diff --git a/src/Runtime/ColdStartTracker.php b/src/Runtime/ColdStartTracker.php
index 49b7dd25a..93d266fb0 100644
--- a/src/Runtime/ColdStartTracker.php
+++ b/src/Runtime/ColdStartTracker.php
@@ -10,6 +10,7 @@
 class ColdStartTracker
 {
     private const FILE = '/tmp/.bref-cold-start';
+    private const PROACTIVE_INITIALIZATION_IDLE_THRESHOLD_SECONDS = 0.05;
 
     private static bool $currentInvocationIsColdStart = false;
     private static ?float $coldStartBeginningTime = null;
@@ -47,6 +48,7 @@ public static function invocationStarted(): void
         // so we are no longer in the cold start invocation anymore
         if (self::$hasFirstInvocationStarted) {
             self::$currentInvocationIsColdStart = false;
+            self::$wasProactiveInitialization = false;
             return;
         }
 
@@ -55,11 +57,11 @@ public static function invocationStarted(): void
         if (self::$currentInvocationIsColdStart) {
             // There was a cold start, let's figure out if it was a proactive initialization
             $timeElapsedSinceColdStartEnded = microtime(true) - self::$coldStartEndedTime;
-            // If more than 100ms have passed since the cold start ended, we can assume the
+            // If more than 50ms have passed since the cold start ended, we can assume the
             // Lambda sandbox was paused/frozen between the cold start and the first invocation
-            // (100ms is an arbitrary value, we could use a lower value but I want to be conservative)
+            // (50ms is an arbitrary value, we could use a lower value but I want to be conservative)
             // That means the Lambda sandbox was initialized proactively
-            self::$wasProactiveInitialization = $timeElapsedSinceColdStartEnded > 0.1;
+            self::$wasProactiveInitialization = $timeElapsedSinceColdStartEnded > self::PROACTIVE_INITIALIZATION_IDLE_THRESHOLD_SECONDS;
         } else {
             // There was no cold start, we are in a warm start
             self::$wasProactiveInitialization = false;
diff --git a/tests/Runtime/ColdStartTrackerTest.php b/tests/Runtime/ColdStartTrackerTest.php
new file mode 100644
index 000000000..9d84646de
--- /dev/null
+++ b/tests/Runtime/ColdStartTrackerTest.php
@@ -0,0 +1,138 @@
+<?php declare(strict_types=1);
+
+namespace Bref\Test\Runtime;
+
+use Bref\Runtime\ColdStartTracker;
+use PHPUnit\Framework\TestCase;
+use ReflectionProperty;
+
+class ColdStartTrackerTest extends TestCase
+{
+    private const COLD_START_FILE = '/tmp/.bref-cold-start';
+
+    protected function setUp(): void
+    {
+        $this->resetColdStartTracker();
+    }
+
+    protected function tearDown(): void
+    {
+        $this->resetColdStartTracker();
+    }
+
+    public function test_first_invocation_after_initialization_is_a_user_facing_cold_start(): void
+    {
+        ColdStartTracker::init();
+        ColdStartTracker::coldStartFinished();
+        ColdStartTracker::invocationStarted();
+
+        $this->assertTrue(ColdStartTracker::currentInvocationIsColdStart());
+        $this->assertTrue(ColdStartTracker::currentInvocationIsUserFacingColdStart());
+        $this->assertFalse(ColdStartTracker::wasProactiveInitialization());
+    }
+
+    public function test_short_idle_time_after_initialization_is_not_a_proactive_initialization(): void
+    {
+        ColdStartTracker::init();
+        ColdStartTracker::coldStartFinished();
+
+        ColdStartTracker::invocationStarted();
+
+        $this->assertTrue(ColdStartTracker::currentInvocationIsUserFacingColdStart());
+        $this->assertFalse(ColdStartTracker::wasProactiveInitialization());
+    }
+
+    public function test_first_invocation_more_than_fifty_milliseconds_after_initialization_is_proactive(): void
+    {
+        ColdStartTracker::init();
+        ColdStartTracker::coldStartFinished();
+
+        usleep(75_000);
+
+        ColdStartTracker::invocationStarted();
+
+        $this->assertTrue(ColdStartTracker::currentInvocationIsColdStart());
+        $this->assertFalse(ColdStartTracker::currentInvocationIsUserFacingColdStart());
+        $this->assertTrue(ColdStartTracker::wasProactiveInitialization());
+    }
+
+    public function test_second_invocation_after_user_facing_cold_start_is_warm(): void
+    {
+        ColdStartTracker::init();
+        ColdStartTracker::coldStartFinished();
+
+        ColdStartTracker::invocationStarted();
+        ColdStartTracker::invocationStarted();
+
+        $this->assertFalse(ColdStartTracker::currentInvocationIsColdStart());
+        $this->assertFalse(ColdStartTracker::currentInvocationIsUserFacingColdStart());
+        $this->assertFalse(ColdStartTracker::wasProactiveInitialization());
+    }
+
+    public function test_proactive_initialization_annotation_does_not_leak_to_warm_invocations(): void
+    {
+        ColdStartTracker::init();
+        ColdStartTracker::coldStartFinished();
+        $this->setColdStartTrackerProperty('coldStartEndedTime', microtime(true) - 0.075);
+
+        ColdStartTracker::invocationStarted();
+
+        $this->assertTrue(ColdStartTracker::currentInvocationIsColdStart());
+        $this->assertFalse(ColdStartTracker::currentInvocationIsUserFacingColdStart());
+        $this->assertTrue(ColdStartTracker::wasProactiveInitialization());
+
+        ColdStartTracker::invocationStarted();
+
+        $this->assertFalse(ColdStartTracker::currentInvocationIsColdStart());
+        $this->assertFalse(ColdStartTracker::currentInvocationIsUserFacingColdStart());
+        $this->assertFalse(ColdStartTracker::wasProactiveInitialization());
+    }
+
+    public function test_process_restart_inside_same_sandbox_is_a_warm_start(): void
+    {
+        touch(self::COLD_START_FILE);
+
+        ColdStartTracker::init();
+        ColdStartTracker::coldStartFinished();
+        ColdStartTracker::invocationStarted();
+
+        $this->assertFalse(ColdStartTracker::currentInvocationIsColdStart());
+        $this->assertFalse(ColdStartTracker::currentInvocationIsUserFacingColdStart());
+        $this->assertFalse(ColdStartTracker::wasProactiveInitialization());
+    }
+
+    public function test_records_cold_start_timestamps(): void
+    {
+        $beforeInit = microtime(true);
+        ColdStartTracker::init();
+        $afterInit = microtime(true);
+
+        $beforeFinished = microtime(true);
+        ColdStartTracker::coldStartFinished();
+        $afterFinished = microtime(true);
+
+        $this->assertGreaterThanOrEqual($beforeInit, ColdStartTracker::getColdStartBeginningTime());
+        $this->assertLessThanOrEqual($afterInit, ColdStartTracker::getColdStartBeginningTime());
+        $this->assertGreaterThanOrEqual($beforeFinished, ColdStartTracker::getColdStartEndedTime());
+        $this->assertLessThanOrEqual($afterFinished, ColdStartTracker::getColdStartEndedTime());
+    }
+
+    private function resetColdStartTracker(): void
+    {
+        if (file_exists(self::COLD_START_FILE)) {
+            unlink(self::COLD_START_FILE);
+        }
+
+        $this->setColdStartTrackerProperty('currentInvocationIsColdStart', false);
+        $this->setColdStartTrackerProperty('coldStartBeginningTime', null);
+        $this->setColdStartTrackerProperty('coldStartEndedTime', null);
+        $this->setColdStartTrackerProperty('hasFirstInvocationStarted', false);
+        $this->setColdStartTrackerProperty('wasProactiveInitialization', false);
+    }
+
+    private function setColdStartTrackerProperty(string $property, mixed $value): void
+    {
+        $reflection = new ReflectionProperty(ColdStartTracker::class, $property);
+        $reflection->setValue(null, $value);
+    }
+}