Swagger not loading - Failed to load API definition: Fetch error undefined

C# Problem Overview

Trying to setup swagger in conjunction with a web application hosted on IIS express. API is built using ASP Net Core. I have followed the instructions prescribed on the relevant microsoft help page regarding Swashbuckle and ASP.NET Core.

Thus far I have got the swagger page to load up and can see that the SwaggerDoc that I have defined is loading, however no API's are present. Currently am getting the following error: >"Fetch error undefined ./swagger/v1/swagger.json"

public class Startup
{

	// This method gets called by the runtime. Use this method to add services to the container.
	public void ConfigureServices(IServiceCollection services)
	{
		// services.AddDbContext<TodoContext>(opt =>
		// opt.UseInMemoryDatabase("TodoList"));
		services.AddMvc()
			.SetCompatibilityVersion(CompatibilityVersion.Version_2_1);

		// Register the Swagger generator, defining 1 or more Swagger documents
		services.AddSwaggerGen(c =>
		{
			c.SwaggerDoc("v1", new Info { Title = "API WSVAP (WebSmartView)", Version = "v1" });
		});
	}

	// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
	public void Configure(IApplicationBuilder app, IHostingEnvironment env)
	{

		// Enable middleware to serve generated Swagger as a JSON endpoint.
		app.UseSwagger();

		// Enable middleware to serve swagger-ui (HTML, JS, CSS, etc.),
		// specifying the Swagger JSON endpoint.
		app.UseSwaggerUI(c =>
		{
			c.SwaggerEndpoint("./swagger/v1/swagger.json", "My API V1");
			c.RoutePrefix = string.Empty;
		});

		app.UseMvc();
	}
}

C# Solutions

Solution 1 - C#

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.

Solution 2 - C#

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:

Swagger configuration errors
Classes with the same name but in different namespaces
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.

Solution 3 - C#

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

Solution 4 - C#

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

Solution 5 - C#

I've come across the same error before, after struggling to find the reason, I discovered that one of my API in one of my controllers have no HTTP verb as an attribute, So I fixed it by putting [HttpGet] on my API. So here is my advice, check your API controllers, maybe you forget the same thing as me!

Take a look at my code, I realized that I should change this :

   public async Task<Product> ProductDetail(int id)
    {
        return await _productQueries.GetProductDetail(id);
    }

to this:

        [Route("ProductDetail")]
        [HttpPost]
        public async Task<Product> ProductDetail(int id)
        {
            return await _productQueries.GetProductDetail(id);
        }

Solution 6 - C#

I had similar issue, I solved it using the Route attribute on the offending controller method:

[HttpGet, Route("Throw")]
public ActionResult<string> Throw()
{
    _logger.LogInformation("I think I am going to throw up");
    throw new NotSupportedException("For testing unhandled exception logging.");
}

I felt that ResolveConflictingActions may potentially sweep a real issue under the rug.

Solution 7 - C#

also I had similar problem in .NET 5.0, I solved below way:

I added this line as attribute over controller:

[Consumes("application/json")]

Solution 8 - C#

I had two issues that caused the same error.

I have two classes with the same name under two different namespaces. Swagger could not reconcile this when generating the swagger doc. To fix it I added the line options.CustomSchemaIds(x => x.FullName); See explanation here

I had a method without an [HttpGet] annotation. Swagger needs the HTTP endpoints to be explicitly defined.

I found both issues by inspecting the Output in visual studio after the API loaded.

Solution 9 - C#

I've been working with .NET 5 and I spent some time trying to understand what was going on.

I got an error like the one below:

Then I resolved this problem by the following:

Open startup.cs file Add the following code in Configure method

    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
        app.UseSwagger(c =>
        {
            c.RouteTemplate = "/swagger/{documentName}/swagger.json";
        });
        app.UseSwaggerUI(c => c.SwaggerEndpoint("/swagger/v1/swagger.json", "API v1"));
    }

And in ConfigureServices method

    services.AddSwaggerGen(c =>
    {
        c.SwaggerDoc("v1", new OpenApiInfo { Title = "API", Version = "v1" });
        c.ResolveConflictingActions(apiDescriptions => apiDescriptions.First());
    });

Thanks to TheCodeBuzz for Resolved: Failed to load API definition (undefined /swagger/v1/swagger.json)

Note the difference between the RouteTemplate string and the SwaggerEndpoint string. One uses {documentName} and the other uses "v1" as a literal.

Solution 10 - C#

I just spent two hours on this issue, but my cause was entirely different, it had NOTHING to do with routes or annotations. I had 2 classes with the same name (but different namespaces): MyProject.Common.ClassName and MyProject.Api.ClassName. Swagger/swashbuckle couldn't tell the difference between the two, so I got that useless error.

Those 2 hours were spent trial-and-error commenting out controllers and endpoints, to finally find 3 endpoints offending endpoints. All 3 endpoints had different routes, different (or no) custom authorization, and different method names. It turned out that all 3 endpoints either accepted a parameter, or returned an object, that contained the API version of my class. Nowhere was the Common version used. Swagger couldn't tell them apart, and puked all over itself.

Why oh why can't Swagger or Swashbuckle provide actual error messages? Would have saved me a couple of hours...

Solution 11 - C#

I just forgot to add HTTP attributes in my controller as soon as I add HTTP attribute it works like a charm for me.

Source : https://www.benday.com/2020/12/16/webapi-core-swagger-failed-to-load-api-definition-error/

Solution 12 - C#

Surely it is one of the Controller's method that is faulty. To get the method, at times you might need to take out all your controllers, Try and insert them one after the other then you will test along to find the Controller with bugs.

For ex. If you have like 3Controllers say

>Controller
>>>AuthController
>>>UserController
>>>HomeController

Take two out of the controllers out and test the controller by adding one controller after each successful testing. With that you will know the controller that has a faulty method.

>Controller
>>>AuthController

If the methods in AuthenController is fine, It will run, If not Check the methods.

>Controller
>>>AuthController
>>>UserController

and carry out the next check on the controller like that of Authen.

Solution 13 - C#

I had the same problem, so I checked it using inspect element on the browser. The "Console" tab shows the file where the problem originated from (v1/swagger/json:1). Opening it by clicking it showed that one of the helper methods I used in my controller was "Public". Changing it to "Private" fixed the problem for me.

This page also has good tips: https://btrehberi.com/swagger-failed-to-load-api-definition-fetch-error-undefined-hatasi-cozumu/yazilim/

Solution 14 - C#

Swagger in my case needed [HttpAction] with all public members in controller. Unfortunately I misspelled constructor name and since it was public, was throwing this error.

Solution 15 - C#

For ASP.NET Core 3.1 I had to ensure the verb were not ambiguous and I found this out by first running the API project without IIS in VS2019 (Green Arrow > left-click the carrot icon and select the name of the project this causes a console window to appear on start up so you can inspect what's happening and see errors).

[HttpGet("MyEndPointA")

Then Swagger is able to generate the documentation correctly.

Solution 16 - C#

Solved issue in dotNet 6! Just change the attribute order of [ApiController]

Solution 17 - C#

Here we go:

I created WEB Controller instead of WEB API Controller. That makes this kind of error. During creation of new Controller, make sure that you created right WEB API controller.

Solution 18 - C#

In my case, there were 2 methods in the Controller class, which had the same annotations, and URL. (Our team was using Entity Framework, ASP.NET and Swagger.)

    [HttpGet("GetMyGreatData/{patientId}")]
    [ValidatePatient]
    public async Task<ActionResult<ServiceResponse<IEnumerable<MyGreatModel>>>> GetMyGreatData(
    [FromRoute] int patientId, int offset = 0, int limit = 0)
    {
        //method details...
    }


    [HttpGet("GetMyGreatData/{patientId}")]
    [ValidatePatient]
    public async Task<ActionResult<ServiceResponse<IEnumerable<MyGreatModel>>>> GetMyGreatData( 
    [FromRoute] int patientId, 
    [FromQuery] DateTimeOffset? startdate = null,
    [FromQuery] DateTimeOffset? endDate = null,
    int offset = 0,
    int limit = 0)
    {
        //method details...
    }

deleting one method solved the issue for me.

Solution 19 - C#

I was having the same issue, the base controller was not decorated with Http and removing that has made it work.

Solution 20 - C#

This error can happen when you deploy an App Service to Azure. I've redeployed the App Service to Azure and the error disappeared.

Solution 21 - C#

When this happened to me, I tracked it down to URL path param having an underscore which it's compatible with the asp generator

Changing this:

/block-content/{machine_name}:

To this

/block-content/{machineName}:

Solved it for me

Solution 22 - C#

<b>make sure the name "v1" matches the path in the swagger endpoint</b>
    <p>
    services.AddSwaggerGen(c =>
       {
     c.SwaggerDoc("v1", new OpenApiInfo { 
Title = "ODAAPP",  
Version = "v1" });
                });
                </p>
                <br/>

app.UseSwaggerUI(c => c.SwaggerEndpoint("/swagger/v1/swagger.json",
                             "ODAAPP v1"));

enter code here

Solution 23 - C#

This will also happen if you use same route for multiple action methods (Overloading is OK)

Solution 24 - C#

In my case, the project was configured to authenticate using identity server 4 using AddPolicy() at startup.cs and there were usages of [Authorize]

I removed the things for startup.cs and usages of [Authorize]

Will update more soon

Solution 25 - C#

In my case I had two identicall inner classes. Extracted them to a single one refactored the namespaces and voilá, all returned to work properly.

Solution 26 - C#

I have experienced the same error when I was using Swagger and also Microsoft.AspNetCore.OData. I usually try to use the latest version - but bringing it down to v 7.5.12 - did solve my issue.

Also adding following to every Action method in the Controller, makes it work with OData v8.x too: [HttpGet], [HttpPost], or [ApiExplorerSettings(IgnoreApi = true)]

Solution 27 - C#

I had a similar Fetch error 404 swagger/v1/swagger.json, when trying to integrate Swagger documentation in ASP.NET Core 3.1 Web API project. I tried almost all of the above suggestions but failed. After an hour of hit-and-trial, I decided to give NSwag a try using this reference, instead of Swashbuckle and it just worked like a charm :)

Solution 28 - C#

I got the similar issues - the root cause is I forgot to add the annotations :-(

Solution 29 - C#

Reasons for this Error

i resolved this issue by this way Use [HttpGet] attribute above the api controller method.

And, because of different versions of swashbuckle, these errors may come.

you should use the correct swagger endpoint url

v1/swagger.json   or swagger/v1/swagger.json

choose above one based on the version you are using.

Note:

Use this url for reference https://myget.org/feed/domaindrivendev/package/nuget/Swashbuckle.AspNetCore.Swagger/6.2.3-preview-1963

Refer the official swagger documentation. lot of information is there with crystal clear documents https://swagger.io/docs/

Solution 30 - C#

> 'Multiple operations with path 'some_path' and method 'GET' '

[HttpGet]
public IActionResult Get()
{
    return Ok(_userService.Get());
}

[HttpGet]
public IActionResult Get(int Id)
{
    return Ok(_userService.Get(Id));
}

Just modify DataAnnotation:

[HttpGet]
public IActionResult Get()
{
    return Ok(_userService.Get());
}

[HttpGet("{Id}"] //HERE
public IActionResult Get(int Id)
{
    return Ok(_userService.Get(Id));
}

Solution 31 - C#

I had the same error and what fixed it was adding the[HttpGet] attributee in my controller

Content Type	Original Author	Original Content on Stackoverflow
Question	Jeremy S.	View Question on Stackoverflow
Solution 1 - C#	Jeremy S.	View Answer on Stackoverflow
Solution 2 - C#	Julio Schurt	View Answer on Stackoverflow
Solution 3 - C#	YOrnstei	View Answer on Stackoverflow
Solution 4 - C#	Mohammed Osman	View Answer on Stackoverflow
Solution 5 - C#	Hamid	View Answer on Stackoverflow
Solution 6 - C#	Arthur	View Answer on Stackoverflow
Solution 7 - C#	Yusif Karimov	View Answer on Stackoverflow
Solution 8 - C#	LazyDog	View Answer on Stackoverflow
Solution 9 - C#	Amar Anondo	View Answer on Stackoverflow
Solution 10 - C#	emery.noel	View Answer on Stackoverflow
Solution 11 - C#	Nafees Sardar	View Answer on Stackoverflow
Solution 12 - C#	Alagbe Sunmbo Amos	View Answer on Stackoverflow
Solution 13 - C#	Kaane Guji	View Answer on Stackoverflow
Solution 14 - C#	Noman Anjum	View Answer on Stackoverflow
Solution 15 - C#	Trevor	View Answer on Stackoverflow
Solution 16 - C#	Abdul Khaliq	View Answer on Stackoverflow
Solution 17 - C#	praguan	View Answer on Stackoverflow
Solution 18 - C#	Onat Korucu	View Answer on Stackoverflow
Solution 19 - C#	Naod Agere	View Answer on Stackoverflow
Solution 20 - C#	Martin Staufcik	View Answer on Stackoverflow
Solution 21 - C#	nigelheap	View Answer on Stackoverflow
Solution 22 - C#	Prassanth Nicholas	View Answer on Stackoverflow
Solution 23 - C#	Wahid Masud	View Answer on Stackoverflow
Solution 24 - C#	Dipon Roy	View Answer on Stackoverflow
Solution 25 - C#	Jackfowl	View Answer on Stackoverflow
Solution 26 - C#	KVN	View Answer on Stackoverflow
Solution 27 - C#	Shakeel Anjum	View Answer on Stackoverflow
Solution 28 - C#	Brian Su	View Answer on Stackoverflow
Solution 29 - C#	Vintage Coder	View Answer on Stackoverflow
Solution 30 - C#	Alex Sandro Martins de Araujo	View Answer on Stackoverflow
Solution 31 - C#	Akinwole Samuel	View Answer on Stackoverflow