diff --git a/archive/src/main/kotlin/org/openrs2/archive/cache/ImportMasterIndexCommand.kt b/archive/src/main/kotlin/org/openrs2/archive/cache/ImportMasterIndexCommand.kt
index d47bcf5b..86912726 100644
--- a/archive/src/main/kotlin/org/openrs2/archive/cache/ImportMasterIndexCommand.kt
+++ b/archive/src/main/kotlin/org/openrs2/archive/cache/ImportMasterIndexCommand.kt
@@ -3,19 +3,24 @@ package org.openrs2.archive.cache
 import com.github.ajalt.clikt.core.CliktCommand
 import com.github.ajalt.clikt.parameters.arguments.argument
 import com.github.ajalt.clikt.parameters.options.default
+import com.github.ajalt.clikt.parameters.options.flag
 import com.github.ajalt.clikt.parameters.options.option
 import com.github.ajalt.clikt.parameters.types.enum
 import com.github.ajalt.clikt.parameters.types.int
 import com.github.ajalt.clikt.parameters.types.path
 import com.google.inject.Guice
+import io.netty.buffer.ByteBuf
 import io.netty.buffer.Unpooled
 import kotlinx.coroutines.runBlocking
 import org.openrs2.archive.ArchiveModule
 import org.openrs2.buffer.use
+import org.openrs2.cache.Js5CompressionType
 import org.openrs2.cache.MasterIndexFormat
 import org.openrs2.cli.instant
 import org.openrs2.inject.CloseableInjector
+import java.io.IOException
 import java.nio.file.Files
+import kotlin.math.min
 
 public class ImportMasterIndexCommand : CliktCommand(name = "import-master-index") {
     private val buildMajor by option().int()
@@ -26,6 +31,7 @@ public class ImportMasterIndexCommand : CliktCommand(name = "import-master-index
     private val url by option()
     private val environment by option().default("live")
     private val language by option().default("en")
+    private val decodeJs5Response by option().flag()
 
     private val game by argument()
     private val format by argument().enum<MasterIndexFormat>()
@@ -40,20 +46,71 @@ public class ImportMasterIndexCommand : CliktCommand(name = "import-master-index
             val importer = injector.getInstance(CacheImporter::class.java)
 
             Unpooled.wrappedBuffer(Files.readAllBytes(input)).use { buf ->
-                importer.importMasterIndex(
-                    buf,
-                    format,
-                    game,
-                    environment,
-                    language,
-                    buildMajor,
-                    buildMinor,
-                    timestamp,
-                    name,
-                    description,
-                    url
-                )
+                if (decodeJs5Response) {
+                    decodeJs5Response(buf)
+                } else {
+                    buf.retain()
+                }.use { decodedBuf ->
+                    importer.importMasterIndex(
+                        decodedBuf,
+                        format,
+                        game,
+                        environment,
+                        language,
+                        buildMajor,
+                        buildMinor,
+                        timestamp,
+                        name,
+                        description,
+                        url
+                    )
+                }
             }
         }
     }
+
+    private fun decodeJs5Response(input: ByteBuf): ByteBuf {
+        input.skipBytes(3) // archive and group
+
+        val compression = input.readUnsignedByte().toInt()
+        val len = input.readInt()
+        if (len < 0) {
+            throw IOException("Length is negative: $len")
+        }
+
+        val lenWithHeader = if (compression == Js5CompressionType.UNCOMPRESSED.ordinal) {
+            len + 5
+        } else {
+            len + 9
+        }
+
+        input.alloc().buffer(lenWithHeader, lenWithHeader).use { output ->
+            output.writeByte(compression)
+            output.writeInt(len)
+
+            var blockLen = 504
+            while (true) {
+                val n = min(blockLen, output.writableBytes())
+                if (input.readableBytes() < n) {
+                    throw IOException("Input truncated (expecting $n bytes, got ${input.readableBytes()})")
+                }
+
+                output.writeBytes(input, n)
+
+                if (!output.isWritable) {
+                    break
+                } else if (!input.isReadable) {
+                    throw IOException("Input truncated (expecting block trailer)")
+                }
+
+                if (input.readUnsignedByte().toInt() != 0xFF) {
+                    throw IOException("Invalid block trailer")
+                }
+
+                blockLen = 511
+            }
+
+            return output.retain()
+        }
+    }
 }