TS Apphost .addParameter() throws Capability 'Aspire.Hosting/addParameter' requires handle of type 'unknown', got 'unknown'

[chrisusher-tt]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

I'm not able to run aspire start if I try and add a Parameter for TS Apphosts.

Expected Behavior

Should be able to use const endpoint = await builder.addParameter("registry-endpoint"); per the docs in the TS Apphost.

Steps To Reproduce

On 13.5.0-preview.1.26277.22+9a9ddbbf2d4a36b9a2af1e13cba8ed8cd29bc6d6

Create a new Apphost, in my case I want to use an existing Cosmos Account and provide values of this account via params

const cosmosName = await builder.addParameter("COSMOS_NAME");
const cosmosResourceGroup = await builder.addParameter("COSMOS_RESOURCE_GROUP");

const cosmosdb = await builder
  .addAzureCosmosDB("cosmos-db")
  .asExisting(cosmosName, {
    resourceGroup: cosmosResourceGroup,
  });


### Exceptions (if any)

[2026-05-28 17:02:37.615] [FAIL] [DetachedAppHost/AppHost] ❌ Capability Error: Capability 'Aspire.Hosting/addParameter' requires handle of type 'unknown', got 'unknown'
[2026-05-28 17:02:37.615] [FAIL] [DetachedAppHost/AppHost]    Code: TYPE_MISMATCH
[2026-05-28 17:02:37.615] [FAIL] [DetachedAppHost/AppHost]    Capability: Aspire.Hosting/addParameter
[2026-05-28 17:02:37.615] [FAIL] [DetachedAppHost/GuestAppHostProject] TypeScript (Node.js) apphost exited with code 1

### Aspire doctor output

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

Aspire
  ✅ Aspire CLI version 13.5.0-preview.1.26277.22 (channel: daily)  

AppHost
  ✅ AppHost version 13.5.0-preview.1.26277.21 (channel: daily) (apphost.ts)  

Container Runtime
  ✅ Docker v29.2.0: running (auto-detected (default)) ← active  

Environment
  ✅ TypeScript AppHost tooling found (pnpm).  
  ✅ HTTPS development certificate is trusted  

Summary: 5 passed, 0 warnings, 0 failed

### Anything else?

I'm on Node 22 on Windows 11 Pro

[chrisusher-tt]

I'd really like to avoid using EnvVars or hardcoding if at all possible, thanks for everything you guys have done so far!!

[joperezr]

🤖 Aspire triage pass — 2026-05-28 17:57 UTC

Area: area-polyglot
Kind: bug
Repro: yes-minimal — see code snippet in the issue body
Affected version(s): 13.5.0-preview.1.26277.22 (latest preview channel)
Severity: S2 — documented TypeScript AppHost API (builder.addParameter) is unusable; no obvious workaround

Urgency: 🟡 vNext — recommend milestone 13.5
Reason: bug surfaced against a 13.5 preview build in the TypeScript AppHost (polyglot) stack; does not affect 13.4 (13.4 ships the TS AppHost in a more constrained shape and the reporter is on the 13.5 daily channel).

Suggested owner(s): @sebastienros (score ~12), backup @IEvangelist (score ~7)
Evidence:

• sebastienros merged Expand TypeScript brownfield workspace validation #17510, Unify ATS withReference exports #15238, Refine polyglot exports for Aspire.Hosting.Azure.Sql #15011 in area-polyglot / Aspire.Hosting.Azure.Sql polyglot exports
• sebastienros recently closed area-polyglot issues Polyglot withHttpEndpointCallback / withEndpointCallback deadlocks when callback body performs nested RPC mutation #17391, Experimentally publish Aspire.Hosting.Integration.Analyzers for integration developers #17264, aspire update for TypeScript AppHost fails with "No code generator found for language: TypeScript" when CLI is older than target package version; leaves project in broken state #17077, Add TypeScript parity for aspire.dev AppHost snippets #17047, Generate TypeScript API documentation from C# XML docs #17044, Add TypeScript generated API compatibility validation #17041, Validate shipped CLI and bundle layout for TypeScript AppHosts #17040, Add stable TypeScript AppHost publish E2E coverage #17038, Expand TypeScript coverage for JavaScript hosting APIs #17037, Expand TypeScript brownfield and workspace validation #17036, Add end-to-end package-manager coverage for TypeScript AppHosts #17033, Add Node LTS CI matrix for TypeScript AppHosts #17030 (last 30d)
• IEvangelist merged Fix TypeScript AppHost deadlock for async callbacks #17508 and closed TypeScript AppHost: async configure callback deadlocks aspire start when stored inside lazy IOptions.Configure (RunAsync runs on NonConcurrentSynchronizationContext) #17487, Add TypeScript AppHost scaffold lint regression coverage #17035, Validate aspire doctor diagnostics for TypeScript AppHost package managers #17032, Document TypeScript AppHost package-manager support policy #17031 in area-polyglot

Confidence: medium
Needs human?: no

Dry-run mode — no labels or milestone applied. Reply /triage apply to promote, or edit and reapply.

[davidfowl]

@sebastienros