diff --git a/src/main/java/net/fabricmc/loom/build/nesting/IncludedJarFactory.java b/src/main/java/net/fabricmc/loom/build/nesting/IncludedJarFactory.java
index 4447d7d3..925f5a7c 100644
--- a/src/main/java/net/fabricmc/loom/build/nesting/IncludedJarFactory.java
+++ b/src/main/java/net/fabricmc/loom/build/nesting/IncludedJarFactory.java
@@ -53,7 +53,7 @@ import org.jetbrains.annotations.Nullable;
 import net.fabricmc.loom.LoomGradleExtension;
 import net.fabricmc.loom.LoomGradlePlugin;
 import net.fabricmc.loom.task.RemapTaskConfiguration;
-import net.fabricmc.loom.util.ZipUtils;
+import net.fabricmc.loom.util.ZipReprocessorUtil;
 import net.fabricmc.loom.util.fmj.FabricModJsonFactory;
 
 public final class IncludedJarFactory {
@@ -163,7 +163,7 @@ public final class IncludedJarFactory {
 
 		try {
 			FileUtils.copyFile(input, tempFile);
-			ZipUtils.add(tempFile.toPath(), "fabric.mod.json", generateModForDependency(metadata).getBytes(StandardCharsets.UTF_8));
+			ZipReprocessorUtil.appendZipEntry(tempFile, "fabric.mod.json", generateModForDependency(metadata).getBytes(StandardCharsets.UTF_8));
 		} catch (IOException e) {
 			throw new UncheckedIOException("Failed to add dummy mod while including %s".formatted(input), e);
 		}
diff --git a/src/main/java/net/fabricmc/loom/util/ZipReprocessorUtil.java b/src/main/java/net/fabricmc/loom/util/ZipReprocessorUtil.java
index 79f0c872..90d486bc 100644
--- a/src/main/java/net/fabricmc/loom/util/ZipReprocessorUtil.java
+++ b/src/main/java/net/fabricmc/loom/util/ZipReprocessorUtil.java
@@ -95,44 +95,80 @@ public class ZipReprocessorUtil {
 			return;
 		}
 
-		try (ZipFile zipFile = new ZipFile(file)) {
+		try (var zipFile = new ZipFile(file)) {
 			ZipEntry[] entries;
 
 			if (reproducibleFileOrder) {
-				entries = zipFile.stream().sorted(Comparator.comparing(ZipEntry::getName, ZipReprocessorUtil::specialOrdering)).toArray(ZipEntry[]::new);
+				entries = zipFile.stream()
+						.sorted(Comparator.comparing(ZipEntry::getName, ZipReprocessorUtil::specialOrdering))
+						.toArray(ZipEntry[]::new);
 			} else {
-				entries = zipFile.stream().toArray(ZipEntry[]::new);
+				entries = zipFile.stream()
+						.toArray(ZipEntry[]::new);
 			}
 
-			ByteArrayOutputStream outZip = new ByteArrayOutputStream(zipFile.size());
+			final var outZip = new ByteArrayOutputStream(entries.length);
 
-			try (ZipOutputStream zipOutputStream = new ZipOutputStream(outZip)) {
+			try (var zipOutputStream = new ZipOutputStream(outZip)) {
 				for (ZipEntry entry : entries) {
 					ZipEntry newEntry = entry;
 
 					if (!preserveFileTimestamps) {
 						newEntry = new ZipEntry(entry.getName());
-						newEntry.setTime(CONSTANT_TIME_FOR_ZIP_ENTRIES);
-						newEntry.setLastModifiedTime(FileTime.fromMillis(CONSTANT_TIME_FOR_ZIP_ENTRIES));
-						newEntry.setLastAccessTime(FileTime.fromMillis(CONSTANT_TIME_FOR_ZIP_ENTRIES));
+						setConstantFileTime(newEntry);
 					}
 
-					zipOutputStream.putNextEntry(newEntry);
-					InputStream inputStream = zipFile.getInputStream(entry);
-					byte[] buf = new byte[1024];
-					int length;
-
-					while ((length = inputStream.read(buf)) > 0) {
-						zipOutputStream.write(buf, 0, length);
-					}
-
-					zipOutputStream.closeEntry();
+					copyZipEntry(zipOutputStream, newEntry, zipFile.getInputStream(entry));
 				}
 			}
 
-			try (FileOutputStream fileOutputStream = new FileOutputStream(file)) {
+			try (var fileOutputStream = new FileOutputStream(file)) {
 				outZip.writeTo(fileOutputStream);
 			}
 		}
 	}
+
+	public static void appendZipEntry(File file, String path, byte[] data) throws IOException {
+		try (var zipFile = new ZipFile(file)) {
+			ZipEntry[] entries = zipFile.stream().toArray(ZipEntry[]::new);
+
+			final var outZip = new ByteArrayOutputStream(entries.length);
+
+			try (var zipOutputStream = new ZipOutputStream(outZip)) {
+				// Copy existing entries
+				for (ZipEntry entry : entries) {
+					copyZipEntry(zipOutputStream, entry, zipFile.getInputStream(entry));
+				}
+
+				// Append the new entry
+				var entry = new ZipEntry(path);
+				setConstantFileTime(entry);
+				zipOutputStream.putNextEntry(entry);
+				zipOutputStream.write(data, 0, data.length);
+				zipOutputStream.closeEntry();
+			}
+
+			try (var fileOutputStream = new FileOutputStream(file)) {
+				outZip.writeTo(fileOutputStream);
+			}
+		}
+	}
+
+	private static void copyZipEntry(ZipOutputStream zipOutputStream, ZipEntry entry, InputStream inputStream) throws IOException {
+		zipOutputStream.putNextEntry(entry);
+		byte[] buf = new byte[1024];
+		int length;
+
+		while ((length = inputStream.read(buf)) > 0) {
+			zipOutputStream.write(buf, 0, length);
+		}
+
+		zipOutputStream.closeEntry();
+	}
+
+	private static void setConstantFileTime(ZipEntry entry) {
+		entry.setTime(ZipReprocessorUtil.CONSTANT_TIME_FOR_ZIP_ENTRIES);
+		entry.setLastModifiedTime(FileTime.fromMillis(ZipReprocessorUtil.CONSTANT_TIME_FOR_ZIP_ENTRIES));
+		entry.setLastAccessTime(FileTime.fromMillis(ZipReprocessorUtil.CONSTANT_TIME_FOR_ZIP_ENTRIES));
+	}
 }
diff --git a/src/test/groovy/net/fabricmc/loom/test/unit/ZipUtilsTest.groovy b/src/test/groovy/net/fabricmc/loom/test/unit/ZipUtilsTest.groovy
index 1c73bed0..95c46916 100644
--- a/src/test/groovy/net/fabricmc/loom/test/unit/ZipUtilsTest.groovy
+++ b/src/test/groovy/net/fabricmc/loom/test/unit/ZipUtilsTest.groovy
@@ -29,7 +29,9 @@ import java.nio.file.Files
 
 import spock.lang.Specification
 
+import net.fabricmc.loom.util.Checksum
 import net.fabricmc.loom.util.Pair
+import net.fabricmc.loom.util.ZipReprocessorUtil
 import net.fabricmc.loom.util.ZipUtils
 
 class ZipUtilsTest extends Specification {
@@ -150,4 +152,24 @@ class ZipUtilsTest extends Specification {
 		then:
 		!result
 	}
+
+	def "append zip entry"() {
+		given:
+		// Create a reproducible input zip
+		def dir = Files.createTempDirectory("loom-zip-test")
+		def zip = Files.createTempFile("loom-zip-test", ".zip")
+		def fileInside = dir.resolve("text.txt")
+		Files.writeString(fileInside, "hello world")
+		ZipUtils.pack(dir, zip)
+		ZipReprocessorUtil.reprocessZip(zip.toFile(), true, false)
+
+		when:
+		// Add an entry to it
+		ZipReprocessorUtil.appendZipEntry(zip.toFile(), "fabric.mod.json", "Some text".getBytes(StandardCharsets.UTF_8))
+
+		then:
+		ZipUtils.unpack(zip, "text.txt") == "hello world".bytes
+		ZipUtils.unpack(zip, "fabric.mod.json") == "Some text".bytes
+		Checksum.sha1Hex(zip) == "232ecda4c770bde8ba618e7a194a4f7b57928dc5"
+	}
 }