diff --git a/build.gradle b/build.gradle
index fa473fe1ce4..bcaf5563441 100644
--- a/build.gradle
+++ b/build.gradle
@@ -419,7 +419,7 @@ subprojects {
             "org.checkerframework.checker.nullness.NullnessChecker",
         ]
         extraJavacArgs = [
-            "-AonlyDefs=^org\\.key_project\\.util\\.lookup",
+            "-AonlyDefs=^org\\.key_project\\.util",
             "-Xmaxerrs", "10000",
             "-Werror",
             "-Aversion",
diff --git a/key.util/src/main/java/org/key_project/util/EqualsModProofIrrelevancyUtil.java b/key.util/src/main/java/org/key_project/util/EqualsModProofIrrelevancyUtil.java
index 9e6e1e7d965..6e8dc28668b 100644
--- a/key.util/src/main/java/org/key_project/util/EqualsModProofIrrelevancyUtil.java
+++ b/key.util/src/main/java/org/key_project/util/EqualsModProofIrrelevancyUtil.java
@@ -10,6 +10,7 @@
  *
  * @author Arne Keller
  */
+@SuppressWarnings("nullness")
 public final class EqualsModProofIrrelevancyUtil {
     private EqualsModProofIrrelevancyUtil() {
 
diff --git a/key.util/src/main/java/org/key_project/util/EqualsModProofIrrelevancyWrapper.java b/key.util/src/main/java/org/key_project/util/EqualsModProofIrrelevancyWrapper.java
index c80c3a8fdfa..92f40585468 100644
--- a/key.util/src/main/java/org/key_project/util/EqualsModProofIrrelevancyWrapper.java
+++ b/key.util/src/main/java/org/key_project/util/EqualsModProofIrrelevancyWrapper.java
@@ -9,6 +9,7 @@
  * @param <T> type to wrap
  * @author Arne Keller
  */
+@SuppressWarnings("nullness")
 public class EqualsModProofIrrelevancyWrapper<T extends EqualsModProofIrrelevancy> {
     /**
      * The wrapped object.
diff --git a/key.util/src/main/java/org/key_project/util/ExtList.java b/key.util/src/main/java/org/key_project/util/ExtList.java
index 8d5bf780754..460a4770592 100644
--- a/key.util/src/main/java/org/key_project/util/ExtList.java
+++ b/key.util/src/main/java/org/key_project/util/ExtList.java
@@ -10,6 +10,7 @@
  * Extends java.util.LinkedList in order to collect elements according to their type.
  * Has facilities to get elements of a certain type ({@link #get(Class)}, {@link #collect(Class)}).
  */
+@SuppressWarnings("nullness")
 public class ExtList extends LinkedList<Object> {
 
     private static final long serialVersionUID = 9182017368310263908L;
diff --git a/key.util/src/main/java/org/key_project/util/Filenames.java b/key.util/src/main/java/org/key_project/util/Filenames.java
index b7122a98e48..05fcbb78483 100644
--- a/key.util/src/main/java/org/key_project/util/Filenames.java
+++ b/key.util/src/main/java/org/key_project/util/Filenames.java
@@ -9,6 +9,7 @@
  * @author Alexander Weigl
  * @version 1 (29.03.19)
  */
+@SuppressWarnings("nullness")
 public class Filenames {
     // =======================================================
     // Methods operating on Strings
diff --git a/key.util/src/main/java/org/key_project/util/LRUCache.java b/key.util/src/main/java/org/key_project/util/LRUCache.java
index 7690a5975b3..3488b9d6db4 100644
--- a/key.util/src/main/java/org/key_project/util/LRUCache.java
+++ b/key.util/src/main/java/org/key_project/util/LRUCache.java
@@ -6,6 +6,7 @@
 /**
  * Simple realisation of an LRU cache.
  */
+@SuppressWarnings("nullness")
 public class LRUCache<K, V> extends LinkedHashMap<K, V> {
 
     /**
diff --git a/key.util/src/main/java/org/key_project/util/RandomName.java b/key.util/src/main/java/org/key_project/util/RandomName.java
index 976e13d7b71..a62cc7c8358 100644
--- a/key.util/src/main/java/org/key_project/util/RandomName.java
+++ b/key.util/src/main/java/org/key_project/util/RandomName.java
@@ -6,6 +6,7 @@
  * @author Alexander Weigl
  * @version 1 (06.12.18)
  */
+@SuppressWarnings("nullness")
 public class RandomName {
     private static final Random random = new Random();
 
diff --git a/key.util/src/main/java/org/key_project/util/Streams.java b/key.util/src/main/java/org/key_project/util/Streams.java
index 6cc8b817209..b10f298b2b2 100644
--- a/key.util/src/main/java/org/key_project/util/Streams.java
+++ b/key.util/src/main/java/org/key_project/util/Streams.java
@@ -4,6 +4,7 @@
 import java.io.IOException;
 import java.io.InputStream;
 
+@SuppressWarnings("nullness")
 public class Streams {
 
     private Streams() {
diff --git a/key.util/src/main/java/org/key_project/util/Strings.java b/key.util/src/main/java/org/key_project/util/Strings.java
index 55e292a1257..1bfa338ea74 100644
--- a/key.util/src/main/java/org/key_project/util/Strings.java
+++ b/key.util/src/main/java/org/key_project/util/Strings.java
@@ -1,5 +1,6 @@
 package org.key_project.util;
 
+import org.jspecify.annotations.NullMarked;
 import org.key_project.util.java.StringUtil;
 
 /**
@@ -8,6 +9,7 @@
  * @author Alexander Weigl
  * @version 1 (29.03.19)
  */
+@NullMarked
 public class Strings {
     /**
      * @deprecated This class has been merged with {@link org.key_project.util.java.StringUtil}.
diff --git a/key.util/src/main/java/org/key_project/util/bean/Bean.java b/key.util/src/main/java/org/key_project/util/bean/Bean.java
index d9d8a085c16..04c5899a2ca 100644
--- a/key.util/src/main/java/org/key_project/util/bean/Bean.java
+++ b/key.util/src/main/java/org/key_project/util/bean/Bean.java
@@ -13,6 +13,7 @@
  * @author Martin Hentschel
  * @see IBean
  */
+@SuppressWarnings("nullness")
 public class Bean implements IBean {
     /**
      * The used {@link PropertyChangeSupport}.
diff --git a/key.util/src/main/java/org/key_project/util/bitops/ImmutableFixedLengthBitSet.java b/key.util/src/main/java/org/key_project/util/bitops/ImmutableFixedLengthBitSet.java
index b8f6c19ecc8..4e1fafae447 100644
--- a/key.util/src/main/java/org/key_project/util/bitops/ImmutableFixedLengthBitSet.java
+++ b/key.util/src/main/java/org/key_project/util/bitops/ImmutableFixedLengthBitSet.java
@@ -9,6 +9,7 @@
  *
  * @author Dominic Scheurer
  */
+@SuppressWarnings("nullness")
 public class ImmutableFixedLengthBitSet {
 
     private boolean[] bitSet = null;
diff --git a/key.util/src/main/java/org/key_project/util/collection/DefaultEdge.java b/key.util/src/main/java/org/key_project/util/collection/DefaultEdge.java
index a2b422b1ca6..3562fa26b76 100644
--- a/key.util/src/main/java/org/key_project/util/collection/DefaultEdge.java
+++ b/key.util/src/main/java/org/key_project/util/collection/DefaultEdge.java
@@ -6,6 +6,7 @@
  *
  * @author Arne Keller
  */
+@SuppressWarnings("nullness")
 public class DefaultEdge implements GraphEdge {
     /**
      * Source node of this edge.
diff --git a/key.util/src/main/java/org/key_project/util/collection/DefaultImmutableMap.java b/key.util/src/main/java/org/key_project/util/collection/DefaultImmutableMap.java
index c903a8a3c90..b81a4d7327d 100644
--- a/key.util/src/main/java/org/key_project/util/collection/DefaultImmutableMap.java
+++ b/key.util/src/main/java/org/key_project/util/collection/DefaultImmutableMap.java
@@ -6,6 +6,7 @@
  * This class implements ImmMap<S,T> and provides a persistent Map. It is a simple implementation
  * like lists
  */
+@SuppressWarnings("nullness")
 public class DefaultImmutableMap<S, T> implements ImmutableMap<S, T> {
 
     /**
diff --git a/key.util/src/main/java/org/key_project/util/collection/DefaultImmutableSet.java b/key.util/src/main/java/org/key_project/util/collection/DefaultImmutableSet.java
index 37b5d60c728..486f3988989 100644
--- a/key.util/src/main/java/org/key_project/util/collection/DefaultImmutableSet.java
+++ b/key.util/src/main/java/org/key_project/util/collection/DefaultImmutableSet.java
@@ -15,6 +15,7 @@
  *
  * @param <T> type of object to store
  */
+@SuppressWarnings("nullness")
 public class DefaultImmutableSet<T> implements ImmutableSet<T> {
 
     /**
diff --git a/key.util/src/main/java/org/key_project/util/collection/DirectedGraph.java b/key.util/src/main/java/org/key_project/util/collection/DirectedGraph.java
index dfc67953ff1..1bd08b623c5 100644
--- a/key.util/src/main/java/org/key_project/util/collection/DirectedGraph.java
+++ b/key.util/src/main/java/org/key_project/util/collection/DirectedGraph.java
@@ -14,6 +14,7 @@
  * @param <E> edge type
  * @author Arne Keller
  */
+@SuppressWarnings("nullness")
 public class DirectedGraph<V, E extends GraphEdge> implements Graph<V, E> {
     /**
      * Set of vertices in this graph.
diff --git a/key.util/src/main/java/org/key_project/util/collection/ImmutableArray.java b/key.util/src/main/java/org/key_project/util/collection/ImmutableArray.java
index 3723550909d..29619b553cd 100644
--- a/key.util/src/main/java/org/key_project/util/collection/ImmutableArray.java
+++ b/key.util/src/main/java/org/key_project/util/collection/ImmutableArray.java
@@ -10,6 +10,7 @@
 import java.util.stream.StreamSupport;
 import javax.annotation.Nonnull;
 
+@SuppressWarnings("nullness")
 public class ImmutableArray<S> implements java.lang.Iterable<S>, java.io.Serializable {
 
     /**
diff --git a/key.util/src/main/java/org/key_project/util/collection/ImmutableLeftistHeap.java b/key.util/src/main/java/org/key_project/util/collection/ImmutableLeftistHeap.java
index 7fd38d909f2..baa197a1f45 100644
--- a/key.util/src/main/java/org/key_project/util/collection/ImmutableLeftistHeap.java
+++ b/key.util/src/main/java/org/key_project/util/collection/ImmutableLeftistHeap.java
@@ -7,6 +7,7 @@
  * This class implements the leftist heap, see &quot;Functional Data Structures&quot; by Chris
  * Okasaki
  */
+@SuppressWarnings("nullness")
 public abstract class ImmutableLeftistHeap<T extends Comparable<T>> implements ImmutableHeap<T> {
 
     /**
diff --git a/key.util/src/main/java/org/key_project/util/collection/ImmutableSLList.java b/key.util/src/main/java/org/key_project/util/collection/ImmutableSLList.java
index 2ac3f8c1fd7..c189524f6fd 100644
--- a/key.util/src/main/java/org/key_project/util/collection/ImmutableSLList.java
+++ b/key.util/src/main/java/org/key_project/util/collection/ImmutableSLList.java
@@ -20,7 +20,7 @@
  * and head with amortized O(1). This will be done later (if necessary).
  */
 
-@SuppressWarnings("unchecked")
+@SuppressWarnings({"unchecked", "nullness"})
 public abstract class ImmutableSLList<T> implements ImmutableList<T> {
 
     /**
diff --git a/key.util/src/main/java/org/key_project/util/collection/ImmutableSet.java b/key.util/src/main/java/org/key_project/util/collection/ImmutableSet.java
index b1c1655c51a..6264ae12d9d 100644
--- a/key.util/src/main/java/org/key_project/util/collection/ImmutableSet.java
+++ b/key.util/src/main/java/org/key_project/util/collection/ImmutableSet.java
@@ -12,6 +12,7 @@
  * offer a public static final variable .<called>nil()
  */
 
+@SuppressWarnings("nullness")
 public interface ImmutableSet<T> extends Iterable<T>, java.io.Serializable {
 
     /**
diff --git a/key.util/src/main/java/org/key_project/util/collection/Immutables.java b/key.util/src/main/java/org/key_project/util/collection/Immutables.java
index 7401b27e682..b37b1c36729 100644
--- a/key.util/src/main/java/org/key_project/util/collection/Immutables.java
+++ b/key.util/src/main/java/org/key_project/util/collection/Immutables.java
@@ -13,6 +13,7 @@
  *
  * @author Mattias Ulbrich
  */
+@SuppressWarnings("nullness")
 public final class Immutables {
 
     private Immutables() {
diff --git a/key.util/src/main/java/org/key_project/util/collection/KeYCollections.java b/key.util/src/main/java/org/key_project/util/collection/KeYCollections.java
index 00acea0741a..11d479a6f91 100644
--- a/key.util/src/main/java/org/key_project/util/collection/KeYCollections.java
+++ b/key.util/src/main/java/org/key_project/util/collection/KeYCollections.java
@@ -14,6 +14,7 @@
  * @author Alexander Weigl
  * @version 1 (29.03.19)
  */
+@SuppressWarnings("nullness")
 public class KeYCollections {
     // =======================================================
     // Methods operating on Arrays
diff --git a/key.util/src/main/java/org/key_project/util/collection/NotUniqueException.java b/key.util/src/main/java/org/key_project/util/collection/NotUniqueException.java
index 340a04eda06..360179ba951 100644
--- a/key.util/src/main/java/org/key_project/util/collection/NotUniqueException.java
+++ b/key.util/src/main/java/org/key_project/util/collection/NotUniqueException.java
@@ -3,6 +3,7 @@
 
 
 /** thrown if a duplicate is being added via addUnique() */
+@SuppressWarnings("nullness")
 public class NotUniqueException extends Exception {
 
     private static final long serialVersionUID = 6565515240836947955L;
diff --git a/key.util/src/main/java/org/key_project/util/collection/PropertiesUtil.java b/key.util/src/main/java/org/key_project/util/collection/PropertiesUtil.java
index b193a693911..d2884d43f9f 100644
--- a/key.util/src/main/java/org/key_project/util/collection/PropertiesUtil.java
+++ b/key.util/src/main/java/org/key_project/util/collection/PropertiesUtil.java
@@ -7,6 +7,7 @@
 import java.nio.charset.StandardCharsets;
 import java.util.Properties;
 
+@SuppressWarnings("nullness")
 public class PropertiesUtil {
 
     private PropertiesUtil() {
diff --git a/key.util/src/main/java/org/key_project/util/helper/FindResources.java b/key.util/src/main/java/org/key_project/util/helper/FindResources.java
index 0ee3215b281..a2365f38712 100644
--- a/key.util/src/main/java/org/key_project/util/helper/FindResources.java
+++ b/key.util/src/main/java/org/key_project/util/helper/FindResources.java
@@ -14,6 +14,7 @@
  * @author Alexander Weigl
  * @version 1 (13.02.19)
  */
+@SuppressWarnings("nullness")
 public final class FindResources {
     /**
      * List directory contents for a resource folder. Not recursive. This is basically a brute-force
diff --git a/key.util/src/main/java/org/key_project/util/helper/HelperClassForUtilityTests.java b/key.util/src/main/java/org/key_project/util/helper/HelperClassForUtilityTests.java
index bf55ef35fcb..d4a855147fc 100644
--- a/key.util/src/main/java/org/key_project/util/helper/HelperClassForUtilityTests.java
+++ b/key.util/src/main/java/org/key_project/util/helper/HelperClassForUtilityTests.java
@@ -5,6 +5,7 @@
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
 
+@SuppressWarnings("nullness")
 public class HelperClassForUtilityTests {
     public static final File RESOURCE_DIRECTORY = FindResources.getTestResourcesDirectory();
 
diff --git a/key.util/src/main/java/org/key_project/util/java/ArrayUtil.java b/key.util/src/main/java/org/key_project/util/java/ArrayUtil.java
index e19552eb30b..03e635d16df 100644
--- a/key.util/src/main/java/org/key_project/util/java/ArrayUtil.java
+++ b/key.util/src/main/java/org/key_project/util/java/ArrayUtil.java
@@ -10,6 +10,7 @@
  *
  * @author Martin Hentschel
  */
+@SuppressWarnings("nullness")
 public final class ArrayUtil {
     /**
      * Forbid instances by this private constructor.
diff --git a/key.util/src/main/java/org/key_project/util/java/CollectionUtil.java b/key.util/src/main/java/org/key_project/util/java/CollectionUtil.java
index 3ce65775909..82a97084db8 100644
--- a/key.util/src/main/java/org/key_project/util/java/CollectionUtil.java
+++ b/key.util/src/main/java/org/key_project/util/java/CollectionUtil.java
@@ -8,6 +8,7 @@
  *
  * @author Martin Hentschel
  */
+@SuppressWarnings("nullness")
 public class CollectionUtil {
     /**
      * The default separator.
diff --git a/key.util/src/main/java/org/key_project/util/java/IOUtil.java b/key.util/src/main/java/org/key_project/util/java/IOUtil.java
index 91a1e6a6c97..37b60c4fa67 100644
--- a/key.util/src/main/java/org/key_project/util/java/IOUtil.java
+++ b/key.util/src/main/java/org/key_project/util/java/IOUtil.java
@@ -18,6 +18,7 @@
  *
  * @author Martin Hentschel
  */
+@SuppressWarnings("nullness")
 public final class IOUtil {
     /**
      * The size of used buffers.
diff --git a/key.util/src/main/java/org/key_project/util/java/IntegerUtil.java b/key.util/src/main/java/org/key_project/util/java/IntegerUtil.java
index 495fd5ae775..1dd354bf163 100644
--- a/key.util/src/main/java/org/key_project/util/java/IntegerUtil.java
+++ b/key.util/src/main/java/org/key_project/util/java/IntegerUtil.java
@@ -1,5 +1,6 @@
 package org.key_project.util.java;
 
+@SuppressWarnings("nullness")
 public final class IntegerUtil {
     /**
      * Forbid instances.
diff --git a/key.util/src/main/java/org/key_project/util/java/MapUtil.java b/key.util/src/main/java/org/key_project/util/java/MapUtil.java
index d6d441fb87a..37712c59b15 100644
--- a/key.util/src/main/java/org/key_project/util/java/MapUtil.java
+++ b/key.util/src/main/java/org/key_project/util/java/MapUtil.java
@@ -12,6 +12,7 @@
  *
  * @author lanzinger
  */
+@SuppressWarnings("nullness")
 public final class MapUtil {
 
     private MapUtil() {}
diff --git a/key.util/src/main/java/org/key_project/util/java/NumberUtil.java b/key.util/src/main/java/org/key_project/util/java/NumberUtil.java
index 2c77796e405..76c34dac6f5 100644
--- a/key.util/src/main/java/org/key_project/util/java/NumberUtil.java
+++ b/key.util/src/main/java/org/key_project/util/java/NumberUtil.java
@@ -5,6 +5,7 @@
  *
  * @author Martin Hentschel
  */
+@SuppressWarnings("nullness")
 public class NumberUtil {
     /**
      * The maximal number of digits of an integer value.
diff --git a/key.util/src/main/java/org/key_project/util/java/WrapUtils.java b/key.util/src/main/java/org/key_project/util/java/WrapUtils.java
index 2c6cdfc4657..09f264ef2f0 100644
--- a/key.util/src/main/java/org/key_project/util/java/WrapUtils.java
+++ b/key.util/src/main/java/org/key_project/util/java/WrapUtils.java
@@ -13,6 +13,7 @@
  *
  * @author Mattias Ulbrich, Mar 2021
  */
+@SuppressWarnings("nullness")
 public class WrapUtils {
 
     /*
diff --git a/key.util/src/main/java/org/key_project/util/java/XMLUtil.java b/key.util/src/main/java/org/key_project/util/java/XMLUtil.java
index 508ad47f6ad..f57110156c7 100644
--- a/key.util/src/main/java/org/key_project/util/java/XMLUtil.java
+++ b/key.util/src/main/java/org/key_project/util/java/XMLUtil.java
@@ -8,6 +8,7 @@
  *
  * @author Martin Hentschel
  */
+@SuppressWarnings("nullness")
 public final class XMLUtil {
     /**
      * Attribute name to store encodings.
diff --git a/key.util/src/main/java/org/key_project/util/java/thread/AbstractRunnableWithException.java b/key.util/src/main/java/org/key_project/util/java/thread/AbstractRunnableWithException.java
index c9d889e5e71..adb9aa9ca68 100644
--- a/key.util/src/main/java/org/key_project/util/java/thread/AbstractRunnableWithException.java
+++ b/key.util/src/main/java/org/key_project/util/java/thread/AbstractRunnableWithException.java
@@ -12,6 +12,7 @@
  * @author Martin Hentschel
  * @see IRunnableWithResult
  */
+@SuppressWarnings("nullness")
 public abstract class AbstractRunnableWithException implements IRunnableWithException {
     /**
      * An occurred exception.
diff --git a/key.util/src/main/java/org/key_project/util/java/thread/AbstractRunnableWithResult.java b/key.util/src/main/java/org/key_project/util/java/thread/AbstractRunnableWithResult.java
index 91ddb7aad27..a05c67b1696 100644
--- a/key.util/src/main/java/org/key_project/util/java/thread/AbstractRunnableWithResult.java
+++ b/key.util/src/main/java/org/key_project/util/java/thread/AbstractRunnableWithResult.java
@@ -12,6 +12,7 @@
  * @author Martin Hentschel
  * @see IRunnableWithResult
  */
+@SuppressWarnings("nullness")
 public abstract class AbstractRunnableWithResult<T> extends AbstractRunnableWithException
         implements IRunnableWithResult<T> {
     /**
diff --git a/key.util/src/main/java/org/key_project/util/lookup/InjectionException.java b/key.util/src/main/java/org/key_project/util/lookup/InjectionException.java
index 863f301d044..2b61eb564c2 100644
--- a/key.util/src/main/java/org/key_project/util/lookup/InjectionException.java
+++ b/key.util/src/main/java/org/key_project/util/lookup/InjectionException.java
@@ -4,6 +4,7 @@
  * @author Alexander Weigl
  * @version 1 (13.01.19)
  */
+@SuppressWarnings("nullness")
 public class InjectionException extends RuntimeException {
     private static final long serialVersionUID = 119998955722036861L;
 
diff --git a/key.util/src/main/java/org/key_project/util/lookup/Lookup.java b/key.util/src/main/java/org/key_project/util/lookup/Lookup.java
index 441e91b69fa..254a6e3cd81 100644
--- a/key.util/src/main/java/org/key_project/util/lookup/Lookup.java
+++ b/key.util/src/main/java/org/key_project/util/lookup/Lookup.java
@@ -24,6 +24,7 @@
  * @version 1 (15.03.19)
  */
 @NullMarked
+@SuppressWarnings("nullness")
 public class Lookup {
     public static final Lookup DEFAULT = new Lookup();
 
diff --git a/key.util/src/main/java/org/key_project/util/reflection/ClassLoaderUtil.java b/key.util/src/main/java/org/key_project/util/reflection/ClassLoaderUtil.java
index 1601f192add..b58cab00727 100644
--- a/key.util/src/main/java/org/key_project/util/reflection/ClassLoaderUtil.java
+++ b/key.util/src/main/java/org/key_project/util/reflection/ClassLoaderUtil.java
@@ -35,6 +35,7 @@
  * @see IClassLoader
  * @see JavaApplicationClassLoader
  */
+@SuppressWarnings("nullness")
 public class ClassLoaderUtil {
     /**
      * The {@link IClassLoader} instance to use.
diff --git a/key.util/src/main/java/org/key_project/util/reflection/JavaApplicationClassLoader.java b/key.util/src/main/java/org/key_project/util/reflection/JavaApplicationClassLoader.java
index b7f07142c5e..38feed62992 100644
--- a/key.util/src/main/java/org/key_project/util/reflection/JavaApplicationClassLoader.java
+++ b/key.util/src/main/java/org/key_project/util/reflection/JavaApplicationClassLoader.java
@@ -10,6 +10,7 @@
  *
  * @author Martin Hentschel
  */
+@SuppressWarnings("nullness")
 public class JavaApplicationClassLoader implements IClassLoader {
     /**
      * {@inheritDoc}