[main] VS Code (CLI?) incorrectly reports "The --apphost option specified a project that does not exist." if an apphost fails to build

[afscrome]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

If you try to run an app host that fails to build, VS Code incorrectly reports that the "Project file does not exist.

But looking at the logs, you can clearly see the project referenced exists.

[2026-08-05 22:53:15.679] [FAIL] [DotNetCliRunner] Failed to get items and properties from project. Exit code was: 1. See debug logs for more details. Stderr: REDACTED\AppHost\AppHost.csproj : error : Could not resolve SDK "Aspire.AppHost.Sdk". Exactly one of the probing messages below indicates why we could not resolve the SDK. Investigate and resolve that message to correctly specify the SDK.

Expected Behavior

The error should report that the project failed to build, not that the project doesn't exist.

Steps To Reproduce

1. Try to run an apphost that doesn't build in VS Code.

(Not 100% sure if it's relevant or not but in my case the failure is a missing SDK)

Exceptions (if any)

[2026-08-05 22:53:14.906] [DBUG] [ProcessExecutionFactory] dotnet(32868) waiting for exit
[2026-08-05 22:53:15.679] [DBUG] [ProcessExecutionFactory] dotnet(32868) exited with code: 1
[2026-08-05 22:53:15.679] [DBUG] [ProcessExecutionFactory] dotnet(32868) output drained
[2026-08-05 22:53:15.679] [FAIL] [DotNetCliRunner] Failed to get items and properties from project. Exit code was: 1. See debug logs for more details. Stderr: REDACTED\AppHost\AppHost.csproj : error : Could not resolve SDK "Aspire.AppHost.Sdk". Exactly one of the probing messages below indicates why we could not resolve the SDK. Investigate and resolve that message to correctly specify the SDK.
REDACTED\AppHost\AppHost.csproj : error :   SDK resolver "Microsoft.DotNet.MSBuildWorkloadSdkResolver" returned null.
REDACTED\AppHost\AppHost.csproj : error :   Unable to find package Aspire.AppHost.Sdk. No packages exist with this id in source(s): AspireLocalHive. PackageSourceMapping is enabled, the following source(s) were not considered: Trayport.
REDACTED\AppHost\AppHost.csproj : error MSB4236: The SDK 'Aspire.AppHost.Sdk/13.5.0-pr.18906.g90007b1b' specified could not be found.
Build failed. Properties, Items, and Target results cannot be obtained. See details in stderr above.
, Stdout: 
[2026-08-05 22:53:15.681] [FAIL] [AspireCliTelemetry] The --apphost option specified a project that does not exist.
Aspire.Cli.Projects.ProjectLocatorException: Project file does not exist.
   at Aspire.Cli.Projects.ProjectLocator.<UseOrFindAppHostProjectFileAsync>d__30.MoveNext() + 0x1645
--- End of stack trace from previous location ---
   at Aspire.Cli.Commands.RunCommand.<ExecuteAsync>d__29.MoveNext() + 0x748
[2026-08-05 22:53:15.689] [DBUG] [ExtensionBackchannel] Starting backchannel connection to Aspire extension at localhost:64547
[2026-08-05 22:53:15.689] [DBUG] [ExtensionBackchannel] Connecting to Aspire Extension backchannel at localhost:64547
[2026-08-05 22:53:15.710] [DBUG] [ExtensionBackchannel] Connected to Aspire Extension backchannel at localhost:64547
[2026-08-05 22:53:15.783] [DBUG] [ExtensionBackchannel] Connected to ExtensionBackchannel at localhost:64547
[2026-08-05 22:53:15.783] [DBUG] [ExtensionBackchannel] Sent error message The --apphost option specified a project that does not exist.
[2026-08-05 22:53:15.784] [INFO] [Stderr] ❌ The --apphost option specified a project that does not exist.
[2026-08-05 22:53:15.784] [DBUG] [ExtensionBackchannel] Sent message See logs at C:\Users\alexanderc\.aspire\logs\cli_20260805T225314_f8224c4b.log

Aspire doctor output

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

Aspire
  ⚠️ Aspire CLI version 13.5.0-preview.1.26405.1 (channel: daily) is out of date. Latest version is 13.5.0-preview.1.26405.3 (channel: prerelease)  
       Run 'aspire update' to update Aspire CLI.       
  ✅ Developer Control Plane (DCP) connection health checks succeeded  

AppHost
  ⚠️ Could not determine AppHost version (src\dotnet\connector\aspire\AppHost\AppHost.csproj)  

.NET SDK
  ✅ .NET 10.0.302 installed (x64)  

Container Runtime
  ⚠️ Docker: installed but not running ← active  
       Start Docker Desktop       

Environment
  ✅ Operating system: Windows 10.0.26100.0  
  ✅ HTTPS development certificate is trusted  

Development Tools
  ✅ Aspire extension for VS Code is installed  

Summary: 5 passed, 3 warnings, 0 failed
For detailed prerequisites: https://aka.ms/aspire-prerequisites

Aspire CLI Installations
========================

╭─────────────────────────────────────────────────────────────┬───────────────────────────────────────────────────────────────────┬──────────┬───────────┬─────────────╮
│ Path                                                        │ Version                                                           │ Channel  │ Route     │ PATH status │
├─────────────────────────────────────────────────────────────┼───────────────────────────────────────────────────────────────────┼──────────┼───────────┼─────────────┤
│ C:\Users\alexanderc\.aspire\bin\aspire.exe (current)        │ 13.5.0-preview.1.26405.1+0182d1671498df3020eea7af5b006eb0bddbafc9 │ daily    │ (unknown) │ active      │
│ C:\Users\alexanderc\.aspire\dogfood\pr-18906\bin\aspire.exe │ 13.5.0-pr.18906.g90007b1b                                         │ pr-18906 │ pr        │ not on PATH │
╰─────────────────────────────────────────────────────────────┴───────────────────────────────────────────────────────────────────┴──────────┴───────────┴─────────────╯

Anything else?

No response

[joperezr]

🤖 Aspire triage pass — 2026-08-05 23:08 UTC

Area: area-vscode-extension, area-cli
Kind: bug
Repro: partial — conceptual steps in the "Steps To Reproduce" section; no minimal AppHost project attached
Affected version(s): 13.5.0-preview.1.26405.1 (daily; latest 13.5.0-preview.1.26405.3)
Severity: S3 — misleading diagnostic. The underlying build genuinely fails (missing Aspire.AppHost.Sdk) and the debug logs surface the real cause, so a workaround (read the logs) exists. The bug is that ProjectLocator reports "project does not exist" instead of "project failed to build".

Urgency: ⚪ backlog — no milestone
Reason: error-message/diagnostic-quality bug (S3), not a regression, workaround exists (build failure is visible in logs). Default to Backlog while 13.5 stabilizes.

Suggested owner(s): James Newton-King (@JamesNK) (area-cli — where the misleading ProjectLocator error originates), backup Adam Ratzman (@adamint) (area-vscode-extension — surfaces the error)
Evidence:

• JamesNK merged Show status while preparing dashboard bundle #18985, Make CLI package metadata prefetching opt-in #18968, Bound aspire doctor checks with timeouts #18969 in area-cli (last few days) and dominates recent area-cli merges
• CODEOWNERS lists James Newton-King (@JamesNK) for /src/Aspire.Cli (home of Aspire.Cli.Projects.ProjectLocator)
• adamint dominates recent area-vscode-extension merges (Pin cacheable-request after npm supply-chain compromise #18997, Fix VS Code file AppHost build ownership #18984, last 24h); CODEOWNERS lists Adam Ratzman (@adamint) for /extension

Confidence: high
Needs human?: no

[afscrome]

Raw CLI gives a slightly different error, although equally misleading. It claims 0 app hosts were found, even though it has the exact path to an apphost from the aspire config file.

[2026-08-05 23:06:34.911] [INFO] [Stdout] ⚠️ Could not build: AppHost.csproj
[2026-08-05 23:06:34.912] [INFO] [ProjectLocator] Found AppHost path 'src/dotnet/connector/aspire/AppHost/AppHost.csproj' from config file in REDACTED
[2026-08-05 23:06:34.913] [DBUG] [ProjectLocator] Found 0 project files.
[2026-08-05 23:06:34.913] [FAIL] [AspireCliTelemetry] No AppHosts were found (there may be AppHost project files with syntax errors/invalid SDK versions).
Aspire.Cli.Projects.ProjectLocatorException: No buildable AppHosts were found, but there may have been unbuildable AppHosts.
   at Aspire.Cli.Projects.ProjectLocator.<UseOrFindAppHostProjectFileAsync>d__30.MoveNext() + 0x16d6
--- End of stack trace from previous location ---
   at Aspire.Cli.Commands.RunCommand.<ExecuteAsync>d__29.MoveNext() + 0x748
[2026-08-05 23:06:34.919] [INFO] [Stderr] ❌ No AppHosts were found (there may be AppHost project files with syntax errors/invalid SDK versions).
[2026-08-05 23:06:34.920] [INFO] [Stderr] 📄 See logs at C:\Users\alexanderc\.aspire\logs\cli_20260805T230632_3c9014b4.log

[adamint]

I can reproduce this through both the raw CLI and the VS Code extension on 13.4.6 and current daily/main.

With a real AppHost project whose Aspire.AppHost.Sdk version cannot be resolved, dotnet build correctly reports the SDK resolution failure. aspire run --apphost <path> instead exits 7 with The --apphost option specified a project that does not exist, even though the CLI debug log immediately before it contains the actual MSBuild error.

Config-file discovery uses the slightly different No AppHosts were found (there may be AppHost project files with syntax errors/invalid SDK versions) message, but the explicit --apphost and VS Code paths still convert an existing-but-unbuildable project into the nonexistent-project error. This remains current.