So...
Avoiding code and returning did the trick!
Instead of using JsonConverter, you need to use the generic version of JsConfig when changing / overriding ServiceStack behavior in a specific class, just attach the following to your services start code, for example.
JsConfig<Claim>.SerializeFn = claim => string.Format("{0}|{1}", claim.Type, claim.Value); JsConfig<Claim>.DeSerializeFn = claimDetails => { var values = claimDetails.Split('|'); return new Claim(values[0], values[1]); };
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