diff --git a/runelite-api/pom.xml b/runelite-api/pom.xml
index 20427e8399..ff91c0ad46 100644
--- a/runelite-api/pom.xml
+++ b/runelite-api/pom.xml
@@ -37,6 +37,11 @@
 	<name>RuneLite API</name>
 
 	<dependencies>
+		<dependency>
+			<groupId>org.slf4j</groupId>
+			<artifactId>slf4j-api</artifactId>
+			<version>1.7.21</version>
+		</dependency>
 		<dependency>
 			<groupId>org.projectlombok</groupId>
 			<artifactId>lombok</artifactId>
diff --git a/runelite-api/src/main/java/net/runelite/api/overlay/OverlayIndex.java b/runelite-api/src/main/java/net/runelite/api/overlay/OverlayIndex.java
new file mode 100644
index 0000000000..6aec875181
--- /dev/null
+++ b/runelite-api/src/main/java/net/runelite/api/overlay/OverlayIndex.java
@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2018, Adam <Adam@sigterm.info>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+ * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package net.runelite.api.overlay;
+
+import java.io.DataInputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.HashSet;
+import java.util.Set;
+import lombok.extern.slf4j.Slf4j;
+
+@Slf4j
+public class OverlayIndex
+{
+	private static final Set<Integer> overlays = new HashSet<>();
+
+	static
+	{
+		InputStream indexStream = OverlayIndex.class.getResourceAsStream("/runelite/index");
+
+		try (DataInputStream in = new DataInputStream(indexStream))
+		{
+			int id;
+			while ((id = in.readInt()) != -1)
+			{
+				overlays.add(id);
+			}
+		}
+		catch (IOException ex)
+		{
+			log.warn("unable to load overlay index", ex);
+		}
+	}
+
+	public static boolean hasOverlay(int indexId, int archiveId)
+	{
+		return overlays.contains(indexId << 16 | archiveId);
+	}
+}
diff --git a/runelite-mixins/src/main/java/net/runelite/mixins/RSIndexDataBaseMixin.java b/runelite-mixins/src/main/java/net/runelite/mixins/RSIndexDataBaseMixin.java
new file mode 100644
index 0000000000..e4c953bd12
--- /dev/null
+++ b/runelite-mixins/src/main/java/net/runelite/mixins/RSIndexDataBaseMixin.java
@@ -0,0 +1,99 @@
+/*
+ * Copyright (c) 2018, Adam <Adam@sigterm.info>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+ * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package net.runelite.mixins;
+
+import com.google.common.hash.HashCode;
+import com.google.common.hash.Hashing;
+import com.google.common.io.BaseEncoding;
+import com.google.common.io.ByteStreams;
+import com.google.common.io.CharStreams;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import net.runelite.api.mixins.Copy;
+import net.runelite.api.mixins.Mixin;
+import net.runelite.api.mixins.Replace;
+import net.runelite.api.overlay.OverlayIndex;
+import static net.runelite.client.callback.Hooks.log;
+import net.runelite.rs.api.RSIndexData;
+import net.runelite.rs.api.RSIndexDataBase;
+
+@Mixin(RSIndexDataBase.class)
+public abstract class RSIndexDataBaseMixin implements RSIndexDataBase
+{
+	@Copy("getConfigData")
+	abstract byte[] rs$getConfigData(int archiveId, int fileId);
+
+	@Replace("getConfigData")
+	public byte[] rl$getConfigData(int archiveId, int fileId)
+	{
+		byte[] rsData = rs$getConfigData(archiveId, fileId);
+		RSIndexData indexData = (RSIndexData) this;
+
+		if (!OverlayIndex.hasOverlay(indexData.getIndex(), archiveId))
+		{
+			return rsData;
+		}
+
+		InputStream in = getClass().getResourceAsStream("/runelite/" + indexData.getIndex() + "/" + archiveId);
+		if (in == null)
+		{
+			log.warn("Missing overlay data for {}/{}", indexData.getIndex(), archiveId);
+			return rsData;
+		}
+
+		HashCode rsDataHash = Hashing.sha256().hashBytes(rsData);
+
+		String rsHash = BaseEncoding.base16().encode(rsDataHash.asBytes());
+
+		InputStream in2 = getClass().getResourceAsStream("/runelite/" + indexData.getIndex() + "/" + archiveId + ".hash");
+		if (in2 == null)
+		{
+			log.warn("Missing hash file for {}/{}", indexData.getIndex(), archiveId);
+			return rsData;
+		}
+
+		try
+		{
+			String replaceHash = CharStreams.toString(new InputStreamReader(in2));
+
+			if (replaceHash.equals(rsHash))
+			{
+				log.debug("Replacing archive {}/{}",
+					indexData.getIndex(), archiveId);
+				return ByteStreams.toByteArray(in);
+			}
+
+			log.warn("Mismatch in overlaid cache archive hash for {}/{}: {} != {}",
+				indexData.getIndex(), archiveId, replaceHash, rsHash);
+		}
+		catch (IOException ex)
+		{
+			log.warn("error checking hash", ex);
+		}
+
+		return rsData;
+	}
+}