diff --git a/AGENTS.md b/AGENTS.md
index 52352b3..c8b31cd 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -31,6 +31,12 @@ Project constraints (do not regress):
   MariaDB/Redis come from docker-compose.yml.
 - Deployment will be Docker-based (compose); do not add systemd/cron unit
   files for scheduling — use the container setup for timers/workers.
+- Same-origin and cross-origin frontends both work: the SPA uses relative
+  URLs unless built with VITE_API_BASE, the backend allows extra origins
+  via CORS_ALLOWED_ORIGINS (plus CSRF_TRUSTED_ORIGINS for the admin), and
+  API media URLs are absolute (built from the request host), so signed
+  files load cross-origin too. TRUST_PROXY_HEADERS=true is required behind
+  a TLS-terminating proxy.
 - No server-side media processing: the home server cannot handle it.
   Compression/optimization runs client-side (WebCodecs + WASM in a worker)
   and the server only applies the result via POST /api/files/J-x/optimize/.
diff --git a/backend/.env.example b/backend/.env.example
index 1ee89e8..1d59693 100644
--- a/backend/.env.example
+++ b/backend/.env.example
@@ -1,6 +1,18 @@
 SECRET_KEY=change-me-to-a-long-random-string
 DEBUG=True
 ALLOWED_HOSTS=localhost,127.0.0.1
+
+# Cross-origin frontends (comma separated). Same-origin keeps working with an
+# empty list; add e.g. https://j621.example.com when the SPA is served from
+# another origin than this API.
+# CORS_ALLOWED_ORIGINS=https://j621.example.com
+# CORS_ALLOW_ALL_ORIGINS=false
+# CORS_ALLOW_CREDENTIALS=false
+# CSRF_TRUSTED_ORIGINS=https://j621.example.com
+# Trust X-Forwarded-Proto/Host from a TLS-terminating reverse proxy so media
+# URLs keep https and the public hostname.
+# TRUST_PROXY_HEADERS=true
+
 # Absolute path to your media folder, or relative to the backend/ folder.
 WATCHED_FOLDER=media/library
 
diff --git a/backend/apps/accounts/serializers.py b/backend/apps/accounts/serializers.py
index 16489f9..ba2f02a 100644
--- a/backend/apps/accounts/serializers.py
+++ b/backend/apps/accounts/serializers.py
@@ -1,10 +1,10 @@
 from pathlib import Path
 
 from django.contrib.auth.password_validation import validate_password
-from django.core import signing
 from rest_framework import serializers
 
-from apps.library.services import MEDIA_FILE_SALT, VIDEO_EXTENSIONS
+from apps.library.services import VIDEO_EXTENSIONS
+from apps.library.services import signed_media_url as signed_library_url
 
 from .models import User
 
@@ -22,11 +22,7 @@ def signed_media_url(request, item):
         and Path(location.rel_path).suffix.lower() in VIDEO_EXTENSIONS
     )
     action = "thumbnail" if is_video else "raw"
-    signature = signing.dumps(
-        {"item": item.id, "user": user.id, "action": action},
-        salt=MEDIA_FILE_SALT,
-    )
-    return f"/api/files/J-{item.id}/{action}/?sig={signature}"
+    return signed_library_url(item, user, action, request=request)
 
 
 class UserSerializer(serializers.ModelSerializer):
diff --git a/backend/apps/library/serializers.py b/backend/apps/library/serializers.py
index 0dea0df..6f5ee6c 100644
--- a/backend/apps/library/serializers.py
+++ b/backend/apps/library/serializers.py
@@ -103,10 +103,17 @@ class MediaItemSerializer(serializers.ModelSerializer):
         return f"J-{obj.id}"
 
     def get_raw_url(self, obj):
-        return signed_media_url(obj, self._request_user(), "raw")
+        return signed_media_url(
+            obj, self._request_user(), "raw", request=self.context.get("request")
+        )
 
     def get_thumbnail_url(self, obj):
-        return signed_media_url(obj, self._request_user(), "thumbnail")
+        return signed_media_url(
+            obj,
+            self._request_user(),
+            "thumbnail",
+            request=self.context.get("request"),
+        )
 
     def get_display_rating(self, obj):
         if obj.rating:
@@ -184,7 +191,9 @@ class TempUploadSerializer(serializers.ModelSerializer):
             {"temp": str(obj.id), "user": user.id},
             salt=UPLOAD_FILE_SALT,
         )
-        return f"/api/uploads/{obj.id}/file/?sig={signature}"
+        url = f"/api/uploads/{obj.id}/file/?sig={signature}"
+        request = self.context.get("request")
+        return request.build_absolute_uri(url) if request is not None else url
 
     def get_preview_url(self, obj):
         """A URL an <img>/<video> tag can load: staged file or library item."""
@@ -202,11 +211,9 @@ class TempUploadSerializer(serializers.ModelSerializer):
             and Path(location.rel_path).suffix.lower() in VIDEO_EXTENSIONS
         )
         action = "thumbnail" if is_video else "raw"
-        signature = signing.dumps(
-            {"item": item.id, "user": user.id, "action": action},
-            salt=MEDIA_FILE_SALT,
+        return signed_media_url(
+            item, user, action, request=self.context.get("request")
         )
-        return f"/api/files/J-{item.id}/{action}/?sig={signature}"
 
 
 class DownloadTaskSerializer(serializers.ModelSerializer):
@@ -292,7 +299,9 @@ class SimilarityCheckSerializer(serializers.ModelSerializer):
             {"check": str(obj.id), "user": user.id},
             salt=UPLOAD_FILE_SALT,
         )
-        return f"/api/similarity/{obj.id}/file/?sig={signature}"
+        url = f"/api/similarity/{obj.id}/file/?sig={signature}"
+        request = self.context.get("request")
+        return request.build_absolute_uri(url) if request is not None else url
 
     def get_expires_at(self, obj):
         ttl = int(getattr(settings, "SIMILARITY_TTL_MINUTES", 30))
diff --git a/backend/apps/library/services.py b/backend/apps/library/services.py
index 43b4d5a..759fc99 100644
--- a/backend/apps/library/services.py
+++ b/backend/apps/library/services.py
@@ -73,16 +73,22 @@ def index_file(path, folder):
     return item, created_item, location, created_location
 
 
-def signed_media_url(item, user, action="raw"):
-    """Media URL that <img>/<video> tags can load for a signed-in user."""
-    base = f"/api/files/J-{item.id}/{action}/"
-    if user is None or not getattr(user, "is_authenticated", False):
-        return base
-    signature = signing.dumps(
-        {"item": item.id, "user": user.id, "action": action},
-        salt=MEDIA_FILE_SALT,
-    )
-    return f"{base}?sig={signature}"
+def signed_media_url(item, user, action="raw", request=None):
+    """Media URL that <img>/<video> tags can load for a signed-in user.
+
+    With a ``request`` the URL is absolute, so the SPA also works when it is
+    served from a different origin; without one it stays relative.
+    """
+    path = f"/api/files/J-{item.id}/{action}/"
+    if user is not None and getattr(user, "is_authenticated", False):
+        signature = signing.dumps(
+            {"item": item.id, "user": user.id, "action": action},
+            salt=MEDIA_FILE_SALT,
+        )
+        path = f"{path}?sig={signature}"
+    if request is None:
+        return path
+    return request.build_absolute_uri(path)
 
 
 def rename_location_to_j_id(item, location):
diff --git a/backend/apps/library/similarity.py b/backend/apps/library/similarity.py
index fc3ea72..96813de 100644
--- a/backend/apps/library/similarity.py
+++ b/backend/apps/library/similarity.py
@@ -119,7 +119,9 @@ class SimilarityCheckViewSet(
         check.md5 = services.compute_md5(check.file.path)
 
         exact = MediaItem.objects.filter(md5=check.md5).first()
-        matches = find_library_matches(check.file.path, limit=12, user=request.user)
+        matches = find_library_matches(
+            check.file.path, limit=12, user=request.user, request=request
+        )
         if exact is not None:
             exact_j_id = f"J-{exact.id}"
             matches = [
diff --git a/backend/apps/library/tools.py b/backend/apps/library/tools.py
index ed93b3c..3852351 100644
--- a/backend/apps/library/tools.py
+++ b/backend/apps/library/tools.py
@@ -99,7 +99,7 @@ def item_brief(item, request=None):
         "location_count": len(locations),
         "uploaded_by": item.uploaded_by.username if item.uploaded_by else None,
         "e621_post_id": item.e621_post_id,
-        "thumbnail_url": signed_media_url(item, user, "thumbnail"),
+        "thumbnail_url": signed_media_url(item, user, "thumbnail", request=request),
     }
 
 
diff --git a/backend/apps/library/uploads.py b/backend/apps/library/uploads.py
index 043d458..f7de195 100644
--- a/backend/apps/library/uploads.py
+++ b/backend/apps/library/uploads.py
@@ -34,7 +34,7 @@ from .tools import HASH_FIELDS, hashes_similarity
 logger = logging.getLogger(__name__)
 
 
-def find_library_matches(path, limit=10, user=None):
+def find_library_matches(path, limit=10, user=None, request=None):
     """Library items visually similar to a staged file."""
     hashes = services.compute_visual_hashes(path)
     if not hashes:
@@ -57,7 +57,9 @@ def find_library_matches(path, limit=10, user=None):
                 "j_id": f"J-{item.id}",
                 "filename": Path(location.rel_path).name if location else item.md5,
                 "similarity": round(similarity * 100, 1),
-                "thumbnail_url": services.signed_media_url(item, user, "thumbnail"),
+                "thumbnail_url": services.signed_media_url(
+                    item, user, "thumbnail", request=request
+                ),
             }
         )
     matches.sort(key=lambda entry: entry["similarity"], reverse=True)
@@ -185,7 +187,9 @@ class TempUploadViewSet(
             temp.library_item = existing
             temp.file.delete(save=False)
         else:
-            matches = find_library_matches(temp.file.path, user=request.user)
+            matches = find_library_matches(
+                temp.file.path, user=request.user, request=request
+            )
             if matches:
                 temp.visual_matches = matches
                 temp.status = TempUpload.STATUS_VISUAL_MATCH
diff --git a/backend/config/settings.py b/backend/config/settings.py
index 5342244..5b55dd7 100644
--- a/backend/config/settings.py
+++ b/backend/config/settings.py
@@ -20,6 +20,38 @@ ALLOWED_HOSTS = [
     if host.strip()
 ]
 
+# Same-origin access always works; list extra frontend origins in
+# CORS_ALLOWED_ORIGINS (comma separated, e.g. https://j621.example.com).
+CORS_ALLOWED_ORIGINS = [
+    origin.strip().rstrip("/")
+    for origin in os.getenv("CORS_ALLOWED_ORIGINS", "").split(",")
+    if origin.strip()
+]
+# Escape hatch for local experiments; never enable against a public server.
+CORS_ALLOW_ALL_ORIGINS = (
+    os.getenv("CORS_ALLOW_ALL_ORIGINS", "false").lower() == "true"
+)
+# The SPA authenticates with an Authorization: Token header, not cookies, so
+# cross-origin requests do not need credentials. Enable this only if a
+# cross-origin client really relies on cookies (e.g. the Django admin).
+CORS_ALLOW_CREDENTIALS = (
+    os.getenv("CORS_ALLOW_CREDENTIALS", "false").lower() == "true"
+)
+# Same list, for session/CSRF-protected endpoints (Django admin) reached from
+# another origin.
+CSRF_TRUSTED_ORIGINS = [
+    origin.strip().rstrip("/")
+    for origin in os.getenv("CSRF_TRUSTED_ORIGINS", "").split(",")
+    if origin.strip()
+]
+
+# Behind a TLS-terminating proxy the backend sees plain http; trust the
+# proxy's X-Forwarded-Proto/Host so absolute media URLs keep https and the
+# public hostname.
+if os.getenv("TRUST_PROXY_HEADERS", "false").lower() == "true":
+    SECURE_PROXY_SSL_HEADER = ("HTTP_X_FORWARDED_PROTO", "https")
+    USE_X_FORWARDED_HOST = True
+
 
 def _git_commit_hash():
     """Short git hash of the running build, mirroring the original app."""
@@ -50,6 +82,7 @@ INSTALLED_APPS = [
     "rest_framework",
     "rest_framework.authtoken",
     "django_filters",
+    "corsheaders",
     "apps.accounts",
     "apps.library",
     "apps.follows",
@@ -58,6 +91,8 @@ INSTALLED_APPS = [
 
 MIDDLEWARE = [
     "django.middleware.security.SecurityMiddleware",
+    # Must sit above CommonMiddleware so preflights are answered early.
+    "corsheaders.middleware.CorsMiddleware",
     "django.contrib.sessions.middleware.SessionMiddleware",
     "django.middleware.common.CommonMiddleware",
     "django.middleware.csrf.CsrfViewMiddleware",
diff --git a/backend/requirements.txt b/backend/requirements.txt
index d3dcda7..102799f 100644
--- a/backend/requirements.txt
+++ b/backend/requirements.txt
@@ -1,6 +1,7 @@
 Django>=6.0,<6.2
 djangorestframework>=3.17
 django-filter>=25.1
+django-cors-headers>=4.9
 Pillow>=11.0
 python-dotenv>=1.0
 requests>=2.32
diff --git a/frontend/src/features/account/AvatarCard.tsx b/frontend/src/features/account/AvatarCard.tsx
index cd9374b..5d79978 100644
--- a/frontend/src/features/account/AvatarCard.tsx
+++ b/frontend/src/features/account/AvatarCard.tsx
@@ -3,7 +3,7 @@ import { Check, ChevronLeft, ChevronRight, Search } from "lucide-react";
 import { useState } from "react";
 
 import { Button, Spinner, inputClass } from "@/components/ui";
-import { api, errorMessage } from "@/lib/api";
+import { api, apiUrl, errorMessage } from "@/lib/api";
 import { cn } from "@/lib/cn";
 import type { MediaItem, Paginated, User } from "@/lib/types";
 import { useAuth } from "@/store/auth";
@@ -112,10 +112,10 @@ function AvatarPickerModal({
                   >
                     <span className="block aspect-square overflow-hidden">
                       <img
-                        src={
+                        src={apiUrl(
                           item.thumbnail_url ??
-                          `/api/files/${item.j_id}/thumbnail/`
-                        }
+                            `/api/files/${item.j_id}/thumbnail/`,
+                        )}
                         alt={item.filename}
                         loading="lazy"
                         className="h-full w-full object-cover"
diff --git a/frontend/src/features/library/DuplicatesPage.tsx b/frontend/src/features/library/DuplicatesPage.tsx
index c62b9e7..b09ad5c 100644
--- a/frontend/src/features/library/DuplicatesPage.tsx
+++ b/frontend/src/features/library/DuplicatesPage.tsx
@@ -3,7 +3,7 @@ import { useMemo, useState } from "react";
 import { Link } from "react-router-dom";
 
 import { Button, Spinner, inputClass } from "@/components/ui";
-import { api, errorMessage } from "@/lib/api";
+import { api, apiUrl, errorMessage } from "@/lib/api";
 import { cn } from "@/lib/cn";
 import { formatBytes } from "@/lib/format";
 import type {
@@ -35,8 +35,9 @@ function MemberCard({
   selected: boolean;
   onToggle: () => void;
 }) {
-  const preview =
-    entry.thumbnail_url ?? `/api/files/${entry.j_id}/thumbnail/`;
+  const preview = apiUrl(
+    entry.thumbnail_url ?? `/api/files/${entry.j_id}/thumbnail/`,
+  );
   return (
     <div
       className={cn(
@@ -250,10 +251,10 @@ export default function DuplicatesPage() {
                 <div className="w-32 shrink-0">
                   <div className="aspect-square overflow-hidden rounded-md bg-ctp-mantle">
                     <img
-                      src={
+                      src={apiUrl(
                         group.thumbnail_url ??
-                        `/api/files/${group.j_id}/thumbnail/`
-                      }
+                          `/api/files/${group.j_id}/thumbnail/`,
+                      )}
                       alt={group.j_id}
                       loading="lazy"
                       className="h-full w-full object-cover"
diff --git a/frontend/src/features/library/LibraryDetail.tsx b/frontend/src/features/library/LibraryDetail.tsx
index 0bc634a..f32324b 100644
--- a/frontend/src/features/library/LibraryDetail.tsx
+++ b/frontend/src/features/library/LibraryDetail.tsx
@@ -8,7 +8,7 @@ import { TagChip } from "@/components/TagChip";
 import { DetailShell } from "@/features/detail/DetailShell";
 import { IqdbCard } from "@/features/library/IqdbCard";
 import { OptimizeModal } from "@/features/optimize/OptimizeModal";
-import { api, errorMessage } from "@/lib/api";
+import { api, apiUrl, errorMessage } from "@/lib/api";
 import { cn } from "@/lib/cn";
 import { isTypingTarget } from "@/lib/dom";
 import { renderDText } from "@/lib/dtext";
@@ -468,7 +468,7 @@ export function LibraryDetail({ jId }: { jId: string }) {
     queryFn: () => api<MediaItem>(`/api/files/${jId}/`),
   });
 
-  const rawUrl = query.data?.raw_url ?? `/api/files/${jId}/raw/`;
+  const rawUrl = apiUrl(query.data?.raw_url ?? `/api/files/${jId}/raw/`);
   const downloadUrl = rawUrl.includes("?")
     ? `${rawUrl}&download=1`
     : `${rawUrl}?download=1`;
diff --git a/frontend/src/features/online/OnlineDetail.tsx b/frontend/src/features/online/OnlineDetail.tsx
index f1b322a..eb6742a 100644
--- a/frontend/src/features/online/OnlineDetail.tsx
+++ b/frontend/src/features/online/OnlineDetail.tsx
@@ -13,7 +13,7 @@ import { Link, useLocation, useNavigate } from "react-router-dom";
 import { Button, EmptyState, Spinner, linkButtonClass } from "@/components/ui";
 import { TagChip } from "@/components/TagChip";
 import { DetailShell } from "@/features/detail/DetailShell";
-import { api, errorMessage } from "@/lib/api";
+import { api, apiUrl, errorMessage } from "@/lib/api";
 import { cn } from "@/lib/cn";
 import { renderDText } from "@/lib/dtext";
 import { isTypingTarget } from "@/lib/dom";
@@ -242,9 +242,11 @@ export function OnlineDetail({ postId }: { postId: number }) {
     post.sample.has && post.sample.url ? post.sample.url : post.file.url;
   const canUpload = Boolean(user?.can_upload);
   const clientDownloadUrl = post.file.url
-    ? `/api/online/file/?url=${encodeURIComponent(post.file.url)}&filename=${encodeURIComponent(
-        `${post.file.md5}.${post.file.ext}`,
-      )}`
+    ? apiUrl(
+        `/api/online/file/?url=${encodeURIComponent(post.file.url)}&filename=${encodeURIComponent(
+          `${post.file.md5}.${post.file.ext}`,
+        )}`,
+      )
     : null;
 
   return (
diff --git a/frontend/src/features/upload/UploadPage.tsx b/frontend/src/features/upload/UploadPage.tsx
index a76ea57..10358fc 100644
--- a/frontend/src/features/upload/UploadPage.tsx
+++ b/frontend/src/features/upload/UploadPage.tsx
@@ -15,7 +15,7 @@ import {
 import { Link } from "react-router-dom";
 
 import { Button, EmptyState, Spinner, inputClass } from "@/components/ui";
-import { api, authHeaders, errorMessage, uploadFile } from "@/lib/api";
+import { api, authHeaders, apiUrl, errorMessage, uploadFile } from "@/lib/api";
 import { cn } from "@/lib/cn";
 import {
   E621Error,
@@ -398,10 +398,10 @@ function MetadataModal({
                           className="flex items-center gap-3 rounded-md border border-ctp-surface0 bg-ctp-surface0/40 p-2"
                         >
                           <img
-                            src={
+                            src={apiUrl(
                               match.thumbnail_url ??
-                              `/api/files/${match.j_id}/thumbnail/`
-                            }
+                                `/api/files/${match.j_id}/thumbnail/`,
+                            )}
                             alt={match.j_id}
                             loading="lazy"
                             className="h-12 w-12 shrink-0 rounded object-cover"
diff --git a/frontend/src/lib/api.ts b/frontend/src/lib/api.ts
index 9b3f1a6..e551eea 100644
--- a/frontend/src/lib/api.ts
+++ b/frontend/src/lib/api.ts
@@ -2,6 +2,19 @@ import type { TempUpload } from "@/lib/types";
 
 const TOKEN_KEY = "j621.token";
 
+/**
+ * Backend origin when the SPA is served from somewhere else (build-time
+ * `VITE_API_BASE`, e.g. `https://j621-api.example.com`). Empty means
+ * same-origin, which is the default and needs no configuration.
+ */
+const API_BASE = String(import.meta.env.VITE_API_BASE ?? "").replace(/\/+$/, "");
+
+/** Absolutise a backend path; external/absolute URLs pass through. */
+export function apiUrl(path: string): string {
+  if (!path.startsWith("/")) return path;
+  return `${API_BASE}${path}`;
+}
+
 export function getToken(): string | null {
   return window.localStorage.getItem(TOKEN_KEY);
 }
@@ -69,7 +82,7 @@ export async function api<T>(path: string, options: ApiOptions = {}): Promise<T>
   const token = getToken();
   if (token) finalHeaders.set("Authorization", `Token ${token}`);
 
-  const response = await fetch(path, { ...rest, headers: finalHeaders, body });
+  const response = await fetch(apiUrl(path), { ...rest, headers: finalHeaders, body });
   if (!response.ok) {
     let data: unknown = null;
     try {
@@ -99,7 +112,7 @@ export function uploadFile<T = TempUpload>(
     }
 
     const request = new XMLHttpRequest();
-    request.open("POST", path);
+    request.open("POST", apiUrl(path));
     const token = getToken();
     if (token) request.setRequestHeader("Authorization", `Token ${token}`);
     request.upload.addEventListener("progress", (event) => {