diff --git a/build.gradle b/build.gradle
index 163d1818e4..7406db8017 100644
--- a/build.gradle
+++ b/build.gradle
@@ -165,7 +165,7 @@ dependencies {
 	implementation libs.night.config.toml
 
 	// Legacy Forge patches
-	implementation libs.lzma
+	implementation libs.xz
 
 	// Testing
 	testImplementation(gradleTestKit())
diff --git a/gradle/libs.versions.toml b/gradle/libs.versions.toml
index af191eda4e..ea4aa3c137 100644
--- a/gradle/libs.versions.toml
+++ b/gradle/libs.versions.toml
@@ -28,7 +28,7 @@ forge-diffpatch = "2.0.7"
 night-config = "3.6.6"
 datafixerupper = "6.0.8"
 at = "1.0.1"
-lzma = "1.3"
+xz = "1.8"
 
 [libraries]
 # Loom compile libraries
@@ -63,7 +63,7 @@ forge-diffpatch = { module = "net.minecraftforge:DiffPatch", version.ref = "forg
 night-config-toml = { module = "com.electronwill.night-config:toml", version.ref = "night-config" }
 datafixerupper = { module = "com.mojang:datafixerupper", version.ref = "datafixerupper" }
 at = { module = "dev.architectury:at", version.ref = "at" }
-lzma = { module = "com.github.jponge:lzma-java", version.ref = "lzma" }
+xz = { module = "org.tukaani:xz", version.ref = "xz" }
 
 [plugins]
 kotlin = { id = "org.jetbrains.kotlin.jvm", version.ref = "kotlin" }
diff --git a/src/main/java/net/fabricmc/loom/LoomGradleExtension.java b/src/main/java/net/fabricmc/loom/LoomGradleExtension.java
index 24f1852b3c..064a1252b8 100644
--- a/src/main/java/net/fabricmc/loom/LoomGradleExtension.java
+++ b/src/main/java/net/fabricmc/loom/LoomGradleExtension.java
@@ -178,8 +178,12 @@ default boolean isForgeLikeAndNotOfficial() {
 		return isForgeLike() && !getMcpConfigProvider().isOfficial();
 	}
 
+	default int getForgeSpec() {
+		return getForgeUserdevProvider().getForgeSpec();
+	}
+
 	default boolean isLegacyForge() {
-		return isForge() && getForgeUserdevProvider().isLegacyForge();
+		return isForge() && getForgeSpec() <= 1;
 	}
 
 	default boolean isModernForge() {
diff --git a/src/main/java/net/fabricmc/loom/configuration/CompileConfiguration.java b/src/main/java/net/fabricmc/loom/configuration/CompileConfiguration.java
index cb0f47c3ca..db4250c165 100644
--- a/src/main/java/net/fabricmc/loom/configuration/CompileConfiguration.java
+++ b/src/main/java/net/fabricmc/loom/configuration/CompileConfiguration.java
@@ -227,13 +227,13 @@ private synchronized void setupMinecraft(ConfigContext configContext) throws Exc
 		// but before MinecraftPatchedProvider.provide.
 		setupDependencyProviders(project, extension);
 
-		if (extension.isLegacyForge()) {
+		if (extension.getForgeSpec() <= 2) {
 			extension.setIntermediateMappingsProvider(GeneratedIntermediateMappingsProvider.class, provider -> {
 				provider.minecraftProvider = minecraftProvider;
 			});
 		}
 
-		if (extension.isForgeLike() && !extension.isLegacyForge()) {
+		if (extension.isForgeLike() && extension.getForgeSpec() > 2) {
 			// Excluded on legacy forge because it pulls in a log4j-api version newer than what forge wants and we don't
 			// need it anyway
 			project.getDependencies().add(Configurations.FORGE_EXTRA, LoomVersions.UNPROTECT.mavenNotation());
diff --git a/src/main/java/net/fabricmc/loom/configuration/providers/forge/ForgeMigratedMappingConfiguration.java b/src/main/java/net/fabricmc/loom/configuration/providers/forge/ForgeMigratedMappingConfiguration.java
index ee4c568bee..1c0bcd6f66 100644
--- a/src/main/java/net/fabricmc/loom/configuration/providers/forge/ForgeMigratedMappingConfiguration.java
+++ b/src/main/java/net/fabricmc/loom/configuration/providers/forge/ForgeMigratedMappingConfiguration.java
@@ -53,7 +53,7 @@ public ForgeMigratedMappingConfiguration(String mappingsIdentifier, Path mapping
 	protected void manipulateMappings(Project project, Path mappingsJar) throws IOException {
 		LoomGradleExtension extension = LoomGradleExtension.get(project);
 
-		if (extension.isLegacyForge()) {
+		if (extension.getForgeSpec() <= 2) {
 			// Legacy forge patches are in official namespace, so if the type of a field is changed by them, then that
 			// is effectively a new field and not traceable to any mapping. Therefore this does not apply to it.
 			return;
diff --git a/src/main/java/net/fabricmc/loom/configuration/providers/forge/ForgeUserdevProvider.java b/src/main/java/net/fabricmc/loom/configuration/providers/forge/ForgeUserdevProvider.java
index e6ac6dc61a..02b18b1a21 100644
--- a/src/main/java/net/fabricmc/loom/configuration/providers/forge/ForgeUserdevProvider.java
+++ b/src/main/java/net/fabricmc/loom/configuration/providers/forge/ForgeUserdevProvider.java
@@ -53,7 +53,7 @@ public class ForgeUserdevProvider extends DependencyProvider {
 	private JsonObject json;
 	private UserdevConfig config;
 	Path joinedPatches;
-	private Boolean isLegacyForge;
+	private Integer forgeSpec;
 
 	public ForgeUserdevProvider(Project project) {
 		super(project);
@@ -88,10 +88,13 @@ public void provide(DependencyInfo dependency) throws Exception {
 
 		try (Reader reader = Files.newBufferedReader(configJson)) {
 			json = new Gson().fromJson(reader, JsonObject.class);
-			isLegacyForge = !json.has("mcp");
+			// Some Forge versions for 1.13.2 specify mcp, but have spec=1. We just "hack" this here.
+			forgeSpec = json.has("mcp") ? Math.max(2, json.get("spec").getAsInt()) : 1;
 
-			if (isLegacyForge) {
+			if (forgeSpec <= 1) {
 				json = createManifestFromForgeGradle2(dependency, json);
+			} else if (forgeSpec <= 2) {
+				addLegacyMCPRepo();
 			}
 
 			config = UserdevConfig.CODEC.parse(JsonOps.INSTANCE, json)
@@ -106,7 +109,7 @@ public void provide(DependencyInfo dependency) throws Exception {
 
 		addDependency(config.universal(), Constants.Configurations.FORGE_UNIVERSAL);
 
-		if (!isLegacyForge && Files.notExists(joinedPatches)) {
+		if (forgeSpec >= 2 && Files.notExists(joinedPatches)) {
 			Files.write(joinedPatches, ZipUtils.unpack(userdevJar.toPath(), config.binpatches()));
 		}
 	}
@@ -177,6 +180,10 @@ private static JsonObject createLegacyRuns() {
 	private static JsonArray createLegacyAts() {
 		JsonArray array = new JsonArray();
 		array.add("merged_at.cfg");
+		array.add("forge_at.cfg");
+		array.add("fml_at.cfg");
+		array.add("src/main/resources/forge_at.cfg");
+		array.add("src/main/resources/fml_at.cfg");
 		return array;
 	}
 
@@ -217,12 +224,12 @@ private void addLegacyMCPRepo() {
 		});
 	}
 
-	public boolean isLegacyForge() {
-		if (isLegacyForge == null) {
+	public int getForgeSpec() {
+		if (forgeSpec == null) {
 			throw new IllegalArgumentException("Not yet resolved.");
 		}
 
-		return isLegacyForge;
+		return forgeSpec;
 	}
 
 	public File getUserdevJar() {
diff --git a/src/main/java/net/fabricmc/loom/configuration/providers/forge/PatchProvider.java b/src/main/java/net/fabricmc/loom/configuration/providers/forge/PatchProvider.java
index 313709a551..043c6b7573 100644
--- a/src/main/java/net/fabricmc/loom/configuration/providers/forge/PatchProvider.java
+++ b/src/main/java/net/fabricmc/loom/configuration/providers/forge/PatchProvider.java
@@ -43,12 +43,11 @@
 import java.util.jar.JarOutputStream;
 import java.util.zip.ZipEntry;
 
-import lzma.sdk.lzma.Decoder;
-import lzma.sdk.lzma.Encoder;
-import lzma.streams.LzmaInputStream;
-import lzma.streams.LzmaOutputStream;
 import org.apache.commons.io.IOUtils;
 import org.gradle.api.Project;
+import org.tukaani.xz.LZMA2Options;
+import org.tukaani.xz.LZMAInputStream;
+import org.tukaani.xz.LZMAOutputStream;
 
 import net.fabricmc.loom.configuration.DependencyInfo;
 import net.fabricmc.loom.configuration.providers.forge.fg2.Pack200Provider;
@@ -100,11 +99,11 @@ private void init() {
 	private void splitAndConvertLegacyPatches(Path joinedLegacyPatches) throws IOException {
 		try (JarInputStream in = new JarInputStream(new ByteArrayInputStream(unpack200Lzma(joinedLegacyPatches)));
 				OutputStream clientFileOut = Files.newOutputStream(clientPatches, CREATE, TRUNCATE_EXISTING);
-				LzmaOutputStream clientLzmaOut = new LzmaOutputStream(clientFileOut, new Encoder());
+				LZMAOutputStream clientLzmaOut = new LZMAOutputStream(clientFileOut, new LZMA2Options(), -1);
 				JarOutputStream clientJarOut = new JarOutputStream(clientLzmaOut);
 				OutputStream serverFileOut = Files.newOutputStream(serverPatches, CREATE, TRUNCATE_EXISTING);
-				LzmaOutputStream serverLzmaOut = new LzmaOutputStream(serverFileOut, new Encoder());
-				JarOutputStream serverJarOut = new JarOutputStream(serverLzmaOut);
+				LZMAOutputStream serverLzmaOut = new LZMAOutputStream(serverFileOut, new LZMA2Options(), -1);
+				JarOutputStream serverJarOut = new JarOutputStream(serverLzmaOut)
 		) {
 			for (JarEntry entry; (entry = in.getNextJarEntry()) != null;) {
 				String name = entry.getName();
@@ -153,7 +152,7 @@ private byte[] unpack200(InputStream in) throws IOException {
 	}
 
 	private byte[] unpack200Lzma(InputStream in) throws IOException {
-		try (LzmaInputStream lzmaIn = new LzmaInputStream(in, new Decoder())) {
+		try (LZMAInputStream lzmaIn = new LZMAInputStream(in)) {
 			return unpack200(lzmaIn);
 		}
 	}
diff --git a/src/main/java/net/fabricmc/loom/configuration/providers/mappings/MappingConfiguration.java b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/MappingConfiguration.java
index f848151233..61673aaaca 100644
--- a/src/main/java/net/fabricmc/loom/configuration/providers/mappings/MappingConfiguration.java
+++ b/src/main/java/net/fabricmc/loom/configuration/providers/mappings/MappingConfiguration.java
@@ -319,7 +319,7 @@ private static void mergeSrg(Project project, Path source, Path target) throws I
 		LoomGradleExtension extension = LoomGradleExtension.get(project);
 
 		// FIXME why is this special case necessary?
-		ForgeMappingsMerger.ExtraMappings extraMappings = extension.isLegacyForge()
+		ForgeMappingsMerger.ExtraMappings extraMappings = extension.getForgeSpec() <= 2
 				? null
 				: ForgeMappingsMerger.ExtraMappings.ofMojmapTsrg(getMojmapSrgFileIfPossible(project));
 
@@ -419,7 +419,8 @@ private void readAndMergeMCP(Project project, ServiceFactory serviceFactory, Min
 
 	private boolean isMCP(Path path) throws IOException {
 		try (FileSystemUtil.Delegate fs = FileSystemUtil.getJarFileSystem(path, false)) {
-			return Files.exists(fs.getPath("fields.csv")) && Files.exists(fs.getPath("methods.csv"));
+			return Files.exists(fs.getPath("fields.csv")) && Files.exists(fs.getPath("methods.csv"))
+				|| (Files.exists(fs.getPath("conf/fields.csv")) && Files.exists(fs.getPath("conf/methods.csv")));
 		}
 	}
 
diff --git a/src/main/java/net/fabricmc/loom/decompilers/cache/ClassEntry.java b/src/main/java/net/fabricmc/loom/decompilers/cache/ClassEntry.java
index 4fe53faa70..64cbf8d67e 100644
--- a/src/main/java/net/fabricmc/loom/decompilers/cache/ClassEntry.java
+++ b/src/main/java/net/fabricmc/loom/decompilers/cache/ClassEntry.java
@@ -50,9 +50,10 @@ public record ClassEntry(String name, List<String> innerClasses, List<String> su
 			throw new IllegalArgumentException("Class name must end with '.class': " + name);
 		}
 
-		if (!name.contains("/")) {
-			throw new IllegalArgumentException("Class name must be in a package: " + name);
-		}
+		// breaks in some 1.7.10 classes:
+		// if (!name.contains("/")) {
+		// 	throw new IllegalArgumentException("Class name must be in a package: " + name);
+		// }
 
 		String className = name.replace(".class", "");
 
diff --git a/src/main/java/net/fabricmc/loom/util/srg/MCPReader.java b/src/main/java/net/fabricmc/loom/util/srg/MCPReader.java
index 19d576c11f..cd55ab16ab 100644
--- a/src/main/java/net/fabricmc/loom/util/srg/MCPReader.java
+++ b/src/main/java/net/fabricmc/loom/util/srg/MCPReader.java
@@ -194,8 +194,11 @@ private void injectMcp(Path mcpJar, Map<String, String> intermediaryToSrgMap, Ma
 
 		try (FileSystemUtil.Delegate fs = FileSystemUtil.getJarFileSystem(mcpJar, false)) {
 			Path fields = fs.getPath("fields.csv");
+			if (!Files.exists(fields)) fields = fs.getPath("conf/fields.csv");
 			Path methods = fs.getPath("methods.csv");
+			if (!Files.exists(methods)) methods = fs.getPath("conf/methods.csv");
 			Path params = fs.getPath("params.csv");
+			if (!Files.exists(params)) params = fs.getPath("conf/params.csv");
 			Pattern paramsPattern = Pattern.compile("p_[^\\d]*(\\d+)_(\\d)+_?");
 
 			try (CSVReader reader = new CSVReader(Files.newBufferedReader(fields, StandardCharsets.UTF_8))) {
diff --git a/src/test/groovy/net/fabricmc/loom/test/integration/forge/ForgeTest.groovy b/src/test/groovy/net/fabricmc/loom/test/integration/forge/ForgeTest.groovy
index 170c93d232..43e566e0fa 100644
--- a/src/test/groovy/net/fabricmc/loom/test/integration/forge/ForgeTest.groovy
+++ b/src/test/groovy/net/fabricmc/loom/test/integration/forge/ForgeTest.groovy
@@ -65,6 +65,7 @@ class ForgeTest extends Specification implements GradleProjectTestTrait {
 		'1.16.5'  | '36.2.4'     | '8'         | '"de.oceanlabs.mcp:mcp_snapshot:20210309-1.16.5"'
 		'1.14.4'  | "28.2.23"    | '8'         | "loom.officialMojangMappings()"
 		'1.14.4'  | "28.2.23"    | '8'         | '"net.fabricmc:yarn:1.14.4+build.18:v2"'
+		'1.13.2'  | "25.0.223"   | '8'         | '"de.oceanlabs.mcp:mcp_stable:47-1.13.2"'
 		'1.12.2'  | "14.23.0.2486" | '8'       | '"de.oceanlabs.mcp:mcp_snapshot:20170615-1.12"'
 		'1.8.9'   | "11.15.1.2318-1.8.9" | '8' | '"de.oceanlabs.mcp:mcp_stable:22-1.8.9"'
 	}