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Fix listRoots() pagination on an empty nextCursor #1275

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Fix listRoots() pagination on an empty nextCursor #1275

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6 changes: 4 additions & 2 deletions mcp-core/src/main/java/io/modelcontextprotocol/server/McpAsyncServerExchange.java
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Original file line number Diff line number Diff line change
Expand Up @@ -207,8 +207,10 @@ public Mono<McpSchema.ListRootsResult> listRoots() {

// @formatter:off
return this.listRoots(McpSchema.FIRST_PAGE)
.expand(result -> (result.nextCursor() != null) ?
this.listRoots(result.nextCursor()) : Mono.empty())
.expand(result -> {
String next = result.nextCursor();
return (next != null && !next.isEmpty()) ? this.listRoots(next) : Mono.empty();
})
.reduceWith(() -> McpSchema.ListRootsResult.builder(new ArrayList<>()).build(),
(allRootsResult, result) -> {
allRootsResult.roots().addAll(result.roots());
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28 changes: 28 additions & 0 deletions mcp-core/src/test/java/io/modelcontextprotocol/server/McpAsyncServerExchangeTests.java
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Original file line number Diff line number Diff line change
Expand Up @@ -160,6 +160,34 @@ void testListRootsWithEmptyResult() {
}).verifyComplete();
}

@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));

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));
}

@Test
void testListRootsWithSpecificCursor() {

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