diff --git a/server/static.ts b/server/static.ts
index f14485e6d..2652cc229 100644
--- a/server/static.ts
+++ b/server/static.ts
@@ -136,6 +136,22 @@ export async function serveStaticFile(
     : 'no-cache'
   const mime = contentType(filePath)
 
+  // `no-cache` means "revalidate before reuse", which only saves bytes when
+  // the response carries a validator. Without one, every editor preview
+  // reload re-downloaded every image and font in full.
+  const validators = fileValidators(file.size, file.lastModified)
+  if (req && isNotModified(req, validators, file.lastModified)) {
+    return new Response(null, {
+      status: 304,
+      headers: {
+        'cache-control': cacheControl,
+        ...validators,
+        // Same `vary` as the 200 this revalidates (compressed bodies only).
+        ...(isCompressible(filePath, file.size) ? { 'vary': 'accept-encoding' } : {}),
+      },
+    })
+  }
+
   const buffer = await file.arrayBuffer()
   const bytes = new Uint8Array(buffer) as ResponseBytes
   const acceptEncoding = req?.headers.get('accept-encoding') ?? null
@@ -154,6 +170,7 @@ export async function serveStaticFile(
         // Tells caches the response varies based on the request encoding,
         // so a gzip-only client doesn't get served a cached brotli payload.
         'vary': 'accept-encoding',
+        ...validators,
       },
     })
   }
@@ -162,10 +179,39 @@ export async function serveStaticFile(
     headers: {
       'content-type': mime,
       'cache-control': cacheControl,
+      ...validators,
     },
   })
 }
 
+/**
+ * Weak ETag from size + mtime: any rewrite of the file (a media replace that
+ * keeps its path, a redeploy) changes it. Weak, because the same validator
+ * covers the identity, gzip and brotli bodies.
+ */
+function fileValidators(size: number, mtimeMs: number): { etag: string; 'last-modified': string } {
+  return {
+    etag: `W/"${size.toString(36)}-${Math.floor(mtimeMs).toString(36)}"`,
+    'last-modified': new Date(mtimeMs).toUTCString(),
+  }
+}
+
+/** RFC 9110 §13.1: `If-None-Match` wins; `If-Modified-Since` only without it. */
+function isNotModified(req: Request, validators: { etag: string }, mtimeMs: number): boolean {
+  if (req.method !== 'GET' && req.method !== 'HEAD') return false
+  const ifNoneMatch = req.headers.get('if-none-match')
+  if (ifNoneMatch !== null) {
+    const opaque = (tag: string) => tag.trim().replace(/^W\//, '')
+    const current = opaque(validators.etag)
+    return ifNoneMatch.split(',').some((tag) => tag.trim() === '*' || opaque(tag) === current)
+  }
+  const ifModifiedSince = req.headers.get('if-modified-since')
+  if (ifModifiedSince === null) return false
+  const since = Date.parse(ifModifiedSince)
+  // HTTP dates have whole-second precision.
+  return Number.isFinite(since) && Math.floor(mtimeMs / 1000) * 1000 <= since
+}
+
 // ---------------------------------------------------------------------------
 // Admin shell — pre-rendered login skeleton for unauthenticated visitors.
 // ---------------------------------------------------------------------------
diff --git a/src/__tests__/server/staticAdmin.test.ts b/src/__tests__/server/staticAdmin.test.ts
index 808460b72..4c81b1782 100644
--- a/src/__tests__/server/staticAdmin.test.ts
+++ b/src/__tests__/server/staticAdmin.test.ts
@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'bun:test'
-import { mkdtempSync, rmSync, writeFileSync, mkdirSync } from 'node:fs'
+import { mkdtempSync, rmSync, writeFileSync, mkdirSync, utimesSync } from 'node:fs'
 import { join } from 'node:path'
 import { tmpdir } from 'node:os'
 import { handleServerRequest } from '../../../server/router'
@@ -209,4 +209,69 @@ describe('self-hosted admin static serving', () => {
       rmSync(uploadsDir, { recursive: true, force: true })
     }
   })
+
+  // `no-cache` uploads must carry a validator, or every editor preview reload
+  // re-downloads every image and font in full.
+  it('answers conditional /uploads requests with 304 until the file changes', async () => {
+    const uploadsDir = mkdtempSync(join(tmpdir(), 'instatic-uploads-'))
+    const get = (headers: Record<string, string> = {}) =>
+      handleServerRequest(new Request('http://localhost/uploads/hero.png', { headers }), {
+        db: fakeDb,
+        uploadsDir,
+      })
+    try {
+      writeFileSync(join(uploadsDir, 'hero.png'), 'image-bytes')
+      utimesSync(join(uploadsDir, 'hero.png'), 1_700_000_000, 1_700_000_000)
+
+      const first = await get()
+      expect(first.status).toBe(200)
+      expect(first.headers.get('cache-control')).toBe('no-cache')
+      const etag = first.headers.get('etag')
+      const lastModified = first.headers.get('last-modified')
+      expect(etag).toMatch(/^W\/".+"$/)
+      expect(lastModified).toBe(new Date(1_700_000_000_000).toUTCString())
+
+      const revalidated = await get({ 'if-none-match': etag! })
+      expect(revalidated.status).toBe(304)
+      expect(await revalidated.text()).toBe('')
+      expect(revalidated.headers.get('etag')).toBe(etag)
+      expect(revalidated.headers.get('x-content-type-options')).toBe('nosniff')
+
+      expect((await get({ 'if-none-match': `"other", ${etag}` })).status).toBe(304)
+      expect((await get({ 'if-modified-since': lastModified! })).status).toBe(304)
+      expect((await get({ 'if-modified-since': new Date(1_600_000_000_000).toUTCString() })).status).toBe(200)
+      // If-None-Match takes precedence over If-Modified-Since.
+      expect((await get({ 'if-none-match': '"stale"', 'if-modified-since': lastModified! })).status).toBe(200)
+
+      // A replace that keeps the path changes the validator.
+      writeFileSync(join(uploadsDir, 'hero.png'), 'new-image-bytes')
+      const changed = await get({ 'if-none-match': etag! })
+      expect(changed.status).toBe(200)
+      expect(changed.headers.get('etag')).not.toBe(etag)
+      expect(await changed.text()).toBe('new-image-bytes')
+    } finally {
+      rmSync(uploadsDir, { recursive: true, force: true })
+    }
+  })
+
+  it('sends validators on immutable admin assets too', async () => {
+    const staticDir = createStaticDir()
+    try {
+      const first = await handleServerRequest(new Request('http://localhost/assets/app.js'), {
+        db: fakeDb,
+        staticDir,
+      })
+      expect(first.headers.get('cache-control')).toBe('public, max-age=31536000, immutable')
+      const etag = first.headers.get('etag')
+      expect(etag).toBeTruthy()
+
+      const revalidated = await handleServerRequest(
+        new Request('http://localhost/assets/app.js', { headers: { 'if-none-match': etag! } }),
+        { db: fakeDb, staticDir },
+      )
+      expect(revalidated.status).toBe(304)
+    } finally {
+      rmSync(staticDir, { recursive: true, force: true })
+    }
+  })
 })