diff --git a/tools/src/main/java/org/bitcoinj/tools/BuildCheckpoints.java b/tools/src/main/java/org/bitcoinj/tools/BuildCheckpoints.java
index df3d9e6b7d4..9f65bcd9ac4 100644
--- a/tools/src/main/java/org/bitcoinj/tools/BuildCheckpoints.java
+++ b/tools/src/main/java/org/bitcoinj/tools/BuildCheckpoints.java
@@ -32,6 +32,7 @@
 import org.bitcoinj.store.MemoryBlockStore;
 import org.bitcoinj.utils.BriefLogFormatter;
 import org.bitcoinj.utils.Threading;
+import org.jspecify.annotations.Nullable;
 import picocli.CommandLine;
 
 import java.io.File;
@@ -50,6 +51,7 @@
 import java.time.temporal.ChronoUnit;
 import java.util.Base64;
 import java.util.List;
+import java.util.Objects;
 import java.util.TreeMap;
 import java.util.concurrent.Callable;
 import java.util.concurrent.Future;
@@ -66,7 +68,7 @@ public class BuildCheckpoints implements Callable<Integer> {
     @CommandLine.Option(names = "--net", description = "Which network to connect to. Valid values: ${COMPLETION-CANDIDATES}. Default: ${DEFAULT-VALUE}")
     private BitcoinNetwork net = BitcoinNetwork.MAINNET;
     @CommandLine.Option(names = "--peer", description = "IP address/domain name for connection instead of localhost.")
-    private String peer = null;
+    @Nullable private String peer = null;
     @CommandLine.Option(names = "--days", description = "How many days to keep as a safety margin. Checkpointing will be done up to this many days ago.")
     private int days = 7;
     @CommandLine.Option(names = "--verbose", description = "Enables logging.")
@@ -74,7 +76,7 @@ public class BuildCheckpoints implements Callable<Integer> {
     @CommandLine.Option(names = "--help", usageHelp = true, description = "Displays program options.")
     private boolean help;
 
-    private static NetworkParameters params;
+    private static @Nullable NetworkParameters params;
 
     public static void main(String[] args) throws Exception {
         int exitCode = new CommandLine(new BuildCheckpoints()).execute(args);
@@ -90,6 +92,7 @@ public Integer call() throws Exception {
 
         final String suffix;
         params = NetworkParameters.of(net);
+        Objects.requireNonNull(params);
         Context.propagate(new Context());
 
         switch (net) {
@@ -156,6 +159,7 @@ public Integer call() throws Exception {
         System.out.println("Checkpointing up to " + TimeUtils.dateTimeFormat(timeAgo));
 
         chain.addNewBestBlockListener(Threading.SAME_THREAD, block -> {
+            Objects.requireNonNull(params);
             int height = block.getHeight();
             if (height % params.getInterval() == 0 && timeAgo.isAfter(block.getHeader().time())) {
                 System.out.println(String.format("Checkpointing block %s at height %d, time %s",
@@ -210,6 +214,7 @@ private static void writeTextualCheckpoints(TreeMap<Integer, StoredBlock> checkp
     }
 
     private static void sanityCheck(File file, int expectedSize) throws IOException {
+        Objects.requireNonNull(params);
         FileInputStream fis = new FileInputStream(file);
         CheckpointManager manager;
         try {
@@ -239,6 +244,7 @@ private static void sanityCheck(File file, int expectedSize) throws IOException
     }
 
     private static void startPeerGroup(PeerGroup peerGroup, InetAddress ipAddress) {
+        Objects.requireNonNull(params);
         final PeerAddress peerAddress = PeerAddress.simple(ipAddress, params.getPort());
         System.out.println("Connecting to " + peerAddress + "...");
         peerGroup.addAddress(peerAddress);
diff --git a/tools/src/main/java/org/bitcoinj/tools/TestFeeLevel.java b/tools/src/main/java/org/bitcoinj/tools/TestFeeLevel.java
index fa00fce9a09..3302a431f92 100644
--- a/tools/src/main/java/org/bitcoinj/tools/TestFeeLevel.java
+++ b/tools/src/main/java/org/bitcoinj/tools/TestFeeLevel.java
@@ -26,8 +26,10 @@
 import org.bitcoinj.wallet.KeyChainGroupStructure;
 import org.bitcoinj.wallet.SendRequest;
 import org.bitcoinj.wallet.Wallet;
+import org.jspecify.annotations.Nullable;
 
 import java.io.File;
+import java.util.Objects;
 import java.util.concurrent.ExecutionException;
 
 /**
@@ -36,7 +38,7 @@
 public class TestFeeLevel {
 
     private static final int NUM_OUTPUTS = 2;
-    private static WalletAppKit kit;
+    private static @Nullable WalletAppKit kit;
 
     public static void main(String[] args) throws Exception {
         BriefLogFormatter.initWithSilentBitcoinJ();
@@ -60,6 +62,7 @@ public static void main(String[] args) throws Exception {
     }
 
     private static void go(Coin feeRateToTest, int numOutputs) throws InterruptedException, ExecutionException, InsufficientMoneyException {
+        Objects.requireNonNull(kit);
         System.out.println("Wallet has " + kit.wallet().getBalance().toFriendlyString()
                 + "; current receive address is " + kit.wallet().currentReceiveAddress());
 
diff --git a/tools/src/main/java/org/bitcoinj/tools/package-info.java b/tools/src/main/java/org/bitcoinj/tools/package-info.java
new file mode 100644
index 00000000000..2f1ff2d79c1
--- /dev/null
+++ b/tools/src/main/java/org/bitcoinj/tools/package-info.java
@@ -0,0 +1,23 @@
+/*
+ * Copyright by the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * <b>bitcoinj<b> tool utility applications.
+ */
+@NullMarked
+package org.bitcoinj.tools;
+
+import org.jspecify.annotations.NullMarked;