Fetch error not found swagger v1 swagger json

Issue Description

Swagger Open API documentation gives the below error in .NET Core API etc.

“Failed to load API definition. Fetch error undefined /swagger/v1/swagger.json”

It is also observed that Swagger API documentation/description works on ‘localhost’ i.e locally but when it runs in publish mode i.e hosted on IIS or Cloud Server, produces the error like “Failed to load API definition” with undefined/swagger/v1/swagger.json error.

Resolution

Before applying this fix, I would recommend you to validate the swagger implementation with the below article,

• ASP.NET Core 3.1 or 5.0 – Add Swagger OpenAPI V3.0 specification documentation

• ASP.NET Core 2.2- Add Swagger v2.0 API documentation

Please make sure you verify the below points steps,

Resolution 1

Swagger Document is defined with proper Title and Version details, as both are required parameters.

Resolution 2

Please make sure the API doesn’t contain any conflicting action. Conflicting action could be using the same routes. To be on the safer side, you can very much use the below flag to control that behavior,

c.ResolveConflictingActions(x => x.First());

Resolution 3

Please make sure all controller methods are attributed with proper HTTP attributes Example- HttpGET or HttpPost etc. Missing this attribute could cause the error.

If the issue still persists then please apply the below fix to resolve the issue.

Resolution 5

Please check if the hosting server allows CORS request processing. Also, its important swagger UI-related resources like CSS or stylesheets are accessible from your server.

Resolution 6

Please, note that Swagger JSON will be exposed at the following route as per default behavior.

"/swagger/{documentName}/swagger.json". 

If you are using a custom route or prefix, then the route MUST include the {documentName} parameter.

To fix the issue, please update the UseSwagger() as below,

app.UseSwagger(c =>
             {
                c.RouteTemplate = "<custom-name>/swagger/{documentName}/swagger.json";
             });

Example,

 c.RouteTemplate = "MyTestService/swagger/{documentName}/swagger.json";  

The above changes also need to be reflected in SwaggerUI middleware. Please update the UseSwaggerUI method will be as follows,

app.UseSwaggerUI(c =>
             {
                 c.SwaggerEndpoint("/<custom-name>/swagger/v1/swagger.json", "TestService");
             });

Example:

 app.UseSwaggerUI(c =>
             {
          c.SwaggerEndpoint("/MyTestService/swagger/v1/swagger.json", "TestService");
             }); 

Please note that in Swaggerendpoint() method ‘documentName’ value is cases sensitive.

In fact, documentName is case sensitive anywhere else if referenced.

This documentName is generally a Group Name associated with API version

Example: It won’t work for V1 but works for v1.

If using RoutePrefix in API then it can be defined as below,

Example only if using RoutePrefix,

c.RoutePrefix= "MyTestService/swagger"

Here MyTestService is my service name.

Please see below the complete implementation,

Finally, Swagger runs successfully locally and in any hosting environment like IIS or Azure Cloud, etc.

For any other issues, please use google chrome dev tools(FUN F12) or Edge Developer tools to verify the exact error causing the issue. Please verify Console and Network tab to validate the exact erros.

Other References:

Add swagger to ASP.NET Core API in simple 2-3 steps:

• Swagger API Documentation in .NET Core 6 or 3.1

• Add Swagger API documentation to .NET Core 2.2

Did I miss anything else in these resolution steps?

Did the above steps resolve your issue? Please sound off your comments below!

Happy Coding !!

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I created an ASP.NET Core 5 Web App for “API” (similar to MVC) with the OpenAPI (aka Swagger) option turned on.

To note: I have nearly 2 decades experience with .NET but this is my first real-world dive into .NET Core/5, so I’m skipping the whole “learning from first principals” and figuring it out as I go off the back of what I already know.
In this case I’m setting up a new project to move some part of an existing ASP.NET 4.6 MVC app into it.

It was all working fine until I wanted to add an OnActionExecuting() override to the Controller, from the IActionFilter interface.
Once I added the interface and the 2 required method the calls to the actual controller, calling the endpoints worked fine. But F5 debug runs of the project, which loaded the https://localhost:xxxxx/swagger/index.html page, would produce the error:

Failed to load API definition.

Errors

Fetch errorundefined /swagger/v1/swagger.json

Which in typical 3rd party framework fashion is fucking useless for consuming and gives use no useful information to work with.

So off to StackOverflow I go.

After a search for OnActionExecuting Fetch errorundefined /swagger/v1/swagger.json I came across this page: https://stackoverflow.com/questions/48450262/asp-net-core-swashbuckle-not-creating-swagger-json-file, specifically the answer https://stackoverflow.com/a/53632914/115704 which mentioned [NoAction]. Things then clicked for me.

The [NoAction] attribute on the 2 interface methods didn’t work for me, but after a bit more searching I found [NonAction] and which solved the problem.

Then I was back up and working with Swagger again.

Screenshots

Code before the fix

(Noting the [NonAction] is not impmented)

The error from Swagger

Honestly, how fucking useless is this an an error? About as good as the classic “an unexpected error occurred” (to which I always exclaim: “no shit, Sherlock!” Like any developer actually expects an error).

Code after the Fix

[NonAction] attributes have been implemented.

Swagger after the fix

And look, Swagger works again.

At the end of the day this was a couple of hours of my night lost.

Partly my fault for implementing a new version of .NET without doing the obligatory 40 hours of training. Also partly Swaggers fault because, well, it pissed me off for not giving a more detailed error message, especially in a development environment.

Simply navigate to https://localhost:{PortNo}/swagger/v1/swagger.json and get much more details about the error message.

I was able to find the error by opening the network tab and looking at the response for swagger.json

I’ve been working with .Net Core 3.1 and I spent some time to find out and understanding what was going on.

The issue can arise from many different reasons:

1. Swagger configuration errors

2. Classes with the same name but in different namespaces

3. Public methods without the rest attribute (Get, Post, etc.)

First, take a look the link below just to check if your setup is ok:

Add Swagger(OpenAPI) API Documentation in ASP.NET Core 3.1

Then,

A good tip to find out the problem is to run the application without to use IISExpress and check the console log. Any error found to generate the documentation will be displayed there.

In my case, the problems was that I had a public method (that should be private) without any rest attribute:

After change the method from public to private I solve the issue.

So after a lot of troubleshooting it came down to basically two things, but I feel that in general this could be helpful to someone else in the future so I’m posting an answer.

First- if ever your stuck with the aforementioned error the best way to actually see whats going on is by adding the following line to your Configure() method

app.UseDeveloperExceptionPage();

Now if you navigate to the ‘swagger/v1/swagger.json’ page you should see some more information which will point you in useful direction.

Second- now for me the error was something along the lines of

‘Multiple operations with path ‘some_path’ and method ‘GET’ ‘

However these API were located inside of dependency libraries so I was unable to apply a solution at the point of definition. As a workaround I found that adding the following line to your ConfigureServices() method resolved the issue

services.AddSwaggerGen(c =>
{
     c.SwaggerDoc("v1", new Info { Title = "API WSVAP (WebSmartView)", Version = "v1" });
     c.ResolveConflictingActions(apiDescriptions => apiDescriptions.First()); //This line
});

Finally- After all that I was able to generate a JSON file but still I wasn’t able to pull up the UI. In order to get this working I had to alter the end point in Configure()

app.UseSwaggerUI(c =>
{
    c.SwaggerEndpoint("./v1/swagger.json", "My API V1"); //originally "./swagger/v1/swagger.json"
});

I’m not sure why this was necessary, although it may be worth noting the web application’s virtual directory is hosted on IIS which might be having an effect.

NOTE: Navigating to swagger/v1/swagger.json will give you more details, for me it was causing issue due to undecorated action. This information is mentioned in comment by @MarkD

Hope this helps someone in the future.