Feature request: WithFrontendConfig API for passing Aspire parameters to SPA frontends

[maddymontaquila]

Is your feature request related to a problem?

Aspire parameters (from AddParameter) are set at runtime, but Vite/React/Angular bake environment variables at build time. There is no built-in way to pass an Aspire parameter to a SPA frontend.

The current workaround requires:

1. Passing the parameter to the API as an env var (WithEnvironment)
2. Writing a custom /api/config/{key} endpoint in the API project
3. Having the SPA fetch from that endpoint at mount time

Every team using AddViteApp / AddNpmApp with parameters will need this pattern. We filed a docs recipe request at microsoft/aspire.dev#394 but believe this should also be a first-class product feature.

Describe the solution you'd like

A WithFrontendConfig API (or similar) that bridges Aspire parameters to SPA frontends:

builder.AddViteApp("frontend", "./src/frontend")
    .WithFrontendConfig("GOOGLE_CLIENT_ID", googleClientId);

This could auto-generate a config endpoint (or inject a __config.json at a well-known path like /__aspire/config) that the SPA can fetch at runtime.

Describe alternatives you've considered

• Manual config endpoint — works but requires boilerplate in every project (our implementation is ~18 lines in ConfigEndpoints.cs)
• Vite plugin that fetches config at build time — defeats the purpose of runtime params
• Documenting the pattern — filed as Add recipe for passing Aspire parameters to SPA frontends (Vite/React runtime config) microsoft/aspire.dev#394, zero engagement so far

Additional context

We're using Aspire 13.2.0-preview.1 with AddViteApp + WithPnpm() and a React 19 frontend. The workaround (config endpoint pattern) has been stable across two sprints and works well — but it's boilerplate that every SPA+Aspire project will reinvent.