Service discovery seems to broken for python/uvicorn resources

[johanndev]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

Reproduction

https://github.com/johanndev/aspire-service-discovery-repro

Description

While investigating why the Scalar integration was unable to load the openapi document of my fastapi app, I discovered that service discovery seems to have stopped working altogether for uvicorn resources.

apphost:

#:sdk Aspire.AppHost.Sdk@13.2.0-preview.1.26109.1
#:package Aspire.Hosting.Python@13.2.0-preview.1.26109.1
#:package Scalar.Aspire@0.8.45

#:project ./mainapi/mainapi.csproj
#:project ./downstreamapi/downstreamapi.csproj

using Scalar.Aspire;

var builder = DistributedApplication.CreateBuilder(args);

var fastapi = builder.AddUvicornApp("fastapi", "./fastapi", "main:app")
    .WithUv()
    .WithExternalHttpEndpoints();

var downstreamapi = builder.AddProject<Projects.downstreamapi>("downstreamapi")
    .WithExternalHttpEndpoints()
    .WithReference(fastapi)
    .WaitFor(fastapi);

var mainapi = builder.AddProject<Projects.mainapi>("mainapi")
    .WithExternalHttpEndpoints()
    .WithReference(fastapi)
    .WaitFor(fastapi)
    .WithReference(downstreamapi)
    .WaitFor(downstreamapi);

var scalar = builder.AddScalarApiReference()
    .WithApiReference(fastapi, options =>
    {
        options
            .AddDocument("openapi", "FastApi OpenAPI")
            .WithOpenApiRoutePattern("/openapi.json");
    })
    .WithApiReference(downstreamapi)
    .WaitFor(mainapi);

builder.Build().Run();

I have to two endpoints in my main api:

• One calls an endpoint in the downstream resource http://downstreamapi/testendpoint <- Works
• One calls an endpoint in the fastapi resource via http://fastapi/testendpoint <- Fails: HttpRequestException: No such host is known. (fastapi:80)

I verified that calling the fastapi endpoint directly works as expected: https://fastapi-aspire-service-discovery-repro.dev.localhost:52163/testendpoint -> 200 OK

Expected Behavior

Calls to a uvicorn app via aspire-assigned DNS-name works.

Steps To Reproduce

• Start the apphost
• Call the test endpoints in the main api:
  • https://mainapi-aspire-service-discovery-repro.dev.localhost:7132/calldownstreamapi
  • https://mainapi-aspire-service-discovery-repro.dev.localhost:7132/callfastapi

The second call fails with HttpRequestException: No such host is known. (fastapi:80)

Exceptions (if any)

No response

.NET Version info

10.0.100

Anything else?

aspire doctor

Aspire Environment Check
========================

.NET SDK
  ✓  .NET 10.0.100 installed (x64)
  ✓  HTTPS development certificate is trusted

Container Runtime
  ✓  Podman detected and running (version 5.7.1)

Summary: 3 passed, 0 warnings, 0 failed

[eerhardt]

@johanndev - what are the environment variables passed to your mainapi? You can see them in the dashboard.

[johanndev]

@eerhardt

fastapi has a http endpoint allocated at https://localhost:51381/

mainapi env variables matches that:
FASTAPI_HTTP=https://localhost:51381/
services__fastapi__http__0=https://localhost:51381/

But the http endpoint contains a https address, which does not seem correct.

The fastapi resource has its PORT env variable set to 51391 - which is the internal port (=where the python process listens) and shouldn't matter for the issue, correct?

Full set of env variables for mainapi

{
  "name": "mainapi-ecbazkdn",
  "displayName": "mainapi",
  "resourceType": "Project",
  "uid": "mainapi-ecbazkdn",
  "state": "Running",
  "creationTimestamp": "2026-02-09T19:13:27+00:00",
  "startTimestamp": "2026-02-09T19:13:27.695+00:00",
  "source": "mainapi.csproj",
  "healthStatus": "Healthy",
  "urls": [
    {
      "name": "https",
      "url": "https://mainapi-aspire-service-discovery-repro.dev.localhost:7132/"
    },
    {
      "name": "https",
      "url": "https://localhost:7132/"
    },
    {
      "name": "http",
      "url": "http://mainapi-aspire-service-discovery-repro.dev.localhost:5008/"
    },
    {
      "name": "http",
      "url": "http://localhost:5008/"
    }
  ],
  "environment": [
    {
      "name": "ASPNETCORE_ENVIRONMENT",
      "value": "Development"
    },
    {
      "name": "ASPNETCORE_HTTPS_PORT",
      "value": "7132"
    },
    {
      "name": "ASPNETCORE_URLS",
      "value": "https://localhost:55879;http://localhost:55880"
    },
    {
      "name": "DOTNET_LAUNCH_PROFILE",
      "value": "https"
    },
    {
      "name": "DOTNET_SYSTEM_CONSOLE_ALLOW_ANSI_COLOR_REDIRECTION",
      "value": "true"
    },
    {
      "name": "DOWNSTREAMAPI_HTTP",
      "value": "http://localhost:5139"
    },
    {
      "name": "DOWNSTREAMAPI_HTTPS",
      "value": "https://localhost:7011"
    },
    {
      "name": "FASTAPI_HTTP",
      "value": "https://localhost:55859"
    },
    {
      "name": "LOGGING__CONSOLE__FORMATTERNAME",
      "value": "simple"
    },
    {
      "name": "OTEL_BLRP_SCHEDULE_DELAY",
      "value": "1000"
    },
    {
      "name": "OTEL_BSP_SCHEDULE_DELAY",
      "value": "1000"
    },
    {
      "name": "OTEL_DOTNET_EXPERIMENTAL_ASPNETCORE_DISABLE_URL_QUERY_REDACTION",
      "value": "true"
    },
    {
      "name": "OTEL_DOTNET_EXPERIMENTAL_HTTPCLIENT_DISABLE_URL_QUERY_REDACTION",
      "value": "true"
    },
    {
      "name": "OTEL_DOTNET_EXPERIMENTAL_OTLP_RETRY",
      "value": "in_memory"
    },
    {
      "name": "OTEL_EXPORTER_OTLP_ENDPOINT",
      "value": "https://localhost:21109"
    },
    {
      "name": "OTEL_EXPORTER_OTLP_HEADERS",
      "value": "x-otlp-api-key=17b7b7f67313162a27a3e46e485c430f"
    },
    {
      "name": "OTEL_EXPORTER_OTLP_PROTOCOL",
      "value": "grpc"
    },
    {
      "name": "OTEL_INSTRUMENTATION_GENAI_CAPTURE_MESSAGE_CONTENT",
      "value": "true"
    },
    {
      "name": "OTEL_METRICS_EXEMPLAR_FILTER",
      "value": "trace_based"
    },
    {
      "name": "OTEL_METRIC_EXPORT_INTERVAL",
      "value": "1000"
    },
    {
      "name": "OTEL_RESOURCE_ATTRIBUTES",
      "value": "service.instance.id=ecbazkdn"
    },
    {
      "name": "OTEL_SERVICE_NAME",
      "value": "mainapi"
    },
    {
      "name": "OTEL_TRACES_SAMPLER",
      "value": "always_on"
    },
    {
      "name": "services__downstreamapi__http__0",
      "value": "http://localhost:5139"
    },
    {
      "name": "services__downstreamapi__https__0",
      "value": "https://localhost:7011"
    },
    {
      "name": "services__fastapi__http__0",
      "value": "https://localhost:55859"
    }
  ],
  "properties": [
    {
      "name": "project.path",
      "value": "D:\\scratch\\aspire-service-discovery-repro\\mainapi\\mainapi.csproj"
    },
    {
      "name": "resource.state",
      "value": "Running"
    },
    {
      "name": "resource.startTime",
      "value": "2026-02-09T19:13:27.6950000Z"
    },
    {
      "name": "resource.stopTime",
      "value": null
    },
    {
      "name": "executable.pid",
      "value": null
    },
    {
      "name": "resource.displayName",
      "value": "mainapi"
    },
    {
      "name": "resource.healthState",
      "value": "Healthy"
    },
    {
      "name": "executable.workDir",
      "value": "D:\\scratch\\aspire-service-discovery-repro\\mainapi"
    },
    {
      "name": "resource.exitCode",
      "value": null
    },
    {
      "name": "resource.createTime",
      "value": "2026-02-09T19:13:27.0000000Z"
    },
    {
      "name": "resource.name",
      "value": "mainapi-ecbazkdn"
    },
    {
      "name": "project.launchProfile",
      "value": "https"
    },
    {
      "name": "resource.uid",
      "value": "mainapi-ecbazkdn"
    },
    {
      "name": "resource.appArgsSensitivity",
      "value": null
    },
    {
      "name": "resource.appArgs",
      "value": null
    },
    {
      "name": "resource.type",
      "value": "Project"
    },
    {
      "name": "executable.args",
      "value": null
    },
    {
      "name": "executable.path",
      "value": "dotnet"
    }
  ],
  "relationships": [
    {
      "type": "Reference",
      "resourceName": "fastapi"
    },
    {
      "type": "WaitFor",
      "resourceName": "fastapi"
    },
    {
      "type": "Reference",
      "resourceName": "downstreamapi"
    },
    {
      "type": "WaitFor",
      "resourceName": "downstreamapi"
    }
  ],
  "commands": [
    {
      "name": "resource-stop",
      "description": "Stop resource"
    },
    {
      "name": "resource-restart",
      "description": "Restart resource. Source code is not recompiled."
    }
  ]
}

[borisf94]

I have the exact same issue.

Using the http__0 endpoint directly.

@johanndev Have you found a workaround?