Fix NPE when adding or removing features on a server with derived capabilities

mcp-core/src/main/java/io/modelcontextprotocol/server/McpAsyncServer.java

@@ -371,7 +371,7 @@ public Mono<Void> addTool(McpServerFeatures.AsyncToolSpecification toolSpecifica
 			this.tools.add(wrappedToolSpecification);
 			logger.debug("Added tool handler: {}", wrappedToolSpecification.tool().name());
 
-			if (this.serverCapabilities.tools().listChanged()) {
+			if (Boolean.TRUE.equals(this.serverCapabilities.tools().listChanged())) {
 				return notifyToolsListChanged();
 			}
 			return Mono.empty();
@@ -519,7 +519,7 @@ public Mono<Void> removeTool(String toolName) {
 		return Mono.defer(() -> {
 			if (this.tools.removeIf(toolSpecification -> toolSpecification.tool().name().equals(toolName))) {
 				logger.debug("Removed tool handler: {}", toolName);
-				if (this.serverCapabilities.tools().listChanged()) {
+				if (Boolean.TRUE.equals(this.serverCapabilities.tools().listChanged())) {
 					return notifyToolsListChanged();
 				}
 			}
@@ -622,7 +622,7 @@ public Mono<Void> addResource(McpServerFeatures.AsyncResourceSpecification resou
 			else {
 				logger.debug("Added resource handler: {}", resourceSpecification.resource().uri());
 			}
-			if (this.serverCapabilities.resources().listChanged()) {
+			if (Boolean.TRUE.equals(this.serverCapabilities.resources().listChanged())) {
 				return notifyResourcesListChanged();
 			}
 			return Mono.empty();
@@ -655,7 +655,7 @@ public Mono<Void> removeResource(String resourceUri) {
 			McpServerFeatures.AsyncResourceSpecification removed = this.resources.remove(resourceUri);
 			if (removed != null) {
 				logger.debug("Removed resource handler: {}", resourceUri);
-				if (this.serverCapabilities.resources().listChanged()) {
+				if (Boolean.TRUE.equals(this.serverCapabilities.resources().listChanged())) {
 					return notifyResourcesListChanged();
 				}
 				return Mono.empty();
@@ -691,7 +691,7 @@ public Mono<Void> addResourceTemplate(
 				logger.debug("Added resource template handler: {}",
 						resourceTemplateSpecification.resourceTemplate().uriTemplate());
 			}
-			if (this.serverCapabilities.resources().listChanged()) {
+			if (Boolean.TRUE.equals(this.serverCapabilities.resources().listChanged())) {
 				return notifyResourcesListChanged();
 			}
 			return Mono.empty();
@@ -891,7 +891,7 @@ public Mono<Void> addPrompt(McpServerFeatures.AsyncPromptSpecification promptSpe
 			else {
 				logger.debug("Added prompt handler: {}", promptSpecification.prompt().name());
 			}
-			if (this.serverCapabilities.prompts().listChanged()) {
+			if (Boolean.TRUE.equals(this.serverCapabilities.prompts().listChanged())) {
 				return this.notifyPromptsListChanged();
 			}
 
@@ -925,7 +925,7 @@ public Mono<Void> removePrompt(String promptName) {
 
 			if (removed != null) {
 				logger.debug("Removed prompt handler: {}", promptName);
-				if (this.serverCapabilities.prompts().listChanged()) {
+				if (Boolean.TRUE.equals(this.serverCapabilities.prompts().listChanged())) {
 					return this.notifyPromptsListChanged();
 				}
 				return Mono.empty();

mcp-test/src/test/java/io/modelcontextprotocol/server/DerivedCapabilitiesRuntimeChangesTests.java

@@ -0,0 +1,118 @@
+/*
+ * Copyright 2026-2026 the original author or authors.
+ */
+
+package io.modelcontextprotocol.server;
+
+import io.modelcontextprotocol.MockMcpServerTransport;
+import io.modelcontextprotocol.MockMcpServerTransportProvider;
+import io.modelcontextprotocol.spec.McpSchema;
+import org.junit.jupiter.api.Test;
+import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatCode;
+
+/**
+ * Runtime add/remove operations on a server whose capabilities were derived from the
+ * features registered at build time (no explicit {@code capabilities(...)}). The derived
+ * capabilities leave {@code listChanged} unset, so no list_changed notification is
+ * expected and the operations must complete normally.
+ */
+class DerivedCapabilitiesRuntimeChangesTests {
+
+	private static final McpSchema.Implementation SERVER_INFO = McpSchema.Implementation.builder("test-server", "1.0.0")
+		.build();
+
+	private static McpServerFeatures.AsyncToolSpecification asyncTool(String name) {
+		return McpServerFeatures.AsyncToolSpecification.builder()
+			.tool(McpSchema.Tool.builder(name).description(name).build())
+			.callHandler((exchange, request) -> Mono.just(McpSchema.CallToolResult.builder().build()))
+			.build();
+	}
+
+	private static McpServerFeatures.AsyncResourceSpecification asyncResource(String uri) {
+		return new McpServerFeatures.AsyncResourceSpecification(McpSchema.Resource.builder(uri, uri).build(),
+				(exchange, request) -> Mono.empty());
+	}
+
+	private static McpServerFeatures.AsyncResourceTemplateSpecification asyncTemplate(String uriTemplate) {
+		return new McpServerFeatures.AsyncResourceTemplateSpecification(
+				McpSchema.ResourceTemplate.builder(uriTemplate, uriTemplate).build(),
+				(exchange, request) -> Mono.empty());
+	}
+
+	private static McpServerFeatures.AsyncPromptSpecification asyncPrompt(String name) {
+		return new McpServerFeatures.AsyncPromptSpecification(McpSchema.Prompt.builder(name).build(),
+				(exchange, request) -> Mono.empty());
+	}
+
+	@Test
+	void addAndRemoveToolWithDerivedCapabilities() {
+		MockMcpServerTransport transport = new MockMcpServerTransport();
+		McpAsyncServer server = McpServer.async(new MockMcpServerTransportProvider(transport))
+			.serverInfo(SERVER_INFO)
+			.tools(asyncTool("initial"))
+			.build();
+
+		assertThat(server.getServerCapabilities().tools().listChanged()).isNull();
+
+		StepVerifier.create(server.addTool(asyncTool("added"))).verifyComplete();
+		StepVerifier.create(server.removeTool("initial")).verifyComplete();
+
+		assertThat(server.listTools().map(McpSchema.Tool::name).collectList().block()).containsExactly("added");
+		assertThat(transport.getAllSentMessages()).isEmpty();
+	}
+
+	@Test
+	void addAndRemoveResourceAndTemplateWithDerivedCapabilities() {
+		MockMcpServerTransport transport = new MockMcpServerTransport();
+		McpAsyncServer server = McpServer.async(new MockMcpServerTransportProvider(transport))
+			.serverInfo(SERVER_INFO)
+			.resources(asyncResource("test://initial"))
+			.build();
+
+		assertThat(server.getServerCapabilities().resources().listChanged()).isNull();
+
+		StepVerifier.create(server.addResource(asyncResource("test://added"))).verifyComplete();
+		StepVerifier.create(server.removeResource("test://initial")).verifyComplete();
+		StepVerifier.create(server.addResourceTemplate(asyncTemplate("test://items/{id}"))).verifyComplete();
+
+		assertThat(transport.getAllSentMessages()).isEmpty();
+	}
+
+	@Test
+	void addAndRemovePromptWithDerivedCapabilities() {
+		MockMcpServerTransport transport = new MockMcpServerTransport();
+		McpAsyncServer server = McpServer.async(new MockMcpServerTransportProvider(transport))
+			.serverInfo(SERVER_INFO)
+			.prompts(asyncPrompt("initial"))
+			.build();
+
+		assertThat(server.getServerCapabilities().prompts().listChanged()).isNull();
+
+		StepVerifier.create(server.addPrompt(asyncPrompt("added"))).verifyComplete();
+		StepVerifier.create(server.removePrompt("initial")).verifyComplete();
+
+		assertThat(transport.getAllSentMessages()).isEmpty();
+	}
+
+	@Test
+	void syncServerAddAndRemoveToolWithDerivedCapabilities() {
+		McpSyncServer server = McpServer.sync(new MockMcpServerTransportProvider(new MockMcpServerTransport()))
+			.serverInfo(SERVER_INFO)
+			.tools(McpServerFeatures.SyncToolSpecification.builder()
+				.tool(McpSchema.Tool.builder("initial").description("initial").build())
+				.callHandler((exchange, request) -> McpSchema.CallToolResult.builder().build())
+				.build())
+			.build();
+
+		assertThatCode(() -> server.addTool(McpServerFeatures.SyncToolSpecification.builder()
+			.tool(McpSchema.Tool.builder("added").description("added").build())
+			.callHandler((exchange, request) -> McpSchema.CallToolResult.builder().build())
+			.build())).doesNotThrowAnyException();
+		assertThatCode(() -> server.removeTool("initial")).doesNotThrowAnyException();
+	}
+
+}