diff --git a/spring-boot-admin-server/pom.xml b/spring-boot-admin-server/pom.xml
index 806c79c8e29..98083582d96 100644
--- a/spring-boot-admin-server/pom.xml
+++ b/spring-boot-admin-server/pom.xml
@@ -156,5 +156,9 @@
             <artifactId>testcontainers-junit-jupiter</artifactId>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-validation</artifactId>
+        </dependency>
     </dependencies>
 </project>
diff --git a/spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/domain/values/Registration.java b/spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/domain/values/Registration.java
index 3f450346bb9..4b1d31ca618 100644
--- a/spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/domain/values/Registration.java
+++ b/spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/domain/values/Registration.java
@@ -23,11 +23,12 @@
 import java.util.LinkedHashMap;
 import java.util.Map;
 
+import jakarta.validation.constraints.AssertTrue;
+import jakarta.validation.constraints.NotBlank;
 import lombok.ToString;
 import org.jspecify.annotations.Nullable;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
-import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
 
 /**
@@ -41,6 +42,7 @@ public final class Registration implements Serializable {
 
 	private static final Logger log = LoggerFactory.getLogger(Registration.class);
 
+	@NotBlank
 	private final String name;
 
 	/**
@@ -55,6 +57,7 @@ public final class Registration implements Serializable {
 	 * Admin to determine the instance status. Example:
 	 * <code>https://example.com/actuator/health</code>
 	 */
+	@NotBlank
 	private final String healthUrl;
 
 	/**
@@ -71,13 +74,6 @@ public final class Registration implements Serializable {
 	@lombok.Builder(builderClassName = "Builder", toBuilder = true)
 	private Registration(String name, @Nullable String managementUrl, String healthUrl, @Nullable String serviceUrl,
 			String source, @lombok.Singular("metadata") Map<String, String> metadata) {
-		Assert.hasText(name, "'name' must not be empty.");
-		Assert.hasText(healthUrl, "'healthUrl' must not be empty.");
-		Assert.isTrue(checkUrl(healthUrl), "'healthUrl' is not valid: " + healthUrl);
-		Assert.isTrue(!StringUtils.hasText(managementUrl) || checkUrl(managementUrl),
-				"'managementUrl' is not valid: " + managementUrl);
-		Assert.isTrue(!StringUtils.hasText(serviceUrl) || checkUrl(serviceUrl),
-				"'serviceUrl' is not valid: " + serviceUrl);
 
 		this.name = name;
 		this.managementUrl = managementUrl;
@@ -128,6 +124,21 @@ public Map<String, String> getMetadata() {
 		return Collections.unmodifiableMap(this.metadata);
 	}
 
+	@AssertTrue(message = "'healthUrl' must be an absolute URL.")
+	public boolean isHealthUrlAbsolute() {
+		return StringUtils.hasText(this.healthUrl) && checkUrl(this.healthUrl);
+	}
+
+	@AssertTrue(message = "'managementUrl' must be an absolute URL when present.")
+	public boolean isManagementUrlAbsoluteWhenPresent() {
+		return !StringUtils.hasText(this.managementUrl) || checkUrl(this.managementUrl);
+	}
+
+	@AssertTrue(message = "'serviceUrl' must be an absolute URL when present.")
+	public boolean isServiceUrlAbsoluteWhenPresent() {
+		return !StringUtils.hasText(this.serviceUrl) || checkUrl(this.serviceUrl);
+	}
+
 	/**
 	 * Checks the syntax of the given URL.
 	 * @param url the URL.
diff --git a/spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/web/InstancesController.java b/spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/web/InstancesController.java
index 109efc0d7a2..2553b146859 100644
--- a/spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/web/InstancesController.java
+++ b/spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/web/InstancesController.java
@@ -21,6 +21,7 @@
 import java.util.Collections;
 import java.util.Map;
 
+import jakarta.validation.Valid;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import org.springframework.http.MediaType;
@@ -74,7 +75,7 @@ public InstancesController(InstanceRegistry registry, InstanceEventStore eventSt
 	 * @return the registered instance id;
 	 */
 	@PostMapping(path = "/instances", consumes = MediaType.APPLICATION_JSON_VALUE)
-	public Mono<ResponseEntity<Map<String, InstanceId>>> register(@RequestBody Registration registration,
+	public Mono<ResponseEntity<Map<String, InstanceId>>> register(@Valid @RequestBody Registration registration,
 			UriComponentsBuilder builder) {
 		Registration withSource = Registration.copyOf(registration).source("http-api").build();
 		LOGGER.debug("Register instance {}", withSource);
diff --git a/spring-boot-admin-server/src/test/java/de/codecentric/boot/admin/server/web/InstancesControllerIntegrationTest.java b/spring-boot-admin-server/src/test/java/de/codecentric/boot/admin/server/web/InstancesControllerIntegrationTest.java
index 664c03d3b90..b67ba4350b4 100644
--- a/spring-boot-admin-server/src/test/java/de/codecentric/boot/admin/server/web/InstancesControllerIntegrationTest.java
+++ b/spring-boot-admin-server/src/test/java/de/codecentric/boot/admin/server/web/InstancesControllerIntegrationTest.java
@@ -17,6 +17,7 @@
 package de.codecentric.boot.admin.server.web;
 
 import java.time.Duration;
+import java.util.List;
 import java.util.Map;
 import java.util.concurrent.atomic.AtomicReference;
 
@@ -33,6 +34,7 @@
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.core.ParameterizedTypeReference;
 import org.springframework.http.MediaType;
+import org.springframework.test.web.reactive.server.EntityExchangeResult;
 import org.springframework.test.web.reactive.server.WebTestClient;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
@@ -95,6 +97,105 @@ void should_return_not_found_when_get_unknown_instance() {
 		this.client.get().uri("/instances/unknown").exchange().expectStatus().isNotFound();
 	}
 
+	@Test
+	void should_reject_invalid_json() {
+		this.client.post()
+			.uri("/instances")
+			.accept(MediaType.APPLICATION_JSON)
+			.contentType(MediaType.APPLICATION_JSON)
+			.bodyValue("{")
+			.exchange()
+			.expectStatus()
+			.isBadRequest();
+	}
+
+	@Test
+	void should_reject_missing_name() {
+		String body = "{ \"healthUrl\": \"http://localhost:" + localPort + "/application/health\" }";
+
+		this.client.post()
+			.uri("/instances")
+			.accept(MediaType.APPLICATION_JSON)
+			.contentType(MediaType.APPLICATION_JSON)
+			.bodyValue(body)
+			.exchange()
+			.expectStatus()
+			.isBadRequest();
+	}
+
+	@Test
+	void should_reject_missing_health_url() {
+		String body = "{ \"name\": \"noname\" }";
+
+		this.client.post()
+			.uri("/instances")
+			.accept(MediaType.APPLICATION_JSON)
+			.contentType(MediaType.APPLICATION_JSON)
+			.bodyValue(body)
+			.exchange()
+			.expectStatus()
+			.isBadRequest();
+	}
+
+	@Test
+	void should_create_distinct_ids_for_different_health_urls() {
+		String id1 = register().block();
+
+		String other = "{ \"name\": \"other\", \"healthUrl\": \"http://localhost:" + localPort + "/other/health\" }";
+		String id2 = registerWithBody(other);
+
+		assertThat(id2).isNotEqualTo(id1);
+
+		assertInstanceById(id1);
+		assertInstanceById(id2);
+	}
+
+	@Test
+	void should_remove_instance_after_delete() {
+		String id = register().block();
+
+		this.client.delete().uri(getLocation(id)).exchange().expectStatus().isNoContent();
+
+		this.client.get().uri(getLocation(id)).exchange().expectStatus().isNotFound();
+
+		this.client.get()
+			.uri("/instances")
+			.exchange()
+			.expectStatus()
+			.isOk()
+			.expectHeader()
+			.contentType(MediaType.APPLICATION_JSON)
+			.expectBody(String.class)
+			.consumeWith((response) -> {
+				DocumentContext json = JsonPath.parse(response.getResponseBody());
+				List<String> ids = json.read("$[?(@.id == '" + id + "')].id");
+				assertThat(ids).isEmpty();
+			});
+
+	}
+
+	@Test
+	void should_not_create_duplicate_instance_on_reregister() {
+		String id = register().block(Duration.ofSeconds(5));
+
+		registerSecondTime(id);
+
+		this.client.get()
+			.uri("/instances")
+			.exchange()
+			.expectStatus()
+			.isOk()
+			.expectHeader()
+			.contentType(MediaType.APPLICATION_JSON)
+			.expectBody(String.class)
+			.consumeWith((response) -> {
+				DocumentContext json = JsonPath.parse(response.getResponseBody());
+				List<String> ids = json.read("$[?(@.id == '" + id + "')].id");
+				assertThat(ids).hasSize(1);
+			});
+
+	}
+
 	@Test
 	void should_return_empty_list() {
 		this.client.get()
@@ -280,6 +381,23 @@ private Mono<String> register() {
 		//@formatter:on
 	}
 
+	private String registerWithBody(String body) {
+		//@formatter:off
+		EntityExchangeResult<Map<String, Object>> result = client.post()
+			.uri("/instances")
+			.accept(MediaType.APPLICATION_JSON).contentType(MediaType.APPLICATION_JSON)
+			.bodyValue(body)
+			.exchange()
+			.expectStatus().isCreated()
+			.expectHeader().contentType(MediaType.APPLICATION_JSON)
+			.expectHeader().valueMatches("location", "http://localhost:" + localPort + "/instances/[0-9a-f]+")
+			.expectBody(responseType)
+			.returnResult();
+		//@formatter:on
+		assertThat(result.getResponseBody()).containsKeys("id");
+		return result.getResponseBody().get("id").toString();
+	}
+
 	private String getLocation(String id) {
 		return "http://localhost:" + localPort + "/instances/" + id;