diff --git a/fleetd/pom.xml b/fleetd/pom.xml
index 8dd8be0..5f5f8b4 100644
--- a/fleetd/pom.xml
+++ b/fleetd/pom.xml
@@ -29,6 +29,7 @@
         <commons-compress.version>1.27.1</commons-compress.version>
         <commons-lang3.version>3.18.0</commons-lang3.version>
         <sqlite-jdbc.version>3.53.4.0</sqlite-jdbc.version>
+        <archunit.version>1.5.0</archunit.version>
     </properties>
 
     <!--
@@ -176,6 +177,14 @@
             <version>${testcontainers.version}</version>
             <scope>test</scope>
         </dependency>
+
+        <!-- fleetd #131: package-boundary and cycle enforcement (PackageCyclesTest). -->
+        <dependency>
+            <groupId>com.tngtech.archunit</groupId>
+            <artifactId>archunit-junit5</artifactId>
+            <version>${archunit.version}</version>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
     <build>
diff --git a/fleetd/src/test/java/dev/ltms/fleet/PackageCyclesTest.java b/fleetd/src/test/java/dev/ltms/fleet/PackageCyclesTest.java
new file mode 100644
index 0000000..46f7b3e
--- /dev/null
+++ b/fleetd/src/test/java/dev/ltms/fleet/PackageCyclesTest.java
@@ -0,0 +1,96 @@
+package dev.ltms.fleet;
+
+import com.tngtech.archunit.base.DescribedPredicate;
+import com.tngtech.archunit.core.domain.JavaClass;
+import com.tngtech.archunit.core.domain.JavaClass.Predicates;
+import com.tngtech.archunit.core.importer.ClassFileImporter;
+import com.tngtech.archunit.core.importer.ImportOption;
+import com.tngtech.archunit.library.dependencies.SliceRule;
+import com.tngtech.archunit.library.dependencies.SlicesRuleDefinition;
+import org.junit.jupiter.api.Test;
+
+/**
+ * fleetd #131 (CB-627): enforce package boundaries with an ArchUnit test instead of a
+ * Maven module split.
+ *
+ * <p>This test fails the build the moment a NEW cycle appears between the top-level
+ * {@code dev.ltms.fleet.*} packages. Today's cycles are recorded below as explicit,
+ * narrow exceptions: each one ignores dependencies between exactly the two named
+ * packages, in both directions, and nothing else. A cycle through any other pair of
+ * packages -- or a brand new pair -- still fails this test.
+ *
+ * <p><b>Main code only.</b> The import excludes test classes
+ * ({@link ImportOption.Predefined#DO_NOT_INCLUDE_TESTS}). Test code legitimately wires
+ * across many packages for setup and mocking; that is not part of the shipped
+ * architecture this rule protects. Verified: importing test classes too pulls in a much
+ * larger, noisier cycle set -- {@code herdr}, {@code member}, {@code peer}, {@code
+ * config}, {@code guard} and {@code placement} all show up in cycles that disappear the
+ * moment test classes are excluded. Scanning off the classpath via {@code
+ * importPackages(...)} (not a hardcoded {@code target/classes} path) also keeps this
+ * test correct regardless of the working directory the build is invoked from.
+ *
+ * <p><b>No package moves here</b> -- ticket #131 is explicit that removing a cycle is
+ * its own, later PR. See the comment on each exception below for which ticket step
+ * removes it.
+ */
+class PackageCyclesTest {
+
+    @Test
+    void packagesAreFreeOfCycles() {
+        var classes = new ClassFileImporter()
+                .withImportOption(ImportOption.Predefined.DO_NOT_INCLUDE_TESTS)
+                .importPackages("dev.ltms.fleet");
+
+        SliceRule rule = SlicesRuleDefinition.slices()
+                .matching("dev.ltms.fleet.(*)..")
+                .should().beFreeOfCycles();
+
+        // fleetd #131 step 1: move ConnectionIdentity so authz stops depending on the
+        // MCP layer. Evidence: auth/CallerResolver.java:3 imports mcp.ConnectionIdentity;
+        // mcp/FleetMcp.java:3-7 imports auth.AuditLog, Authz, CallerResolver, Principal,
+        // Role.
+        rule = ignoreCycle(rule, "auth", "mcp");
+
+        // fleetd #131 step 2: PrimaryRegistry is used by loops in msg; move it, or put
+        // an interface between msg and mcp. Evidence: msg/ReplyPushLoop.java:5 and
+        // msg/LeadHeartbeatLoop.java:5 import mcp.PrimaryRegistry; mcp/FleetMcp.java:15-18
+        // imports msg.LeadChannel, LeadMessage, MessageService, Rendezvous.
+        rule = ignoreCycle(rule, "mcp", "msg");
+
+        // fleetd #131 -- found while implementing this test, NOT one of the ticket's
+        // original three; it names its own follow-up step before removal. Evidence:
+        // inject/CompletionResolver.java:4-5, inject/Injector.java:6 and
+        // inject/TurnListener.java:3 import msg.Rendezvous / msg.TurnToken;
+        // msg/MessageService.java:6 imports inject.Injector.
+        rule = ignoreCycle(rule, "inject", "msg");
+
+        // fleetd #131 -- same as above, its own follow-up. Evidence:
+        // metrics/FleetMetrics.java:3 imports msg.ReplyInbox; msg/MessageService.java:7-8,
+        // msg/LeadHeartbeatLoop.java:6-7 and msg/ReplyPushLoop.java:6-7 import
+        // metrics.FleetMetrics / metrics.Metrics.
+        rule = ignoreCycle(rule, "metrics", "msg");
+
+        // fleetd #131 -- same as above, its own follow-up. Evidence:
+        // session/SessionManager.java:7 imports msg.TurnToken;
+        // msg/LeadHeartbeatLoop.java:8 imports session.MemberSession.
+        rule = ignoreCycle(rule, "msg", "session");
+
+        rule.check(classes);
+    }
+
+    /**
+     * Accepts today's known cycle between two top-level packages, and nothing else.
+     * Ignoring both directions removes exactly this pair from cycle detection; every
+     * other dependency -- including any new one added later, between these same two
+     * packages or any other pair -- is still checked.
+     */
+    private static SliceRule ignoreCycle(SliceRule rule, String packageA, String packageB) {
+        return rule
+                .ignoreDependency(residesIn(packageA), residesIn(packageB))
+                .ignoreDependency(residesIn(packageB), residesIn(packageA));
+    }
+
+    private static DescribedPredicate<JavaClass> residesIn(String topLevelPackage) {
+        return Predicates.resideInAPackage("dev.ltms.fleet." + topLevelPackage + "..");
+    }
+}