diff --git chrome/browser/ui/startup/startup_browser_creator.cc chrome/browser/ui/startup/startup_browser_creator.cc
index ba443f5e8838d..a3f3c250f8006 100644
--- chrome/browser/ui/startup/startup_browser_creator.cc
+++ chrome/browser/ui/startup/startup_browser_creator.cc
@@ -616,6 +616,13 @@ std::optional<ash::KioskAppId> GetAppId(const base::CommandLine& command_line,
 }
 #endif  // BUILDFLAG(IS_CHROMEOS_ASH)
 
+StartupBrowserCreator::ProcessCommandLineCallback*
+GetProcessCommandLineCallback() {
+  static base::NoDestructor<StartupBrowserCreator::ProcessCommandLineCallback>
+      callback;
+  return callback.get();
+}
+
 }  // namespace
 
 StartupProfileMode StartupProfileModeFromReason(
@@ -1479,6 +1486,12 @@ void StartupBrowserCreator::ProcessCommandLineWithProfile(
       {profile, mode}, last_opened_profiles);
 }
 
+// static
+void StartupBrowserCreator::RegisterProcessCommandLineCallback(
+    ProcessCommandLineCallback cb) {
+  *GetProcessCommandLineCallback() = cb;
+}
+
 // static
 void StartupBrowserCreator::ProcessCommandLineAlreadyRunning(
     const base::CommandLine& command_line,
@@ -1488,6 +1501,11 @@ void StartupBrowserCreator::ProcessCommandLineAlreadyRunning(
     return;
   }
 
+  auto* cb = GetProcessCommandLineCallback();
+  if (!cb->is_null() && cb->Run(command_line, cur_dir)) {
+    return;
+  }
+
   Profile* profile = nullptr;
   StartupProfileMode mode =
       StartupProfileModeFromReason(profile_path_info.reason);
diff --git chrome/browser/ui/startup/startup_browser_creator.h chrome/browser/ui/startup/startup_browser_creator.h
index 919bae5ccfeae..5c2dfbf955723 100644
--- chrome/browser/ui/startup/startup_browser_creator.h
+++ chrome/browser/ui/startup/startup_browser_creator.h
@@ -9,6 +9,7 @@
 #include <vector>
 
 #include "base/files/file_path.h"
+#include "base/functional/callback.h"
 #include "base/gtest_prod_util.h"
 #include "base/memory/raw_ptr.h"
 #include "build/build_config.h"
@@ -135,6 +136,13 @@ class StartupBrowserCreator {
              StartupProfileInfo profile_info,
              const Profiles& last_opened_profiles);
 
+  // Registers a callback that will be executed each time
+  // ProcessCommandLineAlreadyRunning is called.
+  using ProcessCommandLineCallback = base::RepeatingCallback<bool(
+      const base::CommandLine& command_line,
+      const base::FilePath& cur_dir)>;
+  static void RegisterProcessCommandLineCallback(ProcessCommandLineCallback cb);
+
   // This function performs command-line handling and is invoked only after
   // start up (for example when we get a start request for another process).
   // |command_line| holds the command line we need to process.