diff --git a/CHANGELOG.md b/CHANGELOG.md
index eecf798..69369fc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## [Unreleased]
+
+### Fixed
+
+- Forgot to declare pure methods
+
 ## 4.1.0 - 2025-08-20
 
 ### Changed
diff --git a/psalm.xml b/psalm.xml
index 510148d..feccc34 100644
--- a/psalm.xml
+++ b/psalm.xml
@@ -14,4 +14,7 @@
             <directory name="vendor" />
         </ignoreFiles>
     </projectFiles>
+    <issueHandlers>
+        <UndefinedAttributeClass errorLevel="suppress" />
+    </issueHandlers>
 </psalm>
diff --git a/src/Handler.php b/src/Handler.php
index 2c6071e..1e985fc 100644
--- a/src/Handler.php
+++ b/src/Handler.php
@@ -11,11 +11,18 @@
 
 final class Handler
 {
+    /**
+     * @psalm-mutation-free
+     */
     private function __construct(
         private Main|Async $implementation,
     ) {
     }
 
+    /**
+     * @psalm-pure
+     */
+    #[\NoDiscard]
     public static function main(): self
     {
         return new self(Main::install());
@@ -43,7 +50,9 @@ public function remove(callable $listener): void
      * instruct the fiber to terminate.
      *
      * @internal
+     * @psalm-mutation-free
      */
+    #[\NoDiscard]
     public function async(?Interceptor $interceptor = null): self
     {
         return new self(
diff --git a/src/Handler/Async.php b/src/Handler/Async.php
index dfb42a0..9211e86 100644
--- a/src/Handler/Async.php
+++ b/src/Handler/Async.php
@@ -15,12 +15,19 @@
  */
 final class Async
 {
+    /**
+     * @psalm-mutation-free
+     */
     private function __construct(
         private Handler $parent,
         private ?Interceptor $interceptor,
     ) {
     }
 
+    /**
+     * @psalm-pure
+     */
+    #[\NoDiscard]
     public static function new(Handler $parent, ?Interceptor $interceptor): self
     {
         return new self($parent, $interceptor);
diff --git a/src/Handler/Main.php b/src/Handler/Main.php
index 05bd1a5..6ca4f99 100644
--- a/src/Handler/Main.php
+++ b/src/Handler/Main.php
@@ -19,6 +19,8 @@
 final class Main
 {
     /**
+     * @psalm-mutation-free
+     *
      * @param Map<Signal, Sequence<callable(Signal, Info): void>> $handlers
      */
     private function __construct(
@@ -28,6 +30,10 @@ private function __construct(
     ) {
     }
 
+    /**
+     * @psalm-pure
+     */
+    #[\NoDiscard]
     public static function install(): self
     {
         return new self(