[mcp] Tool calls return empty results when invoked from external MCP client

[naturzukunft]

Environment

• Spring Tools Version: 5.0.1.RELEASE (Build Id: 202512192126, Revision: ac8ca43)
• IDE: Spring Tool Suite for Eclipse
• OS: TUXEDO OS 24.04 (Ubuntu-based)
• MCP Client: Claude Code CLI (Anthropic)

Problem

MCP tools execute without errors but return empty/no data when called from an external MCP client.

Steps to Reproduce

1. Enable embedded MCP server in STS preferences (AI section)
2. Restart STS with a Spring Boot project open (multi-module Maven project)
3. Configure Claude Code to connect:

   "spring-tools-mcp": { 
     "url": "http://127.0.0.1:50627/sse", 
     "type": "sse" 
   }

4. Call any MCP tool, e.g.:
   • getProjectList() (no parameters)
   • getSpringBootVersion(projectName: "my-project")
   • getRequestMappings(projectName: "my-project")

Expected Behavior

Tools should return data, e.g. getProjectList() returns:

[{"name": "my-spring-project", ...}]

Actual Behavior

• Tools are discovered correctly by the MCP client
• Tool execution completes without error
• Response is empty/null (no data returned)

Additional Context

• MCP connection works (tools are listed and callable)
• Project visible in Package Explorer and Boot Dashboard
• Spring indexing completed
• Tested with different project name variations

Questions

• Is there debug logging available for MCP tool execution?
• Are there specific prerequisites for projects to be recognized by the MCP server?
• Does the MCP server require a specific project structure or import method?

[naturzukunft]

Additional Diagnostic Information

After further testing, I found the following:

Server Log Analysis

The Spring Boot Language Server starts correctly and shows:

• MCP Server initialized on port 50627
• 13 tools registered successfully
• All 19 projects in the workspace are discovered and indexed
• Spring Symbol Index completes successfully

Key Finding: Requests Not Reaching Server

When invoking MCP tools (e.g., getProjectList()), nothing appears in the server log. The tool calls don't seem to reach the server at all.

However, when I kill the client process, the following error appears in the server log:

13:55:56.477 [http-nio-127.0.0.1-50627-exec-5] ERROR i.m.spec.McpServerSession - No handler registered for notification method: notifications/cancelled

This indicates:

1. The SSE connection IS established (cancelled notification was received)
2. But tool call requests may not be processed correctly
3. The notifications/cancelled method is not implemented

Environment Details

• MCP Client config: { "url": "http://127.0.0.1:50627/sse", "type": "sse" }
• Java: 25.0.1 (bundled with STS)
• Spring Boot Language Server: 2.0.1-SNAPSHOT

[martinlippert]

I tried this with Spring Tools 5.0.1 for Eclipse running with two simple projects in the workspace, having the MCP server enabled, and configured in a VSCode session, and the tool calls seem to go through just fine.

Can you try this again with just one simple Maven-based project in your Eclipse / Spring Tools workspace to see if that works?

[martinlippert]

And how did you try to call the tools of the MCP server exactly?

[naturzukunft]

Update: Issue no longer reproducible

I can no longer reproduce the empty results problem. After retesting with the same project, all MCP tools now return correct data:

Tool	Result
getProjectList()	✅ 19 projects detected
getRequestMappings(projectName)	✅ 8 endpoints found
getBeanDetails()	✅ 168 beans with full metadata
findComponentsByStereotype(projectName, stereotype)	✅ Works correctly

What changed:

• Not sure - possibly Eclipse restart, workspace refresh, or something on the Spring Tools side

Environment:

• Eclipse with Spring Tools 4
• Multi-module Maven project (19 modules, Java 25, Spring Boot 3.x)
• MCP connection via Claude Code CLI

Minor observation: getProjectList() response has a typo: projectNane instead of projectName.

Closing this issue as I cannot reproduce it anymore. Feel free to reopen if you need more information.

[martinlippert]

Thanks for the update, glad to hear that it is working now. And thanks for the pointer to the typo, will fix.

[naturzukunft]

Hah, I started a new work session, and now it's not working again. I'll keep an eye on it.

[martinlippert]

Hah, I started a new work session, and now it's not working again. I'll keep an eye on it.

😱 ... That is a little weird. Does it work more reliable when you have a small sample project in your IDE that you work with?

[naturzukunft]

OK, I wanted to take another look at it today. I had an additional test project in the workspace and thought that might be the reason. But today everything is working again, including the test project. So there is one fairly large project and one mini project in the workspace, and MCP is working.