Sharepoint 2013 workflow. Limits with JSON response?

Hi, 

I'm making a service call in a 2013 workflow. The JSON response is something like...
{
    "d": {
        "__type": "DOC_WCF_Service.DocRecord",
        "docTitle": "This is the document title and it has to be just this long.....................", 
        "docType": "Pdf"
    }
}

I can only place the response into a dictionary variable if I keep the docTitle length down to about 60 characters. Above that and the service fails and the workflow cancels, referring to a HTTP 500 code in the error message. 

Can you advise if there are limits with handling service responses, or if there might be some other configuration change I need to address please? 


Thanks

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Not an MVP