Decoding response body "on the fly" in an Okhttp Interceptor

I'm developing an Android App using Retrofit & OkHttp.

The backend i'm facing is of questionable quality:

It bypasses the http-error-code-system for a "homemade" solution. Instead of sending the appropriate error code, it returns a HTTP-200 with a body containing { "errorcode" : 404 }.

Since i can't change the backend, i'd like to cleanup this mess as soon as it enters my app; a Okhttp interceptor seems like the right place:

 okHttpClient
     .addInterceptor(responseBodyInterceptor(json))
     [...]
     .build()


@Throws(BackendError::class)
private fun responseBodyInterceptor(json: Json) = Interceptor { chain ->
    chain.proceed(chain.request()).also { response ->
       
        // retrieving the whole body here - similar to the official logging interceptor 
        val body = response.peekBody(Long.MAX_VALUE).string()

        val bodyObject = json.parseToJsonElement() as? JsonObject

        val errorcode = (bodyObject?.get("errorcode") as? JsonPrimitive)?.content

        if (errorcode != null) throw BackendError(errorcode)
    }
}

I'm now looking for a more elegant way to do this. My issue here is that i'm parsing the complete network response at once. I'd like to parse it somewhat similar to a stream. Is this possible?