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

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

java-sdk/mcp-core/src/main/java/io/modelcontextprotocol/server/McpAsyncServerExchange.java

Lines 141 to 151 in fd00498

    
           public Mono<McpSchema.CreateMessageResult> createMessage(McpSchema.CreateMessageRequest createMessageRequest) { 
        
           	if (this.clientCapabilities == null) { 
        
           		return Mono 
        
           			.error(new IllegalStateException("Client must be initialized. Call the initialize method first!")); 
        
           	} 
        
           	if (this.clientCapabilities.sampling() == null) { 
        
           		return Mono.error(new IllegalStateException("Client must be configured with sampling capabilities")); 
        
           	} 
        
           	return this.session.sendRequest(McpSchema.METHOD_SAMPLING_CREATE_MESSAGE, createMessageRequest, 
        
           			CREATE_MESSAGE_RESULT_TYPE_REF); 
        
           }

listRoots does not do the same

java-sdk/mcp-core/src/main/java/io/modelcontextprotocol/server/McpAsyncServerExchange.java

Lines 227 to 230 in fd00498

    
           public Mono<McpSchema.ListRootsResult> listRoots(String cursor) { 
        
           	return this.session.sendRequest(McpSchema.METHOD_ROOTS_LIST, new McpSchema.PaginatedRequest(cursor), 
        
           			LIST_ROOTS_RESULT_TYPE_REF); 
        
           }

For example, add this

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

I reproduced this on current main at c7fef64f92a99c8c758b1aa634f92868d7c2963b.

A bounded local patch adding the missing initialized-client / roots-capability guard to McpAsyncServerExchange.listRoots(String cursor), plus a regression test asserting that unsupported roots fail before session.sendRequest, passes the project's formatter and focused McpAsyncServerExchangeTests.

I have not opened a PR yet because CONTRIBUTING asks that non-trivial changes be discussed first. If this scope matches what maintainers want for #1067, I can prepare the focused patch and run the broader required validation before submitting.

No production access or credentials were involved.

Hi, I have a fix for McpAsyncServerExchange.listRoots that mirrors the fast-fail capability checks already used by createMessage and createElicitation in the same class. The full mcp-core test suite passes with the change. I noticed PRs are already open for this issue. Before adding a sixth, could a maintainer confirm the desired scope and whether a new PR would help here, or whether you would prefer to pick up one of the existing ones?

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Reject listRoots if not supported by client, without sending any request #1067

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	public Mono<McpSchema.CreateMessageResult> createMessage(McpSchema.CreateMessageRequest createMessageRequest) {
	if (this.clientCapabilities == null) {
	return Mono
	.error(new IllegalStateException("Client must be initialized. Call the initialize method first!"));
	}
	if (this.clientCapabilities.sampling() == null) {
	return Mono.error(new IllegalStateException("Client must be configured with sampling capabilities"));
	}
	return this.session.sendRequest(McpSchema.METHOD_SAMPLING_CREATE_MESSAGE, createMessageRequest,
	CREATE_MESSAGE_RESULT_TYPE_REF);
	}

	public Mono<McpSchema.ListRootsResult> listRoots(String cursor) {
	return this.session.sendRequest(McpSchema.METHOD_ROOTS_LIST, new McpSchema.PaginatedRequest(cursor),
	LIST_ROOTS_RESULT_TYPE_REF);
	}

Reject listRoots if not supported by client, without sending any request #1067

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soyeladice-svg commented on Sep 25, 2026

sushma3690 commented on Oct 7, 2026

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