diff --git a/lib/JIT/InstanceOfHelper.php b/lib/JIT/InstanceOfHelper.php
index a8e417e6461..c5e107c5d48 100644
--- a/lib/JIT/InstanceOfHelper.php
+++ b/lib/JIT/InstanceOfHelper.php
@@ -5,17 +5,30 @@
 namespace PHPCompiler\JIT;
 
 use PHPCompiler\JIT\Builtin\ErrorRaise;
+use PHPCompiler\VM\InstanceOfClassName;
+use PHPCompiler\VM\InstanceOfJitHelper;
 use PHPCfg\Operand;
 use PHPCfg\Operand\Literal;
 use PHPLLVM\Builder;
 use PHPLLVM\Value;
 
 /**
- * instanceof lowering for literal and dynamic class operands (#4339).
+ * instanceof lowering for literal and dynamic class operands (#4339, #10078).
+ *
+ * SSOT: {@see \PHPCompiler\VM\InstanceOfClassName}, {@see \PHPCompiler\VM\InstanceOfJitHelper}
  */
 final class InstanceOfHelper
 {
-    public const ERROR_MESSAGE = 'Class name must be a valid object or a string';
+    public const ERROR_MESSAGE = InstanceOfClassName::ERROR_MESSAGE;
+
+    private const HELPER_PATH = '/VM/InstanceOfJitHelper.php';
+
+    private const VALUE_BOX_RHS_KIND_HELPER = 'PHPCompiler\\VM\\InstanceOfJitHelper::valueBoxRhsKind';
+
+    /** @var list<string> */
+    private const COMPILED_HELPERS = [
+        self::VALUE_BOX_RHS_KIND_HELPER,
+    ];
 
     public static function emit(Context $context, Variable $expr, Operand $classOp): Variable
     {
@@ -28,14 +41,7 @@ public static function emit(Context $context, Variable $expr, Operand $classOp):
 
     private static function emitDynamic(Context $context, Variable $expr, Variable $classVar): Variable
     {
-        if (\in_array($classVar->type, [
-            Variable::TYPE_NATIVE_LONG,
-            Variable::TYPE_NATIVE_DOUBLE,
-            Variable::TYPE_NATIVE_BOOL,
-            Variable::TYPE_NULL,
-            Variable::TYPE_ARRAY,
-            Variable::TYPE_HASHTABLE,
-        ], true)) {
+        if (InstanceOfJitHelper::jitRhsTypeIsInvalidClass($classVar->type)) {
             self::emitInvalidClassRhsError($context);
 
             $i1 = $context->getTypeFromString('int1');
@@ -86,7 +92,8 @@ private static function emitWithBoxedClassVar(Context $context, Variable $expr,
         $typeByte = $context->builder->load(
             $context->builder->structGep($valuePtr, $map['type'])
         );
-        $i8 = $context->getTypeFromString('int8');
+        $rhsKind = self::callValueBoxRhsKind($context, $typeByte);
+        $i32 = $context->getTypeFromString('int32');
 
         $stringBlock = BasicBlockHelper::append($context, 'instanceof_rhs_str');
         $afterString = BasicBlockHelper::append($context, 'instanceof_rhs_after_str');
@@ -96,8 +103,8 @@ private static function emitWithBoxedClassVar(Context $context, Variable $expr,
 
         $isString = $context->builder->icmp(
             Builder::INT_EQ,
-            $typeByte,
-            $i8->constInt(Variable::TYPE_STRING, false)
+            $rhsKind,
+            $i32->constInt(InstanceOfJitHelper::RHS_KIND_STRING, false)
         );
         $context->builder->branchIf($isString, $stringBlock, $afterString);
 
@@ -114,8 +121,8 @@ private static function emitWithBoxedClassVar(Context $context, Variable $expr,
         $context->builder->positionAtEnd($afterString);
         $isObject = $context->builder->icmp(
             Builder::INT_EQ,
-            $typeByte,
-            $i8->constInt(Variable::TYPE_OBJECT, false)
+            $rhsKind,
+            $i32->constInt(InstanceOfJitHelper::RHS_KIND_OBJECT, false)
         );
         $context->builder->branchIf($isObject, $objectBlock, $invalidBlock);
 
@@ -214,4 +221,42 @@ private static function ensureStrcasecmp(Context $context): void
             $context->registerFunction('strcasecmp', $fn);
         }
     }
+
+    private static function ensureValueBoxBridgeLinked(Context $context): void
+    {
+        $abiName = '__instanceof__valueBoxRhsKind';
+        $probe = $context->module->getNamedFunction($abiName);
+        if (null !== $probe && $probe->countBasicBlocks() > 0) {
+            $context->registerFunction($abiName, $probe);
+
+            return;
+        }
+
+        $i8 = $context->getTypeFromString('int8');
+        $i32 = $context->getTypeFromString('int32');
+        JitVmHelperLink::ensureBridge(
+            $context,
+            $abiName,
+            'instanceof_value_box_rhs_kind_entry',
+            [$i8],
+            $i32,
+            self::VALUE_BOX_RHS_KIND_HELPER,
+            self::HELPER_PATH,
+            self::COMPILED_HELPERS,
+            '#10078'
+        );
+        $context->builder->clearInsertionPosition();
+    }
+
+    private static function callValueBoxRhsKind(Context $context, Value $typeByte): Value
+    {
+        self::ensureValueBoxBridgeLinked($context);
+        $fn = $context->lookupFunction('__instanceof__valueBoxRhsKind');
+        $i8 = $context->getTypeFromString('int8');
+
+        return $context->builder->call(
+            $fn,
+            $context->builder->trunc($typeByte, $i8)
+        );
+    }
 }
diff --git a/lib/VM/InstanceOfJitHelper.php b/lib/VM/InstanceOfJitHelper.php
new file mode 100644
index 00000000000..eeb9368b3a4
--- /dev/null
+++ b/lib/VM/InstanceOfJitHelper.php
@@ -0,0 +1,49 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPCompiler\VM;
+
+use PHPCompiler\JIT\Variable as JitVariable;
+
+/**
+ * Lowered into JIT/AOT modules for dynamic instanceof RHS guards (#10078, php-in-PHP).
+ *
+ * php-src: Zend/zend_execute.c — ZEND_INSTANCEOF class operand
+ * SSOT: {@see InstanceOfClassName}
+ */
+final class InstanceOfJitHelper
+{
+    public const RHS_KIND_INVALID = 0;
+    public const RHS_KIND_STRING = 1;
+    public const RHS_KIND_OBJECT = 2;
+
+    /**
+     * JIT-native RHS type is invalid for instanceof class operand (#4339).
+     */
+    public static function jitRhsTypeIsInvalidClass(int $jitType): bool
+    {
+        return \in_array($jitType, [
+            JitVariable::TYPE_NATIVE_LONG,
+            JitVariable::TYPE_NATIVE_DOUBLE,
+            JitVariable::TYPE_NATIVE_BOOL,
+            JitVariable::TYPE_NULL,
+            JitVariable::TYPE_HASHTABLE,
+        ], true);
+    }
+
+    /**
+     * Value-box RHS dispatch: string class name, object class, or invalid (#4339).
+     */
+    public static function valueBoxRhsKind(int $typeByte): int
+    {
+        if (Variable::TYPE_STRING === $typeByte) {
+            return self::RHS_KIND_STRING;
+        }
+        if (Variable::TYPE_OBJECT === $typeByte) {
+            return self::RHS_KIND_OBJECT;
+        }
+
+        return self::RHS_KIND_INVALID;
+    }
+}
diff --git a/test/selfhost/compiler_lib_spine_smoke/main.php b/test/selfhost/compiler_lib_spine_smoke/main.php
index f3e5e97cf14..13ab31119e2 100644
--- a/test/selfhost/compiler_lib_spine_smoke/main.php
+++ b/test/selfhost/compiler_lib_spine_smoke/main.php
@@ -2699,6 +2699,7 @@
 require_once __DIR__.'/../../../lib/VM/InOperatorJitHelper.php';
 require_once __DIR__.'/../../../lib/VM/VmVarFetch.php';
 require_once __DIR__.'/../../../lib/VM/VmVarFetchJitHelper.php';
+require_once __DIR__.'/../../../lib/VM/InstanceOfJitHelper.php';
 require_once __DIR__.'/../../../lib/VM/InstanceOfClassName.php';
 require_once __DIR__.'/../../../lib/VM/InterfaceCheck.php';
 require_once __DIR__.'/../../../lib/VM/InternalStrictArg.php';
diff --git a/test/unit/BootstrapSelfhostBundleTest.php b/test/unit/BootstrapSelfhostBundleTest.php
index aa4095030e6..94ea07457a9 100644
--- a/test/unit/BootstrapSelfhostBundleTest.php
+++ b/test/unit/BootstrapSelfhostBundleTest.php
@@ -24,6 +24,7 @@ final class BootstrapSelfhostBundleTest extends TestCase
         'lib/JIT/CastArrayShared.php',
         'lib/VM/CastJitHelper.php',
         'lib/VM/InOperatorJitHelper.php',
+        'lib/VM/InstanceOfJitHelper.php',
         'lib/VM/VmVarFetch.php',
         'lib/VM/VmVarFetchJitHelper.php',
         'lib/VM/VmFromCallable.php',
diff --git a/test/unit/InstanceOfRuntimeShrinkTest.php b/test/unit/InstanceOfRuntimeShrinkTest.php
new file mode 100644
index 00000000000..39ff44b8006
--- /dev/null
+++ b/test/unit/InstanceOfRuntimeShrinkTest.php
@@ -0,0 +1,59 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPCompiler\Test\Unit;
+
+use PHPCompiler\JIT\Variable as JitVariable;
+use PHPCompiler\VM\InstanceOfClassName;
+use PHPCompiler\VM\InstanceOfJitHelper;
+use PHPCompiler\VM\Variable as VmVariable;
+use PHPUnit\Framework\TestCase;
+
+/** Instanceof JIT routes dynamic RHS guards through InstanceOfJitHelper PHP (#10078). */
+final class InstanceOfRuntimeShrinkTest extends TestCase
+{
+    public function testInstanceOfHelperRoutesValueBoxThroughInstanceOfJitHelper(): void
+    {
+        $source = (string) file_get_contents(__DIR__.'/../../lib/JIT/InstanceOfHelper.php');
+        $this->assertStringContainsString('InstanceOfJitHelper', $source);
+        $this->assertStringContainsString('valueBoxRhsKind', $source);
+        $this->assertStringContainsString('JitVmHelperLink', $source);
+        $this->assertStringContainsString('jitRhsTypeIsInvalidClass', $source);
+    }
+
+    public function testInstanceOfHelperSharesErrorMessageWithVm(): void
+    {
+        $this->assertSame(InstanceOfClassName::ERROR_MESSAGE, \PHPCompiler\JIT\InstanceOfHelper::ERROR_MESSAGE);
+    }
+
+    public function testInstanceOfJitHelperJitRhsTypeIsInvalidClass(): void
+    {
+        $this->assertTrue(InstanceOfJitHelper::jitRhsTypeIsInvalidClass(JitVariable::TYPE_NULL));
+        $this->assertTrue(InstanceOfJitHelper::jitRhsTypeIsInvalidClass(JitVariable::TYPE_NATIVE_LONG));
+        $this->assertTrue(InstanceOfJitHelper::jitRhsTypeIsInvalidClass(JitVariable::TYPE_HASHTABLE));
+        $this->assertFalse(InstanceOfJitHelper::jitRhsTypeIsInvalidClass(JitVariable::TYPE_STRING));
+        $this->assertFalse(InstanceOfJitHelper::jitRhsTypeIsInvalidClass(JitVariable::TYPE_OBJECT));
+        $this->assertFalse(InstanceOfJitHelper::jitRhsTypeIsInvalidClass(JitVariable::TYPE_VALUE));
+    }
+
+    public function testInstanceOfJitHelperValueBoxRhsKind(): void
+    {
+        $this->assertSame(
+            InstanceOfJitHelper::RHS_KIND_STRING,
+            InstanceOfJitHelper::valueBoxRhsKind(VmVariable::TYPE_STRING)
+        );
+        $this->assertSame(
+            InstanceOfJitHelper::RHS_KIND_OBJECT,
+            InstanceOfJitHelper::valueBoxRhsKind(VmVariable::TYPE_OBJECT)
+        );
+        $this->assertSame(
+            InstanceOfJitHelper::RHS_KIND_INVALID,
+            InstanceOfJitHelper::valueBoxRhsKind(VmVariable::TYPE_NULL)
+        );
+        $this->assertSame(
+            InstanceOfJitHelper::RHS_KIND_INVALID,
+            InstanceOfJitHelper::valueBoxRhsKind(VmVariable::TYPE_ARRAY)
+        );
+    }
+}