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Custom Connector for Asynchronous Function

I am trying to build and use a custom connector to call an asynchronous function that is hosted in Azure Function Services. The asynchronous functions returns HTTP 202 accepted and the URL of another function in the same API that can be polled for the status and the final result. It works as intended when I call this function with an HTTP connector but called with a custom connector MS Flow cannot access the of status function and returns HTTP 404 resource not found. I guess this is related to the fact that MS Flow does not call custom connectors directly but via a proxy. Any ideas on how to solve this?



Async function:

module.exports = function (context, req) {
   context.res = {
      status: 202,
         headers: {
              "location": "",
               "retry-after": 10
      body: { result: "try again!"}
module.exports = function (context, req) {
   context.res = {
      status: 200,
      body: { result: { import_status: "done", msg: "Oh yeah, this is the result" } }
Community Support
Community Support

Hi @rekodus,


Have you found the solution to achieve your requirement?


Best regards,


Frequent Visitor

@rekodus @v-yuazh-msft I'm a few years late but this was the top result so hopefully my solution helps others.

Custom Connectors use an APIM Proxy, and the response location header ends up getting re-written with the APIM endpoint. So even though your function app is returning a 202 with the location header properly set the custom connector re-writes that to use the APIM proxy. When you enable the async pattern on the action in a flow Power Automate will try to call that transformed location header and I'm guessing since it's a different endpoint the APIM returns a 404.

To test this out I created a C# function app to mock out the async pattern:

public static async Task<IActionResult> Start([HttpTrigger(AuthorizationLevel.Anonymous, "get", "post", Route = null)] HttpRequest req, ILogger log)
    return new AcceptedResult($"{req.Scheme}://{req.Host}/api/Status", "Please Hold");

public static async Task<IActionResult> Status([HttpTrigger(AuthorizationLevel.Anonymous, "get", "post", Route = null)] HttpRequest req,ILogger log)
    return new OkObjectResult("it worked!");

I created a very simple custom connector with no inputs to call the Start method, swagger as follows: 

swagger: '2.0'
  title: Default title
  description: ''
  version: '1.0'
host: <my function app name>
basePath: /api/
  - https
consumes: []
produces: []
          description: default
          schema: {}
      operationId: Start
      parameters: []
definitions: {}
parameters: {}
responses: {}
securityDefinitions: {}
security: []
tags: []

As described above testing this custom connector shows that the location header is being re-written:


To fix that I can rewrite the location header back to what it was originally with some Custom Connector Code:

public class Script : ScriptBase
    public override async Task<HttpResponseMessage> ExecuteAsync()
        if (this.Context.OperationId == "Start")
            return await this.TransformStartLocationHeader().ConfigureAwait(false);

        return await this.Context.SendAsync(this.Context.Request, this.CancellationToken).ConfigureAwait(false);

    private async Task<HttpResponseMessage> TransformStartLocationHeader()
        HttpResponseMessage response = await this.Context.SendAsync(this.Context.Request, this.CancellationToken).ConfigureAwait(continueOnCapturedContext: false);

        // https://<my function app name>
        var OgReqUri = this.Context.Request.RequestUri;

        var BadLoc = response.Headers.Location.ToString();

        // https://<my function app name>
        var newLocation = $"{OgReqUri.Scheme}://{OgReqUri.Host}/api/{BadLoc.Split('/').Last()}";

        response.Headers.Location = new Uri(newLocation);
        return response;


Now with the async pattern turned on for the card the custom connector will return what’s at the endpoint in the location header.




Note that when we moved this to a function app that required authorization via a query string parameter, we had to add that query string back in the location header for power automate to complete the async pattern without getting a 401.
You also need to use the Azure Function Host key since you'll be talking to two separate functions.

Hope this works for you,


Frequent Visitor

After some more investigation we found a better way to enable async in custom connectors that does not rely on using custom code.
You need to add the endpoint that is defined in the location header of the 202 response to the swagger definition in the custom connector.
in our case that response is a get request to /PDF2TIF_status/23512243564


note line 46 specifies this action as internal, which hides it from being used in a flow.
With this approach the power platform will define an endpoint in APIM for your 202 location and instead of receiving a 404 from APIM it will instead forward the request along as expected.
The authentication is also passed along for you so there's no need to forward that as I noted previously.

New Member

Excellent, with this approach people can easily build custom solution similar to "Wait for Approval".

I'm curious to see what do you did on the other endpoint. I'm currently returning a 202 response with the location header as a relative path, but the custom connector just ends as success. what am I missing?



Frequent Visitor

@murguiar did you enable the async pattern on the custom connector action in your flow?

I certainly did, it's enabled by default. 



Frequent Visitor

I only ever tried with an absolute path, I wonder if you can change your endpoint to be an abslute path for testing, ensure its working as expected and then see how the connector works diffrently with a relative path. I found it easier for me was to have my own test endpoint that would just return a hardcoded location and the endpoint in the location would also return a simple 200 "it worked" type response.

the test functionality in the custom connector editor itself seems to provide better information for debugging then a card in a flow too.

Hope it helps,

Returning an absolute path seems to work, Thanks!

@marcchasse Were you able to find a spec for what the response object schema should be for the resource that get's polled? e.g. is the below correct?



  "status": "Completed",
  "operationId": "12345",
  "result": {
    "data": "Here could be the actual data/result of the operation, such as a download link, operation results, etc.",
    "moreDetails": "Additional details about the operation or results."
  "message": "Your request has been successfully processed."





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