Reject roots listing before sending unsupported client requests

Author: karthiksenv

disclosure.txt

@@ -0,0 +1 @@
+This change was submitted despite me reading the rules and understanding AI contribution guidelines.

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

@@ -225,6 +225,13 @@ public Mono<McpSchema.ListRootsResult> listRoots() {
 	 * @return A Mono that emits the list of roots result containing
 	 */
 	public Mono<McpSchema.ListRootsResult> listRoots(String cursor) {
+		if (this.clientCapabilities == null) {
+			return Mono
+				.error(new IllegalStateException("Client must be initialized. Call the initialize method first!"));
+		}
+		if (this.clientCapabilities.roots() == null) {
+			return Mono.error(new IllegalStateException("Client must be configured with root listing capabilities"));
+		}
 		return this.session.sendRequest(McpSchema.METHOD_ROOTS_LIST, new McpSchema.PaginatedRequest(cursor),
 				LIST_ROOTS_RESULT_TYPE_REF);
 	}

mcp-core/src/test/java/io/modelcontextprotocol/server/McpAsyncServerExchangeTests.java

@@ -17,6 +17,8 @@
 import io.modelcontextprotocol.spec.McpServerSession;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 import org.mockito.Mock;
 import org.mockito.MockitoAnnotations;
 import reactor.core.publisher.Mono;
@@ -29,6 +31,7 @@
 import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.verifyNoInteractions;
 import static org.mockito.Mockito.when;
 
 /**
@@ -85,6 +88,54 @@ void testListRootsWithSinglePage() {
 		}).verifyComplete();
 	}
 
+	@ParameterizedTest
+	@ValueSource(booleans = { false, true })
+	void testListRootsWithoutCapabilities(boolean paginated) {
+		exchange = new McpAsyncServerExchange("testSessionId", mockSession, null, clientInfo,
+				McpTransportContext.EMPTY);
+		when(mockSession.sendRequest(eq(McpSchema.METHOD_ROOTS_LIST), any(McpSchema.PaginatedRequest.class),
+				any(TypeRef.class)))
+			.thenReturn(Mono.just(McpSchema.ListRootsResult.builder(List.of()).build()));
+
+		Mono<McpSchema.ListRootsResult> result = paginated ? exchange.listRoots("cursor") : exchange.listRoots();
+		verifyNoInteractions(mockSession);
+		StepVerifier.create(result)
+			.verifyErrorSatisfies(error -> assertThat(error).isInstanceOf(IllegalStateException.class)
+				.hasMessage("Client must be initialized. Call the initialize method first!"));
+		verifyNoInteractions(mockSession);
+	}
+
+	@ParameterizedTest
+	@ValueSource(booleans = { false, true })
+	void testListRootsWithoutRootsCapability(boolean paginated) {
+		exchange = new McpAsyncServerExchange("testSessionId", mockSession,
+				McpSchema.ClientCapabilities.builder().build(), clientInfo, McpTransportContext.EMPTY);
+		when(mockSession.sendRequest(eq(McpSchema.METHOD_ROOTS_LIST), any(McpSchema.PaginatedRequest.class),
+				any(TypeRef.class)))
+			.thenReturn(Mono.just(McpSchema.ListRootsResult.builder(List.of()).build()));
+
+		Mono<McpSchema.ListRootsResult> result = paginated ? exchange.listRoots("cursor") : exchange.listRoots();
+		verifyNoInteractions(mockSession);
+		StepVerifier.create(result)
+			.verifyErrorSatisfies(error -> assertThat(error).isInstanceOf(IllegalStateException.class)
+				.hasMessage("Client must be configured with root listing capabilities"));
+		verifyNoInteractions(mockSession);
+	}
+
+	@Test
+	void testListRootsWithoutListChangedSupport() {
+		exchange = new McpAsyncServerExchange("testSessionId", mockSession,
+				McpSchema.ClientCapabilities.builder().roots(false).build(), clientInfo, McpTransportContext.EMPTY);
+		McpSchema.ListRootsResult result = McpSchema.ListRootsResult.builder(List.of()).build();
+		when(mockSession.sendRequest(eq(McpSchema.METHOD_ROOTS_LIST), eq(new McpSchema.PaginatedRequest("cursor")),
+				any(TypeRef.class)))
+			.thenReturn(Mono.just(result));
+
+		StepVerifier.create(exchange.listRoots("cursor")).expectNext(result).verifyComplete();
+		verify(mockSession).sendRequest(eq(McpSchema.METHOD_ROOTS_LIST), eq(new McpSchema.PaginatedRequest("cursor")),
+				any(TypeRef.class));
+	}
+
 	@Test
 	void testListRootsWithMultiplePages() {
 

mcp-test/src/main/java/io/modelcontextprotocol/AbstractMcpClientServerIntegrationTests.java

@@ -1129,9 +1129,10 @@ void testRootsWithoutCapability(String clientType) {
 			.tool(Tool.builder("tool1", EMPTY_JSON_SCHEMA).description("tool1 description").build())
 			.callHandler((exchange, request) -> {
 
-				exchange.listRoots(); // try to list roots
+				assertThatThrownBy(exchange::listRoots).isInstanceOf(IllegalStateException.class)
+					.hasMessage("Client must be configured with root listing capabilities");
 
-				return mock(CallToolResult.class);
+				return CallToolResult.builder().addTextContent("Roots capability rejected").build();
 			})
 			.build();
 
@@ -1145,13 +1146,10 @@ void testRootsWithoutCapability(String clientType) {
 
 			assertThat(mcpClient.initialize()).isNotNull();
 
-			// Attempt to list roots should fail
-			try {
-				mcpClient.callTool(McpSchema.CallToolRequest.builder("tool1").arguments(Map.of()).build());
-			}
-			catch (McpError e) {
-				assertThat(e).isInstanceOf(McpError.class).hasMessage("Roots not supported");
-			}
+			CallToolResult result = mcpClient
+				.callTool(McpSchema.CallToolRequest.builder("tool1").arguments(Map.of()).build());
+			assertThat(result.isError()).isFalse();
+			assertThat(result.content()).containsExactly(new McpSchema.TextContent("Roots capability rejected"));
 		}
 		finally {
 			mcpServer.closeGracefully();