aspire publish --publisher docker-compose crashes with 'Sequence contains more than one matching element'

[javiercn]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

Running aspire publish --publisher docker-compose on any project with AddDockerComposeEnvironment fails with an InvalidOperationException because PrepareDeploymentTargetsAsync adds duplicate DeploymentTargetAnnotation entries to compute resources.

From my investigation, it appears that the prepare-deployment-targets-{name} pipeline step executes twice during a publish:

1. Once during ExecuteStepSequentiallyAsync("before-start", ...) in DistributedApplication.StartAsync (because the step declares RequiredBySteps = [BeforeStart] and is included in the transitive dependency closure)
2. Again during the main ExecuteAsync DAG execution in PipelineExecutor

Both runs share the same DistributedApplicationModel, so the second execution adds duplicate annotations to resources that were already annotated in the first pass.

Note: I'm not 100% certain this isn't something I'm doing wrong in my AppHost configuration. It's possible there's a setup step I'm missing or a configuration that would prevent the double-execution. However, I've reproduced this with a minimal AppHost that has nothing beyond AddDockerComposeEnvironment and a single project resource.

Expected Behavior

aspire publish --publisher docker-compose should complete successfully, producing a docker-compose.yaml with all compute resources.

Steps To Reproduce

Minimal AppHost:

var builder = DistributedApplication.CreateBuilder(args);

builder.AddDockerComposeEnvironment("docker-compose")
    .WithProperties(env => env.DashboardEnabled = false);

builder.AddProject<Projects.MyApi>("api");

builder.Build().Run();

Run:

dotnet run -- publish --publisher docker-compose --output-path ./output

Exceptions (if any)

System.InvalidOperationException: Sequence contains more than one matching element
   at System.Linq.ThrowHelper.ThrowMoreThanOneMatchException()
   at System.Linq.Enumerable.SingleOrDefault[TSource](...)
   at Aspire.Hosting.ApplicationModel.ResourceExtensions.GetDeploymentTargetAnnotation(IResource resource, IComputeEnvironment computeEnvironment)
   at Aspire.Hosting.Docker.DockerComposeEnvironmentResource.<>c__DisplayClass...(PipelineStepContext context)

.NET Version info

.NET 10.0 (main branch, as of May 2026)

Anything else?

Workaround: Adding an idempotency guard at the top of PrepareDeploymentTargetsAsync in DockerComposeEnvironmentResource.cs resolves the issue:

private bool _deploymentTargetsPrepared;

private async Task PrepareDeploymentTargetsAsync(PipelineStepContext context)
{
    if (executionContext.IsRunMode) return;
    if (_deploymentTargetsPrepared) return;
    _deploymentTargetsPrepared = true;
    // ... rest of method
}

The issue is in src/Aspire.Hosting.Docker/DockerComposeEnvironmentResource.cs, specifically the step registered at line 68-75 which unconditionally adds DeploymentTargetAnnotation on each invocation.

[joperezr]

🤖 Aspire triage pass — 2026-05-22 20:41 UTC

Area: area-deployment
Kind: bug
Repro: yes-minimal — minimal AppHost + single dotnet run -- publish command provided in the issue body (Steps To Reproduce section)
Affected version(s): main / 13.4.0-dev (latest open milestones: 13.3.x, 13.4, 13.5)
Severity: S2 — aspire publish --publisher docker-compose crashes on the basic happy path; reporter identifies the suspected cause (the prepare-deployment-targets-{name} pipeline step executing twice, once in before-start transitive closure and once in the main DAG) and an in-code workaround, but there is no user-side workaround in the CLI command itself.

Urgency: 🟡 vNext — recommend milestone 13.4
Reason: S2 with no user workaround, but the report targets main/13.4.0-dev rather than a shipped 13.3 build — not framed as a regression vs. a prior release, so this is a fix for the next minor rather than servicing.

Suggested owner(s): @mitchdenny (score ≈ 14), backup @danegsta (score ≈ 4)
Evidence:

• mitchdenny merged Enable Gateway API and AGC in AddAzureKubernetesEnvironment #16990, Add first-class Kubernetes Ingress and Gateway API resources #16487, Add Azure Kubernetes Service (AKS) hosting support #16088, Fix Kubernetes publisher crash on Boolean/scalar environment variable values #15560 in area-deployment over the last 90d (recency-weighted authorship dominant in the area)
• CODEOWNERS lists @danegsta @mitchdenny for src/Aspire.Hosting.Docker/**, which contains the cited DockerComposeEnvironmentResource.cs
• mitchdenny closed Enable Gateway API and Azure Application Gateway for Containers in AddAzureKubernetesEnvironment #16971 as completed in area-deployment (recent)

Confidence: medium
Needs human?: yes — please confirm 13.4 vs 13.3.x. If this regressed vs. a shipped 13.3 build (the docker-compose publisher exists there), this is a strong servicing candidate; the report only cites main, so I'm conservatively routing it to vNext.

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