Assistant doesn't use the tool result

I have a custom tool with oauth credentials set up in vapi.
I can see proper requests and responses on my backend side.
However in the vapi dashboard I can see something like this in message tab:
"name": "getSubmissionDetails",
"role": "tool_callresult",
"time": 1760989282966,
"result": "Success.",
"toolCallId": "call
...",
"secondsFromStart": 5.468

and the assistant doesn't use the information provided by the tool.
The response of my server to tool request is this:
{"results":[{"toolCallId":"call_...","result":"here's a plain text string of the response"}]}
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