diff --git a/deploy/Caddyfile.example b/deploy/Caddyfile.example
index 0bb9357..f25a23a 100644
--- a/deploy/Caddyfile.example
+++ b/deploy/Caddyfile.example
@@ -70,18 +70,57 @@
 
 # Browsers fetch signed object URLs here (S3_DELIVERY_ENDPOINT, ADR 0031).
 {$EKKLESIA_STORAGE_HOSTNAME} {
-	reverse_proxy https://object-storage:8443 {
-		# An S3 signature covers the Host header: pass the original through unchanged, or every signed
-		# URL fails verification.
-		header_up Host {host}
-		header_up -CF-Connecting-IP
-		header_up -X-Real-IP
-		header_up -Forwarded
-		header_up -True-Client-IP
-		transport http {
-			# object-storage presents a certificate from the installation-private CA.
-			tls_trust_pool file /etc/caddy/ekklesia-ca.crt
-			tls_server_name object-storage
+	# Delivery-layer CORS belongs to the deployment, not to the object store: this block decides it,
+	# so it stays the same with any S3 backend behind it. Only the origins whose pages read objects
+	# with fetch() get CORS: the Ekklesia origin, which serves the Dashboard and the displays. An <img> needs none. Every object
+	# URL is still a signed, expiring capability; CORS only decides which pages may read the bytes.
+	@cors_origin header Origin https://{$EKKLESIA_HOSTNAME}
+
+	# A preflight is answered here and never reaches storage. Only GET and HEAD are ever allowed, and
+	# no request header beyond the CORS-safelisted ones.
+	@preflight method OPTIONS
+	handle @preflight {
+		header @cors_origin {
+			Access-Control-Allow-Origin {http.request.header.Origin}
+			Access-Control-Allow-Methods "GET, HEAD"
+			Access-Control-Max-Age 600
+		}
+		header Vary Origin
+		respond 204
+	}
+
+	handle {
+		# Set when the response is written, after the upstream's own CORS headers were removed below.
+		header @cors_origin >Access-Control-Allow-Origin {http.request.header.Origin}
+		header >Vary Origin
+
+		reverse_proxy https://object-storage:8443 {
+			# An S3 signature covers the Host header: pass the original through unchanged, or every
+			# signed URL fails verification.
+			header_up Host {host}
+			header_up -CF-Connecting-IP
+			header_up -X-Real-IP
+			header_up -Forwarded
+			header_up -True-Client-IP
+			# The object store's own CORS answer (SeaweedFS reflects any origin, with every method)
+			# is not the deployment's policy.
+			header_down -Access-Control-Allow-Origin
+			header_down -Access-Control-Allow-Methods
+			header_down -Access-Control-Allow-Headers
+			header_down -Access-Control-Expose-Headers
+			header_down -Access-Control-Allow-Credentials
+			header_down -Access-Control-Max-Age
+			header_down -Vary
+			transport http {
+				# object-storage presents a certificate from the installation-private CA.
+				tls_trust_pool file /etc/caddy/ekklesia-ca.crt
+				tls_server_name object-storage
+				# HTTP/1.1 only. Over HTTP/2, SeaweedFS answers a conditional request with a 304 that
+				# declares a Content-Length it never sends; Caddy then aborts the browser's stream,
+				# which a browser reports as ERR_HTTP2_PROTOCOL_ERROR on every revalidation. Its
+				# HTTP/1.1 304 is clean. Browsers still reach this proxy over HTTP/2 and HTTP/3.
+				versions 1.1
+			}
 		}
 	}
 }
diff --git a/deploy/Caddyfile.split.example b/deploy/Caddyfile.split.example
index ae0ffe4..19196a3 100644
--- a/deploy/Caddyfile.split.example
+++ b/deploy/Caddyfile.split.example
@@ -92,18 +92,61 @@
 
 # Browsers fetch signed object URLs here (S3_DELIVERY_ENDPOINT, ADR 0031).
 {$EKKLESIA_STORAGE_HOSTNAME} {
-	reverse_proxy https://object-storage:8443 {
-		# An S3 signature covers the Host header: pass the original through unchanged, or every signed
-		# URL fails verification.
-		header_up Host {host}
-		header_up -CF-Connecting-IP
-		header_up -X-Real-IP
-		header_up -Forwarded
-		header_up -True-Client-IP
-		transport http {
-			# object-storage presents a certificate from the installation-private CA.
-			tls_trust_pool file /etc/caddy/ekklesia-ca.crt
-			tls_server_name object-storage
+	# Delivery-layer CORS belongs to the deployment, not to the object store: this block decides it,
+	# so it stays the same with any S3 backend behind it. Only the origins whose pages read objects
+	# with fetch() get CORS: the API origin, where displays run, and the Dashboard origin,
+	# where a module's preview may cache images too. An <img> needs none. Every object
+	# URL is still a signed, expiring capability; CORS only decides which pages may read the bytes.
+	@cors_origin {
+		header Origin https://{$EKKLESIA_HOSTNAME}
+		header Origin https://{$EKKLESIA_DASHBOARD_HOSTNAME}
+	}
+
+	# A preflight is answered here and never reaches storage. Only GET and HEAD are ever allowed, and
+	# no request header beyond the CORS-safelisted ones.
+	@preflight method OPTIONS
+	handle @preflight {
+		header @cors_origin {
+			Access-Control-Allow-Origin {http.request.header.Origin}
+			Access-Control-Allow-Methods "GET, HEAD"
+			Access-Control-Max-Age 600
+		}
+		header Vary Origin
+		respond 204
+	}
+
+	handle {
+		# Set when the response is written, after the upstream's own CORS headers were removed below.
+		header @cors_origin >Access-Control-Allow-Origin {http.request.header.Origin}
+		header >Vary Origin
+
+		reverse_proxy https://object-storage:8443 {
+			# An S3 signature covers the Host header: pass the original through unchanged, or every
+			# signed URL fails verification.
+			header_up Host {host}
+			header_up -CF-Connecting-IP
+			header_up -X-Real-IP
+			header_up -Forwarded
+			header_up -True-Client-IP
+			# The object store's own CORS answer (SeaweedFS reflects any origin, with every method)
+			# is not the deployment's policy.
+			header_down -Access-Control-Allow-Origin
+			header_down -Access-Control-Allow-Methods
+			header_down -Access-Control-Allow-Headers
+			header_down -Access-Control-Expose-Headers
+			header_down -Access-Control-Allow-Credentials
+			header_down -Access-Control-Max-Age
+			header_down -Vary
+			transport http {
+				# object-storage presents a certificate from the installation-private CA.
+				tls_trust_pool file /etc/caddy/ekklesia-ca.crt
+				tls_server_name object-storage
+				# HTTP/1.1 only. Over HTTP/2, SeaweedFS answers a conditional request with a 304 that
+				# declares a Content-Length it never sends; Caddy then aborts the browser's stream,
+				# which a browser reports as ERR_HTTP2_PROTOCOL_ERROR on every revalidation. Its
+				# HTTP/1.1 304 is clean. Browsers still reach this proxy over HTTP/2 and HTTP/3.
+				versions 1.1
+			}
 		}
 	}
 }
diff --git a/deploy/README.md b/deploy/README.md
index 42c4618..62d1452 100644
--- a/deploy/README.md
+++ b/deploy/README.md
@@ -178,6 +178,29 @@ the topology and warns when the two hostnames are not siblings.
 The Dashboard learns the API's origin at run time: the API writes its identity base URL into the page
 it serves (`<meta name="ekklesia-api-origin">`). Nothing about the topology is built into the image.
 
+### Storage delivery: CORS is the deployment's
+
+Browsers fetch objects directly from the storage hostname through signed, expiring URLs (ADR 0031).
+**Which pages may read those bytes with `fetch()` is decided by the delivery layer, the proxy, and
+never by the object store.** That policy therefore stays the same with SeaweedFS, R2, or any other
+S3 backend behind it, and an installation that changes backends changes nothing about CORS.
+
+The reference proxy's storage block:
+
+- grants CORS only to the origins whose pages read objects with `fetch()`: the Ekklesia origin in the
+  same-origin topology; the API origin (displays) and the Dashboard origin (a module's preview) in
+  the split topology. An `<img>` needs no CORS and is unaffected;
+- allows `GET` and `HEAD` only, and no request header beyond the CORS-safelisted ones, always with
+  `Vary: Origin`;
+- answers preflight `OPTIONS` requests itself, so they never reach storage;
+- removes the object store's own CORS headers. SeaweedFS's default reflects any origin and
+  advertises `PUT`, `POST`, and `DELETE`.
+
+The proxy reaches storage over **HTTP/1.1 only**. Over HTTP/2, SeaweedFS answers a conditional
+request with a `304` that declares a `Content-Length` it never sends. The proxy then resets the
+browser's stream, and every reload of a cached image fails with `ERR_HTTP2_PROTOCOL_ERROR`.
+Browsers still reach the proxy itself over HTTP/2 and HTTP/3.
+
 ### Behind an existing reverse proxy
 
 Run the base alone and attach `api` and `object-storage` to your proxy's network with an overlay of
@@ -213,6 +236,8 @@ Then make your proxy do what `Caddyfile.example` does:
 - forward the storage hostname to `https://object-storage:8443`, verifying it against
   `/etc/ekklesia/tls/ca.crt` with server name `object-storage`, and **pass the original `Host`
   through unchanged**, or every signed URL fails (ADR 0031).
+- reach `object-storage` over **HTTP/1.1** only, and own the storage hostname's CORS as described in
+  [Storage delivery](#storage-delivery-cors-is-the-deployments).
 
 The proxy's address must be fixed and must never be handed to anything else. With Docker, give its
 network an explicit subnet, limit `ip_range` to part of it, and fix the proxy's address outside that
@@ -288,7 +313,10 @@ deployment:
   platform-only image and a disposable PostgreSQL and SeaweedFS that exist only for that run. It
   follows the first boot above and checks the following through the proxy:
   - readiness and the Dashboard;
-  - a signed object URL fetched through the storage hostname;
+  - a signed object URL fetched through the storage hostname, and the same image loaded twice over
+    HTTP/2, the second time as a conditional request that must be a clean `304`;
+  - storage CORS: exactly the allowed origins, `GET` and `HEAD` only, preflights answered by the
+    proxy, and none of the object store's own CORS headers;
   - the client-address chain, both from an ordinary peer and from the tunnel connector's address;
   - initial-owner sign-in and sign-out;
   - that the API's database connection is TLS 1.3, and that it is refused by a foreign CA and by a
diff --git a/scripts/lib/deploy-smoke-client.mjs b/scripts/lib/deploy-smoke-client.mjs
index 4ec349f..4157d6e 100644
--- a/scripts/lib/deploy-smoke-client.mjs
+++ b/scripts/lib/deploy-smoke-client.mjs
@@ -14,6 +14,8 @@
 // Output is one line per check; any failed check exits non-zero. Nothing printed carries a
 // credential, cookie, signed URL, or response body beyond the fixed shapes asserted here.
 
+import { connect as connectHttp2 } from "node:http2";
+
 const mode = process.argv[2];
 const apiOrigin = `https://${required("SMOKE_HOSTNAME")}`;
 const dashboardOrigin = process.env.SMOKE_DASHBOARD_HOSTNAME
@@ -192,6 +194,46 @@ async function splitCorsChecks() {
   );
 }
 
+/**
+ * One GET over a real HTTP/2 connection. Node's fetch speaks HTTP/1.1, and the failure this guards
+ * against exists only on HTTP/2: a reset stream, which a browser reports as ERR_HTTP2_PROTOCOL_ERROR.
+ */
+function http2Get(url, headers = {}) {
+  return new Promise((resolve) => {
+    const session = connectHttp2(url.origin);
+    const finish = (result) => {
+      clearTimeout(timer);
+      session.close();
+      resolve(result);
+    };
+    const timer = setTimeout(() => finish({ error: "timeout" }), 10_000);
+    session.on("error", () => finish({ error: "session" }));
+    const stream = session.request({
+      ":method": "GET",
+      ":path": `${url.pathname}${url.search}`,
+      ...headers,
+    });
+    let status;
+    let responseHeaders = {};
+    let bytes = 0;
+    stream.on("response", (received) => {
+      status = received[":status"];
+      responseHeaders = received;
+    });
+    stream.on("data", (chunk) => {
+      bytes += chunk.length;
+    });
+    stream.on("error", (error) => finish({ error: error.code ?? "stream" }));
+    stream.on("end", () => finish({ bytes, headers: responseHeaders, status }));
+    stream.end();
+  });
+}
+
+/** The CORS headers of a response, by name: what a browser would act on. */
+function corsOf(response) {
+  return [...response.headers.keys()].filter((name) => name.startsWith("access-control-"));
+}
+
 async function storageDeliveryCheck() {
   // Core's own provider, from the image, exactly as the API composes it: it reaches object-storage
   // over HTTPS on the private network and signs read URLs for the storage hostname.
@@ -238,6 +280,79 @@ async function storageDeliveryCheck() {
       "a delivered object is Cache-Control: private",
     );
     await provider.delete(bucket, "smoke/object.txt");
+
+    // The same image, twice, over HTTP/2: a browser's second load of a signed URL it has cached is a
+    // conditional request, and must come back as a clean 304, not a reset stream.
+    const png = Buffer.from(
+      "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8z8BQDwAEhQGAhKmMIQAAAABJRU5ErkJggg==",
+      "base64",
+    );
+    await provider.put(bucket, "smoke/image.png", png, {
+      contentLength: png.byteLength,
+      contentType: "image/png",
+    });
+    const image = new URL(
+      (await provider.createReadUrl(bucket, "smoke/image.png", { expiresInSeconds: 300 })).url,
+    );
+    const first = await http2Get(image);
+    check(
+      first.status === 200 && first.bytes === png.byteLength && Boolean(first.headers?.etag),
+      "an image loads over HTTP/2 through the proxy, with a validator (200, ETag)",
+    );
+    const revalidated = await http2Get(image, { "if-none-match": first.headers?.etag ?? "" });
+    check(
+      revalidated.status === 304 && revalidated.error === undefined,
+      `the same image loaded again over HTTP/2 revalidates cleanly (${revalidated.status ?? revalidated.error})`,
+    );
+    const sinceModified = await http2Get(image, {
+      "if-modified-since": first.headers?.["last-modified"] ?? "",
+    });
+    check(
+      sinceModified.status === 304 && sinceModified.error === undefined,
+      `a revalidation by date over HTTP/2 is a clean 304 too (${sinceModified.status ?? sinceModified.error})`,
+    );
+
+    // Delivery-layer CORS is the deployment's own, set by the proxy: exactly the origins whose pages
+    // read objects with fetch(), GET and HEAD only, and nothing of the object store's own answer.
+    const fromApi = await fetch(image, { headers: { origin: apiOrigin } });
+    check(
+      fromApi.headers.get("access-control-allow-origin") === apiOrigin &&
+        (fromApi.headers.get("vary") ?? "").toLowerCase().includes("origin") &&
+        corsOf(fromApi).length === 1,
+      "a read from the API origin is allowed for exactly that origin, with Vary: Origin and nothing else",
+    );
+    if (split) {
+      const fromDashboard = await fetch(image, { headers: { origin: dashboardOrigin } });
+      check(
+        fromDashboard.headers.get("access-control-allow-origin") === dashboardOrigin,
+        "a read from the Dashboard origin is allowed for exactly that origin",
+      );
+    }
+    const fromElsewhere = await fetch(image, { headers: { origin: untrustedOrigin } });
+    check(
+      fromElsewhere.status === 200 && corsOf(fromElsewhere).length === 0,
+      "a read from any other origin gets no CORS header at all",
+    );
+    const preflight = await fetch(image, {
+      headers: { "access-control-request-method": "GET", origin: apiOrigin },
+      method: "OPTIONS",
+    });
+    check(
+      preflight.status === 204 &&
+        preflight.headers.get("access-control-allow-origin") === apiOrigin &&
+        preflight.headers.get("access-control-allow-methods") === "GET, HEAD" &&
+        preflight.headers.get("x-amz-request-id") === null,
+      "the proxy answers a preflight itself (204, GET and HEAD only), never the object store",
+    );
+    const foreignPreflight = await fetch(image, {
+      headers: { "access-control-request-method": "GET", origin: untrustedOrigin },
+      method: "OPTIONS",
+    });
+    check(
+      foreignPreflight.status === 204 && corsOf(foreignPreflight).length === 0,
+      "a preflight from any other origin is answered without permission",
+    );
+    await provider.delete(bucket, "smoke/image.png");
   } finally {
     await provider.close();
   }