Webhook call failed. Error: Failed to parse webhook JSON response: Expect message object but got: [Chinese letters] - httprequest

Question

I'm building my own WebhookClient for dialog flow. My code is the following (using Azure Functions, similar to Firebase Functions):
module.exports = async function(context, req) {
const agent = new WebhookClient({ request: context.req, response: context.res });
function welcome(agent) {
agent.add(`Welcome to my agent!!`);
}
let intentMap = new Map();
intentMap.set("Look up person", welcome);
agent.handleRequest(intentMap);
}
I tested the query and the response payload looks like this:
{
"fulfillmentText": "Welcome to my agent!!",
"outputContexts": []
}
And the headers in the response look like this:
Transfer-Encoding: chunked
Content-Type: application/json; charset=utf-8
Server: Microsoft-IIS/10.0
X-Powered-By: ASP.NET
Date: Tue, 11 Dec 2018 18:16:06 GMT
But when I test my bot in dialog flow, it returns the following:
Webhook call failed. Error: Failed to parse webhook JSON response:
Expect message object but got:
"笀ഀ਀  ∀昀甀氀昀椀氀氀洀攀渀琀吀攀砀琀∀㨀 ∀圀攀氀挀漀洀攀 琀漀 洀礀 愀最攀渀琀℀℀∀Ⰰഀ਀  ∀漀甀琀瀀甀琀䌀漀渀琀攀砀琀猀∀㨀 嬀崀ഀ਀紀".
There's Chinese symbols!? Here's a video of me testing it out in DialogFlow: https://imgur.com/yzcj0Kw

Answer 1

I know this should be a comment (as it isn't really an answer), but it's fairly verbose and I didn't want it to get lost in the noise.
I have the same problem using WebAPI on a local machine (using ngrok to tunnel back to Kestrel). A friend of mine has working code (he's hosting in AWS rather than Azure), so I started examining the differences between our responses. I've notice the following:
This occurs with Azure Functions and WebAPI (so it's not that)
The JSON payloads are identical (so it's not that)
Working payload isn't chunked
Working payload doesn't have a content type
As an experiment, I added this code to Startup.cs, in the Configure method:
app.Use(async (context, next) =>
{
var original = context.Response.Body;
var memory = new MemoryStream();
context.Response.Body = memory;
await next();
memory.Seek(0, SeekOrigin.Begin);
if (!context.Response.Headers.ContentLength.HasValue)
{
context.Response.Headers.ContentLength = memory.Length;
context.Response.ContentType = null;
}
await memory.CopyToAsync(original);
});
This code disables response chunking, which is now causing a new and slightly more interesting error for me in the google console:
*Webhook call failed. Error: Failed to parse webhook JSON response: com.google.gson.stream.MalformedJsonException: Unterminated object at line 1 column 94 path $.\u0000\\"\u0000f\u0000u\u0000l\u0000f\u0000i\u0000l\u0000l\u0000m\u0000e\u0000n\u0000t\u0000M\u0000e\u0000s\u0000s\u0000a\u0000g\u0000e\u0000s\u0000\\"\u0000.\
I thought this could be encoding at first, so I stashed my JSON as a string and used the various Encoding classes to convert between them, to no avail.
I fired up Postman and called my endpoint (using the same payload as Google) and I can see the whole response payload correctly - it's almost as if Google's end is terminating the stream part-way through reading...
Hopefully, this additional information will help us figure out what's going on!
Update
After some more digging and various server/lambda configs, I spotted this post here: https://github.com/googleapis/google-cloud-dotnet/issues/2258
It turns out that json.net IS the culprit! I guess it's something to do with the formatters on the way out of the pipeline. In order to prove this, I added this hard-coded response to my POST controller and it worked! :)
return new ContentResult()
{
Content = "{\"fulfillmentText\": null,\"fulfillmentMessages\": [],\"source\": null,\"payload\": {\"google\": {\"expectUserResponse\": false,\"userStorage\": null,\"richResponse\": {\"items\": [{\"simpleResponse\": {\"textToSpeech\": \"Why hello there\",\"ssml\": null,\"displayText\": \"Why hello there\"}}],\"suggestions\": null,\"linkOutSuggestion\": null}}}}",
ContentType = "application/json",
StatusCode = 200
};

Answer 2

Despite the HTTP header saying the charset is utf-8, that is definitely using the utf-16le character set, and then the receiving side is treating them as utf-16be. Given you're running on Azure, it sounds like there is some configuration you need to make in Azure Functions to represent the output as UTF-8 instead of using UTF-16 strings.