+                    {"item_id",         "fc_" + oai_resp_fc_id},
                 }},
             });
         }

tools/server/server-task.h

@@ -110,6 +110,8 @@ struct task_result_state {
     const std::string oai_resp_reasoning_id;
     const std::string oai_resp_message_id;
     std::string oai_resp_fc_id; // function call ID for current args delta
+    int oai_resp_seq_num    = 0; // monotonically increasing per-stream
+    int oai_resp_output_idx = 0; // tracks current output item index
 
     task_result_state(const common_chat_parser_params & chat_parser_params)
         : chat_parser_params(chat_parser_params)
@@ -385,6 +387,7 @@ struct server_task_result_cmpl_final : server_task_result {
         oai_resp_id = state.oai_resp_id;
         oai_resp_reasoning_id = state.oai_resp_reasoning_id;
         oai_resp_message_id = state.oai_resp_message_id;
+        oai_resp_seq_num = state.oai_resp_seq_num;
     }
 
     json to_json_non_oaicompat();
@@ -437,6 +440,8 @@ struct server_task_result_cmpl_partial : server_task_result {
     std::string oai_resp_reasoning_id;
     std::string oai_resp_message_id;
     std::string oai_resp_fc_id;
+    int         oai_resp_seq_num    = 0;
+    int         oai_resp_output_idx = 0;
 
     // for Anthropic API: track if any reasoning content has been generated
     bool anthropic_has_reasoning = false;