diff --git a/pkl-config-java/gradle.lockfile b/pkl-config-java/gradle.lockfile
index c5bb4cf3..2b39bf89 100644
--- a/pkl-config-java/gradle.lockfile
+++ b/pkl-config-java/gradle.lockfile
@@ -22,13 +22,30 @@ com.github.ajalt.mordant:mordant-markdown-jvm:3.0.1=pklCodegenJava
 com.github.ajalt.mordant:mordant-markdown:3.0.1=pklCodegenJava
 com.github.ajalt.mordant:mordant:3.0.1=pklCodegenJava
 com.github.ben-manes.caffeine:caffeine:2.9.3=swiftExportClasspathResolvable
+com.github.ben-manes.caffeine:caffeine:3.0.5=annotationProcessor,testAnnotationProcessor
+com.github.kevinstern:software-and-algorithms:1.0=annotationProcessor,testAnnotationProcessor
+com.google.auto.service:auto-service-annotations:1.0.1=annotationProcessor,testAnnotationProcessor
+com.google.auto.value:auto-value-annotations:1.9=annotationProcessor,testAnnotationProcessor
+com.google.auto:auto-common:1.2.2=annotationProcessor,testAnnotationProcessor
+com.google.errorprone:error_prone_annotation:2.48.0=annotationProcessor,testAnnotationProcessor
 com.google.errorprone:error_prone_annotations:2.28.0=swiftExportClasspathResolvable
+com.google.errorprone:error_prone_annotations:2.48.0=annotationProcessor,testAnnotationProcessor
+com.google.errorprone:error_prone_check_api:2.48.0=annotationProcessor,testAnnotationProcessor
+com.google.errorprone:error_prone_core:2.48.0=annotationProcessor,testAnnotationProcessor
+com.google.googlejavaformat:google-java-format:1.34.1=annotationProcessor,testAnnotationProcessor
+com.google.guava:failureaccess:1.0.3=annotationProcessor,testAnnotationProcessor
+com.google.guava:guava:33.5.0-jre=annotationProcessor,testAnnotationProcessor
+com.google.guava:listenablefuture:9999.0-empty-to-avoid-conflict-with-guava=annotationProcessor,testAnnotationProcessor
+com.google.j2objc:j2objc-annotations:3.1=annotationProcessor,testAnnotationProcessor
+com.google.protobuf:protobuf-java:4.33.2=annotationProcessor,testAnnotationProcessor
 com.palantir.javapoet:javapoet:0.14.0=pklCodegenJava
-io.github.java-diff-utils:java-diff-utils:4.12=kotlinInternalAbiValidation
+com.uber.nullaway:nullaway:0.13.1=annotationProcessor,testAnnotationProcessor
+io.github.eisop:dataflow-errorprone:3.41.0-eisop1=annotationProcessor,testAnnotationProcessor
+io.github.java-diff-utils:java-diff-utils:4.12=annotationProcessor,kotlinInternalAbiValidation,testAnnotationProcessor
 io.leangen.geantyref:geantyref:1.3.16=compileClasspath,runtimeClasspath,testCompileClasspath,testRuntimeClasspath
 io.opentelemetry:opentelemetry-api:1.41.0=swiftExportClasspathResolvable
 io.opentelemetry:opentelemetry-context:1.41.0=swiftExportClasspathResolvable
-javax.inject:javax.inject:1=testCompileClasspath,testRuntimeClasspath
+javax.inject:javax.inject:1=annotationProcessor,testAnnotationProcessor,testCompileClasspath,testRuntimeClasspath
 net.bytebuddy:byte-buddy:1.18.3=testCompileClasspath,testRuntimeClasspath
 net.java.dev.jna:jna:5.14.0=pklCodegenJava
 org.apiguardian:apiguardian-api:1.1.2=testCompileClasspath
@@ -38,6 +55,8 @@ org.bouncycastle:bcpkix-jdk18on:1.80=kotlinBouncyCastleConfiguration
 org.bouncycastle:bcprov-jdk18on:1.80=kotlinBouncyCastleConfiguration
 org.bouncycastle:bcutil-jdk18on:1.80=kotlinBouncyCastleConfiguration
 org.checkerframework:checker-qual:3.43.0=swiftExportClasspathResolvable
+org.checkerframework:checker-qual:3.53.0=annotationProcessor,testAnnotationProcessor
+org.checkerframework:dataflow-nullaway:3.53.0=annotationProcessor,testAnnotationProcessor
 org.graalvm.polyglot:polyglot:25.0.1=pklCodegenJava,runtimeClasspath,testRuntimeClasspath
 org.graalvm.sdk:collections:25.0.1=pklCodegenJava,runtimeClasspath,testRuntimeClasspath
 org.graalvm.sdk:graal-sdk:25.0.1=pklCodegenJava,runtimeClasspath,testRuntimeClasspath
@@ -79,7 +98,7 @@ org.jetbrains.kotlinx:kotlinx-serialization-core:1.7.3=swiftExportClasspathResol
 org.jetbrains:annotations:13.0=kotlinBuildToolsApiClasspath,kotlinCompilerClasspath,kotlinCompilerPluginClasspathMain,kotlinCompilerPluginClasspathTest,kotlinInternalAbiValidation,kotlinKlibCommonizerClasspath,pklCodegenJava,swiftExportClasspathResolvable,testCompileClasspath,testRuntimeClasspath
 org.jetbrains:markdown-jvm:0.7.3=pklCodegenJava
 org.jetbrains:markdown:0.7.3=pklCodegenJava
-org.jspecify:jspecify:1.0.0=pklCodegenJava,runtimeClasspath,testCompileClasspath,testRuntimeClasspath
+org.jspecify:jspecify:1.0.0=annotationProcessor,compileClasspath,pklCodegenJava,runtimeClasspath,testAnnotationProcessor,testCompileClasspath,testRuntimeClasspath
 org.junit.jupiter:junit-jupiter-api:6.0.3=testCompileClasspath,testRuntimeClasspath
 org.junit.jupiter:junit-jupiter-engine:6.0.3=testRuntimeClasspath
 org.junit.jupiter:junit-jupiter-params:6.0.3=testCompileClasspath,testRuntimeClasspath
@@ -90,5 +109,6 @@ org.junit:junit-bom:6.0.3=testCompileClasspath,testRuntimeClasspath
 org.msgpack:msgpack-core:0.9.11=compileClasspath,pklCodegenJava,runtimeClasspath,testCompileClasspath,testRuntimeClasspath
 org.opentest4j:opentest4j:1.3.0=testCompileClasspath,testRuntimeClasspath
 org.organicdesign:Paguro:3.10.3=pklCodegenJava,runtimeClasspath,testRuntimeClasspath
+org.pcollections:pcollections:4.0.1=annotationProcessor,testAnnotationProcessor
 org.snakeyaml:snakeyaml-engine:2.10=pklCodegenJava,runtimeClasspath,testRuntimeClasspath
-empty=annotationProcessor,apiDependenciesMetadata,compileOnlyDependenciesMetadata,fatJar,firstPartySourcesJars,implementationDependenciesMetadata,intransitiveDependenciesMetadata,kotlinCompilerPluginClasspath,kotlinNativeCompilerPluginClasspath,kotlinScriptDefExtensions,shadow,signatures,sourcesJar,testAnnotationProcessor,testApiDependenciesMetadata,testCompileOnlyDependenciesMetadata,testImplementationDependenciesMetadata,testIntransitiveDependenciesMetadata,testKotlinScriptDefExtensions
+empty=apiDependenciesMetadata,compileOnlyDependenciesMetadata,fatJar,firstPartySourcesJars,implementationDependenciesMetadata,intransitiveDependenciesMetadata,kotlinCompilerPluginClasspath,kotlinNativeCompilerPluginClasspath,kotlinScriptDefExtensions,shadow,signatures,sourcesJar,testApiDependenciesMetadata,testCompileOnlyDependenciesMetadata,testImplementationDependenciesMetadata,testIntransitiveDependenciesMetadata,testKotlinScriptDefExtensions
diff --git a/pkl-config-java/pkl-config-java.gradle.kts b/pkl-config-java/pkl-config-java.gradle.kts
index 4203d303..24c2eaef 100644
--- a/pkl-config-java/pkl-config-java.gradle.kts
+++ b/pkl-config-java/pkl-config-java.gradle.kts
@@ -18,6 +18,7 @@ plugins {
   id("pklJavaLibrary")
   id("pklFatJar")
   id("pklPublishLibrary")
+  id("pklJSpecify")
   signing
 }
 
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/Config.java b/pkl-config-java/src/main/java/org/pkl/config/java/Config.java
index 45f59c5d..77552733 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/Config.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/Config.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,6 +18,7 @@ package org.pkl.config.java;
 import java.io.InputStream;
 import java.lang.reflect.Type;
 import java.util.Map;
+import org.jspecify.annotations.Nullable;
 import org.pkl.config.java.mapper.ConversionException;
 import org.pkl.config.java.mapper.ValueMapper;
 import org.pkl.core.Composite;
@@ -55,7 +56,7 @@ public interface Config {
    *
    * @throws ConversionException if the value cannot be converted to the given type
    */
-  <T> T as(Class<T> type);
+  <T extends @Nullable Object> T as(Class<T> type);
 
   /**
    * Converts this node's value to the given {@link Type}.
@@ -64,14 +65,14 @@ public interface Config {
    *
    * @throws ConversionException if the value cannot be converted to the given type
    */
-  <T> T as(Type type);
+  <T extends @Nullable Object> T as(Type type);
 
   /**
    * Converts this node's value to the given {@link JavaType}.
    *
    * @throws ConversionException if the value cannot be converted to the given type
    */
-  <T> T as(JavaType<T> type);
+  <T extends @Nullable Object> T as(JavaType<T> type);
 
   /**
    * Decode a config from the supplied byte array.
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/ConfigEvaluatorBuilder.java b/pkl-config-java/src/main/java/org/pkl/config/java/ConfigEvaluatorBuilder.java
index e2165a59..a5d1a4a3 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/ConfigEvaluatorBuilder.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/ConfigEvaluatorBuilder.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -21,6 +21,7 @@ import java.util.Collection;
 import java.util.List;
 import java.util.Map;
 import java.util.regex.Pattern;
+import org.jspecify.annotations.Nullable;
 import org.pkl.config.java.mapper.ValueMapperBuilder;
 import org.pkl.core.EvaluatorBuilder;
 import org.pkl.core.SecurityManager;
@@ -28,7 +29,6 @@ import org.pkl.core.StackFrameTransformer;
 import org.pkl.core.http.HttpClient;
 import org.pkl.core.project.DeclaredDependencies;
 import org.pkl.core.project.Project;
-import org.pkl.core.util.Nullable;
 
 /** A builder for {@link ConfigEvaluator}s. */
 @SuppressWarnings({"UnusedReturnValue", "unused"})
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/JavaType.java b/pkl-config-java/src/main/java/org/pkl/config/java/JavaType.java
index d1793557..f36e6992 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/JavaType.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/JavaType.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,9 +18,9 @@ package org.pkl.config.java;
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.util.*;
+import org.jspecify.annotations.Nullable;
 import org.pkl.config.java.mapper.Types;
 import org.pkl.core.Pair;
-import org.pkl.core.util.Nullable;
 
 /**
  * Runtime representation of a possibly parameterized Java type. Factory methods are provided to
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ClassRegistry.java b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ClassRegistry.java
index 204a3b45..43218a4b 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ClassRegistry.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ClassRegistry.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,10 +20,10 @@ import java.io.UncheckedIOException;
 import java.util.HashSet;
 import java.util.Properties;
 import java.util.Set;
+import org.jspecify.annotations.Nullable;
 import org.pkl.config.java.InvalidMappingException;
 import org.pkl.core.PClassInfo;
 import org.pkl.core.util.IoUtils;
-import org.pkl.core.util.Nullable;
 
 /**
  * Describes mappings of Pkl class names to their corresponding Java classes.
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ConversionException.java b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ConversionException.java
index f913752f..9414b0b4 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ConversionException.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ConversionException.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -15,13 +15,15 @@
  */
 package org.pkl.config.java.mapper;
 
+import org.jspecify.annotations.Nullable;
+
 /** Thrown when a {@link ValueMapper} conversion fails. */
 public class ConversionException extends RuntimeException {
   public ConversionException(String message) {
     super(message);
   }
 
-  public ConversionException(String message, Throwable cause) {
+  public ConversionException(String message, @Nullable Throwable cause) {
     super(message, cause);
   }
 }
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/Converter.java b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/Converter.java
index 8593d02c..4da8b72b 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/Converter.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/Converter.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -15,6 +15,8 @@
  */
 package org.pkl.config.java.mapper;
 
+import org.jspecify.annotations.Nullable;
+
 /**
  * Converter for a particular source and target type.
  *
@@ -22,7 +24,7 @@ package org.pkl.config.java.mapper;
  * @param <T> the converter's target type
  */
 @FunctionalInterface
-public interface Converter<S, T> {
+public interface Converter<S, T extends @Nullable Object> {
   /**
    * Converts the given value. The given {@link ValueMapper} can be used to convert nested values of
    * composite values (objects, collections, etc.).
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PCollectionToArray.java b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PCollectionToArray.java
index 9fa9bb18..54c2d529 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PCollectionToArray.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PCollectionToArray.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,8 +19,8 @@ import java.lang.reflect.Array;
 import java.lang.reflect.Type;
 import java.util.Collection;
 import java.util.Optional;
+import org.jspecify.annotations.Nullable;
 import org.pkl.core.PClassInfo;
-import org.pkl.core.util.Nullable;
 
 final class PCollectionToArray implements ConverterFactory {
   @Override
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PCollectionToCollection.java b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PCollectionToCollection.java
index 9d75fd8d..30cf00a6 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PCollectionToCollection.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PCollectionToCollection.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,8 +23,8 @@ import java.lang.reflect.Type;
 import java.util.Collection;
 import java.util.Optional;
 import java.util.function.Function;
+import org.jspecify.annotations.Nullable;
 import org.pkl.core.PClassInfo;
-import org.pkl.core.util.Nullable;
 
 class PCollectionToCollection implements ConverterFactory {
   private static final Lookup lookup = MethodHandles.lookup();
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PMapToMap.java b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PMapToMap.java
index a40fdfe9..5167624e 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PMapToMap.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PMapToMap.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -24,8 +24,8 @@ import java.util.Map;
 import java.util.Optional;
 import java.util.Properties;
 import java.util.function.Function;
+import org.jspecify.annotations.Nullable;
 import org.pkl.core.PClassInfo;
-import org.pkl.core.util.Nullable;
 
 class PMapToMap implements ConverterFactory {
   private static final Lookup lookup = MethodHandles.lookup();
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PObjectToDataObject.java b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PObjectToDataObject.java
index a9a19d74..5b80e801 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PObjectToDataObject.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PObjectToDataObject.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -22,10 +22,10 @@ import java.lang.invoke.MethodHandles;
 import java.lang.invoke.MethodHandles.Lookup;
 import java.lang.reflect.*;
 import java.util.*;
+import org.jspecify.annotations.Nullable;
 import org.pkl.core.Composite;
 import org.pkl.core.PClassInfo;
 import org.pkl.core.PObject;
-import org.pkl.core.util.Nullable;
 
 public class PObjectToDataObject implements ConverterFactory {
   private static final Lookup lookup = MethodHandles.lookup();
@@ -156,7 +156,7 @@ public class PObjectToDataObject implements ConverterFactory {
     private final MethodHandle constructorHandle;
     private final Collection<Tuple2<String, Type>> parameters;
     private final PClassInfo<Object>[] cachedPropertyTypes;
-    private final Converter<Object, T>[] cachedConverters;
+    private final @Nullable Converter<Object, T>[] cachedConverters;
 
     ConverterImpl(
         Type targetType,
@@ -172,7 +172,7 @@ public class PObjectToDataObject implements ConverterFactory {
       Arrays.fill(cachedPropertyTypes, PClassInfo.Unavailable);
 
       @SuppressWarnings("unchecked")
-      Converter<Object, T>[] cachedConverters = new Converter[parameters.size()];
+      @Nullable Converter<Object, T>[] cachedConverters = new Converter[parameters.size()];
       this.cachedConverters = cachedConverters;
     }
 
@@ -203,8 +203,9 @@ public class PObjectToDataObject implements ConverterFactory {
             cachedPropertyTypes[i] = cachedPropertyType;
             cachedConverters[i] = valueMapper.getConverter(cachedPropertyType, param.second);
           }
-          assert cachedConverters[i] != null;
-          args[i] = cachedConverters[i].convert(property, valueMapper);
+          var cachedConverter = cachedConverters[i];
+          assert cachedConverter != null;
+          args[i] = cachedConverter.convert(property, valueMapper);
           i += 1;
         } catch (ConversionException e) {
           throw new ConversionException(
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PPairToPair.java b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PPairToPair.java
index 74639569..cf41ed93 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PPairToPair.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PPairToPair.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,9 +18,9 @@ package org.pkl.config.java.mapper;
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.util.Optional;
+import org.jspecify.annotations.Nullable;
 import org.pkl.core.PClassInfo;
 import org.pkl.core.Pair;
-import org.pkl.core.util.Nullable;
 
 final class PPairToPair implements ConverterFactory {
   @Override
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PStringToEnum.java b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PStringToEnum.java
index c86fe295..165eb689 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PStringToEnum.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/PStringToEnum.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -46,12 +46,16 @@ final class PStringToEnum implements ConverterFactory {
       if (enumType == DataSizeUnit.class) {
         for (var value : values) {
           var unit = (DataSizeUnit) value;
-          enumValuesByName.put(CodeGeneratorUtils.toEnumConstantName(unit.getSymbol()), value);
+          var name = CodeGeneratorUtils.toEnumConstantName(unit.getSymbol());
+          assert name != null;
+          enumValuesByName.put(name, value);
         }
       } else if (enumType == DurationUnit.class) {
         for (var value : values) {
           var unit = (DurationUnit) value;
-          enumValuesByName.put(CodeGeneratorUtils.toEnumConstantName(unit.getSymbol()), value);
+          var name = CodeGeneratorUtils.toEnumConstantName(unit.getSymbol());
+          assert name != null;
+          enumValuesByName.put(name, value);
         }
       } else {
         for (Enum<?> value : values) {
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/Reflection.java b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/Reflection.java
index 7782b880..0d873f5a 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/Reflection.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/Reflection.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,7 +20,7 @@ import static java.util.Arrays.stream;
 import io.leangen.geantyref.CaptureType;
 import io.leangen.geantyref.GenericTypeReflector;
 import java.lang.reflect.*;
-import org.pkl.core.util.Nullable;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Reflection utilities for implementing {@link ConverterFactory}s. Mostly covers introspection of
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/Tuple2.java b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/Tuple2.java
index 3230ae42..30e14b3c 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/Tuple2.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/Tuple2.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,7 +16,7 @@
 package org.pkl.config.java.mapper;
 
 import java.util.Objects;
-import org.pkl.core.util.Nullable;
+import org.jspecify.annotations.Nullable;
 
 // avoid name clash with org.pkl.core.Pair
 final class Tuple2<S, T> {
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/TypeMapping.java b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/TypeMapping.java
index 9a7fb33c..8994bb91 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/TypeMapping.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/TypeMapping.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,7 +16,7 @@
 package org.pkl.config.java.mapper;
 
 import java.lang.reflect.Modifier;
-import org.pkl.core.util.Nullable;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Maps a type requested during conversion to the implementation type to be instantiated. The
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ValueMapper.java b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ValueMapper.java
index 60bfc352..b3a77a00 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ValueMapper.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ValueMapper.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,7 @@
 package org.pkl.config.java.mapper;
 
 import java.lang.reflect.Type;
+import org.jspecify.annotations.Nullable;
 import org.pkl.core.PClassInfo;
 import org.pkl.core.PModule;
 
@@ -30,38 +31,40 @@ public interface ValueMapper {
   }
 
   /**
-   * Converts the given Pkl object to the given Java target type. The Pkl object can be an entire
+   * Converts the given Pkl value to the given Java target type. The Pkl value can be an entire
    * {@link PModule} or any value contained therein. See {@link PClassInfo#forValue} for which Java
-   * types are used to represent Pkl objects.
+   * types are used to represent Pkl values.
    *
    * <p>When mapping to a generic target type, a fully parameterized type needs to be passed, e.g.
    * {@code List<String>}. Parameterized type literals can be created using {@link Types}, e.g.
    * {@code Types.listOf(String.class)}.
    *
    * <p>If an error occurs during conversion, or if {@link ValueMapper} does not know how to convert
-   * from the given object to the given target type, a {@link ConversionException} is thrown.
+   * from the given Pkl value to the given target type, a {@link ConversionException} is thrown.
    */
-  <S, T> T map(S model, Type targetType);
+  <S, T extends @Nullable Object> T map(S value, Type targetType);
 
   /**
    * Same as {@link #map(Object, Type)}, except that the target type is narrowed from {@link Type}
    * to {@link Class} to allow for better type inference.
    */
-  default <S, T> T map(S model, Class<T> targetType) {
-    return map(model, (Type) targetType);
+  default <S, T extends @Nullable Object> T map(S value, Class<T> targetType) {
+    return map(value, (Type) targetType);
   }
 
   /**
    * Returns the converter with the given source and target types. Throws {@link
    * ConversionException} if no such converter exists.
    */
-  <S, T> Converter<S, T> getConverter(PClassInfo<S> sourceType, Type targetType);
+  <S, T extends @Nullable Object> Converter<S, T> getConverter(
+      PClassInfo<S> sourceType, Type targetType);
 
   /**
    * Same as {@link #getConverter(PClassInfo, Type)}, except that the target type is narrowed from
    * {@link Type} to {@link Class} to allow for better type inference.
    */
-  default <S, T> Converter<S, T> getConverter(PClassInfo<S> sourceType, Class<T> targetType) {
+  default <S, T extends @Nullable Object> Converter<S, T> getConverter(
+      PClassInfo<S> sourceType, Class<T> targetType) {
     return getConverter(sourceType, (Type) targetType);
   }
 
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ValueMapperImpl.java b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ValueMapperImpl.java
index 3578c6cd..bfb0d6e0 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ValueMapperImpl.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/ValueMapperImpl.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -52,9 +52,9 @@ class ValueMapperImpl implements ValueMapper {
 
   @Override
   @SuppressWarnings("unchecked")
-  public <S, T> T map(S model, Type targetType) {
-    var sourceType = PClassInfo.forValue(model);
-    return (T) getConverter(sourceType, targetType).convert(model, this);
+  public <S, T> T map(S value, Type targetType) {
+    var sourceType = PClassInfo.forValue(value);
+    return (T) getConverter(sourceType, targetType).convert(value, this);
   }
 
   private <S> Class<?> getTargetType(PClassInfo<S> sourceType, Type targetType) {
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/package-info.java b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/package-info.java
index 150ba450..5ca0043b 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/mapper/package-info.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/mapper/package-info.java
@@ -1,4 +1,4 @@
-@NonnullByDefault
+@NullMarked
 package org.pkl.config.java.mapper;
 
-import org.pkl.core.util.NonnullByDefault;
+import org.jspecify.annotations.NullMarked;
diff --git a/pkl-config-java/src/main/java/org/pkl/config/java/package-info.java b/pkl-config-java/src/main/java/org/pkl/config/java/package-info.java
index e08115b8..a80b6d15 100644
--- a/pkl-config-java/src/main/java/org/pkl/config/java/package-info.java
+++ b/pkl-config-java/src/main/java/org/pkl/config/java/package-info.java
@@ -1,4 +1,4 @@
-@NonnullByDefault
+@NullMarked
 package org.pkl.config.java;
 
-import org.pkl.core.util.NonnullByDefault;
+import org.jspecify.annotations.NullMarked;
diff --git a/pkl-config-java/src/test/java/org/pkl/config/java/mapper/PObjectToDataObjectJavaxInjectTest.java b/pkl-config-java/src/test/java/org/pkl/config/java/mapper/PObjectToDataObjectJavaxInjectTest.java
index 89a6f8b1..1fc5f3cb 100644
--- a/pkl-config-java/src/test/java/org/pkl/config/java/mapper/PObjectToDataObjectJavaxInjectTest.java
+++ b/pkl-config-java/src/test/java/org/pkl/config/java/mapper/PObjectToDataObjectJavaxInjectTest.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -22,11 +22,11 @@ import java.util.EnumSet;
 import java.util.Objects;
 import java.util.Set;
 import javax.inject.Named;
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.AfterAll;
 import org.junit.jupiter.api.Test;
 import org.pkl.core.Evaluator;
 import org.pkl.core.PModule;
-import org.pkl.core.util.Nullable;
 
 public class PObjectToDataObjectJavaxInjectTest {
   private static final Evaluator evaluator = Evaluator.preconfigured();
diff --git a/pkl-config-java/src/test/java/org/pkl/config/java/mapper/PObjectToDataObjectTest.java b/pkl-config-java/src/test/java/org/pkl/config/java/mapper/PObjectToDataObjectTest.java
index e16a1fce..efc1784d 100644
--- a/pkl-config-java/src/test/java/org/pkl/config/java/mapper/PObjectToDataObjectTest.java
+++ b/pkl-config-java/src/test/java/org/pkl/config/java/mapper/PObjectToDataObjectTest.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,11 +20,11 @@ import static org.pkl.core.ModuleSource.modulePath;
 
 import java.beans.ConstructorProperties;
 import java.util.*;
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.AfterAll;
 import org.junit.jupiter.api.Test;
 import org.pkl.core.Evaluator;
 import org.pkl.core.PModule;
-import org.pkl.core.util.Nullable;
 
 public class PObjectToDataObjectTest {
   private static final Evaluator evaluator = Evaluator.preconfigured();
diff --git a/pkl-config-java/src/test/java/org/pkl/config/java/mapper/Person.java b/pkl-config-java/src/test/java/org/pkl/config/java/mapper/Person.java
index f8875e47..61016237 100644
--- a/pkl-config-java/src/test/java/org/pkl/config/java/mapper/Person.java
+++ b/pkl-config-java/src/test/java/org/pkl/config/java/mapper/Person.java
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -15,7 +15,7 @@
  */
 package org.pkl.config.java.mapper;
 
-import org.pkl.core.util.Nullable;
+import org.jspecify.annotations.Nullable;
 
 public class Person {
   public final String name;
diff --git a/pkl-config-kotlin/gradle.lockfile b/pkl-config-kotlin/gradle.lockfile
index d143c6b2..72629236 100644
--- a/pkl-config-kotlin/gradle.lockfile
+++ b/pkl-config-kotlin/gradle.lockfile
@@ -80,7 +80,7 @@ org.jetbrains.kotlinx:kotlinx-serialization-core:1.7.3=swiftExportClasspathResol
 org.jetbrains:annotations:13.0=compileClasspath,kotlinBuildToolsApiClasspath,kotlinCompilerClasspath,kotlinCompilerPluginClasspathMain,kotlinCompilerPluginClasspathTest,kotlinInternalAbiValidation,kotlinKlibCommonizerClasspath,pklCodegenKotlin,runtimeClasspath,swiftExportClasspathResolvable,testCompileClasspath,testRuntimeClasspath
 org.jetbrains:markdown-jvm:0.7.3=pklCodegenKotlin
 org.jetbrains:markdown:0.7.3=pklCodegenKotlin
-org.jspecify:jspecify:1.0.0=pklCodegenKotlin,testCompileClasspath,testRuntimeClasspath
+org.jspecify:jspecify:1.0.0=compileClasspath,pklCodegenKotlin,testCompileClasspath,testRuntimeClasspath
 org.junit.jupiter:junit-jupiter-api:6.0.3=testCompileClasspath,testRuntimeClasspath
 org.junit.jupiter:junit-jupiter-engine:6.0.3=testRuntimeClasspath
 org.junit.jupiter:junit-jupiter-params:6.0.3=testCompileClasspath,testRuntimeClasspath
diff --git a/pkl-config-kotlin/src/main/kotlin/org/pkl/config/kotlin/ConfigExtensions.kt b/pkl-config-kotlin/src/main/kotlin/org/pkl/config/kotlin/ConfigExtensions.kt
index 6bf47437..1a783e8b 100644
--- a/pkl-config-kotlin/src/main/kotlin/org/pkl/config/kotlin/ConfigExtensions.kt
+++ b/pkl-config-kotlin/src/main/kotlin/org/pkl/config/kotlin/ConfigExtensions.kt
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -15,10 +15,11 @@
  */
 package org.pkl.config.kotlin
 
+import kotlin.reflect.jvm.javaType
+import kotlin.reflect.typeOf
 import org.pkl.config.java.Config
 import org.pkl.config.java.ConfigEvaluator
 import org.pkl.config.java.ConfigEvaluatorBuilder
-import org.pkl.config.java.JavaType
 import org.pkl.config.java.mapper.ConversionException
 import org.pkl.config.java.mapper.ValueMapperBuilder
 import org.pkl.config.kotlin.mapper.KotlinConversions
@@ -30,23 +31,21 @@ import org.pkl.config.kotlin.mapper.KotlinConverterFactories
  *
  * To allow `null` values, specify a nullable type, for example `to<String?>()`.
  *
- * Kotlin code should prefer this method over [Config. as] for the following reasons:
+ * Kotlin code should prefer this method over [Config.as] for the following reasons:
  * * does not clash with Kotlin's `as` keyword
  * * throws [ConversionException] if conversion to non-nullable type returns `null`
  * * easier to use with parameterized types: `to<List<String>>()` vs.
  *   `as(JavaType.listOf(String::class.java))`
  */
 inline fun <reified T> Config.to(): T {
-  val javaType = object : JavaType<T>() {}
-  // `as T?` may no longer be required after switching to JSpecify
-  val result = `as`<T>(javaType.type) as T?
+  val result = `as`<T>(typeOf<T>().javaType)
   if (result == null && null !is T) {
     throw ConversionException(
       "Expected a non-null value but got `null`. " +
         "To allow null values, convert to a nullable Kotlin type, for example `String?`."
     )
   }
-  return result as T
+  return result
 }
 
 /**
diff --git a/pkl-config-kotlin/src/main/kotlin/org/pkl/config/kotlin/mapper/PPairToKotlinPair.kt b/pkl-config-kotlin/src/main/kotlin/org/pkl/config/kotlin/mapper/PPairToKotlinPair.kt
index cf93536a..edd7409e 100644
--- a/pkl-config-kotlin/src/main/kotlin/org/pkl/config/kotlin/mapper/PPairToKotlinPair.kt
+++ b/pkl-config-kotlin/src/main/kotlin/org/pkl/config/kotlin/mapper/PPairToKotlinPair.kt
@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -48,16 +48,16 @@ internal class PPairToKotlinPair : ConverterFactory {
   ) : Converter<PPair<Any, Any>, Pair<F, S>> {
 
     private var firstCachedType = PClassInfo.Unavailable
-    private var firstCachedConverter: Converter<Any, F>? = null
+    private lateinit var firstCachedConverter: Converter<Any, F>
 
     private var secondCachedType = PClassInfo.Unavailable
-    private var secondCachedConverter: Converter<Any, S>? = null
+    private lateinit var secondCachedConverter: Converter<Any, S>
 
     override fun convert(value: PPair<Any, Any>, valueMapper: ValueMapper): Pair<F, S> {
       val first = value.first
       if (!firstCachedType.isExactClassOf(first)) {
         firstCachedType = PClassInfo.forValue(first)
-        firstCachedConverter = valueMapper.getConverter(firstCachedType, firstTargetType)
+        firstCachedConverter = valueMapper.getConverter<Any, F>(firstCachedType, firstTargetType)
       }
 
       val second = value.second
@@ -67,8 +67,8 @@ internal class PPairToKotlinPair : ConverterFactory {
       }
 
       return Pair(
-        firstCachedConverter!!.convert(first, valueMapper),
-        secondCachedConverter!!.convert(second, valueMapper),
+        firstCachedConverter.convert(first, valueMapper),
+        secondCachedConverter.convert(second, valueMapper),
       )
     }
   }
diff --git a/pkl-gradle/gradle.lockfile b/pkl-gradle/gradle.lockfile
index 3d1cfbeb..4be37e7d 100644
--- a/pkl-gradle/gradle.lockfile
+++ b/pkl-gradle/gradle.lockfile
@@ -63,7 +63,7 @@ org.jetbrains.kotlinx:kotlinx-serialization-bom:1.7.3=swiftExportClasspathResolv
 org.jetbrains.kotlinx:kotlinx-serialization-core-jvm:1.7.3=swiftExportClasspathResolvable
 org.jetbrains.kotlinx:kotlinx-serialization-core:1.7.3=swiftExportClasspathResolvable
 org.jetbrains:annotations:13.0=compileClasspath,kotlinBuildToolsApiClasspath,kotlinCompilerClasspath,kotlinCompilerPluginClasspathMain,kotlinCompilerPluginClasspathTest,kotlinInternalAbiValidation,kotlinKlibCommonizerClasspath,swiftExportClasspathResolvable,testCompileClasspath,testRuntimeClasspath
-org.jspecify:jspecify:1.0.0=testCompileClasspath
+org.jspecify:jspecify:1.0.0=compileClasspath,testCompileClasspath
 org.junit.jupiter:junit-jupiter-api:6.0.3=testCompileClasspath,testRuntimeClasspath
 org.junit.jupiter:junit-jupiter-engine:6.0.3=testCompileClasspath,testRuntimeClasspath
 org.junit.jupiter:junit-jupiter-params:6.0.3=testCompileClasspath,testRuntimeClasspath
diff --git a/pkl-tools/gradle.lockfile b/pkl-tools/gradle.lockfile
index 8afc367f..bdfac7be 100644
--- a/pkl-tools/gradle.lockfile
+++ b/pkl-tools/gradle.lockfile
@@ -55,7 +55,7 @@ org.jline:jline-native:4.0.12=runtimeClasspath,testRuntimeClasspath
 org.jline:jline-reader:4.0.12=runtimeClasspath,testRuntimeClasspath
 org.jline:jline-terminal-jni:4.0.12=runtimeClasspath,testRuntimeClasspath
 org.jline:jline-terminal:4.0.12=runtimeClasspath,testRuntimeClasspath
-org.jspecify:jspecify:1.0.0=runtimeClasspath,testRuntimeClasspath
+org.jspecify:jspecify:1.0.0=compileClasspath,runtimeClasspath,testCompileClasspath,testRuntimeClasspath
 org.msgpack:msgpack-core:0.9.11=runtimeClasspath,testRuntimeClasspath
 org.organicdesign:Paguro:3.10.3=runtimeClasspath,testRuntimeClasspath
 org.snakeyaml:snakeyaml-engine:2.10=runtimeClasspath,testRuntimeClasspath