McpStatelessSyncServer should preemptively register a no-op handler for resources/subscribe

[rombert]

Bug description
Certain non-compliant MCP clients try to subscribe for resource updates even if the MCP server reports no support for subscription.

I have seen this behaviour with Cursor 2.4.27 and 2.4.28.

This leads to the MCP server not intialising in Cursor due to internal errors: Failed to handle request: Missing handler for request type: resources/subscribe" .

Environment
MCP Java SDK 0.17.0
Cursor

Steps to reproduce

I am using a pretty standard MCP server setup

        syncServer = McpServer.sync(transportProvider)
                .serverInfo(config.serverTitle(), serverVersion)
                .jsonMapper(jsonMapper)
                .jsonSchemaValidator(new DefaultJsonSchemaValidator())
                .instructions(config.instructions())
                .completions(completions)
                .capabilities(ServerCapabilities.builder()
                        .tools(false)
                        .prompts(false)
                        .resources(false, false)
                        .completions()
                        .build())
                .build();

When connecting to the server from Cursor the connection fails

2026-02-06 15:15:15.122 [error] Client error for command Streamable HTTP error: Error POSTing to endpoint: (...) "message":"Failed to handle request: Missing handler for request type: resources/subscribe","suppressed":[],"localizedMessage":"Failed to handle request: Missing handler for request type: resources/subscribe"}

Expected behavior

I would expect the server to gracefully handle non-compliant clients that try to subscribe to updates. Failing that, I should be able to register a handler myself.

However, the API does not allow that.

I had to resort to reflectiont to access HttpServletStatelessServerTransport → mcpHandler → requestHandlers and then register a no-op handler

handlers.put(McpSchema.METHOD_RESOURCES_SUBSCRIBE, (context, params) -> Mono.just(Map.of()));

This is brittle and I would welcome either:

• a default handler that prevents this error from happening
• the ability to register handlers directly using a low-level API

Minimal Complete Reproducible example
N/A, sorry. But if the problem is not clear from the report let me know and I'll set something up.