Async PDF rendering with a verified webhook in C# (.NET)

What you need

• An API key. Create one in the app under API keys and export it as FORMFEED_API_KEY. A test key (ff_test_…) is right for trying things out. See the quick start.
• C# (.NET). You need the .NET 8 SDK; the program uses System.Net.Http.Json and System.Text.Json, which ship with it.
• The template. revenue-report from the report example, and its data.json next to the program.
• A secret. 16 to 200 random characters in FORMFEED_WEBHOOK_SECRET, known to both programs. It signs the deliveries of this render, retries included.
• A public URL. Replace https://example.com/hooks/formfeed with your endpoint. While developing, formfeed listen forwards deliveries to localhost without a tunnel.

dotnet new console -o render && cd render   # then replace Program.cs

The program

The request names the template, switches to mode: "async" and says where to report. The answer comes back immediately with the status queued.

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

var data = JsonDocument.Parse(File.ReadAllText("data.json")).RootElement;
var body = new
{
    template = "revenue-report",
    data,
    mode = "async",
    webhook_url = "https://example.com/hooks/formfeed",
    webhook_secret = Environment.GetEnvironmentVariable("FORMFEED_WEBHOOK_SECRET"),
};

using var http = new HttpClient();

// one call to the API: the key goes on each request, never on the client, so no other address sees it
async Task<HttpResponseMessage> Send(HttpMethod method, string url, object? content = null)
{
    using var request = new HttpRequestMessage(method, url) { Content = content is null ? null : JsonContent.Create(content) };
    request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", Environment.GetEnvironmentVariable("FORMFEED_API_KEY"));
    return await http.SendAsync(request);
}

var response = await Send(HttpMethod.Post, "https://api-eu.formfeed.dev/v1/renders", body);
var render = await response.Content.ReadFromJsonAsync<JsonElement>();

// every error is a problem document (RFC 9457) with a title and a detail
if (!response.IsSuccessStatusCode)
{
    Console.Error.WriteLine($"{render.GetProperty("title")}: {(render.TryGetProperty("detail", out var detail) ? detail : "")}");
    return 1;
}

Console.WriteLine($"{render.GetProperty("id")} {render.GetProperty("status")}"); // queued: the webhook reports the rest
return 0;

Run it

It prints the render’s id and queued. A few seconds later your endpoint receives render.completed with the full render, its download_url included, or render.failed with the reason.

export FORMFEED_API_KEY=ff_test_...
export FORMFEED_WEBHOOK_SECRET=$(openssl rand -hex 24)
dotnet run

What is particular about C# (.NET)

• Anonymous objects. The API’s field names are snake_case and are written as they are (webhook_url = …), so no naming policy has to be configured.
• Data without classes. JsonDocument.Parse(…).RootElement embeds the sample data untouched; in your application this is your own record or DTO.
• Key on the request, not the client. The Authorization header is set on each request to the API, never on the HttpClient: the download goes to a signed CDN URL and must not carry your API key.

The receiver

It listens on port 8080, reads the raw body, checks the signature and the age of the timestamp, and only then looks at the JSON. A delivery that fails the check gets 401.

// dotnet new web, then replace Program.cs
using System.Security.Cryptography;
using System.Text;
using System.Text.Json;

var app = WebApplication.CreateBuilder(args).Build();
var secret = Encoding.UTF8.GetBytes(Environment.GetEnvironmentVariable("FORMFEED_WEBHOOK_SECRET")!);

app.MapPost("/hooks/formfeed", async (HttpRequest request) =>
{
    // the signature covers the bytes on the wire: read the raw body, never re-serialised JSON
    using var reader = new StreamReader(request.Body, Encoding.UTF8);
    var body = await reader.ReadToEndAsync();

    // Webhook-Signature: t=<unix seconds>,v1=<hex HMAC-SHA256 of "t.body">
    var signature = request.Headers["Webhook-Signature"].ToString()
        .Split(',', StringSplitOptions.TrimEntries)
        .Select(part => part.Split('=', 2))
        .Where(pair => pair.Length == 2)
        .ToDictionary(pair => pair[0], pair => pair[1]);
    if (!long.TryParse(signature.GetValueOrDefault("t"), out var timestamp)) return Results.Unauthorized();
    if (Math.Abs(DateTimeOffset.UtcNow.ToUnixTimeSeconds() - timestamp) > 300) return Results.Unauthorized();

    var expected = HMACSHA256.HashData(secret, Encoding.UTF8.GetBytes($"{timestamp}.{body}"));
    byte[] given;
    try { given = Convert.FromHexString(signature.GetValueOrDefault("v1") ?? ""); }
    catch (FormatException) { return Results.Unauthorized(); }
    if (!CryptographicOperations.FixedTimeEquals(expected, given)) return Results.Unauthorized();

    var data = JsonDocument.Parse(body).RootElement;
    var render = data.GetProperty("data");
    switch (data.GetProperty("type").GetString())
    {
        case "render.completed": Console.WriteLine($"done {render.GetProperty("id")} {render.GetProperty("download_url")}"); break;
        case "render.failed": Console.WriteLine($"failed {render.GetProperty("id")}"); break;
    }
    return Results.NoContent();
});

app.Run("http://0.0.0.0:8080");

Start the receiver

dotnet new web -o hook && cd hook   # replace Program.cs, then:
FORMFEED_WEBHOOK_SECRET=… dotnet run

Verifying in C# (.NET)

• FixedTimeEquals. CryptographicOperations.FixedTimeEquals compares the two hashes in constant time; SequenceEqual would leak timing.
• Read the stream. Do not bind the body to a model in the endpoint’s signature: model binding parses the JSON, and the signature covers the raw text.

What arrives

A POST with Webhook-Signature: t=<unix seconds>,v1=<hex>, where the hex is HMAC-SHA256 over t, a dot and the raw body, keyed with your secret exactly as you have it, as UTF-8 bytes: the webhook_secret you sent, or a registered endpoint’s whole whsec_… string, prefix included. Every attempt is signed anew with its own t. Webhook-Id stays the same across retries, so it is your deduplication key. data is the render as GET /renders/{id} returns it:

{
  "id": "evt_5b0c8e6f2d4a4c3e9b1a7f6e5d4c3b2a",
  "type": "render.completed",
  "created_at": "2026-09-18T10:42:00Z",
  "workspace_id": "3a6f…",
  "data": {
    "id": "rnd_7da09ab6cd9946b798c187c2a9350852",
    "status": "succeeded",
    "download_url": "https://cdn.formfeed.dev/o/eu/…/document.pdf?exp=…&sig=…",
    "page_count": 2,
    "units": 1,
    …
  }
}

Retries, and what your endpoint owes

• Answer fast. A delivery succeeds with a 2xx within 10 seconds. Store the event and do the work afterwards.
• Retries. Anything else, a redirect included, is retried after 1 minute, 5 minutes, 30 minutes, 2 hours and 12 hours: six attempts in all, over about 14 hours and 40 minutes. The same event can therefore arrive twice: deduplicate by Webhook-Id.
• Without a secret. A webhook_url without webhook_secret is delivered unsigned. For signed deliveries of every render, register an endpoint once in the app or through POST /webhooks; it gets its own secret.
• Try it locally. formfeed listen --forward-to http://localhost:8080/hooks/formfeed relays the events of your test renders to your machine, signed with the session’s own secret rather than webhook_secret: start the receiver with that one, which formfeed listen --print-secret prints (run it before the session, since it ends a running one). formfeed trigger render.completed --template revenue-report makes a real test render whose event you watch arrive. The key needs the webhook:listen scope. See developing webhooks locally.

When the API says no

• One shape for every error. Errors are problem documents (RFC 9457) with type, title, status, detail, a stable code and the request_id to quote to support; the type URL leads to a page about that error. The list is in the error reference.
• The usual ones. 401 unauthorized for a missing or revoked key, 404 template_not_found for a wrong template name, 422 template_runtime_error when the template fails on the data, 429 rate_limited above the plan’s requests per second, 402 quota_exceeded when the Free plan’s units are used up (a paid plan bills extra units up to its hard cap).
• Retry safely. Send an Idempotency-Key header with the POST, and repeat the request with the same key when you do not know whether the first attempt arrived: for 24 hours the key returns the first answer instead of rendering and billing again. The answer is stored once the first attempt has been answered, so do not retry while it may still run; and problems below 500 are stored too, so a corrected request needs a new key.

The job around this program

This guide is one program. What the job needs around it, from the template to the limits and the price, is on the page Report PDF generation.

More C# (.NET) guides

• Generate a PDF invoice from JSON in C# (.NET)
• Create a ZUGFeRD / Factur-X e-invoice in C# (.NET)
• Convert HTML to PDF in C# (.NET)
• Generate Open Graph images in C# (.NET)
• Generate PDFs from a CSV in C# (.NET)
• Merge PDFs and add a password in C# (.NET)

FAQ

Can I poll instead of receiving a webhook?

Yes: GET /renders/{id} until the status is succeeded or failed; the SDKs have waitFor and wait_for for it. A webhook saves the requests and reports sooner. See async and batch.

Why must the raw body be used?

The signature covers the bytes on the wire. Parsing and re-serialising JSON may reorder keys, change number formats or escape characters differently, and the HMAC of that text no longer matches.

What is the five-minute tolerance for?

It stops a captured delivery from being replayed later. It also means the clock of your server must be roughly right.

Async render with a webhook in other languages

• Node.js (TypeScript)
• Python
• PHP
• Go
• Java
• Ruby