API issue on the server

Question

API issue on the server

Scott Huang 3,491

Hi,

No similar issue when calling the project. Why does it appear when going to "https://localhost:7060/swagger/index.html"?

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Damien Pham (WICLOUD CORPORATION) 2,300 Reputation points Microsoft External Staff Moderator

2026-03-11T10:04:03.1533333+00:00
Hello @Scott Huang ,

To help investigate the issue, could you please provide the following:

What happens when you open this URL directly in the browser? https://localhost:7060/swagger/v1/swagger.json

Any errors shown in the Visual Studio output window or application logs when the API starts?

The Swagger configuration from your Program.cs or Startup.cs (sections with AddSwaggerGen, UseSwagger, UseSwaggerUI).

Your .NET / ASP.NET Core version.

In the browser Developer Tools (F12) → Network tab, what status code appears for the request to /swagger/v1/swagger.json?

I look forward to hearing from you.
Scott Huang 3,491 Reputation points

2026-03-12T02:11:52.45+00:00

Here is the code.

I got he issue when going to

https://localhost:7060/swagger/index.html

I do not have the issue when directly running it.

Answer accepted by question author

Damien Pham (WICLOUD CORPORATION) 2,300 Microsoft External Staff Moderator

Hello @Peter_1985 ,

Thank you for the snippet. From the screenshot, you have

if (app.Environment.IsProduction())  
{      
app.UsePathBase("/webapp1"); // Must match your IIS alias  
}

It is configured in Production environment, but the swaggerUI is configured in Development environment. This is likely why you only get the error when go to https://localhost:7060/swagger/index.html

To fix this, you can configure the environment to Production for both.

Hope you can resolve your issue. If you found my response helpful or informative, I would greatly appreciate it if you could follow this guide provide feedback.

Thank you.

Scott Huang 3,491 Reputation points

2026-03-13T01:46:12.7633333+00:00

Can you share the way to configure the environment to Production for both?
Damien Pham (WICLOUD CORPORATION) 2,300 Reputation points Microsoft External Staff Moderator

2026-03-17T08:48:51.42+00:00

Hello,
I see you are unable to choose .NET 10 framework. Which version of visual studio are you using? Since .NET 10 is only supported on Visual studio 2026(refer to this document), you should download Visual Studio 2026 from the official Microsoft website: https://visualstudio.microsoft.com/downloads/.
Scott Huang 3,491 Reputation points

2026-03-17T08:58:16.0266667+00:00

Yes, I am using VS 2022. Really good to use .Net 10 in the long term right?
Damien Pham (WICLOUD CORPORATION) 2,300 Reputation points Microsoft External Staff Moderator

2026-03-17T09:44:25.4033333+00:00

You are correct, using .Net 10 delivers better performance, security, and modern features with a longer support lifecycle compared to older versions. Older frameworks may still work, but they miss optimizations and future compatibility improvements.

3 additional answers

Your answer

Damien Pham (WICLOUD CORPORATION) 2,300 Reputation points Microsoft External Staff Moderator

2026-03-11T10:04:03.1533333+00:00

Hello @Scott Huang ,

To help investigate the issue, could you please provide the following:

What happens when you open this URL directly in the browser? https://localhost:7060/swagger/v1/swagger.json

Any errors shown in the Visual Studio output window or application logs when the API starts?

The Swagger configuration from your Program.cs or Startup.cs (sections with AddSwaggerGen, UseSwagger, UseSwaggerUI).

Your .NET / ASP.NET Core version.

In the browser Developer Tools (F12) → Network tab, what status code appears for the request to /swagger/v1/swagger.json?

I look forward to hearing from you.
Scott Huang 3,491 Reputation points

2026-03-12T02:11:52.45+00:00

Here is the code.

I got he issue when going to

https://localhost:7060/swagger/index.html

I do not have the issue when directly running it.
Scott Huang 3,491 Reputation points

2026-03-13T01:46:12.7633333+00:00

Can you share the way to configure the environment to Production for both?
Damien Pham (WICLOUD CORPORATION) 2,300 Reputation points Microsoft External Staff Moderator

2026-03-17T08:48:51.42+00:00

Hello,
I see you are unable to choose .NET 10 framework. Which version of visual studio are you using? Since .NET 10 is only supported on Visual studio 2026(refer to this document), you should download Visual Studio 2026 from the official Microsoft website: https://visualstudio.microsoft.com/downloads/.
Scott Huang 3,491 Reputation points

2026-03-17T08:58:16.0266667+00:00

Yes, I am using VS 2022. Really good to use .Net 10 in the long term right?
Damien Pham (WICLOUD CORPORATION) 2,300 Reputation points Microsoft External Staff Moderator

2026-03-17T09:44:25.4033333+00:00

You are correct, using .Net 10 delivers better performance, security, and modern features with a longer support lifecycle compared to older versions. Older frameworks may still work, but they miss optimizations and future compatibility improvements.

Answer 1

The screenshot shows that Swagger UI loads, but it can’t fetch the OpenAPI document at /swagger/v1/swagger.json (NetworkError). This usually means the API is running and reachable, but the Swagger endpoint itself is misconfigured or not available at that path.

Typical causes in ASP.NET Core Web API:

Swagger middleware not mapped for the environment
- In Program.cs, Swagger is often only enabled in Development:
```
   if (app.Environment.IsDevelopment())
   {
       app.UseSwagger();
       app.UseSwaggerUI();
   }
```
If the app is running under a different environment (e.g., Production), /swagger/v1/swagger.json won’t exist, and Swagger UI will show a network error.
- Fix: Either run the app with ASPNETCORE_ENVIRONMENT=Development or move app.UseSwagger(); app.UseSwaggerUI(); outside the IsDevelopment block if Swagger should always be available.
Wrong route or document name
- If SwaggerGen is configured with a different document name or route, the UI might still be pointing to /swagger/v1/swagger.json while the actual JSON is at another URL.
- Fix: Ensure the document name in AddSwaggerGen matches the one used in UseSwaggerUI, for example:
```
   builder.Services.AddSwaggerGen(c =>
   {
       c.SwaggerDoc("v1", new() { Title = "WebApp1", Version = "v1" });
   });
   
   app.UseSwagger();
   app.UseSwaggerUI(c =>
   {
       c.SwaggerEndpoint("/swagger/v1/swagger.json", "WebApp1 v1");
   });
```
HTTPS/port mismatch
- If the app is actually listening on a different port or scheme than the one Swagger UI is using, the fetch will fail.
- Fix: Confirm the app is running on https://localhost:7060 and that no reverse proxy or different port is used. Adjust launch profile or Swagger endpoint accordingly.
Middleware order or startup failure
- If there’s an exception during startup before UseSwagger runs, the JSON endpoint won’t be available even though the browser still shows a cached or partial UI.
- Fix: Check application logs/console output for startup errors and ensure app.UseSwagger() is called before app.UseAuthorization()/app.MapControllers().

Because direct calls to the API work, focus on verifying that the Swagger JSON endpoint exists and matches the URL configured in UseSwaggerUI, and that Swagger middleware is enabled for the environment where the app is running.

References:

Answer 2

Bruce (SqlWork.com) 85,416

You don’t specify your .net version. Your code looks like .net 8, but won’t work with later versions. The error means the json url created by swagger gen does not match the url swagger ui is expecting.

Scott Huang 3,491 Reputation points

2026-03-13T02:52:46.8+00:00

It is .Net 8.0.

It would be getting much benefits to switch to higher one (other than .Net 8.0)?

For the issue, what change is needed? More details?
Bruce (SqlWork.com) 85,416 Reputation points

2026-03-13T03:30:14.37+00:00

The later version use a different syntax. See example I posted in earlier thread. I would guess the nuget packages are a later version than this code was written for. Look up how to specify the path for the json and specify in both in gen and ui.
Scott Huang 3,491 Reputation points

2026-03-13T03:35:30.9333333+00:00

Which other thread do you mean?

To the issue, we must use .Net 9 for json-related (within VS2022/26)? Is it fine that we deploy 2 different kinds of projects (like one in .Net and another one with higher one) on the same server?

Bruce (SqlWork.com) 85,416

As .net 9 support ends 11/10/26, you will need to upgrade to .net 10 before then. If it’s a new project, not sure why you would use .net 9 at all.

copy from old thread:

// .net 10 example
var builder = WebApplication.CreateBuilder(args);

builder.Services.AddOpenApi();

var app = builder.Build();

app.UsePathBase("/api");
app.MapOpenApi();
app.UseSwaggerUI(options =>
{
    options.SwaggerEndpoint("/api/openapi/v1.json", "v1");
});

app.MapGet("/", () => "hello world!");
  
app.Run();

Then:

https://abc.com/api/
https://abc.com/api/swagger/index.html

Scott Huang 3,491 Reputation points

2026-03-17T03:17:25.84+00:00

I set up .Net 10 SDK on the server but why is it not available to select below?

Answer 3

Bruce (SqlWork.com) 85,416

You don’t specify your .net version. Your code looks like .net 8, but won’t work with later versions. The error means the json url created by swagger gen does not match the url swagger ui is expecting. Check the overloads to specify the path explicitly.

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