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McpAsyncServerExchange#listRoots() keeps paginating when nextCursor is an empty string #1274
Description
Bug description
McpAsyncServerExchange#listRoots() follows nextCursor until it is null. If a client answers roots/list with "nextCursor": "" instead of omitting the field, the server sends roots/list again with "cursor": "". A client that keeps answering that way makes listRoots() never complete, while the roots of every page accumulate. McpSyncServerExchange#listRoots() calls .block() on it, so the calling thread hangs.
The client had the same problem (#575), fixed in #954: McpAsyncClient stops on next != null && !next.isEmpty() in listTools, listResources, listResourceTemplates and listPrompts. listRoots() on the server exchange is the last aggregate list method in mcp-core without that guard.
Affected: main, 2.1.x, 2.0.x and 1.1.x.
Environment
main @ 54e8b61 (2.2.0-SNAPSHOT). Java 21.0.5, macOS.
Steps to reproduce
- Connect a client whose
roots/listhandler returns its roots with"nextCursor": "". - Call
exchange.listRoots()from a server tool, resource or prompt handler. - The server sends another
roots/listrequest with"cursor": "".
Expected behavior
An empty nextCursor ends pagination, as it does in the client list methods since #954.
Minimal Complete Reproducible example
Failing test in McpAsyncServerExchangeTests (mcp-core):
@Test
void testListRootsStopsOnEmptyNextCursor() {
List<McpSchema.Root> page1Roots = Arrays
.asList(McpSchema.Root.builder("file:///home/user/project1").name("Project 1").build());
List<McpSchema.Root> page2Roots = Arrays
.asList(McpSchema.Root.builder("file:///home/user/project2").name("Project 2").build());
McpSchema.ListRootsResult page1Result = McpSchema.ListRootsResult.builder(page1Roots).nextCursor("").build();
McpSchema.ListRootsResult page2Result = McpSchema.ListRootsResult.builder(page2Roots).build();
when(mockSession.sendRequest(eq(McpSchema.METHOD_ROOTS_LIST), eq(new McpSchema.PaginatedRequest()),
any(TypeRef.class)))
.thenReturn(Mono.just(page1Result));
when(mockSession.sendRequest(eq(McpSchema.METHOD_ROOTS_LIST), eq(new McpSchema.PaginatedRequest("")),
any(TypeRef.class)))
.thenReturn(Mono.just(page2Result));
// Fails on main: a second roots/list request is sent with an empty cursor
StepVerifier.create(exchange.listRoots()).assertNext(result -> {
assertThat(result.roots()).extracting(McpSchema.Root::uri).containsExactly("file:///home/user/project1");
assertThat(result.nextCursor()).isNull();
}).verifyComplete();
verify(mockSession, times(1)).sendRequest(eq(McpSchema.METHOD_ROOTS_LIST),
any(McpSchema.PaginatedRequest.class), any(TypeRef.class));
}
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