CallToolRequest Deserialization Error in McpAsyncServer

[cobach]

Motivation and Context

The MCP Java SDK v0.10.0 has a bug where tools/call requests fail with a deserialization error:

Cannot construct instance of `io.modelcontextprotocol.spec.McpSchema$CallToolRequest` 
(although at least one Creator exists): no String-argument constructor/factory 
method to deserialize from String value

This occurs because McpAsyncServer.toolsCallRequestHandler() attempts to directly convert the params object to CallToolRequest, but params arrives as a LinkedHashMap (generic JSON object) from the transport layer, not as a typed instance.

How Has This Been Tested?

• Created test client using official Node.js SDK that calls tools
• Tested with multiple tools (SystemInformation, DomainAvailability)
• Verified the fix resolves the deserialization error
• All tool calls execute successfully after applying the fix
• Existing tests continue to pass

Test setup:

// Using official Node.js SDK
await client.callTool({
  name: 'any-tool',
  arguments: {}
});

Breaking Changes

No breaking changes. This is a bug fix that maintains the existing API. The fix adds proper type handling for the params object.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

The fix updates McpAsyncServer.toolsCallRequestHandler() to properly handle the conversion:

private McpServerSession.RequestHandler<CallToolResult> toolsCallRequestHandler() {
    return (exchange, params) -> {
        McpSchema.CallToolRequest callToolRequest;
        
        if (params instanceof McpSchema.CallToolRequest) {
            // Already the correct type
            callToolRequest = (McpSchema.CallToolRequest) params;
        }
        else if (params instanceof java.util.Map) {
            // Expected case - params is a Map representing the CallToolRequest
            try {
                callToolRequest = objectMapper.convertValue(params, McpSchema.CallToolRequest.class);
            } catch (Exception e) {
                throw new IllegalArgumentException("Invalid params for tools/call: " + params, e);
            }
        }
        else {
            // Fallback for other types
            try {
                callToolRequest = objectMapper.convertValue(params, McpSchema.CallToolRequest.class);
            } catch (Exception e) {
                throw new IllegalArgumentException("Cannot convert params to CallToolRequest: " + params, e);
            }
        }
        
        // ... rest of the method remains the same
    };
}

This issue affects all tool calls made through the MCP protocol when using the Java SDK as the server.

[tzolov]

@cobach , given, should we class this issue as well?

[cobach]

Yes. Thank you very much.