diff --git a/API.md b/API.md
index 31344ae6..22c60c56 100644
--- a/API.md
+++ b/API.md
@@ -105,6 +105,9 @@ status. A list answers `{ "items", "hasMore" }` and takes `?page=` (from 1) and
 thumbnail, waveform, cover or attachment takes the owner's media token as
 `?token=`.
 
+JSON answers of a kilobyte or more are compressed, in brotli or gzip, for a
+client that accepts it.
+
 ### me
 
 - `GET /api/me` — signed in. The caller's own user.
diff --git a/api/main.ts b/api/main.ts
index 11eb1b20..5e8c2390 100644
--- a/api/main.ts
+++ b/api/main.ts
@@ -3,7 +3,8 @@
  * jobs and listens on {@link LISTEN_HOST}:{@link PORT}, having first signed
  * the users out of other instances if they may no longer sign in there
  * (`FEDERATION.outgoing`). A request goes
- * through the error middleware, the OG tags and CORS, then the routers, one
+ * through the error middleware, the compression of what is answered, the OG
+ * tags and CORS, then the routers, one
  * per resource of the API (`API.md` lists them), and last the built frontend,
  * which answers every path no router did.
  *
@@ -46,6 +47,7 @@ import {
   PORT,
   PUBLIC_URL,
 } from "./config.ts";
+import { compressMiddleware } from "./middleware/compress.ts";
 import { errorMiddleware } from "./middleware/error.ts";
 import { ogMiddleware } from "./middleware/og.ts";
 import { routeStaticFilesFrom } from "./lib/static.ts";
@@ -68,6 +70,7 @@ const app: Application = new Application();
 const cors = oakCors({ origin: ALLOWED_ORIGINS });
 
 app.use(errorMiddleware);
+app.use(compressMiddleware);
 app.use(ogMiddleware);
 // oakCors calls next() without awaiting it when there is no origin, which
 // corrupts the response in Oak's async chain. So it only sees requests from an
diff --git a/api/middleware/compress.ts b/api/middleware/compress.ts
new file mode 100644
index 00000000..4651616d
--- /dev/null
+++ b/api/middleware/compress.ts
@@ -0,0 +1,108 @@
+/**
+ * Compresses what the routes answer in JSON, and the HTML page, for the
+ * browsers that accept it: a page of the feed is some twenty kilobytes as it
+ * is, and a fifth of that compressed. main.ts installs it after the error
+ * middleware, around everything that sets a body. The built frontend's files
+ * aren't its concern: lib/static.ts sends the copies compressed at build time.
+ *
+ * @module
+ */
+
+import type { Context, Next } from "@oak/oak";
+import { promisify } from "node:util";
+import { brotliCompress, constants as zlib, gzip } from "node:zlib";
+
+const brotli = promisify(brotliCompress);
+const gz = promisify(gzip);
+
+/**
+ * Below this, the bytes saved aren't worth the work; it is the threshold the
+ * build compresses the frontend's files from (vite.config.ts).
+ */
+const MIN_BYTES = 1024;
+
+/**
+ * Brotli's quality for a response made on the spot. Past 5 it takes several
+ * times longer for a few percent; the frontend's files, compressed once, are
+ * at 11.
+ */
+const BROTLI_QUALITY = 5;
+
+/**
+ * Returns the coding to answer with for an `Accept-Encoding` header, brotli
+ * before gzip, or null when the client takes neither. A coding refused with
+ * `q=0` isn't taken.
+ */
+function pickCoding(header: string | null): "br" | "gzip" | null {
+  if (!header) return null;
+  const accepted = new Set<string>();
+  for (const part of header.split(",")) {
+    const [name, ...params] = part.trim().toLowerCase().split(";");
+    const q = params.map((p) => p.trim().match(/^q=([\d.]+)$/)?.[1])
+      .find((v) => v !== undefined);
+    if (q === undefined || Number(q) > 0) accepted.add(name.trim());
+  }
+  return accepted.has("br") ? "br" : accepted.has("gzip") ? "gzip" : null;
+}
+
+/**
+ * Returns the text of a body this middleware compresses, or null for any
+ * other: an object or array, which Oak would send as JSON, or a string whose
+ * type its route has set. Files, streams and bytes are left alone, as they
+ * are images, media and files compressed already.
+ */
+function textOf(ctx: Context): string | null {
+  const body = ctx.response.body;
+  if (typeof body === "string") {
+    // Oak guesses an untyped string's type from the string itself, which it
+    // couldn't from its compressed bytes.
+    return ctx.response.type || ctx.response.headers.has("Content-Type")
+      ? body
+      : null;
+  }
+  if (body === null || typeof body !== "object") return null;
+  const proto = Object.getPrototypeOf(body);
+  if (proto !== Object.prototype && !Array.isArray(body)) return null;
+  ctx.response.type = "json";
+  return JSON.stringify(body);
+}
+
+/**
+ * Sends the response's JSON or text compressed, in brotli or gzip as the
+ * request accepts, when it is at least {@link MIN_BYTES} long. A response
+ * already encoded, a `HEAD`, and one marked `no-transform` go out as they are.
+ */
+export async function compressMiddleware(
+  ctx: Context,
+  next: Next,
+): Promise<void> {
+  await next();
+
+  const headers = ctx.response.headers;
+  if (
+    ctx.request.method === "HEAD" || headers.has("Content-Encoding") ||
+    headers.get("Cache-Control")?.includes("no-transform")
+  ) return;
+
+  const text = textOf(ctx);
+  if (text === null) return;
+  const raw = new TextEncoder().encode(text);
+  if (raw.length < MIN_BYTES) return;
+
+  // A cache must not hand the compressed copy to a client that can't read it.
+  headers.append("Vary", "Accept-Encoding");
+  const coding = pickCoding(ctx.request.headers.get("Accept-Encoding"));
+  if (!coding) return;
+
+  const compressed = coding === "br"
+    ? await brotli(raw, {
+      params: {
+        [zlib.BROTLI_PARAM_MODE]: zlib.BROTLI_MODE_TEXT,
+        [zlib.BROTLI_PARAM_QUALITY]: BROTLI_QUALITY,
+        [zlib.BROTLI_PARAM_SIZE_HINT]: raw.length,
+      },
+    })
+    : await gz(raw);
+  headers.set("Content-Encoding", coding);
+  ctx.response.body = new Uint8Array(compressed);
+}
diff --git a/api/test/middleware/compress.test.ts b/api/test/middleware/compress.test.ts
new file mode 100644
index 00000000..ae8847e9
--- /dev/null
+++ b/api/test/middleware/compress.test.ts
@@ -0,0 +1,90 @@
+import { BASE_URL } from "../helpers.ts";
+import { assertEquals } from "@std/assert";
+import { Application } from "@oak/oak";
+import { brotliDecompressSync, gunzipSync } from "node:zlib";
+import { compressMiddleware } from "../../middleware/compress.ts";
+
+const BIG = { success: true, data: { items: Array(200).fill("dump title") } };
+
+function makeApp(): Application {
+  const app = new Application();
+  app.use(compressMiddleware);
+  app.use((ctx) => {
+    const path = ctx.request.url.pathname;
+    if (path === "/big") ctx.response.body = BIG;
+    if (path === "/small") ctx.response.body = { success: true };
+    if (path === "/html") {
+      ctx.response.type = "text/html";
+      ctx.response.body = `<!doctype html>${"<p>gerbeur</p>".repeat(200)}`;
+    }
+    if (path === "/bytes") ctx.response.body = new Uint8Array(4096);
+    if (path === "/encoded") {
+      ctx.response.headers.set("Content-Encoding", "br");
+      ctx.response.type = "text/css";
+      ctx.response.body = "x".repeat(4096);
+    }
+  });
+  return app;
+}
+
+async function get(path: string, acceptEncoding?: string) {
+  const res = await makeApp().handle(
+    new Request(`${BASE_URL}${path}`, {
+      headers: acceptEncoding ? { "Accept-Encoding": acceptEncoding } : {},
+    }),
+  );
+  if (!res) throw new Error("no response");
+  return { res, bytes: new Uint8Array(await res.arrayBuffer()) };
+}
+
+const text = (bytes: Uint8Array) => new TextDecoder().decode(bytes);
+
+Deno.test("compressMiddleware: JSON goes out in brotli, or gzip for a client without it", async () => {
+  const br = await get("/big", "gzip, deflate, br");
+  assertEquals(br.res.headers.get("Content-Encoding"), "br");
+  assertEquals(
+    br.res.headers.get("Content-Type"),
+    "application/json; charset=UTF-8",
+  );
+  assertEquals(br.res.headers.get("Vary"), "Accept-Encoding");
+  assertEquals(JSON.parse(text(brotliDecompressSync(br.bytes))), BIG);
+
+  const gzip = await get("/big", "gzip");
+  assertEquals(gzip.res.headers.get("Content-Encoding"), "gzip");
+  assertEquals(JSON.parse(text(gunzipSync(gzip.bytes))), BIG);
+
+  const refused = await get("/big", "br;q=0, gzip;q=0.5");
+  assertEquals(refused.res.headers.get("Content-Encoding"), "gzip");
+});
+
+Deno.test("compressMiddleware: a client that accepts no coding gets the JSON as it is", async () => {
+  for (const accept of [undefined, "identity", "br;q=0"]) {
+    const { res, bytes } = await get("/big", accept);
+    assertEquals(res.headers.get("Content-Encoding"), null);
+    // A cache still has to tell this answer from the compressed one.
+    assertEquals(res.headers.get("Vary"), "Accept-Encoding");
+    assertEquals(JSON.parse(text(bytes)), BIG);
+  }
+});
+
+Deno.test("compressMiddleware: the HTML page is compressed too", async () => {
+  const { res, bytes } = await get("/html", "br");
+  assertEquals(res.headers.get("Content-Encoding"), "br");
+  assertEquals(res.headers.get("Content-Type"), "text/html; charset=UTF-8");
+  assertEquals(text(brotliDecompressSync(bytes)).startsWith("<!doctype"), true);
+});
+
+Deno.test("compressMiddleware: short answers, bytes and encoded files are left alone", async () => {
+  const small = await get("/small", "br");
+  assertEquals(small.res.headers.get("Content-Encoding"), null);
+  assertEquals(small.res.headers.get("Vary"), null);
+  assertEquals(JSON.parse(text(small.bytes)), { success: true });
+
+  const bytes = await get("/bytes", "br");
+  assertEquals(bytes.res.headers.get("Content-Encoding"), null);
+  assertEquals(bytes.bytes.length, 4096);
+
+  const encoded = await get("/encoded", "br");
+  assertEquals(encoded.res.headers.get("Content-Encoding"), "br");
+  assertEquals(text(encoded.bytes), "x".repeat(4096));
+});