Support classpath groups when using configure on demand. (#1392)

* Support classpath groups when using configure on demand.

* Cleanup

* Work around Gradle 8.14 issue

* Another fix

* Rename plugin

* Fix plugin versioning

* Add some docs

* More fixes

* Ensure backwards compatible.

build.gradle

@@ -237,6 +237,10 @@ gradlePlugin {
 			id = 'fabric-loom'
 			implementationClass = 'net.fabricmc.loom.LoomGradlePlugin'
 		}
+		fabricLoomCompanion {
+			id = 'net.fabricmc.fabric-loom-companion'
+			implementationClass = 'net.fabricmc.loom.LoomCompanionGradlePlugin'
+		}
 	}
 }
 
@@ -292,25 +296,27 @@ publishing {
 				from components.java
 			}
 
-			// Manually crate the plugin marker for snapshot versions
-			snapshotPlugin(MavenPublication) { publication ->
-				groupId = 'fabric-loom'
-				artifactId = 'fabric-loom.gradle.plugin'
-				version = baseVersion + '-SNAPSHOT'
+			gradlePlugin.plugins.forEach { plugin ->
+				// Manually crate the plugin marker for snapshot versions
+				it.create(plugin.id + "SnapshotMarker", MavenPublication) { publication ->
+					groupId = plugin.id
+					artifactId = plugin.id + '.gradle.plugin'
+					version = baseVersion + '-SNAPSHOT'
 
-				pom.withXml({
-					// Based off org.gradle.plugin.devel.plugins.MavenPluginPublishPlugin
-					Element root = asElement()
-					Document document = root.getOwnerDocument()
-					Node dependencies = root.appendChild(document.createElement('dependencies'))
-					Node dependency = dependencies.appendChild(document.createElement('dependency'))
-					Node groupId = dependency.appendChild(document.createElement('groupId'))
-					groupId.setTextContent('net.fabricmc')
-					Node artifactId = dependency.appendChild(document.createElement('artifactId'))
-					artifactId.setTextContent('fabric-loom')
-					Node version = dependency.appendChild(document.createElement('version'))
-					version.setTextContent(baseVersion + '-SNAPSHOT')
-				})
+					pom.withXml({
+						// Based off org.gradle.plugin.devel.plugins.MavenPluginPublishPlugin
+						Element root = asElement()
+						Document document = root.getOwnerDocument()
+						Node dependencies = root.appendChild(document.createElement('dependencies'))
+						Node dependency = dependencies.appendChild(document.createElement('dependency'))
+						Node groupId = dependency.appendChild(document.createElement('groupId'))
+						groupId.setTextContent('net.fabricmc')
+						Node artifactId = dependency.appendChild(document.createElement('artifactId'))
+						artifactId.setTextContent('fabric-loom')
+						Node version = dependency.appendChild(document.createElement('version'))
+						version.setTextContent(project.version)
+					})
+				}
 			}
 		}
 	}

src/main/java/net/fabricmc/loom/LoomCompanionGradlePlugin.java

@@ -0,0 +1,44 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2025 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom;
+
+import org.gradle.api.Plugin;
+import org.gradle.api.Project;
+import org.jetbrains.annotations.NotNull;
+
+import net.fabricmc.loom.configuration.LoomConfigurations;
+import net.fabricmc.loom.task.launch.ExportClasspathTask;
+import net.fabricmc.loom.util.Constants;
+
+public class LoomCompanionGradlePlugin implements Plugin<Project> {
+	public static final String NAME = "net.fabricmc.fabric-loom-companion";
+
+	@Override
+	public void apply(@NotNull Project project) {
+		var exportClassPathTask = project.getTasks().register(Constants.Task.EXPORT_CLASSPATH, ExportClasspathTask.class);
+		project.getConfigurations().register(Constants.Configurations.EXPORTED_CLASSPATH, LoomConfigurations.Role.CONSUMABLE::apply);
+		project.artifacts(artifactHandler -> artifactHandler.add(Constants.Configurations.EXPORTED_CLASSPATH, exportClassPathTask));
+	}
+}

src/main/java/net/fabricmc/loom/LoomGradlePlugin.java

@@ -50,6 +50,7 @@ import net.fabricmc.loom.task.RemapTaskConfiguration;
 import net.fabricmc.loom.util.LibraryLocationLogger;
 
 public class LoomGradlePlugin implements Plugin<PluginAware> {
+	public static final String NAME = "fabric-loom";
 	public static final Gson GSON = new GsonBuilder().setPrettyPrinting().create();
 	public static final String LOOM_VERSION = Objects.requireNonNullElse(LoomGradlePlugin.class.getPackage().getImplementationVersion(), "0.0.0+unknown");
 
@@ -92,5 +93,7 @@ public class LoomGradlePlugin implements Plugin<PluginAware> {
 		for (Class<? extends Runnable> jobClass : SETUP_JOBS) {
 			project.getObjects().newInstance(jobClass).run();
 		}
+
+		project.apply(Map.of("plugin", LoomCompanionGradlePlugin.NAME));
 	}
 }

src/main/java/net/fabricmc/loom/api/ModSettings.java

@@ -24,6 +24,10 @@
 
 package net.fabricmc.loom.api;
 
+import java.io.File;
+import java.util.List;
+import java.util.Map;
+
 import javax.inject.Inject;
 
 import org.gradle.api.Named;
@@ -35,6 +39,8 @@ import org.gradle.api.provider.ListProperty;
 import org.gradle.api.tasks.SourceSet;
 import org.jetbrains.annotations.ApiStatus;
 
+import net.fabricmc.loom.LoomCompanionGradlePlugin;
+import net.fabricmc.loom.configuration.classpathgroups.ExternalClasspathGroup;
 import net.fabricmc.loom.util.gradle.SourceSetHelper;
 import net.fabricmc.loom.util.gradle.SourceSetReference;
 
@@ -49,7 +55,7 @@ public abstract class ModSettings implements Named {
 
 	@Inject
 	public ModSettings() {
-		getModSourceSets().finalizeValueOnRead();
+		getExternalGroups().finalizeValueOnRead();
 		getModFiles().finalizeValueOnRead();
 	}
 
@@ -82,18 +88,46 @@ public abstract class ModSettings implements Named {
 
 	/**
 	 * Add {@link SourceSet}'s output directories from the supplied project to be grouped with the named mod.
+	 * @deprecated Replaced with {@link #sourceSet(String, String)} to avoid passing a project reference.
 	 */
+	@Deprecated
 	public void sourceSet(SourceSet sourceSet, Project project) {
-		getModSourceSets().add(new SourceSetReference(sourceSet, project));
+		ensureCompanion(project);
+
+		sourceSet(sourceSet.getName(), project.getPath());
 	}
 
 	/**
 	 * Add {@link SourceSet}'s output directories from the supplied project to be grouped with the named mod.
 	 *
 	 * @param name the name of the source set
+	 * @deprecated Replaced with {@link #sourceSet(String, String)} to avoid passing a project reference.
 	 */
+	@Deprecated
 	public void sourceSet(String name, Project project) {
-		sourceSet(SourceSetHelper.getSourceSetByName(name, project), project);
+		ensureCompanion(project);
+
+		sourceSet(name, project.getPath());
+	}
+
+	/**
+	 * Add {@link SourceSet}'s output directories from the supplied project to be grouped with the named mod.
+	 *
+	 * <p>If the other project is not a Loom project you must apply the `net.fabricmc.fabric-loom-companion` plugin.
+	 *
+	 * @param sourceSetName the name of the source set
+	 * @param projectPath the path of the project the source set belongs to
+	 */
+	public void sourceSet(String sourceSetName, String projectPath) {
+		if (projectPath.equals(getProject().getPath())) {
+			// Shortcut for source sets in our own project.
+			SourceSetReference ref = new SourceSetReference(SourceSetHelper.getSourceSetByName(sourceSetName, getProject()), getProject());
+			List<File> classpath = SourceSetHelper.getClasspath(ref);
+			getModFiles().from(classpath);
+			return;
+		}
+
+		getExternalGroups().add(new ExternalClasspathGroup(projectPath, sourceSetName));
 	}
 
 	/**
@@ -114,11 +148,10 @@ public abstract class ModSettings implements Named {
 	}
 
 	/**
-	 * List of classpath directories, used to populate the `fabric.classPathGroups` Fabric Loader system property.
-	 * Use the {@link ModSettings#sourceSet} methods to add to this.
+	 * List of {@link ExternalClasspathGroup} that will later be resolved to populate the classpath groups from another Gradle project.
 	 */
 	@ApiStatus.Internal
-	public abstract ListProperty<SourceSetReference> getModSourceSets();
+	public abstract ListProperty<ExternalClasspathGroup> getExternalGroups();
 
 	@Inject
 	public abstract Project getProject();
@@ -127,4 +160,12 @@ public abstract class ModSettings implements Named {
 	public String toString() {
 		return "ModSettings '" + getName() + "'";
 	}
+
+	private void ensureCompanion(Project project) {
+		if (project == getProject()) {
+			return;
+		}
+
+		project.apply(Map.of("plugin", LoomCompanionGradlePlugin.NAME));
+	}
 }

src/main/java/net/fabricmc/loom/configuration/CompileConfiguration.java

@@ -26,7 +26,6 @@ package net.fabricmc.loom.configuration;
 
 import static net.fabricmc.loom.util.Constants.Configurations;
 
-import java.io.File;
 import java.io.IOException;
 import java.io.UncheckedIOException;
 import java.nio.charset.StandardCharsets;
@@ -36,12 +35,13 @@ import java.time.Duration;
 import java.util.Objects;
 import java.util.Optional;
 import java.util.function.Consumer;
-import java.util.stream.Collectors;
 
 import javax.inject.Inject;
 
+import org.gradle.api.Action;
 import org.gradle.api.GradleException;
 import org.gradle.api.Project;
+import org.gradle.api.Task;
 import org.gradle.api.logging.Logger;
 import org.gradle.api.logging.Logging;
 import org.gradle.api.plugins.JavaPlugin;
@@ -71,6 +71,7 @@ import net.fabricmc.loom.configuration.providers.minecraft.mapped.AbstractMapped
 import net.fabricmc.loom.configuration.providers.minecraft.mapped.IntermediaryMinecraftProvider;
 import net.fabricmc.loom.configuration.providers.minecraft.mapped.NamedMinecraftProvider;
 import net.fabricmc.loom.extension.MixinExtension;
+import net.fabricmc.loom.task.service.ClasspathGroupService;
 import net.fabricmc.loom.util.Checksum;
 import net.fabricmc.loom.util.ExceptionUtil;
 import net.fabricmc.loom.util.ProcessUtil;
@@ -266,15 +267,22 @@ public abstract class CompileConfiguration implements Runnable {
 		}
 
 		getProject().getTasks().named(JavaPlugin.TEST_TASK_NAME, Test.class, test -> {
-			String classPathGroups = extension.getMods().stream()
-					.map(modSettings ->
-							SourceSetHelper.getClasspath(modSettings, getProject()).stream()
-									.map(File::getAbsolutePath)
-									.collect(Collectors.joining(File.pathSeparator))
-					)
-					.collect(Collectors.joining(File.pathSeparator+File.pathSeparator));;
+			test.getInputs().property("LoomClassPathGroups", ClasspathGroupService.create(getProject()));
+			test.doFirst(new Action<Task>() {
+				@Override
+				public void execute(Task task) {
+					try (ScopedServiceFactory serviceFactory = new ScopedServiceFactory()) {
+						var options = (ClasspathGroupService.Options) task.getInputs().getProperties().get("LoomClassPathGroups");
+						ClasspathGroupService classpathGroupService = serviceFactory.get(options);
 
-			test.systemProperty("fabric.classPathGroups", classPathGroups);
+						if (classpathGroupService.hasGroups()) {
+							test.systemProperty("fabric.classPathGroups", classpathGroupService.getClasspathGroupsPropertyValue());
+						}
+					} catch (IOException e) {
+						throw new UncheckedIOException("Failed to get classpath groups", e);
+					}
+				}
+			});
 		});
 	}
 

src/main/java/net/fabricmc/loom/configuration/LoomConfigurations.java

@@ -175,7 +175,7 @@ public abstract class LoomConfigurations implements Runnable {
 		getConfigurations().getByName(a, configuration -> configuration.extendsFrom(getConfigurations().getByName(b)));
 	}
 
-	enum Role {
+	public enum Role {
 		NONE(false, false),
 		CONSUMABLE(true, false),
 		RESOLVABLE(false, true);
@@ -188,7 +188,7 @@ public abstract class LoomConfigurations implements Runnable {
 			this.canBeResolved = canBeResolved;
 		}
 
-		void apply(Configuration configuration) {
+		public void apply(Configuration configuration) {
 			configuration.setCanBeConsumed(canBeConsumed);
 			configuration.setCanBeResolved(canBeResolved);
 		}

src/main/java/net/fabricmc/loom/configuration/classpathgroups/ClasspathGroup.java

@@ -0,0 +1,53 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2025 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.configuration.classpathgroups;
+
+import java.io.File;
+import java.io.Serializable;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Set;
+
+import net.fabricmc.loom.api.ModSettings;
+
+public record ClasspathGroup(List<String> paths, List<ExternalClasspathGroup> externalGroups) implements Serializable {
+	public static List<ClasspathGroup> fromModSettings(Set<ModSettings> modSettings) {
+		return modSettings.stream().map(s -> new ClasspathGroup(getPaths(s), s.getExternalGroups().get())).toList();
+	}
+
+	// TODO remove this constructor when updating to Gradle 9.0, works around an issue where config cache cannot serialize immutable lists
+	public ClasspathGroup(List<String> paths, List<ExternalClasspathGroup> externalGroups) {
+		this.paths = new ArrayList<>(paths);
+		this.externalGroups = new ArrayList<>(externalGroups);
+	}
+
+	private static List<String> getPaths(ModSettings modSettings) {
+		return modSettings.getModFiles()
+				.getFiles()
+				.stream()
+				.map(File::getAbsolutePath)
+				.toList();
+	}
+}

src/main/java/net/fabricmc/loom/configuration/classpathgroups/ExternalClasspathGroup.java

@@ -0,0 +1,30 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2025 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.configuration.classpathgroups;
+
+import java.io.Serializable;
+
+public record ExternalClasspathGroup(String projectPath, String sourceSetName) implements Serializable {
+}

src/main/java/net/fabricmc/loom/configuration/classpathgroups/ExternalClasspathGroupDTO.java

@@ -0,0 +1,87 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2025 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.configuration.classpathgroups;
+
+import java.io.File;
+import java.io.IOException;
+import java.io.Serializable;
+import java.io.UncheckedIOException;
+import java.nio.file.Files;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+
+import org.gradle.api.Project;
+import org.gradle.api.tasks.SourceSet;
+import org.gradle.api.tasks.SourceSetContainer;
+
+import net.fabricmc.loom.LoomGradlePlugin;
+import net.fabricmc.loom.util.gradle.SourceSetHelper;
+import net.fabricmc.loom.util.gradle.SourceSetReference;
+
+/**
+ * This object is exported by projects as a json file to be consumed by others to correctly populate the classpath groups.
+ */
+public record ExternalClasspathGroupDTO(String projectPath, Map<String, List<String>> classpaths) implements Serializable {
+	public static ExternalClasspathGroupDTO createFromProject(Project project) {
+		SourceSetContainer sourceSets = SourceSetHelper.getSourceSets(project);
+
+		Map<String, List<String>> classpaths = new HashMap<>();
+
+		for (SourceSet sourceSet : sourceSets) {
+			SourceSetReference ref = new SourceSetReference(sourceSet, project);
+			List<File> classpath = SourceSetHelper.getClasspath(ref);
+			classpaths.put(sourceSet.getName(), classpath.stream().map(File::getAbsolutePath).toList());
+		}
+
+		return new ExternalClasspathGroupDTO(project.getPath(), Collections.unmodifiableMap(classpaths));
+	}
+
+	public static Map<String, ExternalClasspathGroupDTO> resolveExternal(Set<File> files) {
+		Map<String, ExternalClasspathGroupDTO> map = new HashMap<>();
+
+		for (File file : files) {
+			String json;
+
+			try {
+				json = Files.readString(file.toPath());
+			} catch (IOException e) {
+				throw new UncheckedIOException("Failed to read external classpath group file: " + file, e);
+			}
+
+			ExternalClasspathGroupDTO dto = LoomGradlePlugin.GSON.fromJson(json, ExternalClasspathGroupDTO.class);
+			map.put(dto.projectPath(), dto);
+		}
+
+		return Collections.unmodifiableMap(map);
+	}
+
+	public List<String> getForSourceSet(String sourceSetName) {
+		return Objects.requireNonNull(classpaths.get(sourceSetName), "No classpath found for source set: " + sourceSetName);
+	}
+}

src/main/java/net/fabricmc/loom/task/launch/ExportClasspathTask.java

@@ -0,0 +1,93 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2025 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.task.launch;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+
+import javax.inject.Inject;
+
+import org.gradle.api.UncheckedIOException;
+import org.gradle.api.file.RegularFileProperty;
+import org.gradle.api.provider.Property;
+import org.gradle.api.tasks.Input;
+import org.gradle.api.tasks.OutputFile;
+import org.gradle.api.tasks.TaskAction;
+import org.gradle.workers.WorkAction;
+import org.gradle.workers.WorkParameters;
+import org.gradle.workers.WorkQueue;
+import org.gradle.workers.WorkerExecutor;
+
+import net.fabricmc.loom.LoomGradlePlugin;
+import net.fabricmc.loom.configuration.classpathgroups.ExternalClasspathGroupDTO;
+import net.fabricmc.loom.task.AbstractLoomTask;
+
+public abstract class ExportClasspathTask extends AbstractLoomTask {
+	@Input
+	public abstract Property<String> getClasspathDtoJson();
+
+	@OutputFile
+	public abstract RegularFileProperty getOutput();
+
+	@Inject
+	protected abstract WorkerExecutor getWorkerExecutor();
+
+	@Inject
+	public ExportClasspathTask() {
+		getClasspathDtoJson().set(getProject()
+				.provider(() -> ExternalClasspathGroupDTO.createFromProject(getProject()))
+				.map(LoomGradlePlugin.GSON::toJson));
+		getOutput().set(getProject().getLayout().getBuildDirectory().file("export_classpath.json"));
+	}
+
+	@TaskAction
+	public void run() {
+		final WorkQueue workQueue = getWorkerExecutor().noIsolation();
+		workQueue.submit(ExportClassPathWorkAction.class, p -> {
+			p.getClasspathDtoJson().set(getClasspathDtoJson());
+			p.getOutput().set(getOutput());
+		});
+	}
+
+	protected interface ExportClassPathWorkParameters extends WorkParameters {
+		Property<String> getClasspathDtoJson();
+		RegularFileProperty getOutput();
+	}
+
+	public abstract static class ExportClassPathWorkAction implements WorkAction<ExportClassPathWorkParameters> {
+		@Override
+		public void execute() {
+			File outputFile = getParameters().getOutput().getAsFile().get();
+			String json = getParameters().getClasspathDtoJson().get();
+
+			try {
+				Files.writeString(outputFile.toPath(), json);
+			} catch (IOException e) {
+				throw new UncheckedIOException("Failed to write classpath groups", e);
+			}
+		}
+	}
+}

src/main/java/net/fabricmc/loom/task/launch/GenerateDLIConfigTask.java

@@ -42,6 +42,7 @@ import org.gradle.api.logging.configuration.ConsoleOutput;
 import org.gradle.api.provider.Property;
 import org.gradle.api.tasks.Input;
 import org.gradle.api.tasks.InputFile;
+import org.gradle.api.tasks.Nested;
 import org.gradle.api.tasks.Optional;
 import org.gradle.api.tasks.OutputFile;
 import org.gradle.api.tasks.TaskAction;
@@ -51,7 +52,8 @@ import net.fabricmc.loom.LoomGradlePlugin;
 import net.fabricmc.loom.configuration.providers.minecraft.MinecraftVersionMeta;
 import net.fabricmc.loom.configuration.providers.minecraft.mapped.MappedMinecraftProvider;
 import net.fabricmc.loom.task.AbstractLoomTask;
-import net.fabricmc.loom.util.gradle.SourceSetHelper;
+import net.fabricmc.loom.task.service.ClasspathGroupService;
+import net.fabricmc.loom.util.service.ScopedServiceFactory;
 
 public abstract class GenerateDLIConfigTask extends AbstractLoomTask {
 	@Input
@@ -69,10 +71,6 @@ public abstract class GenerateDLIConfigTask extends AbstractLoomTask {
 	@Input
 	protected abstract Property<Boolean> getANSISupportedIDE();
 
-	@Input
-	@Optional
-	protected abstract Property<String> getClassPathGroups();
-
 	@Input
 	protected abstract Property<String> getLog4jConfigPaths();
 
@@ -96,16 +94,16 @@ public abstract class GenerateDLIConfigTask extends AbstractLoomTask {
 	@OutputFile
 	protected abstract RegularFileProperty getDevLauncherConfig();
 
+	@Nested
+	protected abstract Property<ClasspathGroupService.Options> getClasspathGroupOptions();
+
 	public GenerateDLIConfigTask() {
 		getVersionInfoJson().set(LoomGradlePlugin.GSON.toJson(getExtension().getMinecraftProvider().getVersionInfo()));
 		getMinecraftVersion().set(getExtension().getMinecraftProvider().minecraftVersion());
 		getSplitSourceSets().set(getExtension().areEnvironmentSourceSetsSplit());
 		getANSISupportedIDE().set(ansiSupportedIde(getProject()));
 		getPlainConsole().set(getProject().getGradle().getStartParameter().getConsoleOutput() == ConsoleOutput.Plain);
-
-		if (!getExtension().getMods().isEmpty()) {
-			getClassPathGroups().set(buildClassPathGroups(getProject()));
-		}
+		getClasspathGroupOptions().set(ClasspathGroupService.create(getProject()));
 
 		getLog4jConfigPaths().set(getAllLog4JConfigFiles(getProject()));
 
@@ -152,8 +150,12 @@ public abstract class GenerateDLIConfigTask extends AbstractLoomTask {
 			launchConfig.property("fabric.gameJarPath", getCommonGameJarPath().get());
 		}
 
-		if (getClassPathGroups().isPresent()) {
-			launchConfig.property("fabric.classPathGroups", getClassPathGroups().get());
+		try (ScopedServiceFactory serviceFactory = new ScopedServiceFactory()) {
+			ClasspathGroupService classpathGroupService = serviceFactory.get(getClasspathGroupOptions());
+
+			if (classpathGroupService.hasGroups()) {
+				launchConfig.property("fabric.classPathGroups", classpathGroupService.getClasspathGroupsPropertyValue());
+			}
 		}
 
 		//Enable ansi by default for idea and vscode when gradle is not ran with plain console.
@@ -180,19 +182,6 @@ public abstract class GenerateDLIConfigTask extends AbstractLoomTask {
 		};
 	}
 
-	/**
-	 * See: https://github.com/FabricMC/fabric-loader/pull/585.
-	 */
-	private static String buildClassPathGroups(Project project) {
-		return LoomGradleExtension.get(project).getMods().stream()
-				.map(modSettings ->
-						SourceSetHelper.getClasspath(modSettings, project).stream()
-							.map(File::getAbsolutePath)
-							.collect(Collectors.joining(File.pathSeparator))
-				)
-				.collect(Collectors.joining(File.pathSeparator+File.pathSeparator));
-	}
-
 	private static boolean ansiSupportedIde(Project project) {
 		File rootDir = project.getRootDir();
 		return new File(rootDir, ".vscode").exists()

src/main/java/net/fabricmc/loom/task/service/ClasspathGroupService.java

@@ -0,0 +1,162 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2025 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.task.service;
+
+import java.io.File;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.function.Supplier;
+import java.util.stream.Collectors;
+
+import org.gradle.api.NamedDomainObjectContainer;
+import org.gradle.api.Project;
+import org.gradle.api.artifacts.Configuration;
+import org.gradle.api.artifacts.Dependency;
+import org.gradle.api.file.ConfigurableFileCollection;
+import org.gradle.api.provider.ListProperty;
+import org.gradle.api.provider.Provider;
+import org.gradle.api.tasks.Input;
+import org.gradle.api.tasks.InputFiles;
+import org.gradle.api.tasks.Optional;
+
+import net.fabricmc.loom.LoomGradleExtension;
+import net.fabricmc.loom.api.ModSettings;
+import net.fabricmc.loom.configuration.classpathgroups.ClasspathGroup;
+import net.fabricmc.loom.configuration.classpathgroups.ExternalClasspathGroup;
+import net.fabricmc.loom.configuration.classpathgroups.ExternalClasspathGroupDTO;
+import net.fabricmc.loom.util.Constants;
+import net.fabricmc.loom.util.Lazy;
+import net.fabricmc.loom.util.service.Service;
+import net.fabricmc.loom.util.service.ServiceFactory;
+import net.fabricmc.loom.util.service.ServiceType;
+
+public class ClasspathGroupService extends Service<ClasspathGroupService.Options> {
+	public static ServiceType<Options, ClasspathGroupService> TYPE = new ServiceType<Options, ClasspathGroupService>(Options.class, ClasspathGroupService.class);
+
+	public interface Options extends Service.Options {
+		@Input
+		@Optional
+		ListProperty<ClasspathGroup> getClasspathGroups();
+
+		@InputFiles
+		@Optional
+		ConfigurableFileCollection getExternalClasspathGroups();
+	}
+
+	public static Provider<Options> create(Project project) {
+		return TYPE.create(project, options -> {
+			LoomGradleExtension extension = LoomGradleExtension.get(project);
+			NamedDomainObjectContainer<ModSettings> modSettings = extension.getMods();
+
+			if (modSettings.isEmpty()) {
+				return;
+			}
+
+			options.getClasspathGroups().set(ClasspathGroup.fromModSettings(modSettings));
+
+			if (!hasExternalClasspathGroups(modSettings)) {
+				return;
+			}
+
+			List<Dependency> externalDependencies = getExternalDependencies(project, modSettings);
+			Configuration externalClasspathGroups = project.getConfigurations().detachedConfiguration(externalDependencies.toArray(new Dependency[0]));
+			options.getExternalClasspathGroups().from(externalClasspathGroups);
+		});
+	}
+
+	private static boolean hasExternalClasspathGroups(Set<ModSettings> modSettings) {
+		return modSettings.stream()
+			.anyMatch(s ->
+				s.getExternalGroups().isPresent()
+				&& !s.getExternalGroups().get().isEmpty()
+		);
+	}
+
+	private static List<Dependency> getExternalDependencies(Project project, Set<ModSettings> modSettings) {
+		List<String> requiredProjects = modSettings.stream()
+				.flatMap(s -> s.getExternalGroups().get().stream())
+				.map(ExternalClasspathGroup::projectPath)
+				.distinct()
+				.toList();
+
+		List<Dependency> dependencies = new ArrayList<>();
+
+		for (String projectPath : requiredProjects) {
+			Dependency externalDependency = project.getDependencies()
+					.project(Map.of(
+							"path", projectPath,
+							"configuration", Constants.Configurations.EXPORTED_CLASSPATH
+					));
+			dependencies.add(externalDependency);
+		}
+
+		return Collections.unmodifiableList(dependencies);
+	}
+
+	private final Supplier<Map<String, ExternalClasspathGroupDTO>> externalClasspathGroups = Lazy.of(() -> ExternalClasspathGroupDTO.resolveExternal(getOptions().getExternalClasspathGroups().getFiles()));
+
+	public ClasspathGroupService(Options options, ServiceFactory serviceFactory) {
+		super(options, serviceFactory);
+	}
+
+	public List<File> getClasspath(ClasspathGroup classpathGroup) {
+		final List<String> paths = new ArrayList<>();
+
+		for (ExternalClasspathGroup externalGroup : classpathGroup.externalGroups()) {
+			ExternalClasspathGroupDTO dto = externalClasspathGroups.get().get(externalGroup.projectPath());
+
+			if (dto == null) {
+				throw new IllegalStateException("Could not find resolved external classpath group for project: " + externalGroup.projectPath());
+			}
+
+			paths.addAll(dto.getForSourceSet(externalGroup.sourceSetName()));
+		}
+
+		paths.addAll(classpathGroup.paths());
+
+		return paths.stream().map(File::new).toList();
+	}
+
+	/**
+	 * See: https://github.com/FabricMC/fabric-loader/pull/585.
+	 */
+	public String getClasspathGroupsPropertyValue() {
+		return getOptions().getClasspathGroups().get()
+				.stream()
+				.map(group ->
+					getClasspath(group).stream()
+						.map(File::getAbsolutePath)
+						.collect(Collectors.joining(File.pathSeparator))
+				)
+				.collect(Collectors.joining(File.pathSeparator+File.pathSeparator));
+	}
+
+	public boolean hasGroups() {
+		return getOptions().getClasspathGroups().isPresent() && !getOptions().getClasspathGroups().get().isEmpty();
+	}
+}

src/main/java/net/fabricmc/loom/util/Constants.java

@@ -90,6 +90,10 @@ public class Constants {
 		 * Mods to be used by {@link net.fabricmc.loom.task.prod.AbstractProductionRunTask} tasks by default.
 		 */
 		public static final String PRODUCTION_RUNTIME_MODS = "productionRuntimeMods";
+		/**
+		 * Used to query classpath data across project boundaries.
+		 */
+		public static final String EXPORTED_CLASSPATH = "loomExportedClasspath";
 
 		private Configurations() {
 		}
@@ -125,6 +129,7 @@ public class Constants {
 
 	public static final class Task {
 		public static final String PROCESS_INCLUDE_JARS = "processIncludeJars";
+		public static final String EXPORT_CLASSPATH = "exportClasspath";
 
 		private Task() {
 		}

src/main/java/net/fabricmc/loom/util/gradle/GradleUtils.java

@@ -33,6 +33,7 @@ import org.gradle.api.invocation.Gradle;
 import org.gradle.api.provider.Provider;
 
 import net.fabricmc.loom.LoomGradleExtension;
+import net.fabricmc.loom.LoomGradlePlugin;
 
 public final class GradleUtils {
 	private GradleUtils() {
@@ -59,7 +60,7 @@ public final class GradleUtils {
 	}
 
 	public static boolean isLoomProject(Project project) {
-		return project.getPluginManager().hasPlugin("fabric-loom");
+		return project.getPluginManager().hasPlugin(LoomGradlePlugin.NAME);
 	}
 
 	public static Provider<Boolean> getBooleanPropertyProvider(Project project, String key) {

src/main/java/net/fabricmc/loom/util/gradle/SourceSetHelper.java

@@ -52,7 +52,6 @@ import org.jetbrains.annotations.VisibleForTesting;
 import org.xml.sax.InputSource;
 
 import net.fabricmc.loom.LoomGradleExtension;
-import net.fabricmc.loom.api.ModSettings;
 import net.fabricmc.loom.configuration.ide.idea.IdeaUtils;
 import net.fabricmc.loom.util.Constants;
 
@@ -113,18 +112,8 @@ public final class SourceSetHelper {
 		return it.hasNext() ? it.next().getProject() : null;
 	}
 
-	public static List<File> getClasspath(ModSettings modSettings, Project project) {
-		final List<File> files = new ArrayList<>();
-
-		files.addAll(modSettings.getModSourceSets().get().stream()
-				.flatMap(sourceSet -> getClasspath(sourceSet, project).stream())
-				.toList());
-		files.addAll(modSettings.getModFiles().getFiles());
-
-		return Collections.unmodifiableList(files);
-	}
-
-	public static List<File> getClasspath(SourceSetReference reference, Project project) {
+	public static List<File> getClasspath(SourceSetReference reference) {
+		final Project project = reference.project();
 		final List<File> classpath = getGradleClasspath(reference, project);
 
 		classpath.addAll(getIdeaClasspath(reference, project));

src/test/groovy/net/fabricmc/loom/test/integration/MultiProjectTest.groovy

@@ -57,4 +57,19 @@ class MultiProjectTest extends Specification implements GradleProjectTestTrait {
 		where:
 		version << STANDARD_TEST_VERSIONS
 	}
+
+	@Unroll
+	def "classpath groups (gradle #version)"() {
+		setup:
+		def gradle = gradleProject(project: "multiproject", version: version)
+
+		when:
+		def result = gradle.run(tasks: [":generateDLIConfig",])
+
+		then:
+		result.task(":generateDLIConfig").outcome == SUCCESS
+
+		where:
+		version << STANDARD_TEST_VERSIONS
+	}
 }

src/test/resources/projects/multiproject/build.gradle

@@ -5,6 +5,8 @@ plugins {
 }
 
 allprojects {
+	if (it.path == ":javalib") return;
+
 	apply plugin: "fabric-loom"
 
 	version = "1.0.0"
@@ -42,13 +44,14 @@ allprojects {
 loom {
 	mods {
 		core {
-			sourceSet project(':core').sourceSets.main
+			sourceSet("main", ":core")
 		}
 		example {
-			sourceSet project(':example').sourceSets.main
+			sourceSet("main", ":example")
 		}
 		root {
 			sourceSet sourceSets.main
+			sourceSet("main", ":javalib")
 		}
 	}
 }

src/test/resources/projects/multiproject/javalib/build.gradle

@@ -0,0 +1,4 @@
+plugins {
+	id 'java-library'
+	id 'net.fabricmc.fabric-loom-companion'
+}

src/test/resources/projects/multiproject/settings.gradle

@@ -1,4 +1,5 @@
 rootProject.name = "fabric-example-mod"
 
 include 'core'
-include 'example'
+include 'example'
+include 'javalib'