diff --git a/core-it-suite/src/test/java/org/apache/maven/it/MavenITgh13911ReactorAnnotationProcessorTest.java b/core-it-suite/src/test/java/org/apache/maven/it/MavenITgh13911ReactorAnnotationProcessorTest.java
new file mode 100644
index 000000000..fe7d7a504
--- /dev/null
+++ b/core-it-suite/src/test/java/org/apache/maven/it/MavenITgh13911ReactorAnnotationProcessorTest.java
@@ -0,0 +1,63 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.maven.it;
+
+import java.io.File;
+
+import org.apache.maven.shared.verifier.Verifier;
+import org.apache.maven.shared.verifier.util.ResourceExtractor;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Integration test for GH-13911.
+ *
+ * Verifies that an annotation processor (AP) built from source in the same Maven reactor
+ * can be used by a sibling module during its {@code compile} phase. The POM {@code provided}
+ * dependency on the AP module gives Maven the ordering information it needs; no special
+ * reactor-plugin detection (MNG-1088 class) is required.
+ */
+class MavenITgh13911ReactorAnnotationProcessorTest extends AbstractMavenIntegrationTestCase {
+
+ MavenITgh13911ReactorAnnotationProcessorTest() {
+ // Annotation processors via provided dep ordering works since Maven 3.0
+ super("[3.0,)");
+ }
+
+ /**
+ * Verify that Maven correctly orders the AP module before the client module and that the
+ * annotation processor generates its output file.
+ *
+ * @throws Exception in case of failure
+ */
+ @Test
+ void testReactorAnnotationProcessor() throws Exception {
+ File testDir = ResourceExtractor.simpleExtractResources(getClass(), "/gh-13911-reactor-ap");
+
+ Verifier verifier = newVerifier(testDir.getAbsolutePath());
+ verifier.setAutoclean(false);
+ verifier.deleteDirectory("client/target");
+ verifier.deleteArtifacts("org.apache.maven.its.gh13911");
+ verifier.addCliArgument("compile");
+ verifier.execute();
+ verifier.verifyErrorFreeLog();
+
+ // The annotation processor must have run and written its marker resource
+ verifier.verifyFilePresent("client/target/classes/META-INF/generated-by-gh13911.txt");
+ }
+}
diff --git a/core-it-suite/src/test/resources/gh-13911-reactor-ap/client/pom.xml b/core-it-suite/src/test/resources/gh-13911-reactor-ap/client/pom.xml
new file mode 100644
index 000000000..a64ca3258
--- /dev/null
+++ b/core-it-suite/src/test/resources/gh-13911-reactor-ap/client/pom.xml
@@ -0,0 +1,68 @@
+
+
+
+ 4.0.0
+
+
+ org.apache.maven.its.gh13911
+ aggregator
+ 1.0-SNAPSHOT
+
+
+ client
+ jar
+
+ Maven Integration Test :: GH-13911 :: Client (uses reactor AP)
+ Uses the annotation processor built from source in the sibling module.
+ The provided dependency on the processor triggers Maven's reactor ordering.
+
+
+
+ org.apache.maven.its.gh13911
+ processor
+ 1.0-SNAPSHOT
+ provided
+
+
+
+
+
+
+ org.apache.maven.plugins
+ maven-compiler-plugin
+
+
+
+
+ org.apache.maven.its.gh13911
+ processor
+ 1.0-SNAPSHOT
+
+
+
+
+
+
+
diff --git a/core-it-suite/src/test/resources/gh-13911-reactor-ap/client/src/main/java/org/apache/maven/its/gh13911/ClientClass.java b/core-it-suite/src/test/resources/gh-13911-reactor-ap/client/src/main/java/org/apache/maven/its/gh13911/ClientClass.java
new file mode 100644
index 000000000..f5e09fe00
--- /dev/null
+++ b/core-it-suite/src/test/resources/gh-13911-reactor-ap/client/src/main/java/org/apache/maven/its/gh13911/ClientClass.java
@@ -0,0 +1,25 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.maven.its.gh13911;
+
+/**
+ * Client class annotated with the reactor-built annotation processor's annotation.
+ */
+@Gh13911Generated
+public class ClientClass {}
diff --git a/core-it-suite/src/test/resources/gh-13911-reactor-ap/pom.xml b/core-it-suite/src/test/resources/gh-13911-reactor-ap/pom.xml
new file mode 100644
index 000000000..be9a1e1ac
--- /dev/null
+++ b/core-it-suite/src/test/resources/gh-13911-reactor-ap/pom.xml
@@ -0,0 +1,41 @@
+
+
+
+ 4.0.0
+
+ org.apache.maven.its.gh13911
+ aggregator
+ 1.0-SNAPSHOT
+ pom
+
+ Maven Integration Test :: GH-13911 :: Reactor Annotation Processor
+ Verifies that an annotation processor built in the same Maven reactor can be used by a sibling
+ module during its compile phase.
+
+
+ processor
+ client
+
+
+
+ 8
+ 8
+
+
diff --git a/core-it-suite/src/test/resources/gh-13911-reactor-ap/processor/pom.xml b/core-it-suite/src/test/resources/gh-13911-reactor-ap/processor/pom.xml
new file mode 100644
index 000000000..2222738e4
--- /dev/null
+++ b/core-it-suite/src/test/resources/gh-13911-reactor-ap/processor/pom.xml
@@ -0,0 +1,33 @@
+
+
+
+ 4.0.0
+
+
+ org.apache.maven.its.gh13911
+ aggregator
+ 1.0-SNAPSHOT
+
+
+ processor
+ jar
+
+ Maven Integration Test :: GH-13911 :: Annotation Processor
+
diff --git a/core-it-suite/src/test/resources/gh-13911-reactor-ap/processor/src/main/java/org/apache/maven/its/gh13911/Gh13911Generated.java b/core-it-suite/src/test/resources/gh-13911-reactor-ap/processor/src/main/java/org/apache/maven/its/gh13911/Gh13911Generated.java
new file mode 100644
index 000000000..63c5aa368
--- /dev/null
+++ b/core-it-suite/src/test/resources/gh-13911-reactor-ap/processor/src/main/java/org/apache/maven/its/gh13911/Gh13911Generated.java
@@ -0,0 +1,31 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.maven.its.gh13911;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Marker annotation processed by {@link Gh13911Processor} at compile time.
+ */
+@Target(ElementType.TYPE)
+@Retention(RetentionPolicy.SOURCE)
+public @interface Gh13911Generated {}
diff --git a/core-it-suite/src/test/resources/gh-13911-reactor-ap/processor/src/main/java/org/apache/maven/its/gh13911/Gh13911Processor.java b/core-it-suite/src/test/resources/gh-13911-reactor-ap/processor/src/main/java/org/apache/maven/its/gh13911/Gh13911Processor.java
new file mode 100644
index 000000000..a33a52210
--- /dev/null
+++ b/core-it-suite/src/test/resources/gh-13911-reactor-ap/processor/src/main/java/org/apache/maven/its/gh13911/Gh13911Processor.java
@@ -0,0 +1,64 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.maven.its.gh13911;
+
+import javax.annotation.processing.AbstractProcessor;
+import javax.annotation.processing.RoundEnvironment;
+import javax.annotation.processing.SupportedAnnotationTypes;
+import javax.annotation.processing.SupportedSourceVersion;
+import javax.lang.model.SourceVersion;
+import javax.lang.model.element.TypeElement;
+import javax.tools.Diagnostic;
+import javax.tools.FileObject;
+import javax.tools.StandardLocation;
+
+import java.io.IOException;
+import java.io.Writer;
+import java.util.Set;
+
+/**
+ * Annotation processor for the GH-13911 integration test.
+ *
+ * When it finds any type annotated with {@link Gh13911Generated}, it writes a resource file
+ * {@code META-INF/generated-by-gh13911.txt} to the class output directory.
+ */
+@SupportedAnnotationTypes("org.apache.maven.its.gh13911.Gh13911Generated")
+@SupportedSourceVersion(SourceVersion.RELEASE_8)
+public class Gh13911Processor extends AbstractProcessor {
+
+ @Override
+ public boolean process(Set annotations, RoundEnvironment roundEnv) {
+ if (annotations.isEmpty() || roundEnv.processingOver()) {
+ return false;
+ }
+ try {
+ FileObject resource = processingEnv
+ .getFiler()
+ .createResource(StandardLocation.CLASS_OUTPUT, "", "META-INF/generated-by-gh13911.txt");
+ try (Writer writer = resource.openWriter()) {
+ writer.write("[GH-13911] annotation processor ran successfully\n");
+ }
+ } catch (IOException e) {
+ processingEnv
+ .getMessager()
+ .printMessage(Diagnostic.Kind.ERROR, "[GH-13911] failed to write marker: " + e.getMessage());
+ }
+ return true;
+ }
+}
diff --git a/core-it-suite/src/test/resources/gh-13911-reactor-ap/processor/src/main/resources/META-INF/services/javax.annotation.processing.Processor b/core-it-suite/src/test/resources/gh-13911-reactor-ap/processor/src/main/resources/META-INF/services/javax.annotation.processing.Processor
new file mode 100644
index 000000000..5efdbd70a
--- /dev/null
+++ b/core-it-suite/src/test/resources/gh-13911-reactor-ap/processor/src/main/resources/META-INF/services/javax.annotation.processing.Processor
@@ -0,0 +1 @@
+org.apache.maven.its.gh13911.Gh13911Processor