diff --git a/src/app/shared/upload/upload-shell.service.ts b/src/app/shared/upload/upload-shell.service.ts
new file mode 100644
index 0000000..56266c0
--- /dev/null
+++ b/src/app/shared/upload/upload-shell.service.ts
@@ -0,0 +1,87 @@
+import { Injectable, inject } from '@angular/core';
+import { UploadAdapter, XhrUploadRequest, XhrUploadHandle, UPLOAD_ABORTED } from './upload.adapter';
+import { problemDetail } from '@shared/infrastructure/api-error';
+import { UploadMsg, Upload } from './upload.machine';
+
+/**
+ * Transport seam (PRD §6): how upload bytes leave the browser. The shipped impl is
+ * KeepaliveTransport (XHR for progress; poll-on-return covers the background gap).
+ * A ServiceWorkerTransport would set `backgroundSyncAvailable = true` and survive a
+ * closed tab — swapping it in touches only this interface.
+ */
+export interface UploadTransport {
+  readonly backgroundSyncAvailable: boolean;
+  send(req: XhrUploadRequest, onProgress: (pct: number) => void): XhrUploadHandle;
+}
+
+@Injectable({ providedIn: 'root' })
+class KeepaliveTransport implements UploadTransport {
+  private adapter = inject(UploadAdapter);
+  readonly backgroundSyncAvailable = false; // no Service Worker registered in this POC
+  send(req: XhrUploadRequest, onProgress: (pct: number) => void) {
+    return this.adapter.xhrUpload(req, onProgress);
+  }
+}
+
+type Dispatch = (m: UploadMsg) => void;
+
+/**
+ * Orchestrates upload effects: drives the transport and translates outcomes into
+ * domain messages. Holds no UI state itself — the host wizard's reducer owns it.
+ * Effects only; the reducer stays pure.
+ */
+@Injectable({ providedIn: 'root' })
+export class UploadShellService {
+  private transport: UploadTransport = inject(KeepaliveTransport);
+  private adapter = inject(UploadAdapter);
+  private inflight = new Map<string, () => void>(); // localId → cancel
+
+  get backgroundSyncAvailable(): boolean {
+    return this.transport.backgroundSyncAvailable;
+  }
+
+  /** Start (or retry) an upload: queue → progress → complete/failed. */
+  upload(req: XhrUploadRequest, dispatch: Dispatch): void {
+    dispatch({ type: 'UploadQueued', localId: req.localId, backgroundSync: this.backgroundSyncAvailable });
+    const { done, cancel } = this.transport.send(req, (pct) =>
+      dispatch({ type: 'UploadProgress', localId: req.localId, progressPct: pct }),
+    );
+    this.inflight.set(req.localId, cancel);
+    done
+      .then(({ documentId }) => dispatch({ type: 'UploadComplete', localId: req.localId, documentId }))
+      .catch((e) => {
+        if (e !== UPLOAD_ABORTED) {
+          dispatch({ type: 'UploadFailed', localId: req.localId, reason: typeof e === 'string' ? e : '' });
+        }
+      })
+      .finally(() => this.inflight.delete(req.localId));
+  }
+
+  /** Optimistic delete: mark deleting → complete (removed) / failed (reverted). */
+  delete(localId: string, documentId: string, dispatch: Dispatch): void {
+    dispatch({ type: 'UploadDeleting', localId });
+    this.adapter
+      .deleteDocument(documentId)
+      .then(() => dispatch({ type: 'UploadDeleteComplete', localId }))
+      .catch((e) => dispatch({ type: 'UploadDeleteFailed', localId, reason: problemDetail(e, '') }));
+  }
+
+  /** Cancel any in-flight XHRs (e.g. when a category switches to post-delivery). */
+  cancel(localIds: string[]): void {
+    for (const id of localIds) {
+      this.inflight.get(id)?.();
+      this.inflight.delete(id);
+    }
+  }
+
+  /** Poll-on-return: ask the BFF which still-in-flight uploads have arrived. */
+  async pollReturning(uploads: Upload[], dispatch: Dispatch): Promise<void> {
+    const ids = uploads.map((u) => u.localId);
+    if (ids.length === 0) return;
+    const items = await this.adapter.status(ids);
+    const results = items
+      .filter((i) => i.status === 'complete' && i.documentId)
+      .map((i) => ({ localId: i.localId, success: true as const, documentId: i.documentId! }));
+    if (results.length > 0) dispatch({ type: 'BackgroundUploadsReturned', results });
+  }
+}
diff --git a/src/app/shared/upload/upload.adapter.ts b/src/app/shared/upload/upload.adapter.ts
new file mode 100644
index 0000000..ef7ef85
--- /dev/null
+++ b/src/app/shared/upload/upload.adapter.ts
@@ -0,0 +1,125 @@
+import { Injectable, inject, resource } from '@angular/core';
+import {
+  ApiClient,
+  DocumentCategoryDto,
+  UploadStatusItemDto,
+} from '@shared/infrastructure/api-client';
+import { problemDetail } from '@shared/infrastructure/api-error';
+import { environment } from '../../../environments/environment';
+import { DocumentCategory } from './upload.machine';
+
+/** One arrived/known status item from poll-on-return. */
+export interface StatusItem {
+  localId: string;
+  status: string; // 'complete' | 'unknown'
+  documentId?: string;
+}
+
+export interface XhrUploadRequest {
+  localId: string;
+  categoryId: string;
+  wizardId: string;
+  file: File;
+}
+
+/** A live upload: progress is reported via the callback; cancel aborts the XHR. */
+export interface XhrUploadHandle {
+  done: Promise<{ documentId: string }>;
+  cancel: () => void;
+}
+
+/** Sentinel rejection so the caller can tell a user-cancel from a real failure. */
+export const UPLOAD_ABORTED = Symbol('upload-aborted');
+
+/**
+ * Infrastructure: the only place upload HTTP lives. Reads (categories, status,
+ * delete) go through the NSwag client; the multipart POST is hand-written XHR
+ * because the generated client is JSON-only (the endpoint is ExcludeFromDescription)
+ * and we need upload-progress events + cancellation, which fetch can't give us.
+ */
+@Injectable({ providedIn: 'root' })
+export class UploadAdapter {
+  private client = inject(ApiClient);
+
+  categoriesResource(wizardId: string) {
+    return resource({ loader: () => this.client.categories(wizardId).then((r) => (r.categories ?? []).map(toCategory)) });
+  }
+
+  /** Poll-on-return: which client localIds have arrived at the BFF. */
+  status(localIds: string[]): Promise<StatusItem[]> {
+    if (localIds.length === 0) return Promise.resolve([]);
+    return this.client.status(localIds.join(',')).then((r) => (r.results ?? []).map(toStatusItem));
+  }
+
+  /** User delete; throws a ProblemDetails (e.g. 409 once linked) the caller maps. */
+  deleteDocument(documentId: string): Promise<void> {
+    return this.client.uploads(documentId);
+  }
+
+  /**
+   * Multipart upload with progress + cancel. ponytail: XHR (not fetch keepalive) —
+   * progress events need XHR; the keepalive/background gap is covered by
+   * poll-on-return (UploadShellService.pollReturning). Swap in a Service Worker
+   * transport behind UploadTransport for true background sync.
+   */
+  xhrUpload(req: XhrUploadRequest, onProgress: (pct: number) => void): XhrUploadHandle {
+    const xhr = new XMLHttpRequest();
+    const form = new FormData();
+    form.append('file', req.file);
+    form.append('categoryId', req.categoryId);
+    form.append('localId', req.localId);
+    form.append('wizardId', req.wizardId);
+
+    let aborted = false;
+    const done = new Promise<{ documentId: string }>((resolve, reject) => {
+      xhr.upload.addEventListener('progress', (e) => {
+        if (e.lengthComputable) onProgress(Math.round((e.loaded / e.total) * 100));
+      });
+      xhr.addEventListener('load', () => {
+        if (xhr.status >= 200 && xhr.status < 300) {
+          try {
+            resolve({ documentId: JSON.parse(xhr.responseText).documentId });
+          } catch {
+            reject(genericError());
+          }
+        } else {
+          reject(parseError(xhr.responseText));
+        }
+      });
+      xhr.addEventListener('error', () => reject(genericError()));
+      xhr.addEventListener('abort', () => (aborted ? reject(UPLOAD_ABORTED) : reject(genericError())));
+    });
+
+    xhr.open('POST', `${environment.apiBaseUrl}/api/v1/uploads`);
+    xhr.send(form);
+    return { done, cancel: () => ((aborted = true), xhr.abort()) };
+  }
+}
+
+const UPLOAD_FAILED = $localize`:@@upload.failed:Uploaden is niet gelukt. Probeer het opnieuw.`;
+const genericError = (): string => UPLOAD_FAILED;
+const parseError = (body: string): string => {
+  try {
+    return problemDetail(JSON.parse(body), UPLOAD_FAILED);
+  } catch {
+    return genericError();
+  }
+};
+
+/** Wire DTO (all fields optional) → domain. */
+function toCategory(c: DocumentCategoryDto): DocumentCategory {
+  return {
+    categoryId: c.categoryId ?? '',
+    label: c.label ?? '',
+    description: c.description ?? '',
+    required: c.required ?? false,
+    acceptedTypes: c.acceptedTypes ?? [],
+    maxSizeMb: c.maxSizeMb ?? 0,
+    multiple: c.multiple ?? false,
+    allowPostDelivery: c.allowPostDelivery ?? false,
+  };
+}
+
+function toStatusItem(s: UploadStatusItemDto): StatusItem {
+  return { localId: s.localId ?? '', status: s.status ?? 'unknown', documentId: s.documentId ?? undefined };
+}