diff --git a/phpunit.dist.xml b/phpunit.dist.xml
index f2d10a77..131df433 100644
--- a/phpunit.dist.xml
+++ b/phpunit.dist.xml
@@ -53,5 +53,8 @@
     </source>
 
     <extensions>
+        <!-- The streaming controllers call set_time_limit(), and in one
+             PHPUnit process that limit outlives their test. See the class. -->
+        <bootstrap class="App\Tests\Support\PhpUnit\ResetTimeLimitExtension"/>
     </extensions>
 </phpunit>
diff --git a/tests/Support/PhpUnit/ResetTimeLimitExtension.php b/tests/Support/PhpUnit/ResetTimeLimitExtension.php
new file mode 100644
index 00000000..b7f308b1
--- /dev/null
+++ b/tests/Support/PhpUnit/ResetTimeLimitExtension.php
@@ -0,0 +1,47 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Support\PhpUnit;
+
+use PHPUnit\Event\Test\Finished;
+use PHPUnit\Event\Test\FinishedSubscriber;
+use PHPUnit\Runner\Extension\Extension;
+use PHPUnit\Runner\Extension\Facade;
+use PHPUnit\Runner\Extension\ParameterCollection;
+use PHPUnit\TextUI\Configuration\Configuration;
+
+/**
+ * Gives the time limit back after every test.
+ *
+ * Four controllers stream for longer than PHP's default allows and call
+ * set_time_limit() to say so — the JMAP event source, the thread summary, the
+ * compose assistant and the AI warm-up. In a web request that is right: the
+ * limit dies with the request. In the suite it does not, because every test
+ * runs in the one PHPUnit process, and a limit set by a controller test is
+ * still counting for every test after it. On a fast machine the rest of the
+ * suite finished inside it; on a slower one the run died part-way with
+ * "Maximum execution time of 300 seconds exceeded", in whichever unrelated
+ * test happened to be running — CalendarPullerTest one time, GraphFolderMoveTest
+ * the next — and passed again when that test was run on its own.
+ *
+ * In the suite rather than in the controllers: the four calls are correct, and
+ * a seam in production code to hide them from tests would be four places to
+ * get wrong for the sake of one place that is. ResetTimeLimitExtensionTest
+ * holds it.
+ */
+final class ResetTimeLimitExtension implements Extension
+{
+    public function bootstrap(Configuration $configuration, Facade $facade, ParameterCollection $parameters): void
+    {
+        $facade->registerSubscriber(new class implements FinishedSubscriber {
+            public function notify(Finished $event): void
+            {
+                // 0 is the CLI default: no limit. set_time_limit() also
+                // restarts the clock, which is what a limit somebody else set
+                // needs.
+                set_time_limit(0);
+            }
+        });
+    }
+}
diff --git a/tests/Support/PhpUnit/ResetTimeLimitExtensionTest.php b/tests/Support/PhpUnit/ResetTimeLimitExtensionTest.php
new file mode 100644
index 00000000..82243b7a
--- /dev/null
+++ b/tests/Support/PhpUnit/ResetTimeLimitExtensionTest.php
@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Support\PhpUnit;
+
+use PHPUnit\Framework\Attributes\Depends;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * A time limit one test sets is gone by the next one.
+ *
+ * Two tests in a fixed order, because the claim is about what one test leaves
+ * behind for the next: the first does what a streaming controller does, the
+ * second looks. Without ResetTimeLimitExtension registered in phpunit.dist.xml
+ * the second sees the first's 300 seconds and fails.
+ */
+final class ResetTimeLimitExtensionTest extends TestCase
+{
+    public function testAControllerTestSetsALimitTheWayTheStreamingEndpointsDo(): void
+    {
+        set_time_limit(300);
+
+        self::assertSame('300', ini_get('max_execution_time'));
+    }
+
+    #[Depends('testAControllerTestSetsALimitTheWayTheStreamingEndpointsDo')]
+    public function testTheNextTestRunsWithoutIt(): void
+    {
+        self::assertSame('0', ini_get('max_execution_time'), 'a time limit set by one test is still counting in the next');
+    }
+}