Fix WebUI message replay ordering

Treat full log snapshots as authoritative replays by clearing the message DOM when the snapshot starts at log no 0, then render logs in backend order. This prevents streamed responses from staying above earlier welcome/user messages after sync races while keeping incremental updates patch-based.

tests/test_webui_message_ordering_static.py

@@ -0,0 +1,24 @@
+from pathlib import Path
+
+
+PROJECT_ROOT = Path(__file__).resolve().parents[1]
+
+
+def read(*parts: str) -> str:
+    return (PROJECT_ROOT / Path(*parts)).read_text(encoding="utf-8")
+
+
+def test_full_log_replays_replace_existing_message_dom():
+    index_js = read("webui", "index.js")
+    messages_js = read("webui", "js", "messages.js")
+
+    assert "snapshot.logs?.[0]?.no === 0" in index_js
+    assert 'chatHistoryEl.innerHTML = "";' in index_js
+    assert "messages.sort((a, b)" in messages_js
+
+
+def test_message_ordering_fix_does_not_add_renderer_cache_state():
+    messages_js = read("webui", "js", "messages.js")
+
+    assert "_messageCacheByNo" not in messages_js
+    assert "resetMessageRenderState" not in messages_js

webui/index.js

@@ -348,6 +348,10 @@ export async function applySnapshot(snapshot, options = {}) {
 
   if (lastLogVersion != snapshot.log_version) {
     updated = true;
+    if (snapshot.logs?.[0]?.no === 0) {
+      const chatHistoryEl = document.getElementById("chat-history");
+      if (chatHistoryEl) chatHistoryEl.innerHTML = "";
+    }
     await setMessages(snapshot.logs);
     afterMessagesUpdate(snapshot.logs);
   }

webui/js/AGENTS.md

@@ -39,6 +39,7 @@
 - Frontend extension hooks such as `confirm_dialog_after_render` and `get_tool_message_handler` must preserve their mutable context contracts.
 - Sanitize or safely render user/model-provided HTML and markdown.
 - Do not expose secrets in localStorage, console logs, URLs, or WebSocket payloads.
+- Full message snapshots that start at backend log `no` 0 must replace the current message DOM before rendering; incremental snapshots should keep patching existing messages.
 
 ## Work Guidance
 

webui/js/messages.js

@@ -137,6 +137,9 @@ export async function getMessageHandler(type) {
 // entrypoint called from poll/WS communication, this is how all messages are rendered and updated
 // input is raw log format
 export async function setMessages(messages) {
+  messages = Array.isArray(messages) ? [...messages].filter(Boolean) : [];
+  messages.sort((a, b) => (a.no ?? Number.MAX_SAFE_INTEGER) - (b.no ?? Number.MAX_SAFE_INTEGER));
+
   const context = {
     messages,
     history: getChatHistoryEl(),