diff --git a/src/loaders/fabric/fabric-metadata-custom-payload.ts b/src/loaders/fabric/fabric-metadata-custom-payload.ts
new file mode 100644
index 0000000..756c675
--- /dev/null
+++ b/src/loaders/fabric/fabric-metadata-custom-payload.ts
@@ -0,0 +1,174 @@
+import { ACTION_NAME } from "@/action";
+import { Dependency, createDependency } from "@/dependencies";
+import { LoaderType } from "@/loaders/loader-type";
+import { PlatformType } from "@/platforms";
+import { asString } from "@/utils/string-utils";
+import { PartialRecord } from "@/utils/types";
+import { deprecate } from "node:util";
+import { RawFabricMetadata } from "./raw-fabric-metadata";
+
+// TODO: Remove the deprecated stuff in v4.0.
+
+/**
+ * Custom payload for Fabric metadata.
+ */
+export type FabricMetadataCustomPayload = {
+    /**
+     * Indicates whether the mod support Quilt loader or not.
+     *
+     * @deprecated
+     *
+     * Use the universal `loaders` field instead with `["fabric", "quilt"]` to achieve the same result.
+     */
+    quilt?: boolean;
+
+    /**
+     * A list of supported mod loaders.
+     */
+    loaders?: string[];
+
+    /**
+     * A list of mod dependencies.
+     */
+    dependencies?: string[];
+}
+& PartialRecord<PlatformType, string>;
+
+/**
+ * Gets the custom payload from the Fabric metadata.
+ *
+ * @param metadata - The raw Fabric metadata.
+ *
+ * @returns The custom payload attached to the given metadata.
+ */
+export function getFabricMetadataCustomPayload(metadata: RawFabricMetadata): FabricMetadataCustomPayload {
+    return containsLegacyCustomPayloadDefinition(metadata)
+        ? getLegacyFabricMetadataCustomPayload(metadata)
+        : (metadata?.custom?.[ACTION_NAME] || {});
+}
+
+/**
+ * Checks if the metadata contains a legacy custom payload definition.
+ *
+ * @param metadata - The raw Fabric metadata.
+ *
+ * @returns A boolean indicating if the legacy custom payload definition is present.
+ */
+function containsLegacyCustomPayloadDefinition(metadata: RawFabricMetadata): boolean {
+    return !!metadata?.custom?.modmanager;
+}
+
+/**
+ * Gets the legacy custom payload from the Fabric metadata.
+ *
+ * @param metadata - The raw Fabric metadata.
+ *
+ * @returns The custom payload object.
+ */
+function _getLegacyFabricMetadataCustomPayload(metadata: RawFabricMetadata): FabricMetadataCustomPayload {
+    const modManagerPayload = metadata?.custom?.modmanager;
+    const basePayload = metadata?.custom?.[ACTION_NAME];
+    return { ...modManagerPayload, ...basePayload };
+}
+
+/**
+ * Gets the legacy custom payload from the Fabric metadata.
+ *
+ * @param metadata - The raw Fabric metadata.
+ *
+ * @returns The custom payload object.
+ *
+ * @deprecated
+ *
+ * Use `mc-publish` field instead of `modmanager` field.
+ */
+const getLegacyFabricMetadataCustomPayload = deprecate(
+    _getLegacyFabricMetadataCustomPayload,
+    "Use `mc-publish` field instead of `modmanager` field.",
+);
+
+/**
+ * A list of default mod loaders associated with the Fabric loader.
+ */
+const DEFAULT_LOADERS = [LoaderType.FABRIC] as const;
+
+/**
+ * Gets an array of supported mod loaders from the custom payload attached to the Fabric metadata.
+ *
+ * @param payload - The custom payload object.
+ *
+ * @returns An array of supported mod loaders.
+ */
+export function getLoadersFromFabricMetadataCustomPayload(payload: FabricMetadataCustomPayload): string[] {
+    if (containsLegacyLoadersDefinition(payload)) {
+        return getLegacyLoadersFromFabricMetadataCustomPayload(payload);
+    }
+
+    return payload?.loaders || [...DEFAULT_LOADERS];
+}
+
+/**
+ * Checks if the custom payload contains a legacy loaders definition.
+ *
+ * @param payload - The custom payload object.
+ *
+ * @returns A boolean indicating if the legacy loaders definition is present.
+ */
+function containsLegacyLoadersDefinition(payload: FabricMetadataCustomPayload): boolean {
+    return typeof payload?.quilt === "boolean";
+}
+
+/**
+ * Gets an array of supported mod loaders from the legacy custom payload attached to the Fabric metadata.
+ *
+ * @param payload - The legacy custom payload object.
+ *
+ * @returns An array of supported mod loaders.
+ */
+function _getLegacyLoadersFromFabricMetadataCustomPayload(payload: FabricMetadataCustomPayload): string[] {
+    return payload?.quilt ? [LoaderType.FABRIC, LoaderType.QUILT] : [...DEFAULT_LOADERS];
+}
+
+/**
+ * Gets an array of supported mod loaders from the legacy custom payload attached to the Fabric metadata.
+ *
+ * @param payload - The legacy custom payload object.
+ *
+ * @returns An array of supported mod loaders.
+ *
+ * @deprecated
+ *
+ * Use the universal `"loaders": ["fabric", "quilt"]` field instead of `"quilt": true`.
+ */
+const getLegacyLoadersFromFabricMetadataCustomPayload = deprecate(
+    _getLegacyLoadersFromFabricMetadataCustomPayload,
+    "Use the universal `\"loaders\": [\"fabric\", \"quilt\"]` field instead of `\"quilt\": true`",
+);
+
+/**
+ * Gets the dependencies from the custom payload attached to the Fabric metadata.
+ *
+ * @param payload - The custom payload object.
+ *
+ * @returns An array of dependencies included into the custom payload.
+ */
+export function getDependenciesFromFabricMetadataCustomPayload(payload: FabricMetadataCustomPayload): Dependency[] {
+    if (!Array.isArray(payload?.dependencies)) {
+        return [];
+    }
+
+    return payload?.dependencies?.map(x => createDependency(x)).filter(x => x) || [];
+}
+
+/**
+ * Gets the project ID from the custom payload attached to the Fabric metadata based on the given platform.
+ *
+ * @param payload - The custom payload object.
+ * @param platform - The platform for which the project ID is required.
+ *
+ * @returns The project ID as a string, or `undefined` if not found.
+ */
+export function getProjectIdFromFabricMetadataCustomPayload(payload: FabricMetadataCustomPayload, platform: PlatformType): string | undefined {
+    const id = payload?.[platform];
+    return id ? asString(id) : undefined;
+}