diff --git a/CHANGELOG.md b/CHANGELOG.md
index b0a60bb12..a9fff014c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,13 @@
 
 ## [Unreleased]
 
+## [v0.51.179] — 2026-05-30 — Release EY (stage-batch61 — custom-provider reasoning efforts + clearer sidebar tooltips)
+
+### Fixed
+
+- Reasoning effort selector now appears for thinking-capable models served through custom API aggregators (New API, One API, etc.) that use non-standard model naming — bare names like `deepseek-v4-flash` or dot-separated `moonshotai.kimi-k2.5` rather than the OpenRouter-style `vendor/model` slash format. The heuristic now also strips a dot-vendor prefix and recognizes a `thinking`/`reasoning` token anywhere in the model name; plain non-reasoning models stay hidden as before (#3202).
+- Sidebar session row tooltips now explain the fork, prior-turn, child-session, and running/unread status badges, and hovering a truncated chat title shows the full title instead of the old "Double-click to rename" hint (#3203).
+
 ## [v0.51.178] — 2026-05-30 — Release EX (stage-batch60 — parallel sharded CI test runs)
 
 ### Changed
diff --git a/static/sessions.js b/static/sessions.js
index 6ee0bb68a..2ead02c99 100644
--- a/static/sessions.js
+++ b/static/sessions.js
@@ -3339,6 +3339,36 @@ function _sessionTitleForForkParent(parentSid){
   return title;
 }
 
+function _sessionFullTitleTooltip(rawTitle, cleanTitle){
+  const fallback=String(cleanTitle||'Untitled').trim()||'Untitled';
+  const full=String(rawTitle||fallback).trim()||fallback;
+  if(full.startsWith('[SYSTEM:')) return fallback;
+  return full;
+}
+
+function _sessionForkTooltip(parentLabel){
+  const parent=String(parentLabel||'').trim()||'unknown parent';
+  return `Forked conversation — parent: ${parent}`;
+}
+
+function _sessionLineageBadgeTooltip(label, canExpand){
+  const base=String(label||'Prior turns').trim()||'Prior turns';
+  return canExpand
+    ? `${base} — earlier context turns are collapsed here. Click to show or hide them.`
+    : `${base} — earlier context turns are collapsed here.`;
+}
+
+function _sessionChildBadgeTooltip(label){
+  const base=String(label||'Child sessions').trim()||'Child sessions';
+  return `${base} — child conversations spawned from this session. Click to show or hide them.`;
+}
+
+function _sessionStateTooltip({isStreaming=false,hasUnread=false}={}){
+  if(isStreaming) return 'Conversation is running';
+  if(hasUnread) return 'Unread completion';
+  return '';
+}
+
 function _attachChildSessionsToSidebarRows(collapsedRows, rawSessions){
   const rows=(collapsedRows||[]).filter(s=>!_isChildSession(s)).map(s=>({...s}));
   const visibleBySid=new Map();
@@ -4054,7 +4084,7 @@ function renderSessionListFromCache(){
       branchInd.className='session-branch-indicator';
       branchInd.innerHTML=li('git-branch',12);
       const parentLabel=_sessionTitleForForkParent(s.parent_session_id)||_truncatedSessionId(s.parent_session_id);
-      branchInd.title=(typeof t==='function'?t('forked_from'):'Forked from')+' '+parentLabel;
+      branchInd.title=_sessionForkTooltip(parentLabel);
       titleRow.appendChild(branchInd);
     }
     const title=document.createElement('span');
@@ -4063,7 +4093,7 @@ function renderSessionListFromCache(){
     const titleMatched=Boolean(searchQueryRaw&&displayTitle.toLowerCase().includes(searchQueryRaw.toLowerCase()));
     if(titleMatched) _appendHighlightedText(title,displayTitle,searchQueryRaw,'session-search-hit');
     else title.textContent=displayTitle;
-    title.title=readOnly?'Read-only imported session':'Double-click to rename';
+    title.title=_sessionFullTitleTooltip(rawTitle,cleanTitle);
     const tsMs=_sessionTimestampMs(s);
     const ts=document.createElement('span');
     const hasAttentionState=isStreaming||hasUnread||Boolean(attention);
@@ -4099,7 +4129,7 @@ function renderSessionListFromCache(){
       segmentCountEl.className='session-lineage-count'+(canExpandLineageSegments?' expandable':'');
       const segmentLabel=t('session_meta_segments', segmentCount);
       segmentCountEl.textContent=segmentLabel;
-      segmentCountEl.title=segmentLabel;
+      segmentCountEl.title=_sessionLineageBadgeTooltip(segmentLabel,canExpandLineageSegments);
       if(canExpandLineageSegments){
         segmentCountEl.setAttribute('role','button');
         segmentCountEl.setAttribute('tabindex','0');
@@ -4128,7 +4158,7 @@ function renderSessionListFromCache(){
       childCountEl.className='session-child-count';
       const childLabel=t('session_meta_children', childCount);
       childCountEl.textContent=childLabel;
-      childCountEl.title=childLabel;
+      childCountEl.title=_sessionChildBadgeTooltip(childLabel);
       ['pointerdown','pointerup','click'].forEach(ev=>childCountEl.addEventListener(ev,e=>e.stopPropagation()));
       childCountEl.onclick=(e)=>{
         e.stopPropagation();
@@ -4317,6 +4347,7 @@ function renderSessionListFromCache(){
     state.className='session-attention-indicator session-state-indicator'+(isStreaming?' is-streaming':(hasUnread?' is-unread':''))+attentionDotClass;
     if(attention&&attention.title) state.title=attention.title;
     state.setAttribute('aria-hidden','true');
+    state.title=_sessionStateTooltip({isStreaming,hasUnread});
     el.appendChild(state);
     // Single trigger button that opens a shared dropdown menu
     let actions=null;
diff --git a/tests/test_sidebar_tooltips.py b/tests/test_sidebar_tooltips.py
new file mode 100644
index 000000000..a5be58913
--- /dev/null
+++ b/tests/test_sidebar_tooltips.py
@@ -0,0 +1,34 @@
+"""Sidebar tooltip contract tests."""
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).parent.parent.resolve()
+SESSIONS_JS_PATH = REPO_ROOT / "static" / "sessions.js"
+
+
+def _sessions_js() -> str:
+    return SESSIONS_JS_PATH.read_text(encoding="utf-8")
+
+
+def test_session_title_hover_shows_full_title_not_rename_hint():
+    """The truncated sidebar title needs a real full-title tooltip.
+
+    The old "Double-click to rename" title hid the only native hover affordance
+    that can reveal a long chat title when badges/tags squeeze the row.
+    """
+    js = _sessions_js()
+    assert "Double-click to rename" not in js
+    assert "title.title=_sessionFullTitleTooltip(rawTitle,cleanTitle);" in js
+
+
+def test_sidebar_status_badges_have_explanatory_tooltips():
+    """Compact badges/icons must explain what they mean, not repeat the chip text."""
+    js = _sessions_js()
+    assert "function _sessionFullTitleTooltip" in js
+    assert "function _sessionForkTooltip" in js
+    assert "function _sessionLineageBadgeTooltip" in js
+    assert "function _sessionChildBadgeTooltip" in js
+    assert "function _sessionStateTooltip" in js
+    assert "branchInd.title=_sessionForkTooltip(parentLabel);" in js
+    assert "segmentCountEl.title=_sessionLineageBadgeTooltip(segmentLabel,canExpandLineageSegments);" in js
+    assert "childCountEl.title=_sessionChildBadgeTooltip(childLabel);" in js
+    assert "state.title=_sessionStateTooltip({isStreaming,hasUnread});" in js