diff --git a/CLAUDE.md b/CLAUDE.md
index f93091704c..a5849a4edc 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -399,10 +399,6 @@ 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/Assertions.php b/src/Reflection/Assertions.php
index 30f9106b77..208b1f4f82 100644
--- a/src/Reflection/Assertions.php
+++ b/src/Reflection/Assertions.php
@@ -5,10 +5,12 @@
 use PHPStan\PhpDoc\ResolvedPhpDocBlock;
 use PHPStan\PhpDoc\Tag\AssertTag;
 use PHPStan\Type\Type;
+use PHPStan\Type\TypeCombinator;
 use function array_filter;
 use function array_map;
 use function array_merge;
 use function count;
+use function sprintf;
 
 /**
  * Collection of @phpstan-assert annotations on a function or method.
@@ -91,12 +93,76 @@ public function mapTypes(callable $callable): self
 	{
 		$assertTagsCallback = static fn (AssertTag $tag): AssertTag => $tag->withType($callable($tag->getType()));
 
-		return new self(array_map($assertTagsCallback, $this->asserts));
+		return self::create(array_map($assertTagsCallback, $this->asserts));
 	}
 
+	/**
+	 * @deprecated use union() or intersect() instead
+	 */
 	public function intersectWith(Assertions $other): self
 	{
-		return new self(array_merge($this->getAll(), $other->getAll()));
+		return $this->union($other);
+	}
+
+	public function union(Assertions $other): self
+	{
+		if ($this === self::$empty) {
+			return $other;
+		}
+		if ($other === self::$empty) {
+			return $this;
+		}
+
+		return self::create(array_merge($this->getAll(), $other->getAll()));
+	}
+
+	public function intersect(Assertions $other): self
+	{
+		if ($this === self::$empty) {
+			return $other;
+		}
+		if ($other === self::$empty) {
+			return $this;
+		}
+
+		$otherAsserts = $other->getAll();
+		$thisAsserts = $this->getAll();
+
+		$merged = [];
+		foreach ($thisAsserts as $thisAssert) {
+			$key = self::getAssertKey($thisAssert);
+
+			foreach ($otherAsserts as $otherAssert) {
+				if (self::getAssertKey($otherAssert) !== $key) {
+					continue;
+				}
+
+				$merged[] = $thisAssert->withType(TypeCombinator::union($thisAssert->getType(), $otherAssert->getType()));
+			}
+		}
+
+		return self::create($merged);
+	}
+
+	private static function getAssertKey(AssertTag $assert): string
+	{
+		return sprintf(
+			'%s-%s-%s',
+			$assert->getParameter()->describe(),
+			$assert->getIf(),
+			$assert->isNegated() ? '1' : '0',
+		);
+	}
+
+	/**
+	 * @param AssertTag[] $asserts
+	 */
+	private static function create(array $asserts): self
+	{
+		if (count($asserts) === 0) {
+			return self::createEmpty();
+		}
+		return new self($asserts);
 	}
 
 	public static function createEmpty(): self
@@ -116,11 +182,8 @@ public static function createEmpty(): self
 	public static function createFromResolvedPhpDocBlock(ResolvedPhpDocBlock $phpDocBlock): self
 	{
 		$tags = $phpDocBlock->getAssertTags();
-		if (count($tags) === 0) {
-			return self::createEmpty();
-		}
 
-		return new self($tags);
+		return self::create($tags);
 	}
 
 }
diff --git a/src/Reflection/Type/IntersectionTypeMethodReflection.php b/src/Reflection/Type/IntersectionTypeMethodReflection.php
index b1b52f3d33..b31124c17a 100644
--- a/src/Reflection/Type/IntersectionTypeMethodReflection.php
+++ b/src/Reflection/Type/IntersectionTypeMethodReflection.php
@@ -198,7 +198,7 @@ public function getAsserts(): Assertions
 		$assertions = Assertions::createEmpty();
 
 		foreach ($this->methods as $method) {
-			$assertions = $assertions->intersectWith($method->getAsserts());
+			$assertions = $assertions->union($method->getAsserts());
 		}
 
 		return $assertions;
diff --git a/src/Reflection/Type/UnionTypeMethodReflection.php b/src/Reflection/Type/UnionTypeMethodReflection.php
index 135fcfc39d..89557b4e2c 100644
--- a/src/Reflection/Type/UnionTypeMethodReflection.php
+++ b/src/Reflection/Type/UnionTypeMethodReflection.php
@@ -181,7 +181,7 @@ public function getAsserts(): Assertions
 		$assertions = Assertions::createEmpty();
 
 		foreach ($this->methods as $method) {
-			$assertions = $assertions->intersectWith($method->getAsserts());
+			$assertions = $assertions->intersect($method->getAsserts());
 		}
 
 		return $assertions;
diff --git a/tests/PHPStan/Analyser/nsrt/bug-14108.php b/tests/PHPStan/Analyser/nsrt/bug-14108.php
new file mode 100644
index 0000000000..af7667bf16
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-14108.php
@@ -0,0 +1,52 @@
+<?php // lint >= 8.0
+
+namespace Bug14108;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+	public function __construct(private ?string $param)
+	{
+	}
+
+	public function getParam(): ?string
+	{
+		return $this->param;
+	}
+
+	/**
+	 * @phpstan-assert string $this->getParam()
+	 */
+	public function narrowGetParam(): void
+	{
+	}
+}
+
+class Bar
+{
+	public function __construct(private ?int $param)
+	{
+	}
+
+	public function getParam(): ?int
+	{
+		return $this->param;
+	}
+
+	/**
+	 * @phpstan-assert int $this->getParam()
+	 */
+	public function narrowGetParam(): void
+	{
+	}
+}
+
+function test(Foo|Bar $fooOrBar): void
+{
+	assertType('int|string|null', $fooOrBar->getParam());
+
+	$fooOrBar->narrowGetParam();
+
+	assertType('int|string', $fooOrBar->getParam());
+}
diff --git a/tests/PHPStan/Analyser/nsrt/bug-14108b.php b/tests/PHPStan/Analyser/nsrt/bug-14108b.php
new file mode 100644
index 0000000000..738fcdc5cb
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-14108b.php
@@ -0,0 +1,52 @@
+<?php // lint >= 8.0
+
+namespace Bug14108b;
+
+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 narrowGetParam(): void
+	{
+	}
+}
+
+class Bar
+{
+	public function __construct(private ?int $param)
+	{
+	}
+
+	public function getParam(): ?int
+	{
+		return $this->param;
+	}
+
+	/**
+	 * @phpstan-assert int $this->getParam()
+	 */
+	public function narrowGetParam(): void
+	{
+	}
+}
+
+function test(Foo|Bar $fooOrBar): void
+{
+	assertType('int|string|null', $fooOrBar->getParam());
+
+	$fooOrBar->narrowGetParam();
+
+	assertType('int|string|null', $fooOrBar->getParam()); // could be 'int|string'
+}
diff --git a/tests/PHPStan/Analyser/nsrt/bug-14108c.php b/tests/PHPStan/Analyser/nsrt/bug-14108c.php
new file mode 100644
index 0000000000..f95876d99b
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-14108c.php
@@ -0,0 +1,52 @@
+<?php // lint >= 8.0
+
+namespace Bug14108c;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+	public function __construct(private ?string $param)
+	{
+	}
+
+	public function getParam(): ?string
+	{
+		return $this->param;
+	}
+
+	/**
+	 * @phpstan-assert =string $this->getParam()
+	 */
+	public function narrowGetParam(): void
+	{
+	}
+}
+
+class Bar
+{
+	public function __construct(private ?int $param)
+	{
+	}
+
+	public function getParam(): ?int
+	{
+		return $this->param;
+	}
+
+	/**
+	 * @phpstan-assert int $this->getParam()
+	 */
+	public function narrowGetParam(): void
+	{
+	}
+}
+
+function test(Foo|Bar $fooOrBar): void
+{
+	assertType('int|string|null', $fooOrBar->getParam());
+
+	$fooOrBar->narrowGetParam();
+
+	assertType('int|string', $fooOrBar->getParam());
+}
diff --git a/tests/PHPStan/Analyser/nsrt/bug-14108d.php b/tests/PHPStan/Analyser/nsrt/bug-14108d.php
new file mode 100644
index 0000000000..7a04985253
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-14108d.php
@@ -0,0 +1,59 @@
+<?php // lint >= 8.0
+
+namespace Bug14108d;
+
+use function PHPStan\Testing\assertType;
+
+class Foo
+{
+	public function __construct(private ?string $param)
+	{
+	}
+
+	public function getParam(): ?string
+	{
+		return $this->param;
+	}
+
+	/**
+	 * @phpstan-assert-if-true string $this->getParam()
+	 */
+	public function narrowGetParam(): bool
+	{
+	}
+}
+
+class Bar
+{
+	public function __construct(private ?int $param)
+	{
+	}
+
+	public function getParam(): ?int
+	{
+		return $this->param;
+	}
+
+	/**
+	 * @phpstan-assert int $this->getParam()
+	 */
+	public function narrowGetParam(): void
+	{
+	}
+}
+
+function test(Foo|Bar $fooOrBar): void
+{
+	assertType('int|string|null', $fooOrBar->getParam());
+
+	$fooOrBar->narrowGetParam();
+
+	assertType('int|string|null', $fooOrBar->getParam());
+
+	if ($fooOrBar->narrowGetParam()) {
+		assertType('int|string|null', $fooOrBar->getParam()); // could be 'int|string'
+	} else {
+		assertType('int|string|null', $fooOrBar->getParam());
+	}
+	assertType('int|string|null', $fooOrBar->getParam());
+}