diff --git a/apps/client/vite.config.ts b/apps/client/vite.config.ts
index a28329f9..a24d0b83 100644
--- a/apps/client/vite.config.ts
+++ b/apps/client/vite.config.ts
@@ -60,6 +60,14 @@ export default defineConfig(({ mode }) => {
       // precompressed copy (see @fastify/static preCompressed in static.module.ts).
       compression({
         algorithms: ["brotliCompress", "gzip"],
+        // vite-plugin-compression2's default `include` only covers text-ish
+        // bundle output (js/mjs/json/css/html/svg/…). Extend it with the large
+        // VAD binaries copied from public/vad (.wasm ~26MB, .onnx ~2.3MB) so
+        // they are brotli/gzip'd once at build time and served via
+        // @fastify/static preCompressed — otherwise @fastify/compress would
+        // re-brotli them on EVERY request. The default types are repeated here
+        // because setting `include` replaces (does not extend) the default.
+        include: /\.(html|xml|css|json|js|mjs|svg|yaml|yml|toml|wasm|onnx)$/,
         // index.html is rewritten at server boot (window.CONFIG injection); a
         // precompressed copy would go stale — NEVER precompress it.
         exclude: [/index\.html$/],
diff --git a/apps/server/src/core/attachment/attachment.controller.ts b/apps/server/src/core/attachment/attachment.controller.ts
index 73605819..55b68bc4 100644
--- a/apps/server/src/core/attachment/attachment.controller.ts
+++ b/apps/server/src/core/attachment/attachment.controller.ts
@@ -474,6 +474,19 @@ export class AttachmentController {
     const fileSize = Number(attachment.fileSize);
     const rangeHeader = req.headers.range;
 
+    // Opt this download route out of the global @fastify/compress hook.
+    // Attachment bytes are final and mostly binary, so on-the-fly compression
+    // only burns CPU — and on the 206/Range branch it is actively corrupting:
+    // compress decides purely by Content-Type, so for a compressible mime
+    // (application/octet-stream fallback, image/svg+xml, text/*) it would gzip
+    // the byte slice and drop Content-Length while Content-Range still
+    // describes the RAW offsets and the status stays 206. A resuming client
+    // (`curl -C -`, download managers) then appends the encoded bytes as if
+    // raw and ends up with a broken file. @fastify/compress skips whenever the
+    // request carries `x-no-compression` (see its onSend hook), so setting it
+    // here covers both the 200 (full file) and 206 (range) responses.
+    req.headers['x-no-compression'] = 'true';
+
     res.header('Accept-Ranges', 'bytes');
     res.header(
       'Content-Security-Policy',
diff --git a/apps/server/src/integrations/static/static.module.spec.ts b/apps/server/src/integrations/static/static.module.spec.ts
new file mode 100644
index 00000000..ff2ce2fb
--- /dev/null
+++ b/apps/server/src/integrations/static/static.module.spec.ts
@@ -0,0 +1,35 @@
+import { resolveStaticAssetHeaders } from './static.module';
+
+// Unit tests for the static-asset cache classifier extracted from the
+// @fastify/static setHeaders callback (precedent: sandbox.controller.spec.ts).
+describe('resolveStaticAssetHeaders', () => {
+  it('marks a content-hashed /assets/ file immutable and sets Vary', () => {
+    const headers = resolveStaticAssetHeaders(
+      '/app/apps/client/dist/assets/index-a1b2c3.js',
+    );
+    expect(headers['cache-control']).toBe(
+      'public, max-age=31536000, immutable',
+    );
+    expect(headers['vary']).toBe('Accept-Encoding');
+  });
+
+  it('makes index.html always revalidate (never immutable)', () => {
+    const headers = resolveStaticAssetHeaders(
+      '/app/apps/client/dist/index.html',
+    );
+    expect(headers['cache-control']).toBe(
+      'no-cache, no-store, must-revalidate',
+    );
+    expect(headers['vary']).toBe('Accept-Encoding');
+  });
+
+  it('does NOT mark a non-hashed asset immutable but still sets Vary', () => {
+    const headers = resolveStaticAssetHeaders(
+      '/app/apps/client/dist/locales/en.json',
+    );
+    // No immutable cache-control — this path keeps @fastify/static's default
+    // etag/last-modified revalidation.
+    expect(headers['cache-control']).toBeUndefined();
+    expect(headers['vary']).toBe('Accept-Encoding');
+  });
+});
diff --git a/apps/server/src/integrations/static/static.module.ts b/apps/server/src/integrations/static/static.module.ts
index 985864ca..54a7c092 100644
--- a/apps/server/src/integrations/static/static.module.ts
+++ b/apps/server/src/integrations/static/static.module.ts
@@ -5,6 +5,46 @@ import * as fs from 'node:fs';
 import fastifyStatic from '@fastify/static';
 import { EnvironmentService } from '../environment/environment.service';
 
+/**
+ * Resolve the response headers for a statically served client asset.
+ *
+ * Extracted from the @fastify/static `setHeaders` callback so the cache
+ * classification stays a pure, unit-testable function (see
+ * static.module.spec.ts).
+ *
+ * `Vary: Accept-Encoding` is emitted for every static response because
+ * @fastify/static negotiates a precompressed .br/.gz neighbour by the client's
+ * Accept-Encoding but does NOT set Vary itself. Without it a shared/proxy cache
+ * keyed on the URL alone could store the brotli variant and later serve it to a
+ * client that only sent `Accept-Encoding: identity`/gzip → an undecodable body.
+ * This matters most for the immutable /assets/ files, which proxies may keep
+ * for a year.
+ */
+export function resolveStaticAssetHeaders(
+  filePath: string,
+): Record<string, string> {
+  const headers: Record<string, string> = { vary: 'Accept-Encoding' };
+
+  // Content-hashed files under /assets/ never change for a given URL, so they
+  // can be cached forever and skip revalidation entirely.
+  if (filePath.includes('/assets/')) {
+    headers['cache-control'] = 'public, max-age=31536000, immutable';
+    return headers;
+  }
+
+  // index.html is rewritten at boot (window.CONFIG injection) and on every
+  // deploy — it must be revalidated on every load.
+  if (filePath.endsWith('index.html')) {
+    headers['cache-control'] = 'no-cache, no-store, must-revalidate';
+    return headers;
+  }
+
+  // Everything else (locales, vad, icons, manifest) is NOT content-hashed and
+  // changes between deploys, so it keeps @fastify/static's default
+  // etag/last-modified revalidation — do NOT mark it immutable.
+  return headers;
+}
+
 @Module({})
 export class StaticModule implements OnModuleInit {
   constructor(
@@ -76,27 +116,11 @@ export class StaticModule implements OnModuleInit {
         // (see vite-plugin-compression2 in apps/client/vite.config.ts).
         preCompressed: true,
         setHeaders: (res, filePath) => {
-          // Content-hashed files under /assets/ never change for a given URL,
-          // so they can be cached forever and skip revalidation entirely.
-          if (filePath.includes('/assets/')) {
-            res.setHeader(
-              'cache-control',
-              'public, max-age=31536000, immutable',
-            );
-            return;
+          for (const [name, value] of Object.entries(
+            resolveStaticAssetHeaders(filePath),
+          )) {
+            res.setHeader(name, value);
           }
-          // index.html is rewritten at boot (window.CONFIG injection) and on
-          // every deploy — it must be revalidated on every load.
-          if (filePath.endsWith('index.html')) {
-            res.setHeader(
-              'cache-control',
-              'no-cache, no-store, must-revalidate',
-            );
-            return;
-          }
-          // Everything else (locales, vad, icons, manifest) is NOT content-hashed
-          // and changes between deploys, so it keeps @fastify/static's default
-          // etag/last-modified revalidation — do NOT mark it immutable.
         },
       });