Could not connect to an Spring boot MCP server with STDIO

[deleSerna]

Describe the bug
I am newbie in the MCP and wanted to start a MCP server based on spring boot project. I have used a sample spring boot project from here. But when I tried to connect to the MCP sever via npx @modelcontextprotocol/inspector, with the STDIO protocol, I am running into the following issue.
STDIO transport: command=java , args=[-jar, replace-with-full-path/spring_mcp-0.0.1-SNAPSHOT.jar] Error in /stdio route: Error: spawn java ENOENT at ChildProcess._handle.onexit (node:internal/child_process:286:19) at onErrorNT (node:internal/child_process:484:16) at process.processTicksAndRejections (node:internal/process/task_queues:90:21) { errno: -2, code: 'ENOENT', syscall: 'spawn java ', path: 'java ', spawnargs: [ '[-jar', 'replace-with-full-path/spring_mcp-0.0.1-SNAPSHOT.jar]' ] }
I am using modelcontextprotocol/inspector version 0.16.3

To Reproduce
Steps to reproduce the behavior:

1. Build the spring boot sample
2. Start the inspector using either npx @modelcontextprotocol/inspector -- 'java -jar replace-with-full-path/spring_mcp-0.0.1-SNAPSHOT.jar' or npx @modelcontextprotocol/inspector and in the UI fill java as command and provide ["-jar" " replace-with-full-path//spring_mcp-0.0.1-SNAPSHOT.jar"]as the argument. 3 SelectSTDIO` as the transport type.
   4 Press connect button and observe the reported error in the terminal where inspector has started.
   5 Previously I also tried with a StreamableHttp sample but that also had similar issues

Expected behavior
Inspector is able to connect to the MCP server and tools will be listed in the UI.

Logs
If applicable, add logs to help explain your problem.

Additional context

• I have explicitly ran the spring_mcp-0.0.1-SNAPSHOT.jar and made sure that there is no issue with the jar.
• Java is spawned from a sample javascript to make sure that there is no issue in spawning java as a sub process.
• Executer permission has explicitly given to the spring_mcp-0.0.1-SNAPSHOT.jar
• Java version that is used 17.0.15+6-LTS an d Mac OS sequoia 15.5

Therefore, at this point I am exhausted all options to debug the issue. I feel that I am missing something very basic but I could not figure out what is it. Therefore, can some one please shed some light to fix the issue. Happy to provide any other relevant info if it's needed.

[richardkmichael]

👋
In the Inspector UI, the Arguments should not be in wrapped in brackets ("array notation"), just written space-separated, as on the command line.

So if the shell command is: java -jar /path/to/spring_mcp-0.0.1-SNAPSHOT.jar, then the Inspector UI values should be:

Command: java
Arguments: -jar /path/to/spring_mcp-0.0.1-SNAPSHOT.jar

Using npx, so the UI values are pre-filled:

npx @modelcontextprotocol/inspector java -jar /path/to/spring_mcp-0.0.1-SNAPSHOT.jar

[deleSerna]

should not be in wrapped in brackets ("array notation"),`

Tried that but same issue

[richardkmichael]

I've just noticed there seems to be a trailing space in the java command.

[deleSerna]

@richardkmichael Thank you. That's it. After removing the trailing space in the java command ( and also removing the array notation from the arguments), it worked fine.

IMO, it is still a bug as I would expect the command to be trimmed if the extra space is an issue. Therefore, I will keep the issue open.

[richardkmichael]

@deleSerna If this PR lands, it will fix the problem. Sorry about the back-and-forth on this! I shouldn't have glossed over ENOENT 🤦 , I figured it was a problem with java not on PATH which you'd recognize, and the Arguments notation caught my attention.

FWIW, this notation ["-jar" "replace-with-full-path//spring_mcp-0.0.1-SNAPSHOT.jar"] would result in this error in the sidebar:

Error: Could not find or load main class [-jar Caused by: java.lang.ClassNotFoundException: [-jar 

It would be tricky to detect malformed arguments, because there's likely to be a lot of variety. Validity detection would need heuristics, e.g., the unlikely presence of shell-special characters. I'm not sure it would be worth the effort (and potential false-positives), hopefully that error message would've been a sufficient hint?