diff --git a/services/domain/Big.Api/Program.cs b/services/domain/Big.Api/Program.cs
index 466ce1d..f4a21f0 100644
--- a/services/domain/Big.Api/Program.cs
+++ b/services/domain/Big.Api/Program.cs
@@ -37,6 +37,10 @@ builder.Services.AddSingleton(sp => sp.GetRequiredService<IConfiguration>()
 builder.Services.AddSingleton(sp => sp.GetRequiredService<IConfiguration>()
     .GetSection("Acl").Get<AclOptions>()
     ?? throw new InvalidOperationException("Missing configuration section 'Acl'"));
+// clamd defaults to the compose/chart service name; ClamAv__* overrides it (S-29, ADR-0036).
+builder.Services.AddSingleton(sp => sp.GetRequiredService<IConfiguration>()
+    .GetSection("ClamAv").Get<ClamAvOptions>() ?? new ClamAvOptions());
+builder.Services.AddSingleton<IDocumentScanner, ClamdDocumentScanner>();
 
 // The in-memory registration store is shared between the submit endpoint and the worker (ADR-0009).
 builder.Services.AddSingleton<IRegistrationStore, InMemoryRegistrationStore>();
@@ -158,8 +162,8 @@ app.MapPost("/registrations/{id}/withdraw", async (string id, WithdrawRequest bo
 // Provide documents (S-10a): the zorgprofessional supplies the documents their registration is parked
 // waiting for, completing the WachtOpDocumenten task so the process advances to beoordeling (ADR-0017).
 // Owner-scoped by the caller's bsn (the BFF forwards it from the DigiD token); unknown or not-the-
-// caller's is 404 (indistinguishable). Idempotent — completing an already-left wait is a no-op. The
-// real file upload + ZGW storage is S-10b; this endpoint is the trigger that unblocks the process.
+// caller's is 404 (indistinguishable). Idempotent — completing an already-left wait is a no-op. Only a
+// PDF that clamd scans clean is stored and unblocks the process (S-29).
 app.MapPost("/registrations/{id}/documents", async (string id, ProvideDocumentsRequest body, ProvideDocuments provide, CancellationToken ct) =>
 {
     if (!Guid.TryParse(id, out var guid))
@@ -177,8 +181,16 @@ app.MapPost("/registrations/{id}/documents", async (string id, ProvideDocumentsR
     var command = new ProvideDocumentsCommand(
         new RegistrationId(guid), body.Bsn, content,
         body.FileName ?? "diploma.pdf", body.ContentType ?? "application/pdf");
-    var outcome = await provide.HandleAsync(command, ct);
-    return outcome == ProvideDocumentsOutcome.Accepted ? Results.NoContent() : Results.NotFound();
+    // A refused file is 422 with a machine-readable reason the portal words for the citizen; an
+    // unreachable scanner is 503 — retryable, and nothing was stored (S-29, ADR-0036).
+    return await provide.HandleAsync(command, ct) switch
+    {
+        ProvideDocumentsOutcome.Accepted => Results.NoContent(),
+        ProvideDocumentsOutcome.NotAPdf => Results.UnprocessableEntity(new { reason = "not-a-pdf" }),
+        ProvideDocumentsOutcome.Infected => Results.UnprocessableEntity(new { reason = "infected" }),
+        ProvideDocumentsOutcome.ScannerUnavailable => Results.StatusCode(StatusCodes.Status503ServiceUnavailable),
+        _ => Results.NotFound(),
+    };
 });
 
 // The behandelaar's werkbak (S-12): the registrations awaiting beoordeling, read from the open
diff --git a/services/domain/Big.Infrastructure/ClamdDocumentScanner.cs b/services/domain/Big.Infrastructure/ClamdDocumentScanner.cs
index bc6fdc3..fe7666a 100644
--- a/services/domain/Big.Infrastructure/ClamdDocumentScanner.cs
+++ b/services/domain/Big.Infrastructure/ClamdDocumentScanner.cs
@@ -1,10 +1,48 @@
+using System.Buffers.Binary;
+using System.Net.Sockets;
+using System.Text;
 using Big.Application;
 
 namespace Big.Infrastructure;
 
-/// <summary>Scans a document with clamd over its INSTREAM protocol (S-29, ADR-0036).</summary>
+/// <summary>
+/// Scans a document with clamd over its INSTREAM protocol (S-29, ADR-0036): <c>zINSTREAM\0</c>, the
+/// document as one big-endian length-prefixed chunk, a zero-length terminator, then one reply —
+/// <c>stream: OK</c> or <c>stream: &lt;signature&gt; FOUND</c>. Anything else (an ERROR reply, a refused
+/// connection, a timeout) is <see cref="ScanVerdict.Unavailable"/>, so the caller fails closed.
+/// </summary>
 public sealed class ClamdDocumentScanner(ClamAvOptions options) : IDocumentScanner
 {
-    public Task<ScanVerdict> ScanAsync(byte[] content, CancellationToken ct = default)
-        => Task.FromResult(ScanVerdict.Clean);
+    public async Task<ScanVerdict> ScanAsync(byte[] content, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(content);
+        using var timeout = CancellationTokenSource.CreateLinkedTokenSource(ct);
+        timeout.CancelAfter(options.Timeout);
+
+        string reply;
+        try
+        {
+            using var client = new TcpClient();
+            await client.ConnectAsync(options.Host, options.Port, timeout.Token);
+            var stream = client.GetStream();
+
+            var length = new byte[4];
+            BinaryPrimitives.WriteInt32BigEndian(length, content.Length);
+            await stream.WriteAsync("zINSTREAM\0"u8.ToArray(), timeout.Token);
+            await stream.WriteAsync(length, timeout.Token);
+            await stream.WriteAsync(content, timeout.Token);
+            await stream.WriteAsync(new byte[4], timeout.Token);
+
+            using var reader = new StreamReader(stream, Encoding.ASCII);
+            reply = (await reader.ReadToEndAsync(timeout.Token)).TrimEnd('\0', '\n');
+        }
+        catch (Exception e) when (e is SocketException or IOException
+                                  || (e is OperationCanceledException && !ct.IsCancellationRequested))
+        {
+            return ScanVerdict.Unavailable;
+        }
+
+        if (reply == "stream: OK") return ScanVerdict.Clean;
+        return reply.EndsWith(" FOUND", StringComparison.Ordinal) ? ScanVerdict.Infected : ScanVerdict.Unavailable;
+    }
 }