Build a render-preview endpoint

A preview endpoint should render a chart and return the manifest. It should not create a release, reach a cluster, or accept an arbitrary file-system path from a caller.

Install HelmSharp.Chart and HelmSharp.Engine, then resolve the requested chart through an application-owned catalog before calling the renderer:

csharp

app.MapPost("/preview", async (
    PreviewRequest request,
    ChartCatalog charts,
    ValuesCatalog valuesCatalog,
    CancellationToken cancellationToken) =>
{
    var chartPath = charts.GetPath(request.ChartId); // Enforces your allowlist.
    var valuesFilePaths = valuesCatalog.GetPaths(request.ValuesFileIds); // Resolve IDs, never caller-provided paths.
    var chart = await HelmChartLoader.LoadAsync(chartPath, cancellationToken);
    var values = await HelmValues.BuildAsync(
        chart,
        valuesFiles: valuesFilePaths,
        valuesContent: request.ValuesContent,
        setValues: request.SetValues,
        setFileValues: null,
        setStringValues: request.SetStringValues,
        setJsonValues: request.SetJsonValues,
        cancellationToken: cancellationToken);

    var renderer = new HelmTemplateRenderer(
        chart,
        request.ReleaseName,
        request.Namespace,
        values,
        kubeVersion: request.KubeVersion,
        apiVersions: request.ApiVersions,
        isUpgrade: false);

    return Results.Text(renderer.Render(), "text/yaml");
});

ChartCatalog and ValuesCatalog are intentionally application-specific. They might resolve IDs to versioned directories, extracted archives, or a tenant's authorized catalog entries. Keeping path resolution outside the request prevents path traversal and unintended server-side file reads, and makes every preview traceable to exact inputs.

For a production endpoint:

accept values-file IDs rather than paths, limit uploaded/inline values size, and reject override paths your product does not support;
persist the chart version, effective input set, target capabilities, and rendered artifact for later approval;
keep manifest and values access-controlled because either can contain credentials;
call RenderNotes() only when the response has a separate field for human-facing notes.

The review-to-deployment example shows how to turn the stored preview into a cluster-changing operation without replacing this render-only boundary.

Build a render-preview endpoint ​

Build a render-preview endpoint