Frontend MCP Tool Invocation Issue After Reload

[NikshipthYathindra]

Discussion

Hi,
I recently connected my MCP client to my Theia codebase, where Theia is acting as the MCP server. During the initial connection, the client is able to successfully call frontend tools and execute their functions. However, after reloading the Theia frontend, the client is no longer able to execute the frontend tools.

Is this expected behavior, or could there be an issue with how the frontend tools or MCP connection lifecycle is handled after a reload?

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[sdirix]

Hi @NikshipthYathindra,

Yes, this is expected behavior. Each frontend connection gets a fresh UUID as its delegateId , and tools are registered with the pattern {toolName}_{delegateId}). When you reload, the old connection closes and a new connection is established with a new UUID.

I verified this with the Theia example application. Before reload, the example frontend tools were exposed as:

• workspace-info_55e978c0-30b2-45a3-b9f1-93362f08b962
• workspace-files_55e978c0-30b2-45a3-b9f1-93362f08b962

After reload, they became:

• workspace-info_aa654971-8a89-44b8-9af9-8ace34c4322a
• workspace-files_aa654971-8a89-44b8-9af9-8ace34c4322a

So any MCP client holding references to the old tool names will fail after a reload.

If you want stable IDs, you could replace the generateUuid() call with something deterministic or remove it altogether if you always have a single frontend.

We could also consider changing Theia's default behavior to, for example, use the workspace path as the base ID, only appending a counter when multiple frontends have the same workspace open. That would give predictable tool names across reloads. Happy to discuss if you'd like to open a feature request for this.

[NikshipthYathindra]

Hi @sdirix, Thanks for the reply. I tried reconnecting my MCP client to the MCP server after reloading the frontend. I verified that the frontend tools were updated with a new delegateId, and I triggered the tool call using this updated delegateId (not the previous one). However, the tool call still fails. The tools start working only after I restart and reload the frontend.

[sdirix]

I tested this with the Theia example application and the tools work fine after a frontend reload. I was able to list the new tools with the updated delegateId and call them successfully.

Could you describe the exact steps to reproduce your issue? I'm having trouble understanding the scenario:

• If you reload the frontend, you get new delegateIds so you'd need to re-fetch the tool list and use the new tool names
• If the tools are listed correctly with new IDs but still fail when called, that would be a different issue

You wrote the following:

The tools start working only after I restart and reload the frontend.

Do you mean after a backend restart?