diff --git a/static/messages.js b/static/messages.js
index f4d54845..171034ab 100644
--- a/static/messages.js
+++ b/static/messages.js
@@ -5030,9 +5030,21 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
         const isSessionViewed=_isSessionActivelyViewed(activeSid);
         const completedSession=d.session||{session_id:activeSid};
         const completedSid=completedSession.session_id||activeSid;
+        const completedMessageCount=completedSession.message_count != null
+          ? completedSession.message_count
+          : (
+            Array.isArray(completedSession.messages)
+              ? completedSession.messages.length
+              : (
+                (S.session&&((S.session.session_id||activeSid)===completedSid)&&S.session.message_count != null)
+                  ? S.session.message_count
+                  : ((Array.isArray(S.messages)&&S.messages.length)||0)
+              )
+          );
         if(!isSessionViewed && typeof _markSessionCompletionUnread==='function'){
           _markSessionCompletionUnread(completedSid, completedSession.message_count);
         }
+        if(isSessionViewed) _markSessionViewed(completedSid, completedMessageCount);
         _clearOwnerInflightState();
         if(typeof _markSessionCompletedInList==='function'){
           _markSessionCompletedInList(completedSession, activeSid);
@@ -5175,7 +5187,7 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
           // No-reply guard (#373): if agent returned nothing, show inline error
           if(!S.messages.some(m=>m.role==='assistant'&&String(m.content||'').trim())&&!assistantText){removeThinking();S.messages.push({role:'assistant',content:'**No response received.** Check your API key and model selection.'});}
           if(_markerOnlyAssistantError&&typeof showToast==='function') showToast('No response received after context compression. Please retry.',5000,'error');
-          if(isSessionViewed) _markSessionViewed(completedSid, completedSession.message_count ?? S.messages.length);
+          if(isSessionViewed) _markSessionViewed(completedSid, completedSession.message_count ?? completedMessageCount);
           // Cooldown: prevent refreshActiveSessionIfExternallyUpdated from
           // force-reloading immediately after "done" — the event already
           // delivered the final messages and tool calls.
diff --git a/tests/test_issue5273_active_done_viewed_sync.py b/tests/test_issue5273_active_done_viewed_sync.py
new file mode 100644
index 00000000..13424281
--- /dev/null
+++ b/tests/test_issue5273_active_done_viewed_sync.py
@@ -0,0 +1,254 @@
+import json
+import shutil
+import subprocess
+import textwrap
+from pathlib import Path
+
+import pytest
+
+
+ROOT = Path(__file__).resolve().parent.parent
+MESSAGES_JS = (ROOT / "static" / "messages.js").read_text(encoding="utf-8")
+SESSIONS_JS = (ROOT / "static" / "sessions.js").read_text(encoding="utf-8")
+
+
+def _function_body(src: str, name: str) -> str:
+    marker = f"function {name}"
+    start = src.index(marker)
+    brace = src.index("{", start)
+    depth = 0
+    for idx in range(brace, len(src)):
+        ch = src[idx]
+        if ch == "{":
+            depth += 1
+        elif ch == "}":
+            depth -= 1
+            if depth == 0:
+                return src[brace + 1 : idx]
+    raise AssertionError(f"function {name} body not found")
+
+
+def _extract_function(src: str, name: str) -> str:
+    marker = f"function {name}"
+    start = src.index(marker)
+    body = _function_body(src, name)
+    sig = src[start : src.index("{", start)]
+    return f"{sig}{{{body}}}"
+
+
+def _done_handler_body() -> str:
+    marker = "source.addEventListener('done',e=>{"
+    start = MESSAGES_JS.index(marker)
+    brace = MESSAGES_JS.index("{", start)
+    depth = 0
+    for idx in range(brace, len(MESSAGES_JS)):
+        ch = MESSAGES_JS[idx]
+        if ch == "{":
+            depth += 1
+        elif ch == "}":
+            depth -= 1
+            if depth == 0:
+                return MESSAGES_JS[brace + 1 : idx]
+    raise AssertionError("done handler body not found")
+
+
+def _has_pre_list_view_sync() -> bool:
+    body = _done_handler_body()
+    list_call = "_markSessionCompletedInList(completedSession, activeSid);"
+    viewed_call = "_markSessionViewed(completedSid,"
+    assert list_call in body, "done handler must update the cached sidebar row"
+    assert viewed_call in body, "done handler must sync viewed state for active sessions"
+    return body.index(viewed_call) < body.index(list_call)
+
+
+def _run_done_compaction_harness(
+    *,
+    is_session_viewed: bool,
+    include_message_count: bool = True,
+    include_completed_messages: bool = True,
+    session_sid_matches_completed: bool = True,
+    session_message_count: int | None = 4,
+    visible_message_count: int = 4,
+) -> dict:
+    helpers = "\n".join(
+        [
+            _extract_function(MESSAGES_JS, "_markSessionViewed"),
+            _extract_function(SESSIONS_JS, "_getSessionViewedCounts"),
+            _extract_function(SESSIONS_JS, "_saveSessionViewedCounts"),
+            _extract_function(SESSIONS_JS, "_setSessionViewedCount"),
+            _extract_function(SESSIONS_JS, "_getSessionCompletionUnread"),
+            _extract_function(SESSIONS_JS, "_saveSessionCompletionUnread"),
+            _extract_function(SESSIONS_JS, "_markSessionCompletionUnread"),
+            _extract_function(SESSIONS_JS, "_clearSessionCompletionUnread"),
+            _extract_function(SESSIONS_JS, "_hasSessionCompletionUnread"),
+            _extract_function(SESSIONS_JS, "_hasUnreadForSession"),
+            _extract_function(SESSIONS_JS, "_markSessionCompletedInList"),
+        ]
+    )
+    has_pre_list_sync = _has_pre_list_view_sync()
+    harness = textwrap.dedent(
+        f"""
+        const SESSION_VIEWED_COUNTS_KEY='session-viewed-counts';
+        const SESSION_COMPLETION_UNREAD_KEY='session-completion-unread';
+        let _sessionViewedCounts=null;
+        let _sessionCompletionUnread=null;
+        const visibleMessages=Array.from({{length:{visible_message_count}}}, (_, idx)=>({{
+          role:'assistant',
+          content:`message-${{idx + 1}}`,
+        }}));
+        let _allSessions=[{{
+          session_id:'active-before-compact',
+          message_count:3,
+          last_message_at:10,
+          updated_at:10,
+          is_streaming:true,
+        }}];
+        const _sessionStreamingById=new Map();
+        const _sessionListSnapshotById=new Map();
+        const _sessionListSourceById=new Map();
+        const storage=new Map();
+        const localStorage={{
+          getItem(key){{ return storage.has(key) ? storage.get(key) : null; }},
+          setItem(key, value){{ storage.set(key, String(value)); }},
+          removeItem(key){{ storage.delete(key); }},
+        }};
+        function renderSessionListFromCache(){{}}
+        function _forgetObservedStreamingSession(){{}}
+        function _rememberSessionListSource(){{}}
+        {helpers}
+        _setSessionViewedCount('active-after-compact', 3);
+        const activeSid='active-before-compact';
+        const completedSid='active-after-compact';
+        const S={{
+          session:{{session_id:{json.dumps("active-after-compact" if session_sid_matches_completed else "different-session")}}},
+          messages:visibleMessages,
+        }};
+        if ({json.dumps(session_message_count is not None)}) S.session.message_count = {json.dumps(session_message_count)};
+        const completedSession={{
+          session_id:completedSid,
+          updated_at:20,
+          last_message_at:20,
+        }};
+        if ({str(include_completed_messages).lower()}) completedSession.messages = visibleMessages.slice();
+        if ({str(include_message_count).lower()}) completedSession.message_count = 4;
+        const completedMessageCount = completedSession.message_count != null
+          ? completedSession.message_count
+          : (Array.isArray(completedSession.messages)
+              ? completedSession.messages.length
+              : ((S.session&&((S.session.session_id||activeSid)===completedSid)&&S.session.message_count != null)
+                  ? S.session.message_count
+                  : ((Array.isArray(S.messages)&&S.messages.length)||0)));
+        if ({str(is_session_viewed).lower()} && {json.dumps(has_pre_list_sync)}) {{
+          _markSessionViewed(completedSid, completedMessageCount);
+        }}
+        if (!{str(is_session_viewed).lower()}) {{
+          _markSessionCompletionUnread(completedSid, completedSession.message_count);
+        }}
+        _markSessionCompletedInList(completedSession, activeSid);
+        const cacheRow=_allSessions.find(s=>s&&s.session_id===completedSid);
+        const unreadAfterCacheUpdate=_hasUnreadForSession(cacheRow);
+        const viewedCountAfterCacheUpdate=_getSessionViewedCounts()[completedSid] ?? null;
+        if ({str(is_session_viewed).lower()}) {{
+          _markSessionViewed(completedSid, completedSession.message_count ?? completedMessageCount);
+        }}
+        console.log(JSON.stringify({{
+          hasPreListSync:{json.dumps(has_pre_list_sync)},
+          unreadAfterCacheUpdate,
+          viewedCountAfterCacheUpdate,
+          unreadAfterActiveBranchSync:_hasUnreadForSession(cacheRow),
+          hasCompletionUnread:_hasSessionCompletionUnread(completedSid),
+          viewedCount:_getSessionViewedCounts()[completedSid] ?? null,
+          cacheRow,
+        }}));
+        """
+    )
+    result = subprocess.run(
+        ["node", "-e", harness],
+        capture_output=True,
+        text=True,
+        timeout=30,
+        check=False,
+    )
+    assert result.returncode == 0, result.stderr
+    return json.loads(result.stdout.strip())
+
+
+@pytest.mark.skipif(shutil.which("node") is None, reason="node required for behavioral test")
+def test_actively_viewed_done_completion_stays_read_after_sidebar_cache_update():
+    result = _run_done_compaction_harness(is_session_viewed=True)
+
+    assert result["cacheRow"]["session_id"] == "active-after-compact"
+    assert result["cacheRow"]["message_count"] == 4
+    assert result["unreadAfterCacheUpdate"] is False, (
+        "actively viewed done settle must sync viewed_count before the sidebar cache "
+        "re-render sees the higher compacted message_count"
+    )
+    assert result["viewedCountAfterCacheUpdate"] == 4
+    assert result["unreadAfterActiveBranchSync"] is False
+    assert result["hasCompletionUnread"] is False
+    assert result["viewedCount"] == 4
+
+
+@pytest.mark.skipif(shutil.which("node") is None, reason="node required for behavioral test")
+def test_actively_viewed_done_completion_uses_messages_fallback_when_message_count_missing():
+    result = _run_done_compaction_harness(
+        is_session_viewed=True,
+        include_message_count=False,
+    )
+
+    assert result["cacheRow"]["session_id"] == "active-after-compact"
+    assert result["cacheRow"]["message_count"] == 4
+    assert result["unreadAfterCacheUpdate"] is False
+    assert result["viewedCountAfterCacheUpdate"] == 4
+    assert result["unreadAfterActiveBranchSync"] is False
+    assert result["hasCompletionUnread"] is False
+    assert result["viewedCount"] == 4
+
+
+@pytest.mark.skipif(shutil.which("node") is None, reason="node required for behavioral test")
+def test_actively_viewed_done_completion_uses_session_fallback_when_payload_counts_are_missing():
+    result = _run_done_compaction_harness(
+        is_session_viewed=True,
+        include_message_count=False,
+        include_completed_messages=False,
+    )
+
+    assert result["cacheRow"]["session_id"] == "active-after-compact"
+    assert result["cacheRow"]["message_count"] == 3
+    assert result["unreadAfterCacheUpdate"] is False
+    assert result["viewedCountAfterCacheUpdate"] == 4
+    assert result["unreadAfterActiveBranchSync"] is False
+    assert result["hasCompletionUnread"] is False
+    assert result["viewedCount"] == 4
+
+
+@pytest.mark.skipif(shutil.which("node") is None, reason="node required for behavioral test")
+def test_actively_viewed_done_completion_uses_visible_messages_fallback_when_session_count_is_missing():
+    result = _run_done_compaction_harness(
+        is_session_viewed=True,
+        include_message_count=False,
+        include_completed_messages=False,
+        session_sid_matches_completed=False,
+        session_message_count=None,
+    )
+
+    assert result["cacheRow"]["session_id"] == "active-after-compact"
+    assert result["cacheRow"]["message_count"] == 3
+    assert result["unreadAfterCacheUpdate"] is False
+    assert result["viewedCountAfterCacheUpdate"] == 4
+    assert result["unreadAfterActiveBranchSync"] is False
+    assert result["hasCompletionUnread"] is False
+    assert result["viewedCount"] == 4
+
+
+@pytest.mark.skipif(shutil.which("node") is None, reason="node required for behavioral test")
+def test_background_done_completion_stays_unread_after_sidebar_cache_update():
+    result = _run_done_compaction_harness(is_session_viewed=False)
+
+    assert result["cacheRow"]["session_id"] == "active-after-compact"
+    assert result["cacheRow"]["message_count"] == 4
+    assert result["unreadAfterCacheUpdate"] is True
+    assert result["viewedCountAfterCacheUpdate"] == 3
+    assert result["unreadAfterActiveBranchSync"] is True
+    assert result["hasCompletionUnread"] is True
+    assert result["viewedCount"] == 3