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

Question

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();
    }
}

Mini

Can you explain what's going on in the Startup where you're assigning an interface to the Configuration class? – Agueda

Use c.SwaggerEndpoint("/swagger/v1/swagger.json", "My API V1"); – Homovec

jPhizzle - this was left over from previous troubleshooting attempts. I have updated the code. Apologies – Mini

Akash KC - I had tried this originally unfortunately no change. – Mini

What's the error message on the Console tab in the browser dev tools? – Godmother

'Failed to load resource: the server responded with a status of 404 () ... swagger/v1/swagger.json:1 ' – Mini

@Godmother for whatever reason it didn't occur to me to check DevTools... Becuase of your comment I found out that Swagger doesn't like that I have a conflict with two DTOs having the same name. They're in different namespaces but apparently that's not good enough. Anyway, thanks for the idea of where to check! – Brownell

Answer 1

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

Mini

navigating directly to the swagger/v1/swagger.json solved this. An undecorated action. – Solfatara

The navigation to swagger/v1/swagger.json also gave me the solution to my problem - action needed an explicit [HttpGet] attribute – Brufsky

Navigate to your application's swagger.json in your browser has shown the error clearly. localhost:808/swagger/v1/swagger.json – Tabethatabib

adding this line ResolveConflictingActions resolved my issue. – Cranio

I also had change that same endpoint path to "./v1/swagger..." to get it working on the web server. Very strange requirement. The new path works fine in local debug and on the server. Seems like it would be the default from the project template. – Strake

I had a similar case to @Mog0, because of copy-paste from another method, I applied HttpGet attribute for POST method (oversight), so I had two HttpGet methods with the same routing. – Frosty

Answer 2

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.

Dwell

For me it was number three. "Public methods without the rest attribute" You help me very muth. Thanks! – Diffractive

Another reason, which I just encountered, is when the name of the attribute [HttpPost("NAME_HERE") isn't the same as the action name. – Landonlandor

For me, it was 'Classes with the same name but in different namespaces'. I have a .Net Core Api and had a 'User' class in two different namespaces. Renaming the second corrected the issue – Vespers

For me, it is number 3. Thanks. – Rightward

for me it was number 2. I ended up having to smurf name things a little which was a little sad but understandable. – Adolfoadolph

for me, it was number 3. thanks – Roseline

Answer 3

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

Articulate

You are a genius. Just wasted an afternoon not able to find the error thanks. – Capella

Thank you for this, great help in assisting my debugging. – Afebrile

Using dev tools/developer tools is brilliant. Thanks – Percent

Quite Easy and worked for me – Russianize

Answer 4

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

Walking

Thanks!!! my problem was a ODataController with the next error: No media types found in 'Microsoft.AspNet.OData.Formatter.ODataInputFormatter.SupportedMediaTypes – Ectophyte

/swagger/v1/swagger.json not found – Aha

@Aha Did you change the configs of the Swagger? – Walking

None changed. Finally, the error is method duplicated, but I don't know how to troubleshooting for view the error (any output, log, ...) – Aha

Answer 5

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.

Ingaingaberg

Answer 6

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

I added this line as attribute over controller:

[Consumes("application/json")]

Quipster

Answer 7

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);
        } 

Oliver

Answer 8

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.

Ordzhonikidze

Answer 9

I had two issues that caused the same error.

1. 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

2. 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.

Annia

Answer 10

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...

Acierate

Answer 11

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/

Gilliland

Answer 12

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.

Lugsail

Answer 13

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.

Therapist

Answer 14

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/

Paleogeography

Answer 15

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.

Nursling

Answer 16

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.

Tsan

Answer 17

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

Ovarian

Answer 18

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.

Supplicate

Answer 19

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

Modernity

Still a better solution is to decorate the class/controller that has released the error with ApiExplorerSettings (IgnoreApi = true)] – Modernity

Answer 20

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

Expressionism

Answer 21

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

Cockney

Answer 22

<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

Imre

Answer 23

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

Pelias

Answer 24

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

Dulin

Answer 25

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.

Boeotia

Answer 26

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)]

Publication

Answer 27

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 :)

Cirrate

Answer 28

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/

Hunkers

Answer 29

'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));
}

Throat

Answer 30

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

Saturninasaturnine

Answer 31

When using springdoc and you face

Swagger Whitelabel Page Error

or

Swagger Blank Page Error

you can resolve the errors by

Adding springdoc Dependencies:

<dependency>
    <groupId>org.springdoc</groupId>
    <artifactId>springdoc-openapi-ui</artifactId>
    <version>1.6.9</version>
</dependency>

Removing Swagger and Spring Dependencies

Removing SwaggerConfigure or any related Annotation and File

Add these into the Security Config

-- > override configure method and add these

.antMatchers("/v2/api-docs",
             "/configuration/ui",
             "/swagger-resources/**",
             "/configuration/security",
             "/swagger-ui/**",
             "/webjars/**",
             "/v3/api-docs/**")

Lastly, add these in the application.properties file,

spring.mvc.pathmatch.matching-strategy = ANT_PATH_MATCHER

# swagger-ui custom path
springdoc.swagger-ui.path=/swagger-ui.html

Inna

Answer 32

When you have the same name of API Endpoints in the same controller then it will give this error. Like that below in image

Flicker

Answer 33

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

Faller

Which annotation and where? – Grow