diff --git a/jgvariant-tool/pom.xml b/jgvariant-tool/pom.xml
index f4026e4..9d764dc 100644
--- a/jgvariant-tool/pom.xml
+++ b/jgvariant-tool/pom.xml
@@ -59,6 +59,12 @@
       <optional>true</optional>
     </dependency>
     <dependency>
+      <groupId>org.jspecify</groupId>
+      <artifactId>jspecify</artifactId>
+      <scope>provided</scope>
+      <optional>true</optional>
+    </dependency>
+    <dependency>
       <groupId>org.apiguardian</groupId>
       <artifactId>apiguardian-api</artifactId>
       <scope>provided</scope>
diff --git a/jgvariant-tool/src/main/java/eu/mulk/jgvariant/tool/MainCommand.java b/jgvariant-tool/src/main/java/eu/mulk/jgvariant/tool/MainCommand.java
index db724dd..9c3e0f9 100644
--- a/jgvariant-tool/src/main/java/eu/mulk/jgvariant/tool/MainCommand.java
+++ b/jgvariant-tool/src/main/java/eu/mulk/jgvariant/tool/MainCommand.java
@@ -29,8 +29,8 @@
 import java.util.*;
 import java.util.logging.Logger;
 import java.util.stream.IntStream;
-import org.jetbrains.annotations.Nullable;
 import org.jetbrains.annotations.VisibleForTesting;
+import org.jspecify.annotations.Nullable;
 import picocli.AutoComplete;
 import picocli.CommandLine;
 import picocli.CommandLine.*;
diff --git a/jgvariant-tool/src/main/java/eu/mulk/jgvariant/tool/package-info.java b/jgvariant-tool/src/main/java/eu/mulk/jgvariant/tool/package-info.java
index 54a4fff..43dd1bf 100644
--- a/jgvariant-tool/src/main/java/eu/mulk/jgvariant/tool/package-info.java
+++ b/jgvariant-tool/src/main/java/eu/mulk/jgvariant/tool/package-info.java
@@ -9,6 +9,8 @@
  * be used to manipulate GVariant-formatted files from the command line.
  */
 @API(status = API.Status.INTERNAL)
+@NullMarked
 package eu.mulk.jgvariant.tool;
 
 import org.apiguardian.api.API;
+import org.jspecify.annotations.NullMarked;
diff --git a/jgvariant-tool/src/main/java/module-info.java b/jgvariant-tool/src/main/java/module-info.java
index 3f38283..c0f413d 100644
--- a/jgvariant-tool/src/main/java/module-info.java
+++ b/jgvariant-tool/src/main/java/module-info.java
@@ -2,6 +2,8 @@
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
 
+import org.jspecify.annotations.NullMarked;
+
 /**
  * The {@code jgvariant} command line tool.
  *
@@ -68,6 +70,7 @@
  * $ jgvariant ostree summary add-static-delta ./jgvariant-ostree/src/test/resources/ostree/summary f481144629474bd88c106e45ac405ebd75b324b0655af1aec14b31786ae1fd61 31c8835d5c9d2c6687a50091c85142d1b2d853ff416a9fb81b4ee30754510d52
  * }
  */
+@NullMarked
 module eu.mulk.jgvariant.tool {
   requires transitive eu.mulk.jgvariant.ostree;
   requires info.picocli;
@@ -77,6 +80,7 @@
   requires static com.google.errorprone.annotations;
   requires static org.apiguardian.api;
   requires static org.jetbrains.annotations;
+  requires static org.jspecify;
 
   opens eu.mulk.jgvariant.tool to
       info.picocli;