diff --git a/phpunit/code/interface_param_union_narrows_mixed.php b/phpunit/code/interface_param_union_narrows_mixed.php
new file mode 100644
index 00000000..b5690f1c
--- /dev/null
+++ b/phpunit/code/interface_param_union_narrows_mixed.php
@@ -0,0 +1,13 @@
+<?php
+
+interface InterfaceParamUnionNarrowsMixed
+{
+    public function value(mixed $v);
+}
+
+class InterfaceParamUnionNarrowsMixedImpl implements InterfaceParamUnionNarrowsMixed
+{
+    public function value(string|int $v)
+    {
+    }
+}
diff --git a/phpunit/code/interface_param_union_narrows_untyped.php b/phpunit/code/interface_param_union_narrows_untyped.php
new file mode 100644
index 00000000..209ddefb
--- /dev/null
+++ b/phpunit/code/interface_param_union_narrows_untyped.php
@@ -0,0 +1,13 @@
+<?php
+
+interface InterfaceParamUnionNarrowsUntyped
+{
+    public function value($v);
+}
+
+class InterfaceParamUnionNarrowsUntypedImpl implements InterfaceParamUnionNarrowsUntyped
+{
+    public function value(string|int $v)
+    {
+    }
+}
diff --git a/phpunit/src/InheritanceErrorTest.php b/phpunit/src/InheritanceErrorTest.php
index 8731232a..46977a4e 100644
--- a/phpunit/src/InheritanceErrorTest.php
+++ b/phpunit/src/InheritanceErrorTest.php
@@ -58,6 +58,16 @@ class InheritanceErrorTest extends TestCase
         $this->exec('must be compatible', 'inheritance_error_param_covariant_class.php');
     }
 
+    public function testUnionParameterCannotNarrowUntypedParent()
+    {
+        $this->exec('must be compatible', 'interface_param_union_narrows_untyped.php');
+    }
+
+    public function testUnionParameterCannotNarrowMixedParent()
+    {
+        $this->exec('must be compatible', 'interface_param_union_narrows_mixed.php');
+    }
+
     public function testByRefMismatch()
     {
         $this->exec('must be compatible', 'inheritance_error_byref.php');
diff --git a/src/Php/ArgInfo.php b/src/Php/ArgInfo.php
index 93843742..03d528b3 100644
--- a/src/Php/ArgInfo.php
+++ b/src/Php/ArgInfo.php
@@ -25,6 +25,7 @@ class ArgInfo
     public bool $variadic = false;
     public bool $nullable = false;
     public bool $undeclared = false;
+    public bool $explicitMixed = false;
     public bool $property = false;
 
     /**
diff --git a/src/Php/Preprocessor.php b/src/Php/Preprocessor.php
index d9efe66b..ddd37312 100644
--- a/src/Php/Preprocessor.php
+++ b/src/Php/Preprocessor.php
@@ -237,6 +237,14 @@ class Preprocessor extends CompilerBase
         $class = '';
         $type = $this->parseTypeDecl($param->type, self::DECL_TYPE_OF_PARAM, $class);
         $argInfo->undeclared = $param->type === null;
+        if (
+            $param->type !== null
+            && !$param->type instanceof NullableType
+            && !$param->type instanceof UnionType
+            && !$param->type instanceof IntersectionType
+        ) {
+            $argInfo->explicitMixed = in_array(strtolower($this->parseIdentifier($param->type)), ['mixed', 'any'], true);
+        }
         if ($class and !$this->hasInterface($class) and !$this->isAbstractClass($class)) {
             $argInfo->class = $class;
         }
diff --git a/src/Php/Translator.php b/src/Php/Translator.php
index c736f0a6..99e87ed9 100644
--- a/src/Php/Translator.php
+++ b/src/Php/Translator.php
@@ -3358,15 +3358,22 @@ CODE;
     {
         // Child methods may omit parameter types (contravariance — accepting a
         // wider set of inputs is always compatible with the parent contract).
-        if ($childArg->undeclared || $childArg->type === self::TYPE_VAR) {
+        if ($this->isTopParameterType($childArg)) {
             return true;
         }
-        if ($parentArg->undeclared || $parentArg->type === self::TYPE_VAR) {
+        if ($this->isTopParameterType($parentArg)) {
             return false;
         }
-        if ($parentArg->typeCheck || $childArg->typeCheck) {
-            return $parentArg->typeStr === $childArg->typeStr;
+
+        $parentAcceptedTypes = $this->getParameterAcceptedTypes($parentArg);
+        $childAcceptedTypes = $this->getParameterAcceptedTypes($childArg);
+        if ($parentAcceptedTypes !== null || $childAcceptedTypes !== null) {
+            if ($parentAcceptedTypes === null || $childAcceptedTypes === null) {
+                return false;
+            }
+            return $this->isAcceptedTypeSubset($parentAcceptedTypes, $childAcceptedTypes);
         }
+
         if ($childArg->type !== $parentArg->type) {
             return false;
         }
@@ -3382,6 +3389,82 @@ CODE;
         return $this->isInheritedFrom($parentArg->class, $childArg->class);
     }
 
+    private function isTopParameterType(ArgInfo $arg): bool
+    {
+        return $arg->undeclared || $arg->explicitMixed;
+    }
+
+    private function getParameterAcceptedTypes(ArgInfo $arg): ?array
+    {
+        if (!empty($arg->typeCheck)) {
+            return $arg->typeCheck;
+        }
+
+        return match ($arg->type) {
+            self::TYPE_INT => [['kind' => 'isInt']],
+            self::TYPE_FLOAT => [['kind' => 'isFloat']],
+            self::TYPE_BOOL => [['kind' => 'isBool']],
+            self::TYPE_STR => [['kind' => 'isString']],
+            self::TYPE_ARRAY => [['kind' => 'isArray']],
+            self::TYPE_RESOURCE => [['kind' => 'isResource']],
+            self::TYPE_OBJECT => $arg->class
+                ? [['kind' => 'instanceof', 'class' => $arg->class]]
+                : [['kind' => 'isObject']],
+            default => null,
+        };
+    }
+
+    private function isAcceptedTypeSubset(array $parentTypes, array $childTypes): bool
+    {
+        foreach ($parentTypes as $parentType) {
+            if (!$this->isAcceptedTypeCovered($parentType, $childTypes)) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    private function isAcceptedTypeCovered(array $parentType, array $childTypes): bool
+    {
+        foreach ($childTypes as $childType) {
+            if ($this->isAcceptedTypeCompatible($parentType, $childType)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    private function isAcceptedTypeCompatible(array $parentType, array $childType): bool
+    {
+        $parentKind = $parentType['kind'] ?? null;
+        $childKind = $childType['kind'] ?? null;
+
+        if ($parentKind === 'instanceof' && $childKind === 'isObject') {
+            return true;
+        }
+        if ($parentKind !== $childKind) {
+            return false;
+        }
+
+        if ($parentKind === 'allOf') {
+            return $parentType == $childType;
+        }
+
+        if ($parentKind !== 'instanceof') {
+            return true;
+        }
+
+        $parentClass = $parentType['class'] ?? '';
+        $childClass = $childType['class'] ?? '';
+        if ($parentClass === $childClass) {
+            return true;
+        }
+        if ($parentClass === '' || $childClass === '') {
+            return false;
+        }
+        return $this->isInheritedFrom($parentClass, $childClass);
+    }
+
     private function checkInterfaceImplementations(Node\Stmt\Class_|Node\Stmt\Enum_ $classStmt): void
     {
         $classDef = $this->classDef;
diff --git a/tests/core/classes/class_implements_param_type.phpt b/tests/core/classes/class_implements_param_type.phpt
index 55e76b17..07c2f4aa 100644
--- a/tests/core/classes/class_implements_param_type.phpt
+++ b/tests/core/classes/class_implements_param_type.phpt
@@ -1,104 +1,105 @@
---TEST--
-Class implements interface: union type / composite type parameter compatibility
-Tests parameter type contravariance for interface implementation with union types:
- - Child method omitting the type should accept a wider set of inputs.
- - Child method declaring the identical union type should also compile.
- - nullable union (T1|T2|null) and three-way union (T1|T2|T3) are covered.
---FILE--
-<?php
-
-// -------------------------------------------------------
-// Interface with single + union type parameters
-// -------------------------------------------------------
-interface ContractA
-{
-    public function single(string $value);
-    public function union2(string|int $value);
-    public function union3(string|int|float $value);
-    public function nullableUnion(string|int|null $value);
-}
-
-// Child omits every parameter type — must be compatible with all.
-class ImplOmitAll implements ContractA
-{
-    public function single($value)  { var_dump($value); }
-    public function union2($value)  { var_dump($value); }
-    public function union3($value)  { var_dump($value); }
-    public function nullableUnion($value) { var_dump($value); }
-}
-
-// -------------------------------------------------------
-// Interface where child mirrors the union type exactly
-// -------------------------------------------------------
-interface ContractB
-{
-    public function mirror(string|int $x);
-}
-
-class ImplMirror implements ContractB
-{
-    public function mirror(string|int $x)
-    {
-        var_dump($x);
-    }
-}
-
-// -------------------------------------------------------
-// Multiple interfaces with different union types
-// -------------------------------------------------------
-interface ContractC
-{
-    public function a(string|bool $v);
-}
-interface ContractD
-{
-    public function b(int|float $v);
-}
-
-class ImplMulti implements ContractC, ContractD
-{
-    public function a($v) { var_dump($v); }
-    public function b($v) { var_dump($v); }
-}
-
-function main()
-{
-    // ImplOmitAll — omitted types should still pass runtime checks
-    $a = new ImplOmitAll;
-    $a->single('hello');
-    $a->union2('world');
-    $a->union2(42);
-    $a->union3(1);
-    $a->union3('two');
-    $a->union3(3.14);
-    $a->nullableUnion(100);
-    $a->nullableUnion(null);
-
-    // ImplMirror — explicit matching union type
-    $b = new ImplMirror;
-    $b->mirror('ok');
-    $b->mirror(99);
-
-    // ImplMulti — multiple interfaces
-    $c = new ImplMulti;
-    $c->a('yes');
-    $c->a(true);
-    $c->b(123);
-    $c->b(4.56);
-}
-?>
---EXPECT--
-string(5) "hello"
-string(5) "world"
-int(42)
-int(1)
-string(3) "two"
-float(3.14)
-int(100)
-NULL
-string(2) "ok"
-int(99)
-string(3) "yes"
-bool(true)
-int(123)
-float(4.56)
+--TEST--
+Class implements interface: parameter type compatibility
+--FILE--
+<?php
+
+interface ContractA
+{
+    public function single(string $value);
+    public function union2(string|int $value);
+    public function union3(string|int|float $value);
+    public function nullableUnion(string|int|null $value);
+}
+
+class ImplOmitAll implements ContractA
+{
+    public function single($value) { var_dump($value); }
+    public function union2($value) { var_dump($value); }
+    public function union3($value) { var_dump($value); }
+    public function nullableUnion($value) { var_dump($value); }
+}
+
+interface ContractB
+{
+    public function mirror(string|int $x);
+}
+
+class ImplMirror implements ContractB
+{
+    public function mirror(string|int $x)
+    {
+        var_dump($x);
+    }
+}
+
+interface ContractC
+{
+    public function widen(string $v);
+}
+
+class ImplWiden implements ContractC
+{
+    public function widen(string|int $v)
+    {
+        var_dump($v);
+    }
+}
+
+interface ContractD
+{
+    public function a(string|bool $v);
+}
+
+interface ContractE
+{
+    public function b(int|float $v);
+}
+
+class ImplMulti implements ContractD, ContractE
+{
+    public function a($v) { var_dump($v); }
+    public function b($v) { var_dump($v); }
+}
+
+function main()
+{
+    $a = new ImplOmitAll;
+    $a->single('hello');
+    $a->union2('world');
+    $a->union2(42);
+    $a->union3(1);
+    $a->union3('two');
+    $a->union3(3.14);
+    $a->nullableUnion(100);
+    $a->nullableUnion(null);
+
+    $b = new ImplMirror;
+    $b->mirror('ok');
+    $b->mirror(99);
+
+    $c = new ImplWiden;
+    $c->widen('wide');
+
+    $d = new ImplMulti;
+    $d->a('yes');
+    $d->a(true);
+    $d->b(123);
+    $d->b(4.56);
+}
+?>
+--EXPECT--
+string(5) "hello"
+string(5) "world"
+int(42)
+int(1)
+string(3) "two"
+float(3.14)
+int(100)
+NULL
+string(2) "ok"
+int(99)
+string(4) "wide"
+string(3) "yes"
+bool(true)
+int(123)
+float(4.56)