diff --git a/CLAUDE.md b/CLAUDE.md
index f48abb644d..6503317b71 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -373,6 +373,10 @@ When adding or editing PHPDoc comments in this codebase, follow these guidelines
 - Use imperative voice without "Returns the..." preambles when a brief note suffices. Prefer `/** Replaces unresolved TemplateTypes with their bounds. */` over a multi-line block.
 - Preserve `@api` and type tags on their own lines, with no redundant description alongside them.
 
+### UnionTypeMethodReflection and IntersectionTypeMethodReflection parity
+
+When methods are called on union types (`Foo|Bar`), the resolved method reflection is a `UnionTypeMethodReflection` that wraps the individual method reflections. Similarly, `IntersectionTypeMethodReflection` handles intersection types. These two classes must maintain feature parity for things like `getAsserts()`, `getSelfOutType()`, etc. When one class correctly combines member data (e.g. `IntersectionTypeMethodReflection::getAsserts()` iterating over methods and calling `intersectWith()`), the other should do the same rather than returning empty/null. The `Assertions::intersectWith()` method merges assertion tag lists from multiple sources.
+
 ## Important dependencies
 
 - `nikic/php-parser` ^5.7.0 - PHP AST parsing
diff --git a/src/Reflection/Type/UnionTypeMethodReflection.php b/src/Reflection/Type/UnionTypeMethodReflection.php
index f61f27cd5a..135fcfc39d 100644
--- a/src/Reflection/Type/UnionTypeMethodReflection.php
+++ b/src/Reflection/Type/UnionTypeMethodReflection.php
@@ -178,7 +178,13 @@ public function getDocComment(): ?string
 
 	public function getAsserts(): Assertions
 	{
-		return Assertions::createEmpty();
+		$assertions = Assertions::createEmpty();
+
+		foreach ($this->methods as $method) {
+			$assertions = $assertions->intersectWith($method->getAsserts());
+		}
+
+		return $assertions;
 	}
 
 	public function acceptsNamedArguments(): TrinaryLogic
diff --git a/tests/PHPStan/Analyser/nsrt/bug-11441.php b/tests/PHPStan/Analyser/nsrt/bug-11441.php
new file mode 100644
index 0000000000..a6a9bd8e48
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-11441.php
@@ -0,0 +1,58 @@
+<?php // lint >= 8.0
+
+namespace Bug11441;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+	public function __construct(private ?string $param)
+	{
+	}
+
+	public function getParam(): ?string
+	{
+		return $this->param;
+	}
+
+	/**
+	 * @phpstan-assert !null $this->getParam()
+	 */
+	public function checkNotNull(): void
+	{
+		if ($this->getParam() === null) {
+			throw new \Exception();
+		}
+	}
+}
+
+class Bar
+{
+	public function __construct(private ?int $param)
+	{
+	}
+
+	public function getParam(): ?int
+	{
+		return $this->param;
+	}
+
+	/**
+	 * @phpstan-assert !null $this->getParam()
+	 */
+	public function checkNotNull(): void
+	{
+		if ($this->getParam() === null) {
+			throw new \Exception();
+		}
+	}
+}
+
+function test(Foo|Bar $fooOrBar): void
+{
+	assertType('int|string|null', $fooOrBar->getParam());
+
+	$fooOrBar->checkNotNull();
+
+	assertType('int|string', $fooOrBar->getParam());
+}
diff --git a/tests/PHPStan/Analyser/nsrt/bug-13358.php b/tests/PHPStan/Analyser/nsrt/bug-13358.php
new file mode 100644
index 0000000000..43cdeeca2f
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-13358.php
@@ -0,0 +1,45 @@
+<?php declare(strict_types = 1);
+
+namespace Bug13358;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @phpstan-sealed SystemActor|AnonymousVisitorActor
+ */
+abstract class Actor
+{
+	/**
+	 * @phpstan-assert-if-true SystemActor $this
+	 */
+	public function isSystem() : bool
+	{
+		return $this instanceof SystemActor;
+	}
+
+	/**
+	 * @phpstan-assert-if-true AnonymousVisitorActor $this
+	 */
+	public function isAnonymousVisitor() : bool
+	{
+		return $this instanceof AnonymousVisitorActor;
+	}
+}
+
+class SystemActor extends Actor
+{
+}
+
+class AnonymousVisitorActor extends Actor
+{
+}
+
+function (AnonymousVisitorActor|SystemActor $actor): void {
+	assertType('Bug13358\AnonymousVisitorActor|Bug13358\SystemActor', $actor);
+
+	if ($actor->isSystem()) {
+		assertType('Bug13358\SystemActor', $actor);
+	} else {
+		assertType('Bug13358\AnonymousVisitorActor', $actor);
+	}
+};