.NET HTML to PDF in C# without bundling a browser

using System.Net.Http.Headers;
using System.Net.Http.Json;

const string html = """
    <h1>Packing list {{ shipment.number }}</h1>
    <p>Order {{ shipment.order }} for {{ shipment.customer }}, {{ shipment.cartons }} cartons</p>
    <table>
      <thead><tr><th>SKU</th><th>Item</th><th>Qty</th><th>Carton</th></tr></thead>
      <tbody>
      {% for item in items %}
        <tr><td>{{ item.sku }}</td><td>{{ item.name }}</td><td>{{ item.qty }}</td><td>{{ item.carton }}</td></tr>
      {% endfor %}
      </tbody>
    </table>
    """;

using var http = new HttpClient { Timeout = TimeSpan.FromSeconds(75) };
http.DefaultRequestHeaders.Authorization =
    new AuthenticationHeaderValue("Bearer", Environment.GetEnvironmentVariable("CASTPDF_API_KEY"));

var payload = new
{
    html,
    data = new
    {
        shipment = new { number = "PL-55021", order = "SO-18840", customer = "Fjord Outdoor AS", cartons = 3 },
        items = new[]
        {
            new { sku = "TNT-2P-GRN", name = "Two-person tent, green", qty = 4, carton = 1 },
            new { sku = "MAT-INF-L", name = "Inflatable sleeping mat, large", qty = 8, carton = 2 },
            new { sku = "STV-GAS-MINI", name = "Compact gas stove", qty = 6, carton = 3 },
        },
    },
    mode = "print",
    filename = "packing-list-PL-55021",
};

using var response = await http.PostAsJsonAsync("https://api.castpdf.com/v1/pdf", payload);
if (!response.IsSuccessStatusCode)
{
    var problem = await response.Content.ReadFromJsonAsync<ErrorEnvelope>();
    throw new InvalidOperationException($"{(int)response.StatusCode} {problem?.Error?.Code}: {problem?.Error?.Message}");
}

await using (var file = File.Create("packing-list-PL-55021.pdf"))
{
    await response.Content.CopyToAsync(file);
}

var pages = response.Headers.TryGetValues("x-pages", out var values) ? string.Join(",", values) : "?";
Console.WriteLine($"Saved packing-list-PL-55021.pdf, {pages} page(s)");

record ErrorEnvelope(ApiError? Error);
record ApiError(string Code, string Message);

using System.Net.Http.Headers;
using System.Net.Http.Json;
using System.Text.Json;

var json = new JsonSerializerOptions(JsonSerializerDefaults.Web)
{
    PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower,
};

using var http = new HttpClient { Timeout = TimeSpan.FromSeconds(75) };
http.DefaultRequestHeaders.Authorization =
    new AuthenticationHeaderValue("Bearer", Environment.GetEnvironmentVariable("CASTPDF_API_KEY"));

var list = new PackingList(
    new Shipment("PL-55022", "SO-18852", "Kvist Garden Supply", Cartons: 2),
    new[]
    {
        new Item("HOSE-25M", "Garden hose, 25 m", Qty: 6, Carton: 1),
        new Item("RAKE-ALU", "Aluminium rake", Qty: 10, Carton: 2),
    });

var key = $"packing-list-{list.Shipment.Number}";
using var request = new HttpRequestMessage(HttpMethod.Post, "https://api.castpdf.com/v1/pdf")
{
    Content = JsonContent.Create(
        new PdfRequest(Environment.GetEnvironmentVariable("PACKING_TEMPLATE_ID")!, list, key, "url"),
        options: json),
};
request.Headers.Add("Idempotency-Key", key);

using var response = await http.SendAsync(request);
if (!response.IsSuccessStatusCode)
{
    var problem = await response.Content.ReadFromJsonAsync<ErrorEnvelope>(json);
    throw new InvalidOperationException($"{problem?.Error?.Code}: {problem?.Error?.Message}");
}

var link = await response.Content.ReadFromJsonAsync<PdfLink>(json);
Console.WriteLine($"{link!.Url} ({link.Pages} pages), expires {link.ExpiresAt:u}");

record PdfRequest(string TemplateId, PackingList Data, string Filename, string Response);
record PackingList(Shipment Shipment, Item[] Items);
record Shipment(string Number, string Order, string Customer, int Cartons);
record Item(string Sku, string Name, int Qty, int Carton);
record PdfLink(string Id, int Pages, long Bytes, string Url, DateTimeOffset ExpiresAt);
record ErrorEnvelope(ApiError? Error);
record ApiError(string Code, string Message);

using System.Net.Http.Headers;
using System.Net.Http.Json;

public sealed class CastPdfException : Exception
{
    public CastPdfException(int status, string? code, string? message)
        : base($"{status} {code}: {message}")
    {
        Status = status;
        Code = code;
    }

    public int Status { get; }
    public string? Code { get; }
}

public sealed class CastPdfClient
{
    private const string Endpoint = "https://api.castpdf.com/v1/pdf";
    private readonly HttpClient _http;

    public CastPdfClient(HttpClient http) => _http = http;

    public async Task<byte[]> RenderAsync(object payload, string idempotencyKey, CancellationToken ct = default)
    {
        const int maxAttempts = 5;
        for (var attempt = 1; ; attempt++)
        {
            using var request = new HttpRequestMessage(HttpMethod.Post, Endpoint)
            {
                Content = JsonContent.Create(payload),
            };
            request.Headers.Authorization =
                new AuthenticationHeaderValue("Bearer", Environment.GetEnvironmentVariable("CASTPDF_API_KEY"));
            request.Headers.Add("Idempotency-Key", idempotencyKey);

            HttpResponseMessage response;
            try
            {
                response = await _http.SendAsync(request, ct);
            }
            catch (Exception ex) when (ex is HttpRequestException || (ex is TaskCanceledException && !ct.IsCancellationRequested))
            {
                if (attempt == maxAttempts) throw;
                await Task.Delay(TimeSpan.FromSeconds(Math.Pow(2, attempt)), ct);
                continue;
            }

            using (response)
            {
                var status = (int)response.StatusCode;
                if (response.IsSuccessStatusCode)
                {
                    return await response.Content.ReadAsByteArrayAsync(ct);
                }
                if ((status is 409 or 429 or 503) && attempt < maxAttempts)
                {
                    var delay = response.Headers.RetryAfter?.Delta ?? TimeSpan.FromSeconds(Math.Pow(2, attempt));
                    await Task.Delay(delay, ct);
                    continue;
                }

                var problem = await response.Content.ReadFromJsonAsync<ErrorEnvelope>(ct);
                throw new CastPdfException(status, problem?.Error?.Code, problem?.Error?.Message);
            }
        }
    }
}

public sealed record ErrorEnvelope(ApiError? Error);
public sealed record ApiError(string Code, string Message);