**Summary** With webprofiler + tracer enabled, every request throws Exception: Serialization of 'Closure' is not allowed in serialize() from Symfony\\Component\\HttpKernel\\Profiler\\FileProfilerStorage::write() during kernel.terminate. No profile is ever written to storage, so the profiler and toolbar are effectively unusable. Environment - Drupal core: 11.4.1 (PHP 8.3) - drupal/webprofiler: 11.1.1 - drupal/tracer: 1.2.2 **Steps to reproduce** 1. composer require drupal/webprofiler (pulls in drupal/tracer), enable both. 2. Enable the profiler storage and load any front-end page. 3. Observe the exception in dblog/watchdog on every request; sites/default/files/profiler/ contains only empty shard directories — no profile files. **Error / stack trace** Exception: Serialization of 'Closure' is not allowed in serialize() (line 164 of vendor/symfony/http-kernel/Profiler/FileProfilerStorage.php) #0 vendor/symfony/http-kernel/Profiler/FileProfilerStorage.php(164): serialize() #1 vendor/symfony/http-kernel/Profiler/Profiler.php(97): FileProfilerStorage-\>write() #2 web/modules/contrib/webprofiler/src/EventListener/ProfilerListener.php(169): Profiler-\>saveProfile() #3 web/modules/contrib/tracer/src/EventDispatcher/TraceableEventDispatcher.php(155): ProfilerListener-\>onKernelTerminate() **Root cause** FileProfilerStorage::write() serializes the profile's collectors. Profiler::saveProfile() calls lateCollect() on every LateDataCollectorInterface immediately before writing, so the failing data is produced inside EventsDataCollector::lateCollect(). tracer's TraceableEventDispatcher records listeners in two different shapes: - already-called listeners → \['class' =\> …, 'method' =\> …\] - not-called listeners (and any listener still executing) → \['callable' =\> \], where the raw callable is almost always a lazy-loading \\Closure. EventsDataCollector::lateCollect() only reduces the called listeners to class/method and stores the not-called list verbatim, so the raw closures are placed into $this-\>data and serialization fails. There is an additional edge case: during the in-progress kernel.terminate dispatch, getCalledListeners() returns the currently-executing priority-0 listener still in the \['callable' =\> …\] shape (it hasn't been recorded as "called" yet), so a raw closure can end up in the called list too. Any fix must normalize both lists. As a side effect, the "Not called listeners" panel is already broken in this pairing, because classLink() expects class/service keys that the raw \['callable' =\> …\] entries don't have. **Proposed fix** Normalize both the called and not-called listener trees through a single helper in EventsDataCollector::lateCollect(), reducing every entry — whichever shape it arrives in — to serializable class/method/clazz data. Closures (and callables that can't be reflected) render as a plain Closure label. This keeps the collected data serializable and fixes the not-called panel. Patch/MR attached. Verified on Drupal 11.4.1 / webprofiler 11.1.1 / tracer 1.2.2: no exceptions, profiles are written, and the profiler dashboard (including the Events panel) renders. **Possible deeper fix** The underlying issue is that tracer exposes raw closures via getNotCalledListeners()/getCalledListeners(). A more thorough fix could sanitize the tracer output (or exclude the runtime callables from what those getters return), which would let webprofiler drop the workaround.