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The Mono returned by listTools, listResources, listResourceTemplates,
listPrompts and listRoots captured a single mutable accumulator created when
the method was called, so every subscription kept appending to it:
Mono<ListRootsResult> listRoots = exchange.listRoots();
listRoots.block().roots().size(); // 2
listRoots.block().roots().size(); // 4
listRoots.block().roots().size(); // 6
Concurrent subscriptions also raced on the shared ArrayList.
Use reduceWith(Supplier, accumulator) so that a fresh accumulator is created
for every subscription. A single subscription behaves exactly as before.
Kehrlann
approved these changes
Oct 13, 2026
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Motivation and Context
The Mono returned by
listTools,listResources,listResourceTemplates,listPromptsand
listRootsis not safe to subscribe more than once:reduce(seed, accumulator)captures one mutable seed instance created when the method is called, so every
subscription keeps appending to it.
Concurrent subscriptions also race on the shared
ArrayList.Description
Replaced
reduce(seed, accumulator)withreduceWith(Supplier, accumulator)at the fivecall sites, so a fresh accumulator is created per subscription. Behavior for a single
subscription is unchanged.
Fixes #1174
How Has This Been Tested?
McpAsyncServerExchangeTests#testListRootsCanBeResubscribed, which subscribes thesame Mono twice and asserts both subscriptions see the same number of roots. It fails
before the change (2 then 4) and passes after.
AbstractMcpAsyncClientTests, which runsagainst the SSE, stdio and Streamable HTTP transports.
./mvnw -pl mcp-core spring-javaformat:apply./mvnw -pl mcp-core test(73 tests in the affected classes pass)Types of changes