diff --git a/tests/test_webui_message_ordering_static.py b/tests/test_webui_message_ordering_static.py
new file mode 100644
index 000000000..7c9fae509
--- /dev/null
+++ b/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
diff --git a/webui/index.js b/webui/index.js
index de5b64bc3..715166ee1 100644
--- a/webui/index.js
+++ b/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);
   }
diff --git a/webui/js/AGENTS.md b/webui/js/AGENTS.md
index d1b28e95c..53fc1fb3d 100644
--- a/webui/js/AGENTS.md
+++ b/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
 
diff --git a/webui/js/messages.js b/webui/js/messages.js
index 6ed61338e..92d42e15d 100644
--- a/webui/js/messages.js
+++ b/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(),