Bug: attempt_completion toolcall is often written in plaintext json instead of called

[Mushoz]

Tool calls are working extremely well with the proxy now that I fixed my user error for stuff like opening files, editing files, searching the codebase etc. However, the attempt_completion tool call is often being called in plaintext json, eg returned as text in the chat:

{
"name": "attempt_completion",
"arguments": {
"result": "Result text here"
}
}

[Mushoz]

It seems a bit random. It's alternating between working fine, and using plaintext json in the chat. Not sure why it's sometimes working fine, but other times it's not.

[irreg]

This is my current understanding. It is likely that the LLM returned JSON as a message rather than a tool call.

In the JSON for a tool call, arguments are of type string.
Therefore, if the tool call result was returned incorrectly, it should appear as follows:

{
"name": "attempt_completion",
"arguments": "{\"result\": \"Result text here\"}"
}

After replacing the XML-formatted description sent by Roo-Code, the remaining description may have confused the LLM, so I will attempt to improve it.

[irreg]

I would appreciate it if you could check whether it has been improved.

[kpoineal]

I’ll check when I get a chance tonight or tomorrow. Thanks for this!

[Mushoz]

It doesn't call the tool in plain text anymore, but now it often doesn't call the tool at all. Eg, it just gives an answer in regular text without a tool call, and then eventually Roocode complains that the model is not responding with any tool calls.

[irreg]

Currently, I'm unable to reproduce the issue on my end.
There may be specific factors, such as the language, framework, development scale, or MCP server used in vibe coding.

Do you know if there are specific conditions under which it occurs, such as not happening in small-scale tests?
Would it be possible to share information about the environment where it is occurring?

Model name: gpt-oss-120b (URL where the model is published), etc.
LLM server type: llama.cpp (+revision number), etc.