diff --git a/README.md b/README.md
index 2d5f1ac..8e38d47 100644
--- a/README.md
+++ b/README.md
@@ -59,8 +59,24 @@ flowchart LR
     Backup --> Bucket[(Object Storage)]
 ```
 
+Inside the app, each feature package is a module. The dependencies between them are
+declared in each `package-info.java` and checked by a test (Spring Modulith): no cycles,
+and nothing beyond the list.
+
+```mermaid
+flowchart BT
+    catalog --> auth & store
+    reviews --> auth & store
+    stats --> auth & catalog & store
+    collection --> catalog & reviews & store
+    alerts --> auth & collection & store
+    publicpage --> auth & stats
+```
+
 ## Engineering highlights
 
+- **Architecture as a test:** Spring Modulith checks that the feature modules form no
+  cycle and use only the modules each one declares.
 - **Tests against recorded store responses:** WireMock serves real, anonymized responses
   of each store; a weekly workflow runs the same adapters against the live stores to
   catch changes. Testcontainers runs the database tests on a real PostgreSQL. A JaCoCo
diff --git a/docs/decisions/0002-modular-monolith.md b/docs/decisions/0002-modular-monolith.md
index 0320e6a..1cb68ff 100644
--- a/docs/decisions/0002-modular-monolith.md
+++ b/docs/decisions/0002-modular-monolith.md
@@ -14,12 +14,20 @@ One Spring Boot application (a monolith), with one package per feature:
 `store`, `collection`, `stats`, `reviews`, `alerts`, `publicpage`, `auth`.
 Each package owns its classes; others use it only through its public services.
 
+Since 2026-10-28 this is checked, not only intended: Spring Modulith treats each
+top-level package as a module, and `ModularityTests` fails on a cycle between modules
+or on a dependency a module's `package-info.java` doesn't allow
+(`@ApplicationModule(allowedDependencies = ...)`). `store` and `auth` depend on no other
+module; the others list theirs.
+
 ## Consequences
 
 - One thing to build, test, deploy and debug.
 - The packages can become separate services later if one part needs to scale on its own.
 - Packaging by feature (not by layer: `controllers/`, `services/`, `repositories/`)
   keeps everything about reviews in one place.
+- A new dependency between features is a one-line change to a `package-info.java`,
+  visible in review, instead of an import nobody notices.
 
 ## Alternatives considered
 
diff --git a/pom.xml b/pom.xml
index 892b9ac..b53e6ce 100644
--- a/pom.xml
+++ b/pom.xml
@@ -31,6 +31,7 @@
 		<jsoup.version>1.23.2</jsoup.version>
 		<resilience4j.version>2.4.0</resilience4j.version>
 		<springdoc.version>3.1.1</springdoc.version>
+		<spring-modulith.version>2.1.1</spring-modulith.version>
 		<!-- springdoc 3.1.1 brings Swagger UI 5.32.14, whose DOMPurify (3.4.13) has known
 		     vulnerabilities (ZAP flagged it); 5.33.2 updates it to 3.4.16. Drop this once
 		     springdoc brings a newer Swagger UI itself. -->
@@ -50,6 +51,13 @@
 	</properties>
 	<dependencyManagement>
 		<dependencies>
+			<dependency>
+				<groupId>org.springframework.modulith</groupId>
+				<artifactId>spring-modulith-bom</artifactId>
+				<version>${spring-modulith.version}</version>
+				<type>pom</type>
+				<scope>import</scope>
+			</dependency>
 			<dependency>
 				<groupId>org.webjars</groupId>
 				<artifactId>swagger-ui</artifactId>
@@ -194,6 +202,17 @@
 			<version>${wiremock.version}</version>
 			<scope>test</scope>
 		</dependency>
+		<!-- @ApplicationModule in each package-info: which modules a module may use. -->
+		<dependency>
+			<groupId>org.springframework.modulith</groupId>
+			<artifactId>spring-modulith-api</artifactId>
+		</dependency>
+		<!-- ModularityTests: checks those rules and that modules form no cycle. -->
+		<dependency>
+			<groupId>org.springframework.modulith</groupId>
+			<artifactId>spring-modulith-starter-test</artifactId>
+			<scope>test</scope>
+		</dependency>
 	</dependencies>
 
 	<build>
diff --git a/src/main/java/com/mccprogrammer/extpulse/alerts/package-info.java b/src/main/java/com/mccprogrammer/extpulse/alerts/package-info.java
index 2a07b0b..9b94c5a 100644
--- a/src/main/java/com/mccprogrammer/extpulse/alerts/package-info.java
+++ b/src/main/java/com/mccprogrammer/extpulse/alerts/package-info.java
@@ -2,5 +2,10 @@
  * Alerts: rules a user sets (low ratings, install spikes, new versions, a listing that
  * stopped being collected, a weekly summary), checked after every collection and sent by
  * email or to a Discord channel, never twice for the same thing.
+ * <p>
+ * May use only auth, collection, store (checked by {@code ModularityTests}).
  */
+@ApplicationModule(allowedDependencies = { "auth", "collection", "store" })
 package com.mccprogrammer.extpulse.alerts;
+
+import org.springframework.modulith.ApplicationModule;
diff --git a/src/main/java/com/mccprogrammer/extpulse/auth/package-info.java b/src/main/java/com/mccprogrammer/extpulse/auth/package-info.java
index b3bc506..512a23c 100644
--- a/src/main/java/com/mccprogrammer/extpulse/auth/package-info.java
+++ b/src/main/java/com/mccprogrammer/extpulse/auth/package-info.java
@@ -1,4 +1,9 @@
 /**
  * Accounts, login and JWT tokens (Spring Security).
+ * <p>
+ * Depends on nothing else in ExtPulse.
  */
+@ApplicationModule(allowedDependencies = {})
 package com.mccprogrammer.extpulse.auth;
+
+import org.springframework.modulith.ApplicationModule;
diff --git a/src/main/java/com/mccprogrammer/extpulse/catalog/package-info.java b/src/main/java/com/mccprogrammer/extpulse/catalog/package-info.java
index 2f66b60..cb58d06 100644
--- a/src/main/java/com/mccprogrammer/extpulse/catalog/package-info.java
+++ b/src/main/java/com/mccprogrammer/extpulse/catalog/package-info.java
@@ -1,5 +1,10 @@
 /**
  * What ExtPulse tracks: publishers, their extensions and each extension's listings on the
  * stores. The other packages read listings from here.
+ * <p>
+ * May use only auth, store (checked by {@code ModularityTests}).
  */
+@ApplicationModule(allowedDependencies = { "auth", "store" })
 package com.mccprogrammer.extpulse.catalog;
+
+import org.springframework.modulith.ApplicationModule;
diff --git a/src/main/java/com/mccprogrammer/extpulse/collection/package-info.java b/src/main/java/com/mccprogrammer/extpulse/collection/package-info.java
index a879801..d71962c 100644
--- a/src/main/java/com/mccprogrammer/extpulse/collection/package-info.java
+++ b/src/main/java/com/mccprogrammer/extpulse/collection/package-info.java
@@ -1,5 +1,10 @@
 /**
  * Daily collection: the scheduled job that asks every store adapter for stats and
  * reviews, saves one snapshot per listing per day and records each run.
+ * <p>
+ * May use only catalog, reviews, store (checked by {@code ModularityTests}).
  */
+@ApplicationModule(allowedDependencies = { "catalog", "reviews", "store" })
 package com.mccprogrammer.extpulse.collection;
+
+import org.springframework.modulith.ApplicationModule;
diff --git a/src/main/java/com/mccprogrammer/extpulse/publicpage/package-info.java b/src/main/java/com/mccprogrammer/extpulse/publicpage/package-info.java
index c0ccb5d..3fa939e 100644
--- a/src/main/java/com/mccprogrammer/extpulse/publicpage/package-info.java
+++ b/src/main/java/com/mccprogrammer/extpulse/publicpage/package-info.java
@@ -1,4 +1,9 @@
 /**
  * Public portfolio page and the combined installs badge, served without login.
+ * <p>
+ * May use only auth, stats (checked by {@code ModularityTests}).
  */
+@ApplicationModule(allowedDependencies = { "auth", "stats" })
 package com.mccprogrammer.extpulse.publicpage;
+
+import org.springframework.modulith.ApplicationModule;
diff --git a/src/main/java/com/mccprogrammer/extpulse/reviews/package-info.java b/src/main/java/com/mccprogrammer/extpulse/reviews/package-info.java
index 66a0b04..fca1286 100644
--- a/src/main/java/com/mccprogrammer/extpulse/reviews/package-info.java
+++ b/src/main/java/com/mccprogrammer/extpulse/reviews/package-info.java
@@ -1,4 +1,9 @@
 /**
  * Reviews from every store in one inbox, with answered / unanswered status.
+ * <p>
+ * May use only auth, store (checked by {@code ModularityTests}).
  */
+@ApplicationModule(allowedDependencies = { "auth", "store" })
 package com.mccprogrammer.extpulse.reviews;
+
+import org.springframework.modulith.ApplicationModule;
diff --git a/src/main/java/com/mccprogrammer/extpulse/stats/package-info.java b/src/main/java/com/mccprogrammer/extpulse/stats/package-info.java
index 4b05a3e..75d7bca 100644
--- a/src/main/java/com/mccprogrammer/extpulse/stats/package-info.java
+++ b/src/main/java/com/mccprogrammer/extpulse/stats/package-info.java
@@ -1,5 +1,10 @@
 /**
  * Statistics read from the saved snapshots: combined totals, daily and weekly gains,
  * split by store.
+ * <p>
+ * May use only auth, catalog, store (checked by {@code ModularityTests}).
  */
+@ApplicationModule(allowedDependencies = { "auth", "catalog", "store" })
 package com.mccprogrammer.extpulse.stats;
+
+import org.springframework.modulith.ApplicationModule;
diff --git a/src/main/java/com/mccprogrammer/extpulse/store/package-info.java b/src/main/java/com/mccprogrammer/extpulse/store/package-info.java
index c2b6c5f..57a13ef 100644
--- a/src/main/java/com/mccprogrammer/extpulse/store/package-info.java
+++ b/src/main/java/com/mccprogrammer/extpulse/store/package-info.java
@@ -1,5 +1,10 @@
 /**
  * Store adapters: one {@code StoreSource} per store (VS Code Marketplace, Open VSX,
  * Chrome Web Store). The only code that talks to the stores.
+ * <p>
+ * Depends on nothing else in ExtPulse.
  */
+@ApplicationModule(allowedDependencies = {})
 package com.mccprogrammer.extpulse.store;
+
+import org.springframework.modulith.ApplicationModule;
diff --git a/src/test/java/com/mccprogrammer/extpulse/ModularityTests.java b/src/test/java/com/mccprogrammer/extpulse/ModularityTests.java
new file mode 100644
index 0000000..d69c3c6
--- /dev/null
+++ b/src/test/java/com/mccprogrammer/extpulse/ModularityTests.java
@@ -0,0 +1,20 @@
+package com.mccprogrammer.extpulse;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.modulith.core.ApplicationModules;
+
+/**
+ * The package-per-feature structure, checked by Spring Modulith: each top-level package
+ * is a module, modules form no cycle, and a module uses only the ones its package-info
+ * allows ({@code @ApplicationModule(allowedDependencies = ...)}). A new dependency
+ * between features is then a change to that list, made on purpose and reviewed.
+ */
+class ModularityTests {
+
+	@Test
+	void modulesUseOnlyTheModulesTheyAreAllowedTo() {
+		ApplicationModules.of(ExtpulseApplication.class).verify();
+	}
+
+}