diff --git a/.github/workflows/github-packages.yml b/.github/workflows/github-packages.yml
new file mode 100644
index 0000000..3a1cd4b
--- /dev/null
+++ b/.github/workflows/github-packages.yml
@@ -0,0 +1,21 @@
+name: GitHub release
+
+on:
+  push:
+    tags:
+      - 'v*.*.*'
+
+permissions:
+  contents: write
+
+jobs:
+  version:
+    name: Detect version
+    uses: ./.github/workflows/version.yml
+  publish:
+    name: Publish
+    needs: version
+    uses: ./.github/workflows/build.yml
+    with:
+      version: ${{ needs.version.outputs.version }}
+      task: publish
diff --git a/api/build.gradle.kts b/api/build.gradle.kts
new file mode 100644
index 0000000..652084c
--- /dev/null
+++ b/api/build.gradle.kts
@@ -0,0 +1,38 @@
+plugins {
+    id("java-convention")
+    `maven-publish`
+}
+
+java {
+    withSourcesJar()
+    withJavadocJar()
+}
+
+publishing {
+    fun env(name: String) = System.getenv(name) ?: name
+    repositories {
+        maven {
+            name = "GitHubPackages"
+            url = uri("https://maven.pkg.github.com/${env("GITHUB_REPOSITORY")}")
+            credentials {
+                username = env("GITHUB_ACTOR")
+                password = env("GITHUB_TOKEN")
+            }
+        }
+    }
+    publications {
+        create<MavenPublication>("mavenJava") {
+            from(components["java"])
+            pom {
+                name = "MapModCompanion (API)"
+                artifactId = "mapmodcompanion-api"
+                licenses {
+                    license {
+                        name = "MIT License"
+                        url = "https://opensource.org/licenses/MIT"
+                    }
+                }
+            }
+        }
+    }
+}
diff --git a/api/src/main/java/com/turikhay/mc/mapmodcompanion/Channels.java b/api/src/main/java/com/turikhay/mc/mapmodcompanion/Channels.java
new file mode 100644
index 0000000..786c8b6
--- /dev/null
+++ b/api/src/main/java/com/turikhay/mc/mapmodcompanion/Channels.java
@@ -0,0 +1,16 @@
+package com.turikhay.mc.mapmodcompanion;
+
+/**
+ * Utility holder for network channel identifiers used by various map
+ * modifications.
+ */
+public interface Channels {
+    /** Plugin channel used by Xaero's Minimap. */
+    String XAERO_MINIMAP_CHANNEL = "xaerominimap:main";
+    /** Plugin channel used by Xaero's World Map. */
+    String XAERO_WORLDMAP_CHANNEL = "xaeroworldmap:main";
+    /** Official MapModCompanion channel used to query world identifiers. */
+    String WORLDID_CHANNEL = "worldinfo:world_id";
+    /** Legacy channel name supported for backwards compatibility. */
+    String WORLDID_LEGACY_CHANNEL = "world_id";
+}
diff --git a/api/src/main/java/com/turikhay/mc/mapmodcompanion/Id.java b/api/src/main/java/com/turikhay/mc/mapmodcompanion/Id.java
new file mode 100644
index 0000000..d19f954
--- /dev/null
+++ b/api/src/main/java/com/turikhay/mc/mapmodcompanion/Id.java
@@ -0,0 +1,92 @@
+package com.turikhay.mc.mapmodcompanion;
+
+/**
+ * Represents a world identifier.
+ * <p>
+ * Implementations are immutable and return a new instance when a different id
+ * is required.
+ */
+public interface Id {
+
+    /**
+     * Flag value used by VoxelMap-style packets.
+     * <p>
+     * When present at the start of a packet, the value {@code 42} signals that
+     * the payload follows the VoxelMap format.
+     */
+    int MAGIC_MARKER = 42;
+
+    /**
+     * Returns the numeric id value.
+     */
+    int getId();
+
+    /**
+     * Creates a copy of this id with a different numeric value.
+     *
+     * @param id new value
+     * @return new id instance
+     */
+    Id withIdUnchecked(int id);
+
+    /**
+     * Type-safe wrapper around {@link #withIdUnchecked(int)}.
+     *
+     * <pre>{@code
+     * StandardId newId = existing.withId(5);
+     * }</pre>
+     *
+     * @param id new value
+     * @param <IdType> concrete {@link Id} type
+     * @return new id instance
+     */
+    @SuppressWarnings("unchecked")
+    default <IdType> IdType withId(int id) {
+        return (IdType) withIdUnchecked(id);
+    }
+
+    /**
+     * Converts raw packet bytes into an {@link Id} instance.
+     * <p>
+     * Implementations are typically stateless and expose a shared
+     * {@code instance()} method so they can be reused:
+     *
+     * <pre>{@code
+     * PrefixedId id = PrefixedId.Deserializer.instance().deserialize(data);
+     * }</pre>
+     *
+     * @param <IdType> type of id produced
+     */
+    interface Deserializer<IdType extends Id> {
+        /**
+         * Deserializes an id from raw bytes.
+         *
+         * @param data raw packet data
+         * @return parsed id
+         * @throws MalformedPacketException if the data cannot be parsed
+         */
+        IdType deserialize(byte[] data) throws MalformedPacketException;
+    }
+
+    /**
+     * Produces raw packet bytes from an {@link Id} instance.
+     * <p>
+     * Like deserializers, serializers commonly provide a shared
+     * {@code instance()} for reuse:
+     *
+     * <pre>{@code
+     * byte[] data = PrefixedId.Serializer.instance().serialize(id);
+     * }</pre>
+     *
+     * @param <IdType> type of id to serialize
+     */
+    interface Serializer<IdType extends Id> {
+        /**
+         * Serializes the supplied id to a byte array.
+         *
+         * @param id id to encode
+         * @return encoded packet bytes
+         */
+        byte[] serialize(IdType id);
+    }
+}
diff --git a/api/src/main/java/com/turikhay/mc/mapmodcompanion/IdBlender.java b/api/src/main/java/com/turikhay/mc/mapmodcompanion/IdBlender.java
new file mode 100644
index 0000000..de087fd
--- /dev/null
+++ b/api/src/main/java/com/turikhay/mc/mapmodcompanion/IdBlender.java
@@ -0,0 +1,30 @@
+package com.turikhay.mc.mapmodcompanion;
+
+import java.util.Objects;
+
+/**
+ * Combines two world identifiers into a single deterministic value.
+ */
+public interface IdBlender {
+
+    /**
+     * A simple implementation that hashes the provided ids using
+     * {@link Objects#hash(Object...)}.
+     */
+    IdBlender DEFAULT = new IdBlender() {
+        @Override
+        public <IdType extends Id> IdType blend(IdType id, int anotherId) {
+            return id.withId(Objects.hash(id.getId(), anotherId));
+        }
+    };
+
+    /**
+     * Produces a new id based on two input ids.
+     *
+     * @param id        base id
+     * @param anotherId id to blend with
+     * @param <IdType>  concrete {@link Id} type
+     * @return blended identifier
+     */
+    <IdType extends Id> IdType blend(IdType id, int anotherId);
+}
diff --git a/common/src/main/java/com/turikhay/mc/mapmodcompanion/LevelMapProperties.java b/api/src/main/java/com/turikhay/mc/mapmodcompanion/LevelMapProperties.java
similarity index 68%
rename from common/src/main/java/com/turikhay/mc/mapmodcompanion/LevelMapProperties.java
rename to api/src/main/java/com/turikhay/mc/mapmodcompanion/LevelMapProperties.java
index 826d468..854a7cf 100644
--- a/common/src/main/java/com/turikhay/mc/mapmodcompanion/LevelMapProperties.java
+++ b/api/src/main/java/com/turikhay/mc/mapmodcompanion/LevelMapProperties.java
@@ -2,10 +2,23 @@
 
 import java.io.*;
 
+/**
+ * Binary format used by Xaero's Minimap and World Map to communicate the
+ * world id.
+ */
 public interface LevelMapProperties {
+
+    /**
+     * Deserializes standard id packets.
+     *
+     * <pre>{@code
+     * StandardId id = LevelMapProperties.Deserializer.instance().deserialize(data);
+     * }</pre>
+     */
     class Deserializer implements Id.Deserializer<StandardId> {
         private static Deserializer INSTANCE;
 
+        /** {@inheritDoc} */
         @Override
         public StandardId deserialize(byte[] data) throws MalformedPacketException {
             DataInputStream in = new DataInputStream(new ByteArrayInputStream(data));
@@ -20,19 +33,36 @@ public StandardId deserialize(byte[] data) throws MalformedPacketException {
             }
         }
 
+        /**
+         * Returns a shared instance of the deserializer.
+         */
         public static Deserializer instance() {
             return INSTANCE == null ? INSTANCE = new Deserializer() : INSTANCE;
         }
     }
 
+    /**
+     * Serializes standard ids into packet byte arrays.
+     *
+     * <pre>{@code
+     * byte[] data = LevelMapProperties.Serializer.instance().serialize(id);
+     * }</pre>
+     */
     class Serializer implements Id.Serializer<StandardId> {
         private static Serializer INSTANCE;
 
+        /** {@inheritDoc} */
         @Override
         public byte[] serialize(StandardId id) {
             return serialize(id.getId());
         }
 
+        /**
+         * Encodes the supplied id into the binary representation.
+         *
+         * @param id numeric world id
+         * @return encoded packet bytes
+         */
         public byte[] serialize(int id) {
             ByteArrayOutputStream array = new ByteArrayOutputStream();
             try(DataOutputStream out = new DataOutputStream(array)) {
@@ -44,6 +74,9 @@ public byte[] serialize(int id) {
             return array.toByteArray();
         }
 
+        /**
+         * Returns a shared instance of the serializer.
+         */
         public static Serializer instance() {
             return INSTANCE == null ? INSTANCE = new Serializer() : INSTANCE;
         }
diff --git a/api/src/main/java/com/turikhay/mc/mapmodcompanion/LightweightException.java b/api/src/main/java/com/turikhay/mc/mapmodcompanion/LightweightException.java
new file mode 100644
index 0000000..67b8ce3
--- /dev/null
+++ b/api/src/main/java/com/turikhay/mc/mapmodcompanion/LightweightException.java
@@ -0,0 +1,33 @@
+package com.turikhay.mc.mapmodcompanion;
+
+import javax.annotation.Nullable;
+
+/**
+ * A lightweight {@link Exception} implementation that does not populate the
+ * stack trace.
+ * <p>
+ * This is useful when the exception is part of the regular control flow and
+ * the overhead of capturing a stack trace is undesirable.
+ */
+public class LightweightException extends Exception {
+
+    /**
+     * Creates a new exception with the provided message and optional cause
+     * without filling in the stack trace.
+     *
+     * @param message human readable description of the problem
+     * @param cause   underlying cause or {@code null}
+     */
+    public LightweightException(String message, @Nullable Throwable cause) {
+        super(message, cause, false, false);
+    }
+
+    /**
+     * Creates a new exception with the provided message.
+     *
+     * @param message human readable description of the problem
+     */
+    public LightweightException(String message) {
+        this(message, null);
+    }
+}
diff --git a/api/src/main/java/com/turikhay/mc/mapmodcompanion/MalformedPacketException.java b/api/src/main/java/com/turikhay/mc/mapmodcompanion/MalformedPacketException.java
new file mode 100644
index 0000000..50b5354
--- /dev/null
+++ b/api/src/main/java/com/turikhay/mc/mapmodcompanion/MalformedPacketException.java
@@ -0,0 +1,28 @@
+package com.turikhay.mc.mapmodcompanion;
+
+import javax.annotation.Nullable;
+
+/**
+ * Indicates that a network packet could not be parsed.
+ */
+public class MalformedPacketException extends LightweightException {
+
+    /**
+     * Creates a new exception with the provided message and optional cause.
+     *
+     * @param message human readable description of the problem
+     * @param cause   underlying cause or {@code null}
+     */
+    public MalformedPacketException(String message, @Nullable Throwable cause) {
+        super(message, cause);
+    }
+
+    /**
+     * Creates a new exception with the provided message.
+     *
+     * @param message human readable description of the problem
+     */
+    public MalformedPacketException(String message) {
+        super(message);
+    }
+}
diff --git a/common/src/main/java/com/turikhay/mc/mapmodcompanion/PrefixedId.java b/api/src/main/java/com/turikhay/mc/mapmodcompanion/PrefixedId.java
similarity index 73%
rename from common/src/main/java/com/turikhay/mc/mapmodcompanion/PrefixedId.java
rename to api/src/main/java/com/turikhay/mc/mapmodcompanion/PrefixedId.java
index 568f1a5..373e2bb 100644
--- a/common/src/main/java/com/turikhay/mc/mapmodcompanion/PrefixedId.java
+++ b/api/src/main/java/com/turikhay/mc/mapmodcompanion/PrefixedId.java
@@ -4,36 +4,62 @@
 import java.nio.charset.StandardCharsets;
 import java.util.Objects;
 
+/**
+ * Identifier encoded with a zero-prefixed length and optional magic marker.
+ * When present, the marker has the value {@link Id#MAGIC_MARKER} (42) which
+ * flags a VoxelMap-style packet.
+ * <p>
+ * This format is used by VoxelMap and other mods that implement the same
+ * packet structure. The {@link Deserializer} and {@link Serializer} nested
+ * classes can be used to convert between the packet representation and this
+ * class.
+*/
 public class PrefixedId implements Id {
     private final int padding;
     private final boolean usesMagicByte;
     private final int id;
 
+    /**
+     * Creates a new prefixed id.
+     *
+     * @param padding       number of leading zero bytes before the marker
+     * @param usesMagicByte whether the {@link #MAGIC_MARKER} is present
+     * @param id            numeric world id
+     */
     public PrefixedId(int padding, boolean usesMagicByte, int id) {
         this.padding = padding;
         this.usesMagicByte = usesMagicByte;
         this.id = id;
     }
 
+    /**
+     * Deprecated constructor that assumes the packet uses the magic marker.
+     */
     @Deprecated
     public PrefixedId(int padding, int id) {
         this(padding, true, id);
     }
 
+    /** {@inheritDoc} */
     @Override
     public int getId() {
         return id;
     }
 
+    /**
+     * Number of zero bytes prepended to the packet.
+     */
     public int getPadding() {
         return padding;
     }
 
+    /** {@inheritDoc} */
     @Override
     public PrefixedId withIdUnchecked(int id) {
         return new PrefixedId(this.padding, this.usesMagicByte, id);
     }
 
+    /** {@inheritDoc} */
     @Override
     public boolean equals(Object o) {
         if (this == o) return true;
@@ -42,11 +68,13 @@ public boolean equals(Object o) {
         return padding == that.padding && usesMagicByte == that.usesMagicByte && id == that.id;
     }
 
+    /** {@inheritDoc} */
     @Override
     public int hashCode() {
         return Objects.hash(padding, usesMagicByte, id);
     }
 
+    /** {@inheritDoc} */
     @Override
     public String toString() {
         return "PrefixedId{" +
@@ -56,9 +84,17 @@ public String toString() {
                 '}';
     }
 
+    /**
+     * Deserializes prefixed id packets.
+     *
+     * <pre>{@code
+     * PrefixedId id = PrefixedId.Deserializer.instance().deserialize(data);
+     * }</pre>
+     */
     public static class Deserializer implements Id.Deserializer<PrefixedId> {
         private static Deserializer INSTANCE;
 
+        /** {@inheritDoc} */
         @Override
         public PrefixedId deserialize(byte[] data) throws MalformedPacketException {
             DataInputStream in = new DataInputStream(new ByteArrayInputStream(data));
@@ -101,14 +137,25 @@ public PrefixedId deserialize(byte[] data) throws MalformedPacketException {
             }
         }
 
+        /**
+         * Returns a shared instance of the deserializer.
+         */
         public static Deserializer instance() {
             return INSTANCE == null ? INSTANCE = new Deserializer() : INSTANCE;
         }
     }
 
+    /**
+     * Serializes {@link PrefixedId} instances into packet byte arrays.
+     *
+     * <pre>{@code
+     * byte[] data = PrefixedId.Serializer.instance().serialize(id);
+     * }</pre>
+     */
     public static class Serializer implements Id.Serializer<PrefixedId> {
         private static Serializer INSTANCE;
 
+        /** {@inheritDoc} */
         @Override
         public byte[] serialize(PrefixedId id) {
             ByteArrayOutputStream array = new ByteArrayOutputStream();
@@ -117,7 +164,7 @@ public byte[] serialize(PrefixedId id) {
                     out.writeByte(0);      // packetId, or prefix
                 }
                 if (id.usesMagicByte) {
-                    out.writeByte(MAGIC_MARKER); // 42 (literally)
+                    out.writeByte(MAGIC_MARKER); // VoxelMap-style flag (42)
                 }
                 byte[] data = String.valueOf(id.getId()).getBytes(StandardCharsets.UTF_8);
                 out.write(data.length);      // length
@@ -128,6 +175,9 @@ public byte[] serialize(PrefixedId id) {
             return array.toByteArray();
         }
 
+        /**
+         * Returns a shared instance of the serializer.
+         */
         public static Serializer instance() {
             return INSTANCE == null ? INSTANCE = new Serializer() : INSTANCE;
         }
diff --git a/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/PrefixedIdRequest.java b/api/src/main/java/com/turikhay/mc/mapmodcompanion/PrefixedIdRequest.java
similarity index 62%
rename from spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/PrefixedIdRequest.java
rename to api/src/main/java/com/turikhay/mc/mapmodcompanion/PrefixedIdRequest.java
index 31fc33e..7d7ffb2 100644
--- a/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/PrefixedIdRequest.java
+++ b/api/src/main/java/com/turikhay/mc/mapmodcompanion/PrefixedIdRequest.java
@@ -1,6 +1,4 @@
-package com.turikhay.mc.mapmodcompanion.spigot;
-
-import com.turikhay.mc.mapmodcompanion.*;
+package com.turikhay.mc.mapmodcompanion;
 
 import javax.annotation.Nullable;
 import java.io.ByteArrayInputStream;
@@ -11,19 +9,48 @@
 
 import static com.turikhay.mc.mapmodcompanion.Id.MAGIC_MARKER;
 
+/**
+ * Description of a packet sent by VoxelMap-style clients to request the
+ * server's {@code world_id}.
+ *
+ * <p>The packet itself has no internal structure that influences how the
+ * server responds. The {@link #parse(byte[], Integer)} method examines the raw
+ * bytes and sets markers such as zero-padding length and the presence of the
+ * magic marker {@link Id#MAGIC_MARKER}. These markers indicate which client
+ * family and version range issued the request, allowing the server to craft a
+ * matching {@link PrefixedId} via {@link #constructId(int)}.</p>
+ *
+ * <pre>{@code
+ * PrefixedIdRequest request = PrefixedIdRequest.parse(data, protocolVersion);
+ * PrefixedId id = request.constructId(1337);
+ * }</pre>
+ */
 public class PrefixedIdRequest {
     private final int padding;
     private final boolean usesMagicByte;
 
+    /**
+     * Creates a new request descriptor.
+     *
+     * @param padding       number of leading zero bytes
+     * @param usesMagicByte whether the {@link Id#MAGIC_MARKER} is present
+     */
     public PrefixedIdRequest(int padding, boolean usesMagicByte) {
         this.padding = padding;
         this.usesMagicByte = usesMagicByte;
     }
 
+    /**
+     * Constructs a {@link PrefixedId} using this request's parameters.
+     *
+     * @param id numeric world id
+     * @return {@link PrefixedId} matching the request
+     */
     public PrefixedId constructId(int id) {
         return new PrefixedId(padding, usesMagicByte, id);
     }
 
+    /** {@inheritDoc} */
     @Override
     public boolean equals(Object o) {
         if (this == o) return true;
@@ -32,11 +59,13 @@ public boolean equals(Object o) {
         return padding == that.padding && usesMagicByte == that.usesMagicByte;
     }
 
+    /** {@inheritDoc} */
     @Override
     public int hashCode() {
         return Objects.hash(padding, usesMagicByte);
     }
 
+    /** {@inheritDoc} */
     @Override
     public String toString() {
         return "PrefixedIdRequest{" +
@@ -45,6 +74,16 @@ public String toString() {
                 '}';
     }
 
+    /**
+     * Parses a VoxelMap-style request packet and sets markers describing the
+     * client family and version range.
+     *
+     * @param payload         raw packet bytes
+     * @param protocolVersion client's protocol version or {@code null} if unknown;
+     *                        see {@link ProtocolVersion} for common constants
+     * @return parsed request descriptor
+     * @throws MalformedPacketException if the payload cannot be parsed
+     */
     public static PrefixedIdRequest parse(byte[] payload, @Nullable Integer protocolVersion) throws MalformedPacketException {
         int padding = -1;
         try (DataInputStream in = new DataInputStream(new ByteArrayInputStream(payload))) {
@@ -70,7 +109,7 @@ public static PrefixedIdRequest parse(byte[] payload, @Nullable Integer protocol
         }
         switch (padding) {
             case 1:
-                if (protocolVersion != null && protocolVersion <= ProtocolVersion.MINECRAFT_1_16_3) {
+                if (protocolVersion != null && protocolVersion <= ProtocolVersion.MINECRAFT_1_16_3) { // 1.16.3 and below
                     // VoxelMap Forge 1.13.2 - 1.16.3
                     return new PrefixedIdRequest(1, false);
                 }
@@ -79,7 +118,7 @@ public static PrefixedIdRequest parse(byte[] payload, @Nullable Integer protocol
                 // JourneyMap 1.16.5+
                 return new PrefixedIdRequest(padding, true);
             case 3:
-                // VoxelMap LiteLoader 1.18.9 - 1.12.2
+                // VoxelMap LiteLoader 1.8.9 - 1.12.2
                 // VoxelMap Fabric 1.14.4 - 1.19.x
                 return new PrefixedIdRequest(0, true);
             default:
diff --git a/api/src/main/java/com/turikhay/mc/mapmodcompanion/ProtocolVersion.java b/api/src/main/java/com/turikhay/mc/mapmodcompanion/ProtocolVersion.java
new file mode 100644
index 0000000..175119a
--- /dev/null
+++ b/api/src/main/java/com/turikhay/mc/mapmodcompanion/ProtocolVersion.java
@@ -0,0 +1,18 @@
+package com.turikhay.mc.mapmodcompanion;
+
+/**
+ * Well-known Minecraft protocol version numbers.
+ *
+ * <p>These constants can be used to interpret version-specific nuances in
+ * packet formats.</p>
+ */
+public interface ProtocolVersion {
+    /** Protocol version number for Minecraft 1.13.2. */
+    int MINECRAFT_1_13_2 = 404;
+    /** Protocol version number for Minecraft 1.14.4. */
+    int MINECRAFT_1_14_4 = 498;
+    /** Protocol version number for Minecraft 1.15.2. */
+    int MINECRAFT_1_15_2 = 578;
+    /** Protocol version number for Minecraft 1.16.3. */
+    int MINECRAFT_1_16_3 = 753;
+}
diff --git a/common/src/main/java/com/turikhay/mc/mapmodcompanion/StandardId.java b/api/src/main/java/com/turikhay/mc/mapmodcompanion/StandardId.java
similarity index 70%
rename from common/src/main/java/com/turikhay/mc/mapmodcompanion/StandardId.java
rename to api/src/main/java/com/turikhay/mc/mapmodcompanion/StandardId.java
index cb2ee03..6eccc01 100644
--- a/common/src/main/java/com/turikhay/mc/mapmodcompanion/StandardId.java
+++ b/api/src/main/java/com/turikhay/mc/mapmodcompanion/StandardId.java
@@ -2,22 +2,35 @@
 
 import java.util.Objects;
 
+/**
+ * Plain numeric {@link Id} implementation used by Xaero's mods.
+ */
 public class StandardId implements Id {
     private final int id;
 
+    /**
+     * Creates a new id with the given value.
+     *
+     * @param id numeric world id
+     */
     public StandardId(int id) {
         this.id = id;
     }
 
+    /**
+     * Returns the numeric world id.
+     */
     public int getId() {
         return id;
     }
 
+    /** {@inheritDoc} */
     @Override
     public StandardId withIdUnchecked(int id) {
         return new StandardId(id);
     }
 
+    /** {@inheritDoc} */
     @Override
     public boolean equals(Object o) {
         if (this == o) return true;
@@ -26,11 +39,13 @@ public boolean equals(Object o) {
         return id == that.id;
     }
 
+    /** {@inheritDoc} */
     @Override
     public int hashCode() {
         return Objects.hash(id);
     }
 
+    /** {@inheritDoc} */
     @Override
     public String toString() {
         return "StandardId{" +
diff --git a/common/src/main/java/com/turikhay/mc/mapmodcompanion/package-info.java b/api/src/main/java/com/turikhay/mc/mapmodcompanion/package-info.java
similarity index 57%
rename from common/src/main/java/com/turikhay/mc/mapmodcompanion/package-info.java
rename to api/src/main/java/com/turikhay/mc/mapmodcompanion/package-info.java
index 8b192c9..bf23e44 100644
--- a/common/src/main/java/com/turikhay/mc/mapmodcompanion/package-info.java
+++ b/api/src/main/java/com/turikhay/mc/mapmodcompanion/package-info.java
@@ -1,7 +1,10 @@
+/**
+ * Public API exposing helper classes for querying world identifiers.
+ */
 @ParametersAreNonnullByDefault
 @ReturnValuesAreNonnullByDefault
 package com.turikhay.mc.mapmodcompanion;
 
 import edu.umd.cs.findbugs.annotations.ReturnValuesAreNonnullByDefault;
 
-import javax.annotation.ParametersAreNonnullByDefault;
\ No newline at end of file
+import javax.annotation.ParametersAreNonnullByDefault;
diff --git a/common/src/test/java/IdBlenderTest.java b/api/src/test/java/IdBlenderTest.java
similarity index 100%
rename from common/src/test/java/IdBlenderTest.java
rename to api/src/test/java/IdBlenderTest.java
diff --git a/common/src/test/java/LevelMapPropertiesPacketDeserializerTest.java b/api/src/test/java/LevelMapPropertiesPacketDeserializerTest.java
similarity index 100%
rename from common/src/test/java/LevelMapPropertiesPacketDeserializerTest.java
rename to api/src/test/java/LevelMapPropertiesPacketDeserializerTest.java
diff --git a/common/src/test/java/LevelMapPropertiesPacketSerializerTest.java b/api/src/test/java/LevelMapPropertiesPacketSerializerTest.java
similarity index 100%
rename from common/src/test/java/LevelMapPropertiesPacketSerializerTest.java
rename to api/src/test/java/LevelMapPropertiesPacketSerializerTest.java
diff --git a/common/src/test/java/PrefixedIdPacketDeserializerTest.java b/api/src/test/java/PrefixedIdPacketDeserializerTest.java
similarity index 100%
rename from common/src/test/java/PrefixedIdPacketDeserializerTest.java
rename to api/src/test/java/PrefixedIdPacketDeserializerTest.java
diff --git a/spigot/src/test/java/com/turikhay/mc/mapmodcompanion/spigot/PrefixedIdRequestParserTest.java b/api/src/test/java/PrefixedIdRequestParserTest.java
similarity index 93%
rename from spigot/src/test/java/com/turikhay/mc/mapmodcompanion/spigot/PrefixedIdRequestParserTest.java
rename to api/src/test/java/PrefixedIdRequestParserTest.java
index 237bb39..d2f46be 100644
--- a/spigot/src/test/java/com/turikhay/mc/mapmodcompanion/spigot/PrefixedIdRequestParserTest.java
+++ b/api/src/test/java/PrefixedIdRequestParserTest.java
@@ -1,12 +1,10 @@
-package com.turikhay.mc.mapmodcompanion.spigot;
-
 import com.turikhay.mc.mapmodcompanion.*;
 import org.junit.jupiter.api.Test;
 
 import java.util.Arrays;
 import java.util.List;
 
-import static com.turikhay.mc.mapmodcompanion.spigot.PrefixedIdRequest.parse;
+import static com.turikhay.mc.mapmodcompanion.PrefixedIdRequest.parse;
 import static org.junit.jupiter.api.Assertions.*;
 
 class PrefixedIdRequestParserTest {
diff --git a/spigot/src/test/java/com/turikhay/mc/mapmodcompanion/spigot/PrefixedIdRequestTest.java b/api/src/test/java/PrefixedIdRequestTest.java
similarity index 91%
rename from spigot/src/test/java/com/turikhay/mc/mapmodcompanion/spigot/PrefixedIdRequestTest.java
rename to api/src/test/java/PrefixedIdRequestTest.java
index a357119..1013c0b 100644
--- a/spigot/src/test/java/com/turikhay/mc/mapmodcompanion/spigot/PrefixedIdRequestTest.java
+++ b/api/src/test/java/PrefixedIdRequestTest.java
@@ -1,6 +1,5 @@
-package com.turikhay.mc.mapmodcompanion.spigot;
-
 import com.turikhay.mc.mapmodcompanion.PrefixedId;
+import com.turikhay.mc.mapmodcompanion.PrefixedIdRequest;
 import org.junit.jupiter.api.Test;
 
 import static org.junit.jupiter.api.Assertions.*;
diff --git a/buildSrc/build.gradle.kts b/buildSrc/build.gradle.kts
index 8346d7a..7f1a8b4 100644
--- a/buildSrc/build.gradle.kts
+++ b/buildSrc/build.gradle.kts
@@ -5,6 +5,10 @@ plugins {
 
 repositories {
     gradlePluginPortal()
+    maven {
+        name = "Fabric"
+        url = uri("https://maven.fabricmc.net/")
+    }
 }
 
 gradlePlugin {
@@ -28,4 +32,5 @@ dependencies {
     implementation(libs.semver.get().toString())
     implementation("com.modrinth.minotaur:Minotaur:2.+")
     implementation(with(libs.plugins.hangar.get()) { "io.papermc:hangar-publish-plugin:$version" })
+    implementation(with(libs.plugins.fabric.loom.get()) { "net.fabricmc:fabric-loom:$version" })
 }
diff --git a/bungee/build.gradle.kts b/bungee/build.gradle.kts
index ede6885..2be0f7d 100644
--- a/bungee/build.gradle.kts
+++ b/bungee/build.gradle.kts
@@ -29,6 +29,7 @@ tasks {
 }
 
 dependencies {
+    implementation(project(":api"))
     implementation(project(":common"))
     implementation(libs.bstats.bungeecord)
     compileOnly(libs.bungeecord.api)
diff --git a/common/build.gradle.kts b/common/build.gradle.kts
index 090e9b5..13f0186 100644
--- a/common/build.gradle.kts
+++ b/common/build.gradle.kts
@@ -1,3 +1,8 @@
 plugins {
     id("java-convention")
 }
+
+dependencies {
+    implementation(project(":api"))
+    compileOnly(libs.slf4j)
+}
diff --git a/common/src/main/java/com/turikhay/mc/mapmodcompanion/Channels.java b/common/src/main/java/com/turikhay/mc/mapmodcompanion/Channels.java
deleted file mode 100644
index f5c2af3..0000000
--- a/common/src/main/java/com/turikhay/mc/mapmodcompanion/Channels.java
+++ /dev/null
@@ -1,8 +0,0 @@
-package com.turikhay.mc.mapmodcompanion;
-
-public interface Channels {
-    String XAERO_MINIMAP_CHANNEL = "xaerominimap:main";
-    String XAERO_WORLDMAP_CHANNEL = "xaeroworldmap:main";
-    String WORLDID_CHANNEL = "worldinfo:world_id";
-    String WORLDID_LEGACY_CHANNEL = "world_id";
-}
diff --git a/common/src/main/java/com/turikhay/mc/mapmodcompanion/Id.java b/common/src/main/java/com/turikhay/mc/mapmodcompanion/Id.java
deleted file mode 100644
index 284318f..0000000
--- a/common/src/main/java/com/turikhay/mc/mapmodcompanion/Id.java
+++ /dev/null
@@ -1,22 +0,0 @@
-package com.turikhay.mc.mapmodcompanion;
-
-public interface Id {
-    int MAGIC_MARKER = 42;
-
-    int getId();
-
-    Id withIdUnchecked(int id);
-
-    @SuppressWarnings("unchecked")
-    default <IdType> IdType withId(int id) {
-        return (IdType) withIdUnchecked(id);
-    }
-
-    interface Deserializer<IdType extends Id> {
-        IdType deserialize(byte[] data) throws MalformedPacketException;
-    }
-
-    interface Serializer<IdType extends Id> {
-        byte[] serialize(IdType id);
-    }
-}
diff --git a/common/src/main/java/com/turikhay/mc/mapmodcompanion/IdBlender.java b/common/src/main/java/com/turikhay/mc/mapmodcompanion/IdBlender.java
deleted file mode 100644
index e005ecd..0000000
--- a/common/src/main/java/com/turikhay/mc/mapmodcompanion/IdBlender.java
+++ /dev/null
@@ -1,14 +0,0 @@
-package com.turikhay.mc.mapmodcompanion;
-
-import java.util.Objects;
-
-public interface IdBlender {
-    IdBlender DEFAULT = new IdBlender() {
-        @Override
-        public <IdType extends Id> IdType blend(IdType id, int anotherId) {
-            return id.withId(Objects.hash(id.getId(), anotherId));
-        }
-    };
-
-    <IdType extends Id> IdType blend(IdType id, int anotherId);
-}
diff --git a/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/IdRegistry.java b/common/src/main/java/com/turikhay/mc/mapmodcompanion/IdRegistry.java
similarity index 85%
rename from spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/IdRegistry.java
rename to common/src/main/java/com/turikhay/mc/mapmodcompanion/IdRegistry.java
index ea3b0b0..6ddffa9 100644
--- a/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/IdRegistry.java
+++ b/common/src/main/java/com/turikhay/mc/mapmodcompanion/IdRegistry.java
@@ -1,9 +1,4 @@
-package com.turikhay.mc.mapmodcompanion.spigot;
-
-import com.turikhay.mc.mapmodcompanion.IdLookup;
-import com.turikhay.mc.mapmodcompanion.PrefixLogger;
-import com.turikhay.mc.mapmodcompanion.VerboseLogger;
-import org.bukkit.World;
+package com.turikhay.mc.mapmodcompanion;
 
 import java.util.HashMap;
 import java.util.Map;
@@ -11,7 +6,7 @@
 import java.util.logging.Logger;
 
 public interface IdRegistry {
-    int getId(World world);
+    int getId(WorldInfo world);
 
     class CacheableRegistry implements IdRegistry {
         private final Map<String, Integer> cache = new HashMap<>();
@@ -22,7 +17,7 @@ public CacheableRegistry(IdRegistry delegate) {
         }
 
         @Override
-        public int getId(World world) {
+        public int getId(WorldInfo world) {
             return cache.computeIfAbsent(world.getName(), s -> delegate.getId(world));
         }
 
@@ -46,7 +41,7 @@ public ConvertingRegistry(VerboseLogger parent, IdLookup lookup, IdRegistry dele
         }
 
         @Override
-        public int getId(World world) {
+        public int getId(WorldInfo world) {
             Optional<Integer> byName = lookup.findMatch(world.getName());
             if (byName.isPresent()) {
                 logger.fine("Found override: " + world.getName() + " -> " + byName.get());
@@ -77,7 +72,7 @@ public ConstantRegistry(int id) {
         }
 
         @Override
-        public int getId(World world) {
+        public int getId(WorldInfo world) {
             return id;
         }
 
@@ -91,7 +86,7 @@ public String toString() {
 
     class DynamicUUIDRegistry implements IdRegistry {
         @Override
-        public int getId(World world) {
+        public int getId(WorldInfo world) {
             return world.getUID().hashCode();
         }
     }
diff --git a/common/src/main/java/com/turikhay/mc/mapmodcompanion/LightweightException.java b/common/src/main/java/com/turikhay/mc/mapmodcompanion/LightweightException.java
deleted file mode 100644
index 72fbc10..0000000
--- a/common/src/main/java/com/turikhay/mc/mapmodcompanion/LightweightException.java
+++ /dev/null
@@ -1,13 +0,0 @@
-package com.turikhay.mc.mapmodcompanion;
-
-import javax.annotation.Nullable;
-
-public class LightweightException extends Exception {
-    public LightweightException(String message, @Nullable Throwable cause) {
-        super(message, cause, false, false);
-    }
-
-    public LightweightException(String message) {
-        this(message, null);
-    }
-}
diff --git a/common/src/main/java/com/turikhay/mc/mapmodcompanion/MalformedPacketException.java b/common/src/main/java/com/turikhay/mc/mapmodcompanion/MalformedPacketException.java
deleted file mode 100644
index 6bdb40d..0000000
--- a/common/src/main/java/com/turikhay/mc/mapmodcompanion/MalformedPacketException.java
+++ /dev/null
@@ -1,13 +0,0 @@
-package com.turikhay.mc.mapmodcompanion;
-
-import javax.annotation.Nullable;
-
-public class MalformedPacketException extends LightweightException {
-    public MalformedPacketException(String message, @Nullable Throwable cause) {
-        super(message, cause);
-    }
-
-    public MalformedPacketException(String message) {
-        super(message);
-    }
-}
diff --git a/common/src/main/java/com/turikhay/mc/mapmodcompanion/Slf4jLogger.java b/common/src/main/java/com/turikhay/mc/mapmodcompanion/Slf4jLogger.java
new file mode 100644
index 0000000..a82a676
--- /dev/null
+++ b/common/src/main/java/com/turikhay/mc/mapmodcompanion/Slf4jLogger.java
@@ -0,0 +1,31 @@
+package com.turikhay.mc.mapmodcompanion;
+
+import org.slf4j.Logger;
+
+public class Slf4jLogger implements ILogger {
+    private final Logger logger;
+
+    public Slf4jLogger(Logger logger) {
+        this.logger = logger;
+    }
+
+    @Override
+    public void fine(String message) {
+        logger.debug("{}", message);
+    }
+
+    @Override
+    public void info(String message) {
+        logger.info("{}", message);
+    }
+
+    @Override
+    public void warn(String message, Throwable t) {
+        logger.warn("{}", message, t);
+    }
+
+    @Override
+    public void error(String message, Throwable t) {
+        logger.error("{}", message, t);
+    }
+}
diff --git a/common/src/main/java/com/turikhay/mc/mapmodcompanion/WorldInfo.java b/common/src/main/java/com/turikhay/mc/mapmodcompanion/WorldInfo.java
new file mode 100644
index 0000000..cb7646a
--- /dev/null
+++ b/common/src/main/java/com/turikhay/mc/mapmodcompanion/WorldInfo.java
@@ -0,0 +1,42 @@
+package com.turikhay.mc.mapmodcompanion;
+
+import java.util.Objects;
+import java.util.UUID;
+
+public class WorldInfo {
+    private final UUID uuid;
+    private final String name;
+
+    public WorldInfo(UUID uuid, String name) {
+        this.uuid = uuid;
+        this.name = name;
+    }
+
+    public UUID getUID() {
+        return uuid;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (o == null || getClass() != o.getClass()) return false;
+        WorldInfo worldInfo = (WorldInfo) o;
+        return Objects.equals(uuid, worldInfo.uuid) && Objects.equals(name, worldInfo.name);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(uuid, name);
+    }
+
+    @Override
+    public String toString() {
+        return "WorldInfo{" +
+                "uuid=" + uuid +
+                ", name='" + name + '\'' +
+                '}';
+    }
+}
diff --git a/fabric/.gitignore b/fabric/.gitignore
new file mode 100644
index 0000000..7e3b24e
--- /dev/null
+++ b/fabric/.gitignore
@@ -0,0 +1 @@
+run/
diff --git a/fabric/1.21/build.gradle.kts b/fabric/1.21/build.gradle.kts
new file mode 100644
index 0000000..e8aa5ea
--- /dev/null
+++ b/fabric/1.21/build.gradle.kts
@@ -0,0 +1,33 @@
+plugins {
+    id("fabric-loom")
+}
+
+java {
+    sourceCompatibility = JavaVersion.VERSION_17
+    targetCompatibility = JavaVersion.VERSION_17
+}
+
+dependencies {
+    implementation(project(":api"))
+    implementation(project(":common"))
+
+    // To change the versions see the gradle.properties file
+    minecraft("com.mojang:minecraft:${properties["minecraft_version"]}")
+    mappings("net.fabricmc:yarn:${properties["yarn_mappings"]}:v2")
+    modImplementation("net.fabricmc:fabric-loader:${properties["loader_version"]}")
+
+    // Fabric API. This is technically optional, but you probably want it anyway.
+    modImplementation("net.fabricmc.fabric-api:fabric-api:${properties["fabric_version"]}")
+}
+
+loom {
+    accessWidenerPath = file("src/main/resources/mapmodcompanion.accesswidener")
+}
+
+tasks {
+    processResources {
+        filesMatching("fabric.mod.json") {
+            expand("version" to version)
+        }
+    }
+}
diff --git a/fabric/1.21/gradle.properties b/fabric/1.21/gradle.properties
new file mode 100644
index 0000000..eb48e44
--- /dev/null
+++ b/fabric/1.21/gradle.properties
@@ -0,0 +1,8 @@
+# Fabric Properties
+# check these on https://fabricmc.net/develop
+minecraft_version=1.21.8
+yarn_mappings=1.21.8+build.1
+loader_version=0.17.2
+
+# Fabric API
+fabric_version=0.131.0+1.21.8
diff --git a/fabric/1.21/src/main/java/com/turikhay/mc/mapmodcompanion/fabric/Channels.java b/fabric/1.21/src/main/java/com/turikhay/mc/mapmodcompanion/fabric/Channels.java
new file mode 100644
index 0000000..bf9f88f
--- /dev/null
+++ b/fabric/1.21/src/main/java/com/turikhay/mc/mapmodcompanion/fabric/Channels.java
@@ -0,0 +1,15 @@
+package com.turikhay.mc.mapmodcompanion.fabric;
+
+import net.minecraft.util.Identifier;
+
+import static com.turikhay.mc.mapmodcompanion.Channels.*;
+
+public interface Channels {
+    Identifier WORLD_ID = of(WORLDID_CHANNEL);
+    Identifier XAERO_MINIMAP = of(XAERO_MINIMAP_CHANNEL);
+    Identifier XAERO_WORLDMAP = of(XAERO_WORLDMAP_CHANNEL);
+
+    static Identifier of(String channel) {
+        return Identifier.tryParse(channel);
+    }
+}
diff --git a/fabric/1.21/src/main/java/com/turikhay/mc/mapmodcompanion/fabric/LevelMapPropertiesPayload.java b/fabric/1.21/src/main/java/com/turikhay/mc/mapmodcompanion/fabric/LevelMapPropertiesPayload.java
new file mode 100644
index 0000000..5e453fc
--- /dev/null
+++ b/fabric/1.21/src/main/java/com/turikhay/mc/mapmodcompanion/fabric/LevelMapPropertiesPayload.java
@@ -0,0 +1,57 @@
+package com.turikhay.mc.mapmodcompanion.fabric;
+
+import net.fabricmc.fabric.api.networking.v1.PayloadTypeRegistry;
+import net.minecraft.network.RegistryByteBuf;
+import net.minecraft.network.codec.PacketCodec;
+import net.minecraft.network.packet.CustomPayload;
+
+public class LevelMapPropertiesPayload {
+
+    private interface LevelIdPayload extends CustomPayload {
+        int id();
+    }
+
+    public record Minimap(int id) implements LevelIdPayload {
+        public static final Id<Minimap> ID = new Id<>(Channels.XAERO_MINIMAP);
+
+        @Override
+        public Id<? extends CustomPayload> getId() {
+            return ID;
+        }
+    }
+
+    public record WorldMap(int id) implements LevelIdPayload {
+        public static final Id<WorldMap> ID = new Id<>(Channels.XAERO_WORLDMAP);
+
+        @Override
+        public Id<? extends CustomPayload> getId() {
+            return ID;
+        }
+    }
+
+    public static void register() {
+        register(Minimap.ID);
+        register(WorldMap.ID);
+    }
+
+    private static <T extends LevelIdPayload> void register(CustomPayload.Id<T> id) {
+        PayloadTypeRegistry.playS2C().register(id, new Codec<>());
+    }
+
+    public static class Codec<T extends LevelIdPayload> implements PacketCodec<RegistryByteBuf, T> {
+
+        @Override
+        public T decode(RegistryByteBuf buf) {
+            throw new UnsupportedOperationException();
+        }
+
+        @Override
+        public void encode(RegistryByteBuf buf, T value) {
+            buf.writeByte(0);
+            buf.writeInt(value.id());
+        }
+    }
+
+    private LevelMapPropertiesPayload() {
+    }
+}
diff --git a/fabric/1.21/src/main/java/com/turikhay/mc/mapmodcompanion/fabric/MapModCompanion.java b/fabric/1.21/src/main/java/com/turikhay/mc/mapmodcompanion/fabric/MapModCompanion.java
new file mode 100644
index 0000000..7c0fa8d
--- /dev/null
+++ b/fabric/1.21/src/main/java/com/turikhay/mc/mapmodcompanion/fabric/MapModCompanion.java
@@ -0,0 +1,84 @@
+package com.turikhay.mc.mapmodcompanion.fabric;
+
+import com.turikhay.mc.mapmodcompanion.IdRegistry;
+import com.turikhay.mc.mapmodcompanion.WorldInfo;
+import net.fabricmc.api.ModInitializer;
+import net.fabricmc.fabric.api.networking.v1.ServerPlayNetworking;
+
+import net.minecraft.server.network.ServerPlayerEntity;
+import net.minecraft.server.world.ServerWorld;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.IOException;
+import java.util.UUID;
+import java.util.function.Consumer;
+
+public class MapModCompanion implements ModInitializer {
+    public static final String MOD_ID = "mapmodcompanion";
+    public static final Logger LOGGER = LoggerFactory.getLogger(MOD_ID);
+
+    public static final boolean
+            xaeroMinimap = enableIfMissing("xaero.minimap.XaeroMinimap"),
+            xaeroWorldMap = enableIfMissing("xaero.map.WorldMap");
+
+    private static MapModCompanion INSTANCE;
+
+    private IdRegistry registry;
+
+    @Override
+    public void onInitialize() {
+        LevelMapPropertiesPayload.register();
+        registry = new IdRegistry.DynamicUUIDRegistry();
+        INSTANCE = this;
+        // TODO add overrides support
+        // then
+        // TODO make pre-1.20.5 support
+        // TODO adapt tests_e2e to also test fabric servers
+    }
+
+    public void sendLevelData(ServerPlayerEntity player, ServerWorld world) {
+        int id = registry.getId(toWorldInfo(world));
+        if (xaeroMinimap) {
+            ServerPlayNetworking.send(player, new LevelMapPropertiesPayload.Minimap(id));
+        }
+        if (xaeroWorldMap) {
+            ServerPlayNetworking.send(player, new LevelMapPropertiesPayload.WorldMap(id));
+        }
+    }
+
+    public static void run(String taskDescription, Consumer<MapModCompanion> task) {
+        MapModCompanion instance = INSTANCE;
+        if (instance == null) {
+            throw new IllegalStateException("MapModCompanion has not been initialized");
+        }
+        try {
+            task.accept(instance);
+        } catch (Exception e) {
+            LOGGER.error("Error running the task: {}", taskDescription, e);
+        }
+    }
+
+    public static WorldInfo toWorldInfo(ServerWorld world) {
+        UUID uuid;
+        try {
+            uuid = WorldUID.getOrCreate(world);
+        } catch (IOException e) {
+            throw new RuntimeException("Failed to read or write CraftBukkit-style uid.dat file", e);
+        }
+        return new WorldInfo(uuid, world.worldProperties.getLevelName());
+    }
+
+    private static boolean enableIfMissing(String className) {
+        boolean enable = false;
+        try {
+            Class.forName(className);
+        } catch (ClassNotFoundException e) {
+            enable = true;
+        }
+        if (!enable) {
+            LOGGER.warn("Found {} in the classpath", className);
+        }
+        return enable;
+    }
+}
diff --git a/fabric/1.21/src/main/java/com/turikhay/mc/mapmodcompanion/fabric/WorldUID.java b/fabric/1.21/src/main/java/com/turikhay/mc/mapmodcompanion/fabric/WorldUID.java
new file mode 100644
index 0000000..44be89c
--- /dev/null
+++ b/fabric/1.21/src/main/java/com/turikhay/mc/mapmodcompanion/fabric/WorldUID.java
@@ -0,0 +1,50 @@
+package com.turikhay.mc.mapmodcompanion.fabric;
+
+import net.minecraft.server.world.ServerWorld;
+
+import java.io.*;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.UUID;
+
+// CraftBukkit-style uid.dat file reader/writer
+public class WorldUID {
+    public static UUID getOrCreate(ServerWorld world) throws IOException {
+        Path worldDir = getWorldDir(world);
+        return getOrCreate(worldDir);
+    }
+
+    static Path getWorldDir(ServerWorld world) {
+        return world.getServer().session.getDirectory().path();
+    }
+
+    static UUID getOrCreate(Path worldDir) throws IOException {
+        return getOrCreate(worldDir, UUID.randomUUID());
+    }
+
+    static UUID getOrCreate(Path worldDir, UUID fallbackUid) throws IOException {
+        Path uidFile = getUidFile(worldDir);
+        if (Files.exists(uidFile)) {
+            return read(uidFile);
+        }
+        write(uidFile, fallbackUid);
+        return fallbackUid;
+    }
+
+    static Path getUidFile(Path worldDir) {
+        return worldDir.resolve("uid.dat");
+    }
+
+    static UUID read(Path uidFile) throws IOException {
+        try (DataInputStream inputStream = new DataInputStream(Files.newInputStream(uidFile))) {
+            return new UUID(inputStream.readLong(), inputStream.readLong());
+        }
+    }
+
+    static void write(Path uidFile, UUID uuid) throws IOException {
+        try (DataOutputStream outputStream = new DataOutputStream(Files.newOutputStream(uidFile))) {
+            outputStream.writeLong(uuid.getMostSignificantBits());
+            outputStream.writeLong(uuid.getLeastSignificantBits());
+        }
+    }
+}
diff --git a/fabric/1.21/src/main/java/com/turikhay/mc/mapmodcompanion/fabric/mixin/PlayerManagerMixin.java b/fabric/1.21/src/main/java/com/turikhay/mc/mapmodcompanion/fabric/mixin/PlayerManagerMixin.java
new file mode 100644
index 0000000..0493e4c
--- /dev/null
+++ b/fabric/1.21/src/main/java/com/turikhay/mc/mapmodcompanion/fabric/mixin/PlayerManagerMixin.java
@@ -0,0 +1,18 @@
+package com.turikhay.mc.mapmodcompanion.fabric.mixin;
+
+import com.turikhay.mc.mapmodcompanion.fabric.MapModCompanion;
+import net.minecraft.server.PlayerManager;
+import net.minecraft.server.network.ServerPlayerEntity;
+import net.minecraft.server.world.ServerWorld;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Inject;
+import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
+
+@Mixin(PlayerManager.class)
+public class PlayerManagerMixin {
+    @Inject(method = "sendWorldInfo", at = @At("HEAD"))
+    private void mapmodcompanion$sendWorldId(ServerPlayerEntity player, ServerWorld world, CallbackInfo ci) {
+        MapModCompanion.run("$sendWorldId", mmc -> mmc.sendLevelData(player, world));
+    }
+}
diff --git a/fabric/1.21/src/main/resources/fabric.mod.json b/fabric/1.21/src/main/resources/fabric.mod.json
new file mode 100644
index 0000000..3e420bb
--- /dev/null
+++ b/fabric/1.21/src/main/resources/fabric.mod.json
@@ -0,0 +1,22 @@
+{
+  "schemaVersion": 1,
+  "id": "mapmodcompanion",
+  "version": "${version}",
+  "name": "MapModCompanion",
+  "license": "MIT",
+  "environment": "*",
+  "entrypoints": {
+    "main": [
+      "com.turikhay.mc.mapmodcompanion.fabric.MapModCompanion"
+    ]
+  },
+  "depends": {
+    "fabricloader": ">=0.16.14",
+    "minecraft": "~1.21.8",
+    "java": ">=21",
+    "fabric-api": "*"
+  },
+  "mixins": [
+    "mapmodcompanion.mixins.json"
+  ]
+}
diff --git a/fabric/1.21/src/main/resources/mapmodcompanion.accesswidener b/fabric/1.21/src/main/resources/mapmodcompanion.accesswidener
new file mode 100644
index 0000000..1863376
--- /dev/null
+++ b/fabric/1.21/src/main/resources/mapmodcompanion.accesswidener
@@ -0,0 +1,3 @@
+accessWidener   v2      named
+accessible      field   net/minecraft/server/world/ServerWorld worldProperties Lnet/minecraft/world/level/ServerWorldProperties;
+accessible      field   net/minecraft/server/MinecraftServer session Lnet/minecraft/world/level/storage/LevelStorage$Session;
diff --git a/fabric/1.21/src/main/resources/mapmodcompanion.mixins.json b/fabric/1.21/src/main/resources/mapmodcompanion.mixins.json
new file mode 100644
index 0000000..2abe692
--- /dev/null
+++ b/fabric/1.21/src/main/resources/mapmodcompanion.mixins.json
@@ -0,0 +1,8 @@
+{
+  "required": true,
+  "package": "com.turikhay.mc.mapmodcompanion.fabric.mixin",
+  "compatibilityLevel": "JAVA_17",
+  "server": [
+    "PlayerManagerMixin"
+  ]
+}
diff --git a/fabric/build.gradle.kts b/fabric/build.gradle.kts
new file mode 100644
index 0000000..9f7b6c7
--- /dev/null
+++ b/fabric/build.gradle.kts
@@ -0,0 +1,3 @@
+plugins {
+    base
+}
\ No newline at end of file
diff --git a/gradle/libs.versions.toml b/gradle/libs.versions.toml
index 706506b..418ad36 100644
--- a/gradle/libs.versions.toml
+++ b/gradle/libs.versions.toml
@@ -9,6 +9,8 @@ jackson = "2.16.1"
 semver = "1.3.0"
 hangar = "0.1.3"
 bungeecord-api = "1.20-R0.2"
+fabric-loom = "1.11-SNAPSHOT"
+slf4j = "2.0.17"
 
 [libraries]
 spotbugs-annotations = { module = "com.github.spotbugs:spotbugs-annotations", version.ref = "spotbugs" }
@@ -22,8 +24,10 @@ velocity-api = { module = "com.velocitypowered:velocity-api", version.ref = "vel
 jackson-core = { module = "com.fasterxml.jackson.core:jackson-core", version.ref = "jackson" }
 semver = { module = "net.swiftzer.semver:semver", version.ref = "semver" }
 bungeecord-api = { module = "net.md-5:bungeecord-api", version.ref = "bungeecord-api" }
+slf4j = { module = "org.slf4j:slf4j-api", version.ref = "slf4j" }
 
 
 [plugins]
 shadow = { id = "com.gradleup.shadow", version.ref = "shadow" }
 hangar = { id = "io.papermc.hangar-publish-plugin", version.ref = "hangar" }
+fabric-loom = { id = "fabric-loom", version.ref = "fabric-loom" }
diff --git a/settings.gradle.kts b/settings.gradle.kts
index 0200f75..afd54c9 100644
--- a/settings.gradle.kts
+++ b/settings.gradle.kts
@@ -1,8 +1,10 @@
 rootProject.name = "mapmodcompanion"
 
 include(":common")
+include(":api")
 include(":bungee")
 include(":spigot")
 include(":velocity")
+include(":fabric:1.21")
 include(":packages")
 include(":packages:single")
diff --git a/spigot/build.gradle.kts b/spigot/build.gradle.kts
index bb9ed39..b2fa80b 100644
--- a/spigot/build.gradle.kts
+++ b/spigot/build.gradle.kts
@@ -39,6 +39,7 @@ val spigot_version: String by project
 val protocolLib_version: String by project
 
 dependencies {
+    implementation(project(":api"))
     implementation(project(":common"))
     implementation(libs.bstats.bukkit)
 
diff --git a/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/LevelIdHandler.java b/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/LevelIdHandler.java
index 3d92294..61dfaee 100644
--- a/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/LevelIdHandler.java
+++ b/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/LevelIdHandler.java
@@ -8,6 +8,8 @@
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
+import static com.turikhay.mc.mapmodcompanion.spigot.MapModCompanion.toWorldInfo;
+
 public class LevelIdHandler implements Handler, PluginMessageListener {
     private final Logger logger;
     private final String channelName;
@@ -35,7 +37,7 @@ public void cleanUp() {
     @Override
     public void onPluginMessageReceived(String channel, Player player, byte[] requestBytes) {
         Integer protocolVersion = plugin.getProtocolLib().map(lib -> lib.getProtocolVersion(player)).orElse(null);
-        int id = plugin.getRegistry().getId(player.getWorld());
+        int id = plugin.getRegistry().getId(toWorldInfo(player.getWorld()));
         PrefixedIdRequest request;
         try {
             request = PrefixedIdRequest.parse(requestBytes, protocolVersion);
diff --git a/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/MapModCompanion.java b/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/MapModCompanion.java
index 016c2ae..b316dc4 100644
--- a/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/MapModCompanion.java
+++ b/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/MapModCompanion.java
@@ -210,4 +210,8 @@ void checkEnabled(String configPath) throws InitializationException {
             throw new InitializationException("disabled in the config");
         }
     }
+
+    public static WorldInfo toWorldInfo(World world) {
+        return new WorldInfo(world.getUID(), world.getName());
+    }
 }
diff --git a/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/ProtocolVersion.java b/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/ProtocolVersion.java
deleted file mode 100644
index aff3d38..0000000
--- a/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/ProtocolVersion.java
+++ /dev/null
@@ -1,8 +0,0 @@
-package com.turikhay.mc.mapmodcompanion.spigot;
-
-public interface ProtocolVersion {
-    int MINECRAFT_1_13_2 = 404;
-    int MINECRAFT_1_14_4 = 498;
-    int MINECRAFT_1_15_2 = 578;
-    int MINECRAFT_1_16_3 = 753;
-}
diff --git a/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/XaeroHandler.java b/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/XaeroHandler.java
index 83f8c16..9a71093 100644
--- a/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/XaeroHandler.java
+++ b/spigot/src/main/java/com/turikhay/mc/mapmodcompanion/spigot/XaeroHandler.java
@@ -19,6 +19,8 @@
 import java.util.concurrent.TimeUnit;
 import java.util.logging.Logger;
 
+import static com.turikhay.mc.mapmodcompanion.spigot.MapModCompanion.toWorldInfo;
+
 public class XaeroHandler implements Handler, Listener {
     private final Logger logger;
     private final String configPath;
@@ -64,7 +66,7 @@ public void onWorldChanged(PlayerChangedWorldEvent event) {
     private void sendPacket(PlayerEvent event, Type type) {
         Player p = event.getPlayer();
         World world = p.getWorld();
-        int id = plugin.getRegistry().getId(world);
+        int id = plugin.getRegistry().getId(toWorldInfo(world));
         byte[] payload = LevelMapProperties.Serializer.instance().serialize(id);
         SendPayloadTask task = new SendPayloadTask(logger, plugin, p.getUniqueId(), channelName, payload, world.getUID());
         int repeatTimes = plugin.getConfig().getInt(
diff --git a/velocity/build.gradle.kts b/velocity/build.gradle.kts
index 2899094..1cb0ec1 100644
--- a/velocity/build.gradle.kts
+++ b/velocity/build.gradle.kts
@@ -15,6 +15,7 @@ repositories {
 }
 
 dependencies {
+    implementation(project(":api"))
     implementation(project(":common"))
     implementation(libs.bstats.velocity)
 
diff --git a/velocity/src/main/java/com/turikhay/mc/mapmodcompanion/velocity/MapModCompanion.java b/velocity/src/main/java/com/turikhay/mc/mapmodcompanion/velocity/MapModCompanion.java
index d5a17b9..b46477b 100644
--- a/velocity/src/main/java/com/turikhay/mc/mapmodcompanion/velocity/MapModCompanion.java
+++ b/velocity/src/main/java/com/turikhay/mc/mapmodcompanion/velocity/MapModCompanion.java
@@ -171,27 +171,7 @@ private Path getConfigFile() {
 
     private static final String CONFIG_PATH = "/config_velocity.toml";
 
-    private static ILogger ofSlf4j(Logger logger) {
-        return new ILogger() {
-            @Override
-            public void fine(String message) {
-                logger.debug("{}", message);
-            }
-
-            @Override
-            public void info(String message) {
-                logger.info("{}", message);
-            }
-
-            @Override
-            public void warn(String message, Throwable t) {
-                logger.warn("{}", message, t);
-            }
-
-            @Override
-            public void error(String message, Throwable t) {
-                logger.error("{}", message, t);
-            }
-        };
+    private static Slf4jLogger ofSlf4j(Logger logger) {
+        return new Slf4jLogger(logger);
     }
 }