Reject listRoots if not supported by client, without sending any request

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
82/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Quiet
Tech stack
java
Domain
api

Research direction

Read mcp-core/src/main/java/io/modelcontextprotocol/server/McpAsyncServerExchange.java, especially the existing capability check around lines 141-151 and the listRoots entry point around lines 227-230. Confirm that an unsupported roots capability is rejected before any request is sent, with the specified error behavior, then run the relevant project tests.

Written by the indexing model from the issue text.

Description

enhancement good first issue P3

Some requests in McpAsyncServerExchange make a point of failing fast if functionality not supported, such the condition on line 146

https://github.com/modelcontextprotocol/java-sdk/blob/fd004989b9484c9b81be6b03463396797b354804/mcp-core/src/main/java/io/modelcontextprotocol/server/McpAsyncServerExchange.java#L141-L151

listRoots does not do the same

https://github.com/modelcontextprotocol/java-sdk/blob/fd004989b9484c9b81be6b03463396797b354804/mcp-core/src/main/java/io/modelcontextprotocol/server/McpAsyncServerExchange.java#L227-L230

For example, add this

if (this.clientCapabilities.roots() == null) { 
    return Mono.error(new IllegalStateException("Client must be configured with root listing capabilities")); 
}
Dominant language
Java
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