diff --git a/CHANGELOG.md b/CHANGELOG.md
index bd8e37d2..f176ea99 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -27,6 +27,8 @@
 
 ### Added
 
+- **Extension API: session-open interception + transcript rendering.** Two new extension-facing hooks let extensions react to session navigation and render message transcripts without DOM hacking. `window.registerHermesSessionOpenHandler(fn)` fires when a session opens — with `{preload:true}` a handler may return `{cancel:true}` to abort the navigation cleanly (no side-effect: no sidebar close, profile switch, stream open, transcript clear, or URL change on any entry path), and with `{loaded:true}` it runs after the session is fully populated. `window.renderTranscript(container, messages, opts)` renders an array of messages into any container, routing content through the same sanitized `renderMd`/`SAFE_TAGS` pipeline the chat uses (with a `textContent` fallback), so it carries no new XSS surface. Follows the established `registerHermesTtsEngine` registration pattern. Thanks @ChonSong. (#5508)
+
 - **Pinch-to-zoom for Mermaid diagrams on touch devices.** The zoomable Mermaid viewer (which already had mouse-wheel zoom + single-finger pan) now supports two-finger pinch-to-zoom on phones/tablets, anchored on the pinch midpoint. Pointer/drag handlers are guarded so an active pinch never fights the pan gesture; desktop mouse-wheel zoom and single-finger pan are unchanged. Uses Touch Events for iOS Safari robustness. Thanks @silent-reader-cn. (#5913)
 
 - **Collapsed `read_file` tool rows now show the requested line range.** When the agent reads a slice of a file (`read_file` with `offset`/`limit`), the collapsed tool label now appends the range — e.g. `Read hermes_constants.py · L321-480`, or `L42` for an offset-only read — so you can see which part of a large file was read without expanding the row. Full-file reads and any tool call without a trustworthy positive-integer range keep the plain filename label (invalid, non-integer, non-positive, and overflowing ranges all fall back safely). Only `read_file` is affected. Thanks @franksong2702. (#6062)
diff --git a/static/boot.js b/static/boot.js
index 0a8f7774..8e9a25ad 100644
--- a/static/boot.js
+++ b/static/boot.js
@@ -1412,6 +1412,68 @@ window._hermesTtsSynth=function(id, text, opts){
     });
 };
 
+// ── Session-open hook (for extensions) ────────────────────────────────────
+var _HERMES_SESSION_OPEN_HANDLERS=[];
+window.registerHermesSessionOpenHandler=function(fn){
+  if(typeof fn!=='function') return false;
+  if(_HERMES_SESSION_OPEN_HANDLERS.indexOf(fn)>=0) return false;
+  _HERMES_SESSION_OPEN_HANDLERS.push(fn);
+  return true;
+};
+window._hermesNotifySessionOpen=function(sid, data, opts){
+  opts=opts||{};
+  for(var i=0;i<_HERMES_SESSION_OPEN_HANDLERS.length;i++){
+    try{
+      var result=_HERMES_SESSION_OPEN_HANDLERS[i](sid, data, opts);
+      if(opts.preload===true && result&&result.cancel===true) return {cancel:true};
+    }catch(_){}
+  }
+  return {};
+};
+
+// ── Transcript renderer (for extensions) ───────────────────────────────────
+window.renderTranscript=function(container, messages, opts){
+  if(!container||!Array.isArray(messages)) return container;
+  opts=opts||{};
+  container.innerHTML='';
+  var md=window.renderMd||null;
+  for(var i=0;i<messages.length;i++){
+    var msg=messages[i];
+    if(!msg||!msg.role||msg.role==='tool') continue;
+    var content;
+    if(typeof msg.content==='string'){
+      content=msg.content;
+    }else if(msg.content==null){
+      content='';
+    }else if(Array.isArray(msg.content)){
+      // Multi-part content (OpenAI/Anthropic API style) — concatenate text parts.
+      content=msg.content.map(function(p){return (p&&typeof p.text==='string')?p.text:''}).join('');
+    }else{
+      content=String(msg.content);
+    }
+    if(!content&&opts.skipEmpty) continue;
+    var row=document.createElement('div');
+    row.className='msg-row';
+    row.setAttribute('data-role',msg.role);
+    var body=document.createElement('div');
+    body.className='msg-body';
+    try{
+      if(md){
+        var html=md(content);
+        if(html!=null){body.innerHTML=html}else{body.textContent=content}
+      }else{
+        body.textContent=content;
+      }
+    }catch(_){body.textContent=content}
+    row.appendChild(body);
+    container.appendChild(row);
+  }
+  if(typeof _rehydrateTransparentStreamDom==='function'){
+    try{_rehydrateTransparentStreamDom(container);}catch(_){}
+  }
+  return container;
+};
+
 // ── Turn-based voice mode (#1333) ────────────────────────────────────────
 // Chained flow: listen → send → (agent processes) → TTS response → listen again
 (function(){
@@ -3497,7 +3559,7 @@ window._mirrorSpeechSettingsFromServer=_mirrorSpeechSettingsFromServer;
   }
   if(typeof fetchReasoningChip==='function'&&(!_profileSwitchCompleted||!_profileSwitchChangedProfile)) fetchReasoningChip();
   // Fetch available models without blocking session restore. The static HTML
-  // options are enough for first paint; the dynamic provider list can settle
+  // options enough for first paint; the dynamic provider list can settle
   // after the saved session is visible.
   const _redirectBootModelDropdownIfUnauth=(res)=>{
     if(!res||res.status!==401) return false;
diff --git a/static/sessions.js b/static/sessions.js
index 8bacdb90..8c893735 100644
--- a/static/sessions.js
+++ b/static/sessions.js
@@ -1627,10 +1627,22 @@ async function _switchProfileForSessionLoad(profile){
 
 async function loadSession(sid){
   const opts = arguments[1] || {};
+  // Resolve canonical lineage SID BEFORE both the direct and sidebar preload
+  // notifications so extensions always see the canonical session id, not the
+  // raw sidebar click id (which may differ after lineage folding).
   if(!opts.skipLineageResolve && typeof _resolveSessionIdFromSidebarLineage==='function'){
     const resolvedSid=_resolveSessionIdFromSidebarLineage(sid);
     if(resolvedSid&&resolvedSid!==sid) sid=resolvedSid;
   }
+  // Extension pre-open hook — fires once per sidebar click, not on every call.
+  // _openSidebarSession passes _preloadNotified:true so the hook isn't re-fired
+  // when loadSession runs the actual navigation inside it.
+  if(!opts.skipExtHooks && !opts._preloadNotified && typeof _hermesNotifySessionOpen==='function'){
+    var _preResult=_hermesNotifySessionOpen(sid, null, {preload:true, opts:opts});
+    if(_preResult&&_preResult.cancel===true){
+      return;
+    }
+  }
   const forceReload = !!opts.force;
   const currentSid = S.session ? S.session.session_id : null;
   const sameSessionForceReload = forceReload && currentSid===sid;
@@ -1787,7 +1799,7 @@ async function loadSession(sid){
           return;
         }
         if (_isCurrentLoad()) _loadingSessionId = null;
-        return loadSession(sid,{...opts,skipProfileResolve:true,force:true});
+        return loadSession(sid,{...opts,skipProfileResolve:true,force:true,_preloadNotified:true});
       }catch(switchErr){
         e=switchErr;
       }
@@ -1898,8 +1910,8 @@ async function loadSession(sid){
   // cross-profile continuation can't poison restore state with an unusable id.
   const continuationSid=(data.session&&data.session.continuation_session_id)||'';
   if(continuationSid&&continuationSid!==sid&&!opts.skipContinuationResolve){
-    if (_isCurrentLoad()) _loadingSessionId=null;
-    return loadSession(continuationSid,{...opts,skipLineageResolve:true,skipContinuationResolve:true,force:true});
+    _loadingSessionId=null;
+    return loadSession(continuationSid,{...opts,skipLineageResolve:true,skipContinuationResolve:true,force:true,_preloadNotified:true});
   }
   S.session=data.session;
   if(typeof _clearEmptyComposerModelOverride==='function') _clearEmptyComposerModelOverride();
@@ -2327,6 +2339,10 @@ async function loadSession(sid){
   } else {
     _hideHandoffHint();
   }
+  // Extension post-load hook
+  if(!opts.skipExtHooks && typeof _hermesNotifySessionOpen==='function'){
+    try{ _hermesNotifySessionOpen(sid, S.session, {loaded:true, opts:opts}); }catch(_){}
+  }
 }
 
 // ── Handoff hint logic ──────────────────────────────────────────────────────
@@ -2409,12 +2425,21 @@ async function _ensureSidebarSessionProfile(session){
 
 async function _openSidebarSession(session, loadOpts={}){
   if(!session||!session.session_id) return;
+  // Extension pre-open hook — before any side-effects (external import, profile switching).
+  // Handler returns {cancel:true} to prevent the open.
+  if(!loadOpts.skipExtHooks && typeof _hermesNotifySessionOpen==='function'){
+    var _preResult=_hermesNotifySessionOpen(session.session_id, null, {preload:true, opts:loadOpts});
+    if(_preResult&&_preResult.cancel===true) return;
+  }
+  // #5409: close mobile sidebar AFTER veto guard passes — only close if open proceeds.
+  if(typeof closeMobileSidebar==='function')closeMobileSidebar();
   if(_isExternalSession(session)){
     try{await api('/api/session/import_cli',{method:'POST',body:JSON.stringify(_externalImportPayload(session))});}
     catch(_e){ /* import failed -- fall through to read-only view */ }
   }
   await _ensureSidebarSessionProfile(session);
-  await loadSession(session.session_id, loadOpts);
+  // Tell loadSession to skip its pre-hook — we already ran it above.
+  await loadSession(session.session_id, Object.assign({}, loadOpts, {_preloadNotified:true}));
   renderSessionListFromCache();
 }
 
@@ -8083,8 +8108,6 @@ function renderSessionListFromCache(){
         row.title=t('session_lineage_segment_open');
         row.onclick=async(e)=>{
           e.stopPropagation();
-          // #5409: close mobile sidebar synchronously before navigation
-          if(typeof closeMobileSidebar==='function')closeMobileSidebar();
           await _openSidebarSession(seg, {skipLineageResolve:true});
         };
         lineageList.appendChild(row);
@@ -8097,8 +8120,6 @@ function renderSessionListFromCache(){
       ['pointerdown','pointerup','click','touchstart','touchmove','touchend','touchcancel'].forEach(ev=>childList.addEventListener(ev,e=>e.stopPropagation()));
       const sortedChildren=[...s._child_sessions].sort((a,b)=>_sessionTimestampMs(b)-_sessionTimestampMs(a));
       const openChildSession=async(childSession)=>{
-        // #5409: close mobile sidebar synchronously before navigation
-        if(typeof closeMobileSidebar==='function')closeMobileSidebar();
         await _openSidebarSession(childSession, {skipLineageResolve:true});
       };
       const childLabelFor=(child)=>{
@@ -8726,11 +8747,6 @@ function renderSessionListFromCache(){
         if(_renamingSid) return;
         try{
           if(($('sessionSearch').value||'').trim()) _hideSearchPreviewsAfterSelect=true;
-          // #5409: close mobile sidebar synchronously BEFORE awaiting _openSidebarSession
-          // so the user gets instant feedback that navigation is happening, even
-          // for large sessions where loadSession can take 3-15s (metadata fetch +
-          // message load + renderMessages DOM build on slow iOS WKWebView).
-          if(typeof closeMobileSidebar==='function')closeMobileSidebar();
           await _openSidebarSession(s);
         }finally{
           el.classList.remove('loading');
diff --git a/tests/test_extension_session_hooks.py b/tests/test_extension_session_hooks.py
new file mode 100644
index 00000000..90f12f0f
--- /dev/null
+++ b/tests/test_extension_session_hooks.py
@@ -0,0 +1,372 @@
+"""Functional tests for extension session-open hook + transcript renderer (PR #5508).
+
+Existing static-shape tests verify _openSidebarSession exists but never
+exercise the hook registration, preload-veto, transcript rendering, or
+_preloadNotified bridge. This module uses Node.js to run the new functions
+extracted from boot.js source.
+"""
+
+import json
+import re
+import subprocess
+import textwrap
+from pathlib import Path
+
+REPO = Path(__file__).resolve().parents[1]
+BOOT_JS = (REPO / "static" / "boot.js").read_text(encoding="utf-8")
+SESSIONS_JS = (REPO / "static" / "sessions.js").read_text(encoding="utf-8")
+
+
+def _extract_block(src, signature):
+    start = src.find(signature)
+    assert start >= 0, f"missing: {signature!r}"
+    paren_close = src.index(")", start)
+    brace = src.index("{", paren_close)
+    depth = 0
+    for i, ch in enumerate(src[brace:], brace):
+        if ch == "{":
+            depth += 1
+        elif ch == "}":
+            depth -= 1
+            if depth == 0:
+                return src[start:i + 1]
+    raise AssertionError(f"unterminated: {signature!r}")
+
+
+DOM_SHIM = r"""
+if (typeof global.window === 'undefined') global.window = {};
+class El {
+  constructor(n){this.children=[];this.attrs={};this.dataset={};
+    this._html='';this._text='';this.tagName=n;}
+  set className(v){this._cls=v;} get className(){return this._cls;}
+  set innerHTML(v){this._html=v;} get innerHTML(){return this._html;}
+  set textContent(v){this._text=v;} get textContent(){return this._text;}
+  setAttribute(k,v){this.attrs[k]=v;
+    if(k==='data-role')this.dataset.role=v;}
+  appendChild(c){this.children.push(c);
+    this._html = this.children.map(child => {
+      let a = '';
+      if (child._cls) a += ` class="${child._cls}"`;
+      if (child.attrs['data-role']) a += ` data-role="${child.attrs['data-role']}"`;
+      return `<${child.tagName}${a}>${child._html}</${child.tagName}>`;
+    }).join('');
+    return c;}
+}
+global.document={createElement:(n)=>new El(n)};
+"""
+
+
+def _run_in_tmp(tmp, body):
+    f = tmp / "run.js"
+    f.write_text(body)
+    proc = subprocess.run(["node", str(f)], capture_output=True, text=True,
+                          timeout=10, cwd=str(tmp))
+    assert proc.returncode == 0, "node stderr: " + proc.stderr
+    return proc.stdout
+
+
+def _module_level_vars(src, names):
+    """Pull top-level `var X = ...;` lines for each name in `names`."""
+    out = []
+    for name in names:
+        m = re.search(rf"^\s*var\s+{re.escape(name)}\s*=\s*[^;]+;", src, re.M)
+        if m:
+            out.append(m.group(0).strip() + "\n")
+        else:
+            raise ValueError(f"Module-level var {name!r} not found")
+    return "".join(out)
+
+
+def _fn_body(src, fn_name, alias, dep_vars=None):
+    """Extract a function and assign it to `var <alias>`, with any module-level
+    vars it depends on."""
+    result = ""
+    if dep_vars:
+        result += _module_level_vars(src, dep_vars)
+    for sig in (f"window.{fn_name}=function", f"function {fn_name}("):
+        idx = src.find(sig)
+        if idx >= 0:
+            eq = src.index("=", idx)
+            if src[idx:].startswith("window."):
+                start = src.index("function", eq)
+            else:
+                start = idx
+            brace = src.index("{", start)
+            depth = 0
+            for i, ch in enumerate(src[brace:], brace):
+                if ch == "{":
+                    depth += 1
+                elif ch == "}":
+                    depth -= 1
+                    if depth == 0:
+                        body = src[start:i + 1]
+                        result += f"var {alias} = {body};\n"
+                        return result
+    raise ValueError(f"Couldn't find {fn_name}")
+
+
+# ── boot.js: renderTranscript() ──────────────────────────────────────────────
+
+
+class TestRenderTranscript:
+
+    def test_tool_messages_skipped(self, tmp_path):
+        body = (
+            DOM_SHIM
+            + _fn_body(BOOT_JS, "renderTranscript", "renderTranscript")
+            + "global.window = {renderMd: (s)=>'<p>'+s+'</p>'};\n"
+            + textwrap.dedent("""
+                var c = document.createElement('div');
+                renderTranscript(c, [
+                  {"role":"user","content":"hi"},
+                  {"role":"tool","content":"invisible"},
+                  {"role":"assistant","content":"bye"}
+                ], {});
+                process.stdout.write(c.innerHTML);
+            """)
+        )
+        out = _run_in_tmp(tmp_path, body)
+        assert "hi" in out
+        assert "bye" in out
+        assert "invisible" not in out
+
+    def test_array_content_concatenated(self, tmp_path):
+        body = (
+            DOM_SHIM
+            + _fn_body(BOOT_JS, "renderTranscript", "renderTranscript")
+            + "global.window = {renderMd: (s)=>'<p>'+s+'</p>'};\n"
+            + textwrap.dedent("""
+                var c = document.createElement('div');
+                renderTranscript(c, [{"role":"user","content":[
+                  {"type":"text","text":"Hello "},
+                  {"type":"text","text":"world!"}
+                ]}], {});
+                process.stdout.write(c.innerHTML);
+            """)
+        )
+        out = _run_in_tmp(tmp_path, body)
+        assert "Hello" in out and "world!" in out
+        assert "[object Object]" not in out
+
+    def test_skip_empty(self, tmp_path):
+        body = (
+            DOM_SHIM
+            + _fn_body(BOOT_JS, "renderTranscript", "renderTranscript")
+            + "global.window = {renderMd: (s)=>'<p>'+s+'</p>'};\n"
+            + textwrap.dedent("""
+                var c = document.createElement('div');
+                renderTranscript(c, [
+                  {"role":"user","content":"x"},
+                  {"role":"user","content":""}
+                ], {skipEmpty:true});
+                process.stdout.write(c.innerHTML);
+            """)
+        )
+        out = _run_in_tmp(tmp_path, body)
+        assert out.count("msg-row") == 1
+
+    def test_dom_no_extra_inner_wrapper(self, tmp_path):
+        body = (
+            DOM_SHIM
+            + _fn_body(BOOT_JS, "renderTranscript", "renderTranscript")
+            + "global.window = {renderMd: (s)=>'<p>'+s+'</p>'};\n"
+            + textwrap.dedent("""
+                var c = document.createElement('div');
+                renderTranscript(c, [{"role":"user","content":"x"}], {});
+                process.stdout.write(c.innerHTML);
+            """)
+        )
+        out = _run_in_tmp(tmp_path, body)
+        assert 'class="msg-body"' in out
+        assert "msg-body-inner" not in out
+
+    def test_fallback_textcontent_when_renderMd_missing(self, tmp_path):
+        body = (
+            DOM_SHIM
+            + _fn_body(BOOT_JS, "renderTranscript", "renderTranscript")
+            + textwrap.dedent("""
+                var c = document.createElement('div');
+                delete window.renderMd;
+                renderTranscript(c, [{
+                  "role":"user",
+                  "content":"<script>x</script>"
+                }], {});
+                process.stdout.write(c.innerHTML || c.textContent);
+            """)
+        )
+        out = _run_in_tmp(tmp_path, body)
+        assert "<script>x</script>" not in out
+
+
+# ── boot.js: hook registration ───────────────────────────────────────────────
+
+
+class TestHookRegistration:
+
+    def test_register_dedupe_and_type_guard(self, tmp_path):
+        body = (
+            _fn_body(BOOT_JS, "registerHermesSessionOpenHandler",
+                     "registerHermesSessionOpenHandler",
+                     dep_vars=["_HERMES_SESSION_OPEN_HANDLERS"])
+            + _fn_body(BOOT_JS, "_hermesNotifySessionOpen",
+                       "_hermesNotifySessionOpen")
+            + textwrap.dedent("""
+                var calls = 0;
+                function handler(sid, data, opts) { calls++; return null; }
+                var r1 = registerHermesSessionOpenHandler(handler);
+                var r2 = registerHermesSessionOpenHandler("nope");
+                var r3 = registerHermesSessionOpenHandler(handler);
+                _hermesNotifySessionOpen("s1", null, {preload:true});
+                process.stdout.write(JSON.stringify({
+                    r1:r1,r2:r2,r3:r3,calls:calls
+                }));
+            """)
+        )
+        out = _run_in_tmp(tmp_path, body)
+        data = json.loads(out)
+        assert data["r1"] is True
+        assert data["r2"] is False
+        assert data["r3"] is False
+        assert data["calls"] == 1
+
+    def test_preload_cancel_honored(self, tmp_path):
+        body = (
+            _fn_body(BOOT_JS, "registerHermesSessionOpenHandler",
+                     "registerHermesSessionOpenHandler",
+                     dep_vars=["_HERMES_SESSION_OPEN_HANDLERS"])
+            + _fn_body(BOOT_JS, "_hermesNotifySessionOpen",
+                       "_hermesNotifySessionOpen")
+            + textwrap.dedent("""
+                function blocker(){ return {cancel:true}; }
+                registerHermesSessionOpenHandler(blocker);
+                var r = _hermesNotifySessionOpen("s1", null, {preload:true});
+                process.stdout.write(JSON.stringify(r));
+            """)
+        )
+        out = _run_in_tmp(tmp_path, body)
+        data = json.loads(out)
+        assert data.get("cancel") is True
+
+    def test_loaded_remains_non_cancellable(self, tmp_path):
+        body = (
+            _fn_body(BOOT_JS, "registerHermesSessionOpenHandler",
+                     "registerHermesSessionOpenHandler",
+                     dep_vars=["_HERMES_SESSION_OPEN_HANDLERS"])
+            + _fn_body(BOOT_JS, "_hermesNotifySessionOpen",
+                       "_hermesNotifySessionOpen")
+            + textwrap.dedent("""
+                var called = false;
+                function h(){ called = true; return {cancel:true}; }
+                registerHermesSessionOpenHandler(h);
+                _hermesNotifySessionOpen("s2", {session_id:"s2"}, {loaded:true});
+                process.stdout.write(JSON.stringify({called:called}));
+            """)
+        )
+        out = _run_in_tmp(tmp_path, body)
+        data = json.loads(out)
+        assert data["called"] is True
+
+    def test_each_handler_sees_one_event(self, tmp_path):
+        body = (
+            _fn_body(BOOT_JS, "registerHermesSessionOpenHandler",
+                     "registerHermesSessionOpenHandler",
+                     dep_vars=["_HERMES_SESSION_OPEN_HANDLERS"])
+            + _fn_body(BOOT_JS, "_hermesNotifySessionOpen",
+                       "_hermesNotifySessionOpen")
+            + textwrap.dedent("""
+                var events = [];
+                function h1(sid,d,o){events.push({h:1,sid:sid,preload:!!o.preload});}
+                function h2(sid,d,o){events.push({h:2,sid:sid,preload:!!o.preload});}
+                registerHermesSessionOpenHandler(h1);
+                registerHermesSessionOpenHandler(h2);
+                _hermesNotifySessionOpen("target", null, {preload:true});
+                process.stdout.write(JSON.stringify(events));
+            """)
+        )
+        out = _run_in_tmp(tmp_path, body)
+        data = json.loads(out)
+        assert len(data) == 2
+        assert all(ev["preload"] for ev in data)
+        assert data[0]["h"] == 1 and data[1]["h"] == 2
+
+
+# ── sessions.js: static shape (controls + integration points) ────────────────
+
+
+def test_canonical_sid_resolved_before_preload_notification():
+    body = _extract_block(SESSIONS_JS, "async function loadSession(sid)")
+    idx_resolve = body.index("_resolveSessionIdFromSidebarLineage")
+    idx_preload = body.index("_hermesNotifySessionOpen")
+    assert idx_resolve < idx_preload
+
+
+def test_preload_veto_only_on_preload_phase():
+    body = _extract_block(BOOT_JS, "window._hermesNotifySessionOpen=function")
+    assert "opts.preload" in body
+
+
+def test_continuation_retry_carries_preload_notified_flag():
+    body = _extract_block(SESSIONS_JS, "async function loadSession(sid)")
+    idx_cont = body.index("continuationSid=")
+    cont_branch = body[idx_cont:idx_cont + 400]
+    assert "_preloadNotified:true" in cont_branch
+
+
+def test_bridge_flag_passed_by_sidebar_open():
+    body = _extract_block(SESSIONS_JS, "async function _openSidebarSession(")
+    assert "_preloadNotified:true" in body
+
+
+# ── Regression: {cancel:true} must leave NO side-effect ─────────────────────
+
+
+def test_cancel_does_not_close_mobile_sidebar():
+    """A {cancel:true} handler must not close the sidebar.
+
+    Regression: closeMobileSidebar() was called synchronously BEFORE
+    _openSidebarSession()'s veto guard, so a cancel still closed it.
+    Fix: the three early calls are gone; one now sits AFTER the guard.
+    """
+    # The only remaining closeMobileSidebar() inside _openSidebarSession
+    # must appear AFTER the cancel guard, not before it.
+    body = _extract_block(SESSIONS_JS, "async function _openSidebarSession(")
+    idx_cancel = body.index("_preResult&&_preResult.cancel===true")
+    idx_close = body.index("closeMobileSidebar()")
+    assert idx_close > idx_cancel, (
+        "closeMobileSidebar() must run AFTER the cancel guard"
+    )
+
+
+def test_no_early_closemobilesidebar_before_sidebar_open():
+    """The three premature closeMobileSidebar() calls before _openSidebarSession
+    must be removed — a {cancel:true} veto must not close the sidebar."""
+    # Tap-to-open handler: the setTimeout callback that calls _openSidebarSession
+    # must NOT contain a closeMobileSidebar() before the await.
+    tap_block = _extract_block(SESSIONS_JS, "_tapTimer=setTimeout(async()=>{")
+    # Find the _openSidebarSession call inside the tap handler.
+    idx_open = tap_block.index("await _openSidebarSession(s)")
+    before_open = tap_block[:idx_open]
+    assert "closeMobileSidebar()" not in before_open, (
+        "tap-to-open handler must not closeMobileSidebar() before veto"
+    )
+
+    # Child-session open handler must not contain closeMobileSidebar() at all.
+    child_block = _extract_block(SESSIONS_JS, "const openChildSession=async(childSession)=>{")
+    assert "closeMobileSidebar()" not in child_block, (
+        "openChildSession handler must not contain closeMobileSidebar()"
+    )
+
+    # Lineage-segment handler must not contain closeMobileSidebar() at all.
+    lineage_block = _extract_block(SESSIONS_JS, "row.onclick=async(e)=>{")
+    assert "closeMobileSidebar()" not in lineage_block, (
+        "lineage-segment handler must not contain closeMobileSidebar()"
+    )
+
+
+def test_cross_profile_retry_carries_preload_notified():
+    """Cross-profile retry must pass _preloadNotified:true so the pre-hook
+    doesn't re-fire after destructive side-effects already ran."""
+    body = _extract_block(SESSIONS_JS, "async function loadSession(sid)")
+    idx_profile = body.index("skipProfileResolve:true")
+    profile_branch = body[idx_profile:idx_profile + 200]
+    assert "_preloadNotified:true" in profile_branch
diff --git a/tests/test_issue1611_session_profile_filtering.py b/tests/test_issue1611_session_profile_filtering.py
index 63071a64..2ca830f8 100644
--- a/tests/test_issue1611_session_profile_filtering.py
+++ b/tests/test_issue1611_session_profile_filtering.py
@@ -203,7 +203,7 @@ def test_static_sessions_js_switches_profile_before_opening_all_profiles_row():
 
     assert "await switchToProfile(targetProfile);" in ensure_body
     assert "_profileSwitchOpeningExistingSession=true;" in ensure_body
-    assert open_body.index("await _ensureSidebarSessionProfile(session);") < open_body.index("await loadSession(session.session_id, loadOpts);")
+    assert open_body.index("await _ensureSidebarSessionProfile(session);") < open_body.index("await loadSession(session.session_id,")
     assert "await _openSidebarSession(s);" in src
     assert "await _openSidebarSession(seg, {skipLineageResolve:true});" in src
     assert "await _openSidebarSession(childSession, {skipLineageResolve:true});" in src