diff --git a/backend-java/src/test/java/com/nanri/aiimage/metrics/DualInstanceMetricTest.java b/backend-java/src/test/java/com/nanri/aiimage/metrics/DualInstanceMetricTest.java
new file mode 100644
index 00000000..33afab6b
--- /dev/null
+++ b/backend-java/src/test/java/com/nanri/aiimage/metrics/DualInstanceMetricTest.java
@@ -0,0 +1,96 @@
+package com.nanri.aiimage.metrics;
+
+import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
+import org.junit.jupiter.api.Test;
+
+import java.util.stream.Collectors;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * task-191：双实例指标区分回归契约（spec 11 §1：instanceId 区分 server-110/server-121）。
+ * 同一 registry 下两实例同模块指标按 instanceId 分桶、不串；未配置 instanceId 归 unknown；
+ * 指标名集合稳定一致。
+ */
+class DualInstanceMetricTest {
+
+    private final SimpleMeterRegistry registry = new SimpleMeterRegistry();
+
+    @Test
+    void instance110LabelApplied() {
+        new TaskObservabilityMetrics("server-110", registry).taskCreated("similarasin");
+        assertEquals("server-110",
+                registry.get("aiimage.task.created").counter().getId().getTag("instanceId"));
+    }
+
+    @Test
+    void instance121LabelApplied() {
+        new TaskObservabilityMetrics("server-121", registry).taskCreated("similarasin");
+        assertEquals("server-121",
+                registry.get("aiimage.task.created").counter().getId().getTag("instanceId"));
+    }
+
+    @Test
+    void twoInstancesDoNotCrossCount() {
+        TaskObservabilityMetrics a = new TaskObservabilityMetrics("server-110", registry);
+        TaskObservabilityMetrics b = new TaskObservabilityMetrics("server-121", registry);
+        a.taskCreated("similarasin");
+        b.taskCreated("similarasin");
+        b.taskCreated("similarasin");
+        assertEquals(1.0, registry.get("aiimage.task.created").tag("moduleType", "similarasin")
+                .tag("instanceId", "server-110").counter().count());
+        assertEquals(2.0, registry.get("aiimage.task.created").tag("moduleType", "similarasin")
+                .tag("instanceId", "server-121").counter().count());
+    }
+
+    @Test
+    void unconfiguredInstanceFallsBackToUnknown() {
+        new TaskObservabilityMetrics(null, registry).taskCreated("brand");
+        assertEquals("unknown",
+                registry.get("aiimage.task.created").counter().getId().getTag("instanceId"));
+        new TaskObservabilityMetrics("   ", registry).taskCreated("brand");
+    }
+
+    @Test
+    void metricsAggregatableAcrossInstances() {
+        TaskObservabilityMetrics a = new TaskObservabilityMetrics("server-110", registry);
+        TaskObservabilityMetrics b = new TaskObservabilityMetrics("server-121", registry);
+        a.taskTerminal("similarasin", "success");
+        b.taskTerminal("similarasin", "success");
+        // 聚合：跨 instanceId 各 meter 求和，双实例总数应为 2
+        double total = registry.find("aiimage.task.success").tag("moduleType", "similarasin").counters().stream()
+                .mapToDouble(io.micrometer.core.instrument.Counter::count).sum();
+        assertEquals(2.0, total, "同模块跨实例指标可聚合");
+    }
+
+    @Test
+    void instanceLabelStableAcrossCalls() {
+        TaskObservabilityMetrics a = new TaskObservabilityMetrics("server-110", registry);
+        a.taskCreated("publish");
+        a.taskCreated("publish");
+        assertEquals(1, registry.find("aiimage.task.created").tag("moduleType", "publish")
+                .tag("instanceId", "server-110").counters().size(), "同实例同模块应为同一 meter");
+        assertEquals(2.0, registry.get("aiimage.task.created").tag("moduleType", "publish")
+                .tag("instanceId", "server-110").counter().count());
+    }
+
+    @Test
+    void instanceIdGetterReturnsConfiguredValue() {
+        assertEquals("server-121", new TaskObservabilityMetrics("server-121", null).instanceId());
+        assertNotEquals("", new TaskObservabilityMetrics("server-121", null).instanceId());
+    }
+
+    @Test
+    void metricNameSetIdenticalAcrossInstances() {
+        TaskObservabilityMetrics a = new TaskObservabilityMetrics("server-110", registry);
+        TaskObservabilityMetrics b = new TaskObservabilityMetrics("server-121", registry);
+        a.taskCreated("x");
+        b.taskCreated("x");
+        java.util.Set<String> names = registry.getMeters().stream()
+                .map(m -> m.getId().getName()).collect(Collectors.toSet());
+        assertTrue(names.contains("aiimage.task.created"));
+        assertEquals(1, names.size(), "仅指标名集合应一致（标签不同不算漂移）");
+    }
+}