NullPointerException for tools that return void.

[krishnanmms]

For tools that return void or null, it results in an NPE; since the output of the tool method is processed without checking for null.

In class McpServerTool (refer to NPE comment),

 @Override
  public McpSchema.CallToolResult apply(McpSyncServerExchange ex, McpSchema.CallToolRequest req) {

    Object result;
    boolean isError = false;
    try {
      Map<String, Object> arguments = req.arguments();
      List<Object> convertedParams = converter.convertAllParameters(methodCache, arguments);
      // Use cached method for invocation
      result = methodCache.getMethod().invoke(instance, convertedParams.toArray());
    } catch (Exception e) {
      log.error("Error invoking tool method: {}", methodCache.getMethodSignature(), e);
      result = e + ": " + e.getMessage();
      isError = true;
    }
    McpSchema.Content content = new McpSchema.TextContent(result.toString()); // NPE : result, can be null
    return new McpSchema.CallToolResult(List.of(content), isError);
  }

I am fairly new to this library (and to MCP). Apologies in case I have made a mistake in reporting this issue.

[codeboyzhou]

@krishnanmms Good catch! Thank you for your feedback. Generally speaking, most MCP tools will return valid feedback content to LLMs. However, the situation of returning nullable or void should also be taken into consideration, at least should not get the NPE, so I enhanced here and now (released v0.8.2) it will return the content This tool returned nullable or void instead of throwing NPE. Please feel free to let me know if you encounter any other issues.

[krishnanmms]

Thanks for your prompt fix. Tried with 0.8.2 and the problem seems to be fixed.

I had number of my tool methods (fire & forget) that returned void, and I stumbled into this problem (wasn't intentionally trying to break it). As you have pointed out, this may not be a best practice.