src/main/java/io/github/hectorvent/floci/services/cloudwatch/dashboards/CloudWatchDashboardsService.java

@@ -179,29 +179,82 @@ public class CloudWatchDashboardsService {
     }
 
     public Map<String, String> listTagsForResource(String resourceArn, String region) {
-        return findByArn(resourceArn, region).map(Dashboard::getTags).orElse(Map.of());
+        return requireDashboard(resourceArn, region).getTags();
     }
 
     public void tagResource(String resourceArn, Map<String, String> tags, String region) {
-        findByArn(resourceArn, region).ifPresent(dashboard -> {
-            dashboard.getTags().putAll(tags);
-            dashboardStore.put(key(region, dashboard.getDashboardName()), dashboard);
-        });
+        Dashboard dashboard = requireDashboard(resourceArn, region);
+        dashboard.getTags().putAll(tags);
+        dashboardStore.put(key(region, dashboard.getDashboardName()), dashboard);
     }
 
     public void untagResource(String resourceArn, List<String> tagKeys, String region) {
-        findByArn(resourceArn, region).ifPresent(dashboard -> {
-            tagKeys.forEach(dashboard.getTags()::remove);
-            dashboardStore.put(key(region, dashboard.getDashboardName()), dashboard);
-        });
+        Dashboard dashboard = requireDashboard(resourceArn, region);
+        tagKeys.forEach(dashboard.getTags()::remove);
+        dashboardStore.put(key(region, dashboard.getDashboardName()), dashboard);
+    }
+
+    /**
+     * Resolves the dashboard an ARN names, or reports that nothing does. The tag operations
+     * declare {@code ResourceNotFoundException}, which is a different shape from the
+     * {@code ResourceNotFound} that {@code GetDashboard} declares,
+     * so this is not the {@link #notFound(String)} used by the rest of this service.
+     */
+    private Dashboard requireDashboard(String resourceArn, String region) {
+        return findByArn(resourceArn, region).orElseThrow(() ->
+                new AwsException("ResourceNotFoundException",
+                        "Dashboard does not exist: " + resourceArn, 404));
     }
 
     private java.util.Optional<Dashboard> findByArn(String resourceArn, String region) {
         return dashboardStore.scan(k -> k.startsWith(region + "::")).stream()
-                .filter(d -> d.getDashboardArn() != null && d.getDashboardArn().equals(resourceArn))
+                .filter(d -> namesDashboard(resourceArn, d.getDashboardArn()))
                 .findFirst();
     }
 
+    /**
+     * Whether {@code candidate} is an ARN for the dashboard whose own ARN is {@code stored}.
+     *
+     * <p>Both the region-ful and the regionless form resolve, deliberately, because the two
+     * disagree here and a caller may hold either. Dashboards are a global resource and AWS
+     * documents their ARN without a region on both the TagResource and the ListTagsForResource
+     * pages, {@code arn:aws:cloudwatch::<account-id>:dashboard/<dashboard-name>}, while Floci
+     * mints a region-ful one. That divergence used to be invisible on this path: an ARN
+     * matching nothing was a silent success answering an empty tag map. Now that it is an
+     * error, a caller sending the documented ARN for a dashboard that exists would be told the
+     * dashboard does not, so the lookup accepts both shapes.
+     *
+     * <p>What Floci mints is left alone here on purpose. Clients may already hold the ARNs it
+     * has handed out, so bringing the minted shape into line with AWS is a separate change
+     * from making the lookup tolerant, and only the second one is needed to keep this path
+     * correct.
+     *
+     * <p>Only an absent region is accepted in place of the dashboard's own, so an ARN naming
+     * some other region is still a miss rather than resolving to a same-named dashboard
+     * elsewhere.
+     */
+    private static boolean namesDashboard(String candidate, String stored) {
+        if (candidate == null || stored == null) {
+            return false;
+        }
+        return candidate.equals(stored) || candidate.equals(withoutRegion(stored));
+    }
+
+    /**
+     * Blanks the region field of an ARN, turning
+     * {@code arn:aws:cloudwatch:us-east-1:123:dashboard/x} into
+     * {@code arn:aws:cloudwatch::123:dashboard/x}. An ARN with fewer than six fields is
+     * returned unchanged, since there is no region field to blank.
+     */
+    private static String withoutRegion(String arn) {
+        String[] fields = arn.split(":", 6);
+        if (fields.length < 6) {
+            return arn;
+        }
+        fields[3] = "";
+        return String.join(":", fields);
+    }
+
     private static String key(String region, String dashboardName) {
         return region + "::" + dashboardName;
     }

src/main/java/io/github/hectorvent/floci/services/cloudwatch/metrics/CloudWatchMetricsService.java

@@ -332,35 +332,43 @@ public class CloudWatchMetricsService {
         LOG.infov("SetAlarmState: {0} -> {1}", alarmName, stateValue);
     }
 
-    public Map<String, String> listTagsForResource(String resourceArn, String region) {
+    /**
+     * Resolves the alarm an ARN names, or reports that nothing does.
+     *
+     * <p>CloudWatch's three tag operations each declare {@code ResourceNotFoundException},
+     * which is a different shape from the {@code ResourceNotFound} that {@code SetAlarmState}
+     * and {@code GetDashboard} declare; the SDK maps the two codes to two exception classes,
+     * so the tag path uses the longer one rather than the code used elsewhere in this service.
+     *
+     * <p>The message names the ARN rather than asserting it was an alarm, because both tag
+     * handlers route every ARN that is not a dashboard and not a metric stream here. That
+     * includes kinds AWS considers taggable and Floci does not serve, a Contributor Insights
+     * {@code insight-rule/} ARN being the one AWS documents; reporting that no resource
+     * matches the ARN is true of those, where "alarm not found" would not be.
+     */
+    private MetricAlarm requireAlarm(String resourceArn, String region) {
         return alarmStore.scan(k -> k.startsWith(region + "::"))
                 .stream()
-                .filter(a -> resourceArn.equals(a.getAlarmArn()))
+                .filter(a -> a.getAlarmArn() != null && a.getAlarmArn().equals(resourceArn))
                 .findFirst()
-                .map(MetricAlarm::getTags)
-                .orElse(Map.of());
+                .orElseThrow(() -> new AwsException("ResourceNotFoundException",
+                        "No CloudWatch resource matches the ARN " + resourceArn + ".", 404));
+    }
+
+    public Map<String, String> listTagsForResource(String resourceArn, String region) {
+        return requireAlarm(resourceArn, region).getTags();
     }
 
     public void tagResource(String resourceArn, Map<String, String> tags, String region) {
-        alarmStore.scan(k -> k.startsWith(region + "::"))
-                .stream()
-                .filter(a -> resourceArn.equals(a.getAlarmArn()))
-                .findFirst()
-                .ifPresent(alarm -> {
-                    alarm.getTags().putAll(tags);
-                    alarmStore.put(region + "::" + alarm.getAlarmName(), alarm);
-                });
+        MetricAlarm alarm = requireAlarm(resourceArn, region);
+        alarm.getTags().putAll(tags);
+        alarmStore.put(region + "::" + alarm.getAlarmName(), alarm);
     }
 
     public void untagResource(String resourceArn, List<String> tagKeys, String region) {
-        alarmStore.scan(k -> k.startsWith(region + "::"))
-                .stream()
-                .filter(a -> resourceArn.equals(a.getAlarmArn()))
-                .findFirst()
-                .ifPresent(alarm -> {
-                    tagKeys.forEach(alarm.getTags()::remove);
-                    alarmStore.put(region + "::" + alarm.getAlarmName(), alarm);
-                });
+        MetricAlarm alarm = requireAlarm(resourceArn, region);
+        tagKeys.forEach(alarm.getTags()::remove);
+        alarmStore.put(region + "::" + alarm.getAlarmName(), alarm);
     }
 
     // ──────────────────────────── Helpers ────────────────────────────

src/main/java/io/github/hectorvent/floci/services/cloudwatch/metricstreams/CloudWatchMetricStreamsService.java

@@ -196,21 +196,30 @@ public class CloudWatchMetricStreamsService {
     }
 
     public Map<String, String> listTagsForResource(String resourceArn, String region) {
-        return findByArn(resourceArn, region).map(MetricStream::getTags).orElse(Map.of());
+        return requireStream(resourceArn, region).getTags();
     }
 
     public void tagResource(String resourceArn, Map<String, String> tags, String region) {
-        findByArn(resourceArn, region).ifPresent(stream -> {
-            stream.getTags().putAll(tags);
-            streamStore.put(key(region, stream.getName()), stream);
-        });
+        MetricStream stream = requireStream(resourceArn, region);
+        stream.getTags().putAll(tags);
+        streamStore.put(key(region, stream.getName()), stream);
     }
 
     public void untagResource(String resourceArn, List<String> tagKeys, String region) {
-        findByArn(resourceArn, region).ifPresent(stream -> {
-            tagKeys.forEach(stream.getTags()::remove);
-            streamStore.put(key(region, stream.getName()), stream);
-        });
+        MetricStream stream = requireStream(resourceArn, region);
+        tagKeys.forEach(stream.getTags()::remove);
+        streamStore.put(key(region, stream.getName()), stream);
+    }
+
+    /**
+     * Resolves the stream an ARN names, or reports that nothing does. The tag operations declare
+     * {@code ResourceNotFoundException}, the same code {@code GetMetricStream} already answers
+     * with, so an unknown ARN here reads the same way an unknown name does.
+     */
+    private MetricStream requireStream(String resourceArn, String region) {
+        return findByArn(resourceArn, region).orElseThrow(() ->
+                new AwsException("ResourceNotFoundException",
+                        "Metric stream " + resourceArn + " does not exist.", 404));
     }
 
     private Optional<MetricStream> findByArn(String resourceArn, String region) {

src/test/java/io/github/hectorvent/floci/services/cloudwatch/CloudWatchTagUnknownResourceIntegrationTest.java

@@ -0,0 +1,349 @@
+package io.github.hectorvent.floci.services.cloudwatch;
+
+import io.github.hectorvent.floci.testing.RestAssuredJsonUtils;
+import io.quarkus.test.junit.QuarkusTest;
+import io.restassured.specification.RequestSpecification;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+import static io.restassured.RestAssured.given;
+import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.not;
+
+/**
+ * Tagging a CloudWatch resource whose ARN names nothing has to fail, on every one of the three
+ * tag operations and for every taggable kind CloudWatch owns.
+ *
+ * <p>Each kind is checked twice: once on an ARN that resolves, so the operation is known to work
+ * at all, and once on the same ARN with the resource name swapped for one that was never created.
+ * The two ARNs are byte-identical in shape, so "the ARN was malformed" cannot explain the second
+ * answer. Without that pairing a test asserting only the failure would also pass against a build
+ * whose tag operations were broken outright.
+ *
+ * <p>The failure to look for is {@code ResourceNotFoundException}. CloudWatch's model declares
+ * that code, not the shorter {@code ResourceNotFound} that {@code GetDashboard} and
+ * {@code SetAlarmState} answer with, for {@code TagResource}, {@code UntagResource} and
+ * {@code ListTagsForResource}; the SDK maps the two to different exception classes, so a caller
+ * catching the documented one would not see the other.
+ */
+@QuarkusTest
+class CloudWatchTagUnknownResourceIntegrationTest {
+
+    private static final String CW_SCOPE =
+            "AWS4-HMAC-SHA256 Credential=test/20260205/us-east-1/monitoring/aws4_request";
+    private static final String LOGS_SCOPE =
+            "AWS4-HMAC-SHA256 Credential=test/20260205/us-east-1/logs/aws4_request";
+
+    private static final String JSON_1_0 = "application/x-amz-json-1.0";
+    private static final String JSON_1_1 = "application/x-amz-json-1.1";
+
+    private static final String PREFIX = "arn:aws:cloudwatch:us-east-1:000000000000:";
+    private static final String FIREHOSE_ARN =
+            "arn:aws:firehose:us-east-1:000000000000:deliverystream/metrics";
+    private static final String ROLE_ARN = "arn:aws:iam::000000000000:role/metric-stream-role";
+
+    @BeforeAll
+    static void configureRestAssured() {
+        RestAssuredJsonUtils.configureAwsContentTypes();
+    }
+
+    private static RequestSpecification query(String action) {
+        return given()
+                .contentType("application/x-www-form-urlencoded")
+                .header("Authorization", CW_SCOPE)
+                .formParam("Action", action);
+    }
+
+    private static RequestSpecification json(String contentType, String scope, String target, String body) {
+        return given()
+                .contentType(contentType)
+                .header("Authorization", scope)
+                .header("X-Amz-Target", target)
+                .body(body);
+    }
+
+    private static RequestSpecification metricsJson(String target, String body) {
+        return json(JSON_1_0, CW_SCOPE, "GraniteServiceVersion20100801." + target, body);
+    }
+
+    private static RequestSpecification logsJson(String target, String body) {
+        return json(JSON_1_1, LOGS_SCOPE, "Logs_20140328." + target, body);
+    }
+
+    /** The three Query tag operations against one ARN, asserting the same outcome for each. */
+    private static void assertQueryTagOpsNotFound(String arn) {
+        query("TagResource")
+                .formParam("ResourceARN", arn)
+                .formParam("Tags.member.1.Key", "env")
+                .formParam("Tags.member.1.Value", "prod")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(404)
+                .body("ErrorResponse.Error.Code", equalTo("ResourceNotFoundException"));
+
+        query("UntagResource")
+                .formParam("ResourceARN", arn)
+                .formParam("TagKeys.member.1", "env")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(404)
+                .body("ErrorResponse.Error.Code", equalTo("ResourceNotFoundException"));
+
+        query("ListTagsForResource")
+                .formParam("ResourceARN", arn)
+            .when()
+                .post("/")
+            .then()
+                .statusCode(404)
+                .body("ErrorResponse.Error.Code", equalTo("ResourceNotFoundException"));
+    }
+
+    /** The same three operations against an ARN that does resolve, read back through a list. */
+    private static void assertQueryTagOpsSucceed(String arn) {
+        query("TagResource")
+                .formParam("ResourceARN", arn)
+                .formParam("Tags.member.1.Key", "env")
+                .formParam("Tags.member.1.Value", "prod")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(200);
+
+        query("ListTagsForResource")
+                .formParam("ResourceARN", arn)
+            .when()
+                .post("/")
+            .then()
+                .statusCode(200)
+                .body("ListTagsForResourceResponse.ListTagsForResourceResult.Tags.member.Key",
+                        equalTo("env"))
+                .body("ListTagsForResourceResponse.ListTagsForResourceResult.Tags.member.Value",
+                        equalTo("prod"));
+
+        query("UntagResource")
+                .formParam("ResourceARN", arn)
+                .formParam("TagKeys.member.1", "env")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(200);
+
+        query("ListTagsForResource")
+                .formParam("ResourceARN", arn)
+            .when()
+                .post("/")
+            .then()
+                .statusCode(200)
+                .body(not(containsString("<Key>env</Key>")));
+    }
+
+    @Test
+    void alarmTagOperationsRejectAnArnThatNamesNoAlarm() {
+        query("PutMetricAlarm")
+                .formParam("AlarmName", "tag-unknown-alarm")
+                .formParam("MetricName", "CPUUtilization")
+                .formParam("Namespace", "AWS/EC2")
+                .formParam("ComparisonOperator", "GreaterThanThreshold")
+                .formParam("EvaluationPeriods", "1")
+                .formParam("Period", "60")
+                .formParam("Statistic", "Average")
+                .formParam("Threshold", "80")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(200);
+
+        assertQueryTagOpsSucceed(PREFIX + "alarm:tag-unknown-alarm");
+        assertQueryTagOpsNotFound(PREFIX + "alarm:tag-unknown-alarm-nosuch");
+    }
+
+    @Test
+    void dashboardTagOperationsRejectAnArnThatNamesNoDashboard() {
+        query("PutDashboard")
+                .formParam("DashboardName", "tag-unknown-dashboard")
+                .formParam("DashboardBody", "{\"widgets\":[]}")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(200);
+
+        assertQueryTagOpsSucceed(PREFIX + "dashboard/tag-unknown-dashboard");
+        assertQueryTagOpsNotFound(PREFIX + "dashboard/tag-unknown-dashboard-nosuch");
+    }
+
+    @Test
+    void metricStreamTagOperationsRejectAnArnThatNamesNoStream() {
+        query("PutMetricStream")
+                .formParam("Name", "tag-unknown-stream")
+                .formParam("FirehoseArn", FIREHOSE_ARN)
+                .formParam("RoleArn", ROLE_ARN)
+                .formParam("OutputFormat", "json")
+                .formParam("IncludeFilters.member.1.Namespace", "AWS/EC2")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(200);
+
+        assertQueryTagOpsSucceed(PREFIX + "metric-stream/tag-unknown-stream");
+        assertQueryTagOpsNotFound(PREFIX + "metric-stream/tag-unknown-stream-nosuch");
+    }
+
+    /**
+     * The dashboard ARN AWS documents carries no region, and the one Floci mints does. A
+     * caller sending the documented form for a dashboard that exists must not be told the
+     * dashboard does not exist, which is what raising on a miss would otherwise have made of
+     * a shape that used to fall through to an empty tag map.
+     */
+    @Test
+    void theRegionlessDashboardArnAwsDocumentsAlsoResolves() {
+        query("PutDashboard")
+                .formParam("DashboardName", "tag-regionless-dashboard")
+                .formParam("DashboardBody", "{\"widgets\":[]}")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(200);
+
+        assertQueryTagOpsSucceed("arn:aws:cloudwatch::000000000000:dashboard/tag-regionless-dashboard");
+
+        // Still only an absent region: another region's ARN remains a miss.
+        assertQueryTagOpsNotFound(
+                "arn:aws:cloudwatch:eu-west-1:000000000000:dashboard/tag-regionless-dashboard");
+    }
+
+    /**
+     * Both handlers route every ARN that is not a dashboard and not a metric stream to the
+     * alarm path, so a Contributor Insights ARN, which AWS documents as taggable and Floci
+     * does not serve, lands there. Reporting it as not found is right; calling it an alarm
+     * that was not found is not, so the message names the ARN instead.
+     */
+    @Test
+    void anArnOfAKindFlociDoesNotServeIsReportedAgainstTheArn() {
+        query("TagResource")
+                .formParam("ResourceARN", PREFIX + "insight-rule/no-such-rule")
+                .formParam("Tags.member.1.Key", "env")
+                .formParam("Tags.member.1.Value", "prod")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(404)
+                .body("ErrorResponse.Error.Code", equalTo("ResourceNotFoundException"))
+                .body("ErrorResponse.Error.Message",
+                        containsString("insight-rule/no-such-rule"))
+                .body("ErrorResponse.Error.Message",
+                        not(containsString("Alarm")));
+    }
+
+    /**
+     * CloudWatch Metrics serves the same three operations over JSON as well as Query, and
+     * AGENTS.md requires the two handlers not to drift. A fix applied to only one of them
+     * would leave the CLI and SDK v3 route still answering 200.
+     */
+    @Test
+    void theJsonProtocolRejectsTheSameArns() {
+        metricsJson("PutMetricAlarm", """
+                {"AlarmName": "tag-unknown-json-alarm", "MetricName": "CPUUtilization",
+                 "Namespace": "AWS/EC2", "ComparisonOperator": "GreaterThanThreshold",
+                 "EvaluationPeriods": 1, "Period": 60, "Statistic": "Average", "Threshold": 80}""")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(200);
+
+        String real = PREFIX + "alarm:tag-unknown-json-alarm";
+        String ghost = PREFIX + "alarm:tag-unknown-json-alarm-nosuch";
+
+        metricsJson("TagResource",
+                "{\"ResourceARN\": \"" + real + "\", \"Tags\": [{\"Key\": \"env\", \"Value\": \"prod\"}]}")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(200);
+
+        metricsJson("ListTagsForResource", "{\"ResourceARN\": \"" + real + "\"}")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(200)
+                .body("Tags[0].Key", equalTo("env"))
+                .body("Tags[0].Value", equalTo("prod"));
+
+        metricsJson("TagResource",
+                "{\"ResourceARN\": \"" + ghost + "\", \"Tags\": [{\"Key\": \"env\", \"Value\": \"prod\"}]}")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(404)
+                .body("__type", containsString("ResourceNotFoundException"));
+
+        metricsJson("UntagResource", "{\"ResourceARN\": \"" + ghost + "\", \"TagKeys\": [\"env\"]}")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(404)
+                .body("__type", containsString("ResourceNotFoundException"));
+
+        metricsJson("ListTagsForResource", "{\"ResourceARN\": \"" + ghost + "\"}")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(404)
+                .body("__type", containsString("ResourceNotFoundException"));
+    }
+
+    /**
+     * CloudWatch Logs serves its own TagResource, UntagResource and ListTagsForResource over
+     * a separate handler, and already rejects an ARN that names no log group. Pinning it keeps
+     * the two halves of CloudWatch's tagging from diverging again in the other direction.
+     */
+    @Test
+    void logGroupTagOperationsRejectAnArnThatNamesNoGroup() {
+        String real = "arn:aws:logs:us-east-1:000000000000:log-group:tag-unknown-group:*";
+        String ghost = "arn:aws:logs:us-east-1:000000000000:log-group:tag-unknown-group-nosuch:*";
+
+        logsJson("CreateLogGroup", "{\"logGroupName\": \"tag-unknown-group\"}")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(200);
+
+        logsJson("TagResource",
+                "{\"resourceArn\": \"" + real + "\", \"tags\": {\"env\": \"prod\"}}")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(200);
+
+        logsJson("ListTagsForResource", "{\"resourceArn\": \"" + real + "\"}")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(200)
+                .body("tags.env", equalTo("prod"));
+
+        logsJson("TagResource",
+                "{\"resourceArn\": \"" + ghost + "\", \"tags\": {\"env\": \"prod\"}}")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(400)
+                .body("__type", containsString("ResourceNotFoundException"));
+
+        logsJson("UntagResource", "{\"resourceArn\": \"" + ghost + "\", \"tagKeys\": [\"env\"]}")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(400)
+                .body("__type", containsString("ResourceNotFoundException"));
+
+        logsJson("ListTagsForResource", "{\"resourceArn\": \"" + ghost + "\"}")
+            .when()
+                .post("/")
+            .then()
+                .statusCode(400)
+                .body("__type", containsString("ResourceNotFoundException"));
+    }
+}

src/test/java/io/github/hectorvent/floci/services/cloudwatch/dashboards/CloudWatchDashboardsServiceTest.java

@@ -8,6 +8,7 @@ import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
 import java.util.List;
+import java.util.Map;
 
 import static org.junit.jupiter.api.Assertions.*;
 
@@ -116,10 +117,10 @@ class CloudWatchDashboardsServiceTest {
 
     @Test
     void tagsGivenOnCreateAreReadableByArn() {
-        service.putDashboard("ops", BODY, java.util.Map.of("team", "platform"), REGION);
+        service.putDashboard("ops", BODY, Map.of("team", "platform"), REGION);
         String arn = service.getDashboard("ops", REGION).getDashboardArn();
 
-        assertEquals(java.util.Map.of("team", "platform"), service.listTagsForResource(arn, REGION));
+        assertEquals(Map.of("team", "platform"), service.listTagsForResource(arn, REGION));
         assertTrue(CloudWatchDashboardsService.isDashboardArn(arn));
     }
 
@@ -129,12 +130,12 @@ class CloudWatchDashboardsServiceTest {
      */
     @Test
     void tagsOnAReplacingPutDoNotOverwriteTheExistingOnes() {
-        service.putDashboard("ops", BODY, java.util.Map.of("team", "platform"), REGION);
+        service.putDashboard("ops", BODY, Map.of("team", "platform"), REGION);
         String arn = service.getDashboard("ops", REGION).getDashboardArn();
 
-        service.putDashboard("ops", BODY, java.util.Map.of("team", "someone-else"), REGION);
+        service.putDashboard("ops", BODY, Map.of("team", "someone-else"), REGION);
 
-        assertEquals(java.util.Map.of("team", "platform"), service.listTagsForResource(arn, REGION));
+        assertEquals(Map.of("team", "platform"), service.listTagsForResource(arn, REGION));
     }
 
     @Test
@@ -142,11 +143,75 @@ class CloudWatchDashboardsServiceTest {
         service.putDashboard("ops", BODY, REGION);
         String arn = service.getDashboard("ops", REGION).getDashboardArn();
 
-        service.tagResource(arn, java.util.Map.of("env", "dev"), REGION);
-        assertEquals(java.util.Map.of("env", "dev"), service.listTagsForResource(arn, REGION));
+        service.tagResource(arn, Map.of("env", "dev"), REGION);
+        assertEquals(Map.of("env", "dev"), service.listTagsForResource(arn, REGION));
 
         service.untagResource(arn, List.of("env"), REGION);
-        assertEquals(java.util.Map.of(), service.listTagsForResource(arn, REGION));
+        assertEquals(Map.of(), service.listTagsForResource(arn, REGION));
+    }
+
+    /**
+     * AWS documents the dashboard ARN without a region and Floci mints one with a region, so
+     * both forms have to resolve. Before the tag operations raised on a miss this divergence
+     * was invisible here: the documented ARN matched nothing and got an empty tag map back.
+     */
+    @Test
+    void bothTheRegionlessAndTheRegionfulDashboardArnResolve() {
+        service.putDashboard("ops", BODY, REGION);
+        String regionful = service.getDashboard("ops", REGION).getDashboardArn();
+        String regionless = "arn:aws:cloudwatch::000000000000:dashboard/ops";
+
+        assertEquals("arn:aws:cloudwatch:us-east-1:000000000000:dashboard/ops", regionful);
+        assertTrue(CloudWatchDashboardsService.isDashboardArn(regionless));
+
+        service.tagResource(regionless, Map.of("env", "dev"), REGION);
+        // Read back through the other form: one dashboard, reachable either way.
+        assertEquals(Map.of("env", "dev"), service.listTagsForResource(regionful, REGION));
+        assertEquals(Map.of("env", "dev"), service.listTagsForResource(regionless, REGION));
+
+        service.untagResource(regionless, List.of("env"), REGION);
+        assertEquals(Map.of(), service.listTagsForResource(regionful, REGION));
+    }
+
+    /**
+     * Only an absent region stands in for the dashboard's own. An ARN naming a different
+     * region must not resolve to the same-named dashboard in this one.
+     */
+    @Test
+    void anArnNamingAnotherRegionDoesNotResolve() {
+        service.putDashboard("ops", BODY, REGION);
+        String elsewhere = "arn:aws:cloudwatch:eu-west-1:000000000000:dashboard/ops";
+
+        AwsException e = assertThrows(AwsException.class,
+                () -> service.listTagsForResource(elsewhere, REGION));
+        assertEquals("ResourceNotFoundException", e.getErrorCode());
+    }
+
+    /**
+     * An ARN naming no dashboard is an error on all three operations, not a silent no-op. The
+     * ARN used here differs from a working one only in the dashboard name, so the outcome
+     * cannot be explained by the ARN failing to parse as a dashboard ARN.
+     */
+    @Test
+    void tagOperationsRejectAnArnThatNamesNoDashboard() {
+        service.putDashboard("ops", BODY, REGION);
+        String arn = service.getDashboard("ops", REGION).getDashboardArn();
+        String ghost = arn.replace("dashboard/ops", "dashboard/nosuch");
+        assertTrue(CloudWatchDashboardsService.isDashboardArn(ghost));
+
+        for (Runnable call : List.<Runnable>of(
+                () -> service.tagResource(ghost, Map.of("env", "prod"), REGION),
+                () -> service.untagResource(ghost, List.of("env"), REGION),
+                () -> service.listTagsForResource(ghost, REGION))) {
+            AwsException e = assertThrows(AwsException.class, call::run);
+            assertEquals("ResourceNotFoundException", e.getErrorCode());
+            assertEquals(404, e.getHttpStatus());
+        }
+
+        // The failures wrote nothing: the real dashboard still carries only what it was given.
+        assertEquals(Map.of(), service.listTagsForResource(arn, REGION));
+        service.tagResource(arn, Map.of("env", "dev"), REGION);
+        assertEquals(Map.of("env", "dev"), service.listTagsForResource(arn, REGION));
     }
 
     @Test

src/test/java/io/github/hectorvent/floci/services/cloudwatch/metrics/CloudWatchMetricsTagsTest.java

@@ -2,6 +2,7 @@ package io.github.hectorvent.floci.services.cloudwatch.metrics;
 
 import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.databind.ObjectMapper;
+import io.github.hectorvent.floci.core.common.AwsException;
 import io.github.hectorvent.floci.core.common.RegionResolver;
 import io.github.hectorvent.floci.core.storage.InMemoryStorage;
 import io.github.hectorvent.floci.services.cloudwatch.dashboards.CloudWatchDashboardsService;
@@ -83,6 +84,34 @@ class CloudWatchMetricsTagsTest {
         assertNull(tags.get("team"));
     }
 
+    /**
+     * An ARN naming no alarm is an error on all three operations, not a silent no-op. The ARN
+     * used here differs from a working one only in the alarm name, so the outcome cannot be
+     * explained by the ARN failing to parse.
+     */
+    @Test
+    void tagOperationsRejectAnArnThatNamesNoAlarm() {
+        MetricAlarm alarm = new MetricAlarm();
+        alarm.setAlarmName("test-alarm");
+        alarm.setAlarmArn("arn:aws:cloudwatch:us-east-1:000000000000:alarm:test-alarm");
+        service.putMetricAlarm(alarm, REGION);
+        String ghost = "arn:aws:cloudwatch:us-east-1:000000000000:alarm:nosuch";
+
+        for (Runnable call : List.<Runnable>of(
+                () -> service.tagResource(ghost, Map.of("env", "prod"), REGION),
+                () -> service.untagResource(ghost, List.of("env"), REGION),
+                () -> service.listTagsForResource(ghost, REGION))) {
+            AwsException e = assertThrows(AwsException.class, call::run);
+            assertEquals("ResourceNotFoundException", e.getErrorCode());
+            assertEquals(404, e.getHttpStatus());
+        }
+
+        // The failures wrote nothing: the real alarm still carries only what it was given.
+        assertEquals(Map.of(), service.listTagsForResource(alarm.getAlarmArn(), REGION));
+        service.tagResource(alarm.getAlarmArn(), Map.of("env", "dev"), REGION);
+        assertEquals(Map.of("env", "dev"), service.listTagsForResource(alarm.getAlarmArn(), REGION));
+    }
+
     @Test
     void tagsViaPutMetricAlarmQuery() {
         MultivaluedMap<String, String> params = new MultivaluedHashMap<>();

src/test/java/io/github/hectorvent/floci/services/cloudwatch/metricstreams/CloudWatchMetricStreamsServiceTest.java

@@ -232,4 +232,29 @@ class CloudWatchMetricStreamsServiceTest {
         service.untagResource(arn, List.of("team"), REGION);
         assertFalse(service.listTagsForResource(arn, REGION).containsKey("team"));
     }
+
+    /**
+     * An ARN naming no stream is an error on all three operations, not a silent no-op. The ARN
+     * used here differs from a working one only in the stream name, so the outcome cannot be
+     * explained by the ARN failing to parse as a metric stream ARN.
+     */
+    @Test
+    void tagOperationsRejectAnArnThatNamesNoStream() {
+        String arn = service.putMetricStream(stream("ops"), REGION).getArn();
+        String ghost = arn.replace("metric-stream/ops", "metric-stream/nosuch");
+        assertTrue(CloudWatchMetricStreamsService.isMetricStreamArn(ghost));
+
+        for (Runnable call : List.<Runnable>of(
+                () -> service.tagResource(ghost, Map.of("env", "prod"), REGION),
+                () -> service.untagResource(ghost, List.of("env"), REGION),
+                () -> service.listTagsForResource(ghost, REGION))) {
+            AwsException e = assertThrows(AwsException.class, call::run);
+            assertEquals("ResourceNotFoundException", e.getErrorCode());
+            assertEquals(404, e.getHttpStatus());
+        }
+
+        // The failures wrote nothing: the real stream still carries only what it was given.
+        service.tagResource(arn, Map.of("team", "platform"), REGION);
+        assertEquals(Map.of("team", "platform"), service.listTagsForResource(arn, REGION));
+    }
 }