diff --git a/src/Process/ForkedChildTerminator.php b/src/Process/ForkedChildTerminator.php
index 0452f589d89..0ccfb2a97c9 100644
--- a/src/Process/ForkedChildTerminator.php
+++ b/src/Process/ForkedChildTerminator.php
@@ -2,8 +2,8 @@
 
 namespace PHPStan\Process;
 
+use PHPStan\Turbo\ForkedChild;
 use PHPStan\Turbo\TurboExtensionEnabler;
-use PHPStanTurbo\Runtime;
 use function register_shutdown_function;
 
 /**
@@ -25,9 +25,10 @@
  * Everything the parent reads from the child, the results over the socket
  * and the captured output, has gone through unbuffered fds by then.
  *
- * PHP itself has no _exit(); the turbo extension provides it, and fork mode
- * from a phar requires the extension anyway (see ForkParallelChecker). A
- * source checkout forking without it keeps the plain exit().
+ * PHP itself has no _exit(); the turbo extension provides it
+ * (ForkedChild::exitImmediately()), and fork mode from a phar requires the
+ * extension anyway (see ForkParallelChecker). A source checkout forking
+ * without it keeps the plain exit().
  */
 final class ForkedChildTerminator
 {
@@ -39,7 +40,7 @@ public static function install(): void
 		}
 
 		register_shutdown_function(static function (): void {
-			Runtime::exitImmediately();
+			ForkedChild::exitImmediately();
 		});
 	}
 
diff --git a/src/Turbo/ForkedChild.php b/src/Turbo/ForkedChild.php
new file mode 100644
index 00000000000..d7b86e9e696
--- /dev/null
+++ b/src/Turbo/ForkedChild.php
@@ -0,0 +1,30 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Turbo;
+
+/**
+ * What a pcntl_fork()ed child needs from the turbo extension. With the
+ * extension active (TurboExtensionEnabler::isActive()), the native class
+ * replaces this one.
+ *
+ * @api
+ */
+#[ShadowedByTurboExtension(implementation: __DIR__ . '/../../turbo-ext/src/ForkedChild.cpp')]
+final class ForkedChild
+{
+
+	/**
+	 * Ends the child with _exit() and the engine's exit status, skipping
+	 * destructors and the module shutdown of every extension the child
+	 * inherited - one waiting for threads the child never had wedges there.
+	 * Register it as the child's last shutdown function.
+	 *
+	 * @return never
+	 * @throws TurboNotActiveException
+	 */
+	public static function exitImmediately()
+	{
+		throw new TurboNotActiveException();
+	}
+
+}
diff --git a/src/Turbo/TurboNotActiveException.php b/src/Turbo/TurboNotActiveException.php
new file mode 100644
index 00000000000..3b046e4bd3f
--- /dev/null
+++ b/src/Turbo/TurboNotActiveException.php
@@ -0,0 +1,20 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Turbo;
+
+use Exception;
+
+/**
+ * Thrown by a method only the active turbo extension implements.
+ *
+ * @api
+ */
+final class TurboNotActiveException extends Exception
+{
+
+	public function __construct()
+	{
+		parent::__construct('The turbo extension is not active.');
+	}
+
+}
diff --git a/turbo-ext/README.md b/turbo-ext/README.md
index a0027e02397..ea55ddd0b21 100644
--- a/turbo-ext/README.md
+++ b/turbo-ext/README.md
@@ -110,11 +110,16 @@ hardcoded in the collector), so a renamed class updates the map on the next
 autoloader dump. Shadowed classes are never referenced that way — the native
 code holds their class entries itself and instantiates them directly.
 
-Two more `Runtime` entry points serve fork mode (see `ForkParallelChecker`):
-`enablePharForkGuard()` gives each pcntl_fork()ed worker a private cursor on
-the running phar's fd (`PharForkGuard.cpp`), and `exitImmediately()` —
-`_exit()` with the engine's exit status — is how `ForkedChildTerminator` ends
-a forked worker. A forked child inherits every loaded extension but none of
+Fork mode (see `ForkParallelChecker`) uses two more entry points:
+`Runtime::enablePharForkGuard()` gives each pcntl_fork()ed worker a private
+cursor on the running phar's fd (`PharForkGuard.cpp`), and
+`exitImmediately()` — `_exit()` with the engine's exit status — is how
+`ForkedChildTerminator` ends a forked worker. `exitImmediately()` lives on
+`PHPStan\Turbo\ForkedChild`, a shadowed class that is part of PHPStan's
+`@api` (`ForkedChild.cpp`), because a program that forks its own workers
+needs it too; its PHP twin throws `TurboNotActiveException`. The `Runtime`
+methods cannot move there: `TurboExtensionEnabler` calls them before
+shadowing is active. A forked child inherits every loaded extension but none of
 their threads, so PHP's full teardown can wedge in a fork-unsafe extension's
 module shutdown (ext-grpc without `grpc.enable_fork_support`); a forked child
 that does not exec() must `_exit()` instead.
diff --git a/turbo-ext/analysis-stub.php b/turbo-ext/analysis-stub.php
index 0d3993c805e..607656b0207 100644
--- a/turbo-ext/analysis-stub.php
+++ b/turbo-ext/analysis-stub.php
@@ -37,8 +37,4 @@ public static function trustTypesUnder(string $prefix): bool
 	{
 	}
 
-	public static function exitImmediately(): never
-	{
-	}
-
 }
diff --git a/turbo-ext/src/ForkedChild.cpp b/turbo-ext/src/ForkedChild.cpp
new file mode 100644
index 00000000000..a467b8d826d
--- /dev/null
+++ b/turbo-ext/src/ForkedChild.cpp
@@ -0,0 +1,63 @@
+/*
+ * PHPStanTurbo\ForkedChild — native implementation of
+ * PHPStan\Turbo\ForkedChild, what a pcntl_fork()ed child needs from the
+ * extension, as part of PHPStan's API. Unlike PHPStanTurbo\Runtime
+ * (main.cpp), which TurboExtensionEnabler calls before shadowing is active,
+ * this class only exists under its PHP name once activateShadowing() ran;
+ * the PHP twin throws TurboNotActiveException.
+ */
+
+#include "support.h"
+#include "generated/ForkedChild.h"
+
+namespace sigs = ptdecl::ForkedChild::sig;
+
+#ifdef PHP_WIN32
+#include <process.h>
+#else
+#include <unistd.h>
+#endif
+
+static zend_class_entry *pt_ce_forked_child = nullptr;
+
+/* {{{ registration */
+
+#include "reg.h"
+
+PT_MINIT_REGISTRATION(pt_register_forked_child)
+{
+	reg::Class cls("PHPStan\\Turbo\\ForkedChild");
+	ptdecl::ForkedChild::declareClass(cls);
+	ptdecl::ForkedChild::declareProperties(cls);
+
+	/* exitImmediately() — _exit() for a pcntl_fork()ed worker;
+	 * ForkedChildTerminator registers it as the child's last shutdown
+	 * function.
+	 *
+	 * A forked child inherits the whole parent process, every loaded
+	 * extension with whatever background threads it started included — but
+	 * fork() copies only the calling thread. PHP's exit() then runs
+	 * destructors and each extension's module shutdown, and an extension
+	 * whose shutdown waits for its threads to check out (ext-grpc's
+	 * grpc_shutdown() without grpc.enable_fork_support, for one) waits
+	 * forever for threads the child never had: the worker has delivered its
+	 * results, the parent keeps polling waitpid(), and the run hangs at 100%.
+	 * A forked child that does not exec() must end with _exit() — no
+	 * destructors, no module shutdown, no atexit handlers; that teardown is
+	 * the parent's. The shutdown functions have run by then, so the crash
+	 * report (ForkedChildCrashReporter) is written, and everything the parent
+	 * reads — the results over the socket, the captured output — went
+	 * through unbuffered fds.
+	 *
+	 * The status is the engine's: what exit() was given, 255 after a fatal
+	 * error, 0 otherwise. */
+	cls.method(sigs::exitImmediately, [](INTERNAL_FUNCTION_PARAMETERS) {
+		ZEND_PARSE_PARAMETERS_NONE();
+
+		_exit(EG(exit_status));
+	});
+
+	cls.shadow(&pt_ce_forked_child);
+}
+
+/* }}} */
diff --git a/turbo-ext/src/generated/ForkedChild.h b/turbo-ext/src/generated/ForkedChild.h
new file mode 100644
index 00000000000..79f871a2662
--- /dev/null
+++ b/turbo-ext/src/generated/ForkedChild.h
@@ -0,0 +1,39 @@
+/* Generated by turbo-ext/bin/generate-declarations.php from
+ * src/Turbo/ForkedChild.php — do not edit. */
+
+#ifndef PHPSTANTURBO_GENERATED_FORKED_CHILD_H
+#define PHPSTANTURBO_GENERATED_FORKED_CHILD_H
+
+#include "../reg.h"
+
+namespace ptdecl::ForkedChild {
+
+inline void declareClass(reg::Class &cls)
+{
+	cls.final();
+}
+
+/* the properties the class declares itself, in declaration order (a used trait's come from its registrar) */
+inline void declareProperties(reg::Class &cls)
+{
+	(void) cls;
+}
+
+/* the string and parameter tables the signatures below index into (see reg::Sig) */
+namespace sigtab {
+inline constexpr char strings[] =
+	"exitImmediately"; /* 0 */
+inline constexpr reg::PackedArg args[] = {
+	reg::packed(0, 0), /* placeholder: C++ has no empty arrays */
+};
+using Sig = reg::Sig<strings, args>;
+} // namespace sigtab
+
+/* the signatures of the methods the class declares itself (a used trait's are in the trait's header) */
+namespace sig {
+inline constexpr sigtab::Sig exitImmediately = { { 0 /* exitImmediately */, 0, 0, 0, reg::NoArg, ZEND_ACC_PUBLIC | ZEND_ACC_STATIC } };
+} // namespace sig
+
+} // namespace ptdecl::ForkedChild
+
+#endif
diff --git a/turbo-ext/src/main.cpp b/turbo-ext/src/main.cpp
index bd8ac04380d..53220507058 100644
--- a/turbo-ext/src/main.cpp
+++ b/turbo-ext/src/main.cpp
@@ -15,13 +15,6 @@
 #include "Engine.h"
 #include "version.h"
 
-#ifdef PHP_WIN32
-#include <process.h>
-#else
-#include <unistd.h>
-#endif
-
-
 /* PHPStanTurbo\Runtime::configure() — cold-path configuration entry point.
  * TurboExtensionEnabler passes the generated class map (derived from the
  * ReferencedByTurboExtension attributes) that the native code resolves
@@ -107,33 +100,6 @@ static void ZEND_FASTCALL runtimeTrustTypesUnder(INTERNAL_FUNCTION_PARAMETERS)
 	RETURN_BOOL(pt_trusted_types_set_prefix(prefix));
 }
 
-/* PHPStanTurbo\Runtime::exitImmediately() — _exit() for a pcntl_fork()ed
- * worker; ForkedChildTerminator registers it as the child's last shutdown
- * function.
- *
- * A forked child inherits the whole parent process, every loaded extension
- * with whatever background threads it started included — but fork() copies
- * only the calling thread. PHP's exit() then runs destructors and each
- * extension's module shutdown, and an extension whose shutdown waits for
- * its threads to check out (ext-grpc's grpc_shutdown() without
- * grpc.enable_fork_support, for one) waits forever for threads the child
- * never had: the worker has delivered its results, the parent keeps
- * polling waitpid(), and the run hangs at 100%. A forked child that does
- * not exec() must end with _exit() — no destructors, no module shutdown, no
- * atexit handlers; that teardown is the parent's. The shutdown functions
- * have run by then, so the crash report (ForkedChildCrashReporter) is
- * written, and everything the parent reads — the results over the socket,
- * the captured output — went through unbuffered fds.
- *
- * The status is the engine's: what exit() was given, 255 after a fatal
- * error, 0 otherwise. */
-static void ZEND_FASTCALL runtimeExitImmediately(INTERNAL_FUNCTION_PARAMETERS)
-{
-	ZEND_PARSE_PARAMETERS_NONE();
-
-	_exit(EG(exit_status));
-}
-
 /* Whether the shared core loaded next to this library carries the same
  * version: a mismatched pair (a torn update of the distributed files) must
  * never run together — the extension then registers nothing and reports
@@ -201,7 +167,6 @@ static PHP_MINIT_FUNCTION(phpstan_turbo)
 	runtime.method("isShadowing", reg::PublicStatic, 0, {}, runtimeIsShadowing, &returnsBool);
 	runtime.method("enablePharForkGuard", reg::PublicStatic, 1, { reg::stringArg("pharPath") }, runtimeEnablePharForkGuard);
 	runtime.method("trustTypesUnder", reg::PublicStatic, 1, { reg::stringArg("prefix") }, runtimeTrustTypesUnder, &returnsBool);
-	runtime.method("exitImmediately", reg::PublicStatic, 0, {}, runtimeExitImmediately);
 	runtime.register_();
 
 	pt_minit_registrations_run();
diff --git a/turbo-ext/tests/exit-immediately.php b/turbo-ext/tests/exit-immediately.php
index eefb96e2afd..b6f0697ca90 100644
--- a/turbo-ext/tests/exit-immediately.php
+++ b/turbo-ext/tests/exit-immediately.php
@@ -1,7 +1,7 @@
 <?php declare(strict_types=1);
 
 /**
- * Test for Runtime::exitImmediately().
+ * Test for ForkedChild::exitImmediately().
  *
  * A pcntl_fork()ed child inherits the parent's whole process, and PHP's
  * exit() runs the full teardown there: destructors, then every extension's
@@ -17,9 +17,12 @@
  * Two passes over the same child:
  *   1. control, plain exit() - the child must still be alive at the deadline
  *      (proves the workload hangs)
- *   2. exitImmediately() from a shutdown function - the child must be gone
- *      well within the deadline, with exit()'s status, the earlier shutdown
- *      function's marker written and the destructor's not
+ *   2. the native exitImmediately() from a shutdown function - the child must
+ *      be gone well within the deadline, with exit()'s status, the earlier
+ *      shutdown function's marker written and the destructor's not
+ *
+ * The PHP twin, which runs without the active extension, must throw
+ * TurboNotActiveException instead.
  *
  * Run: php -d extension=$PWD/phpstan_turbo.so tests/exit-immediately.php
  */
@@ -27,15 +30,14 @@
 const DEADLINE_SECONDS = 3;
 const EXIT_STATUS = 7;
 
-if (!extension_loaded('phpstan_turbo')) {
-	fwrite(STDERR, "phpstan_turbo extension is not loaded\n");
-	exit(1);
-}
 if (!function_exists('pcntl_fork')) {
 	echo "SKIP: pcntl is not available\n";
 	exit(0);
 }
 
+// declares the native class as PHPStanTurbo\ForkedChild next to the PHP twin
+require __DIR__ . '/activate-prefixed.php';
+
 final class WedgedTeardown
 {
 
@@ -57,9 +59,10 @@ public function __destruct()
 }
 
 /**
+ * @param class-string|null $terminator
  * @return array{exited: bool, status: int|null, markers: string}
  */
-function forkAndExit(bool $terminate): array
+function forkAndExit(?string $terminator): array
 {
 	$markers = tmpfile();
 	if ($markers === false) {
@@ -78,9 +81,9 @@ function forkAndExit(bool $terminate): array
 		register_shutdown_function(static function () use ($markers): void {
 			fwrite($markers, "shutdown\n");
 		});
-		if ($terminate) {
-			register_shutdown_function(static function (): void {
-				PHPStanTurbo\Runtime::exitImmediately();
+		if ($terminator !== null) {
+			register_shutdown_function(static function () use ($terminator): void {
+				$terminator::exitImmediately();
 			});
 		}
 		exit(EXIT_STATUS);
@@ -110,11 +113,19 @@ function forkAndExit(bool $terminate): array
 	return ['exited' => $exited, 'status' => $status, 'markers' => $written];
 }
 
-$control = forkAndExit(false);
+$control = forkAndExit(null);
 printf("control (exit):      exited %s, markers %s\n", $control['exited'] ? 'yes' : 'no', json_encode($control['markers']));
 
-$terminated = forkAndExit(true);
-printf("exitImmediately():   exited %s, status %s, markers %s\n", $terminated['exited'] ? 'yes' : 'no', var_export($terminated['status'], true), json_encode($terminated['markers']));
+$terminated = forkAndExit(\PHPStanTurbo\ForkedChild::class);
+printf("native:              exited %s, status %s, markers %s\n", $terminated['exited'] ? 'yes' : 'no', var_export($terminated['status'], true), json_encode($terminated['markers']));
+
+$twinThrew = false;
+try {
+	\PHPStan\Turbo\ForkedChild::exitImmediately();
+} catch (\PHPStan\Turbo\TurboNotActiveException) {
+	$twinThrew = true;
+}
+printf("PHP twin:            threw TurboNotActiveException %s\n", $twinThrew ? 'yes' : 'no');
 
 if ($control['exited'] || $control['markers'] !== "shutdown\ndestructor\n") {
 	fwrite(STDERR, "FAIL: the control child did not wedge in its destructor - the workload no longer models the hang\n");
@@ -132,5 +143,9 @@ function forkAndExit(bool $terminate): array
 	fwrite(STDERR, "FAIL: unexpected teardown ran in the child\n");
 	exit(1);
 }
+if (!$twinThrew) {
+	fwrite(STDERR, "FAIL: the PHP twin's exitImmediately() did not throw TurboNotActiveException\n");
+	exit(1);
+}
 
 echo "ALL OK\n";
diff --git a/turbo-ext/tests/smoke.php b/turbo-ext/tests/smoke.php
index a26c086997e..db61783b772 100644
--- a/turbo-ext/tests/smoke.php
+++ b/turbo-ext/tests/smoke.php
@@ -4659,6 +4659,7 @@ public function getComments(): array
 	\PHPStan\Cache\ArenaCache::class => 'arena-smoke.php',
 	\PHPStan\Parser\ParserRunner::class => 'parser-corpus.php',
 	\PHPStan\Reflection\BetterReflection\SourceLocator\PhpFileCleaner::class => 'php-file-cleaner-corpus.php',
+	\PHPStan\Turbo\ForkedChild::class => 'exit-immediately.php',
 	\PHPStan\Reflection\BetterReflection\SourceLocator\SymbolFinderInFiles::class => 'symbol-finder-corpus.php',
 	\PHPStan\Analyser\ExprHandler\Helper\EarlyTerminatingCallHelper::class => 'walk-trace.php',
 	\PHPStan\Analyser\ExprHandler\Helper\MethodCallReturnTypeHelper::class => 'walk-trace.php',