diff --git a/CLAUDE.md b/CLAUDE.md
index 5aa3f2e..7f5bcb5 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -20,7 +20,7 @@ The macro renders a button row below the block's LiaScript code editor and an if
 
 ```bash
 node scripts/check-macro.mjs   # node --check on every <script> block of README.md
-npm ci && npm run check:ready  # the first program on "ready", in Chrome: @onload on a test page with a stand-in lia-editor and a stub embed
+npm ci && npm run check:ready  # in Chrome, @onload on a test page (stand-in lia-editor, stub embed): the first program on "ready"; «IDE-Link kopieren» writes ?p= (raw deflate, base64url), ?project= without fflate
 npm run check:player [visits]  # the same with the real LiaScript player and the live embed (scripts/player/course.md); not in CI
 ```
 
diff --git a/README.md b/README.md
index f108dca..12de9d7 100644
--- a/README.md
+++ b/README.md
@@ -187,10 +187,13 @@ window.__glpythonMount = (args, send) => {
   }
 
   // «IDE-Link kopieren» (and «In IDE öffnen») hand the block's code to the
-  // glpython IDE as a copy, as a share link of the IDE (?project=, lz-string-compressed JSON). The link
-  // is built while the learner taps, without waiting: iPad Safari allows the
-  // clipboard and a new tab only inside the tap's own handler. So lz-string
-  // (the IDE's version) is loaded once per page in advance.
+  // glpython IDE as a copy, as a share link of the IDE: its compact form ?p=
+  // (the JSON's UTF-8 bytes in raw deflate, base64url without padding, as
+  // the IDE's «Teilen» writes it), or - where fflate doesn't load - the old
+  // ?project= (lz-string), which the IDE reads too. The link is built while
+  // the learner taps, without waiting: iPad Safari allows the clipboard and a
+  // new tab only inside the tap's own handler. So fflate and lz-string (the
+  // IDE's versions) are loaded once per page in advance.
   const ideUrl = "https://glpython-edu.github.io/"
   window.__glpythonLzString ??= new Promise((resolve, reject) => {
     const script = document.createElement("script")
@@ -202,6 +205,16 @@ window.__glpythonMount = (args, send) => {
     script.onerror = reject
     document.head.append(script)
   })
+  window.__glpythonFflate ??= new Promise((resolve, reject) => {
+    const script = document.createElement("script")
+    script.src = "https://cdn.jsdelivr.net/npm/fflate\u00400.8.3/umd/index.js"
+    script.integrity = "sha384-ETfVLWAUsU3zp3SpnQsPDbAyrF6jYv2w8aNYo6KtY2JuyxXhzYLOL7e0ATbLSIHP"
+    script.crossOrigin = "anonymous"
+    // A page with an AMD loader (define.amd) gets no global: then lz-string.
+    script.onload = () => window.fflate?.deflateSync ? resolve(window.fflate) : reject(new Error("fflate"))
+    script.onerror = reject
+    document.head.append(script)
+  })
 
   const sessions = window.__glpythonSessions ??= new Map()
   const existingSession = sessions.get(sessionId)
@@ -228,6 +241,9 @@ window.__glpythonMount = (args, send) => {
   let chosenFile = null
   // Filled once loaded, for building an IDE link without waiting.
   let lzString = null
+  // fflate once loaded; whether it failed (then lz-string writes the link).
+  let fflate = null
+  let fflateFailed = false
 
   const showError = (message) => {
     if (!error) return
@@ -340,6 +356,14 @@ window.__glpythonMount = (args, send) => {
   }
   libraries.then(showLibraries)
   window.__glpythonLzString.then((library) => { lzString = library; attach() }, () => {})
+  window.__glpythonFflate.then((library) => { fflate = library; attach() }, () => { fflateFailed = true; attach() })
+  // Ready to build a link: the compact form, or the old one without fflate.
+  const linkReady = () => fflate !== null || (fflateFailed && lzString !== null)
+  const base64Url = (bytes) => {
+    let binary = ""
+    for (let index = 0; index < bytes.length; index += 0x8000) binary += String.fromCharCode(...bytes.subarray(index, index + 0x8000))
+    return btoa(binary).replace(/\+/g, "-").replace(/\//g, "_").replace(/=+$/, "")
+  }
 
   // A program with input() where the embed can't wait for an answer (no
   // cross-origin isolation, as in LiaScript): with "ide", the way to the
@@ -390,7 +414,7 @@ window.__glpythonMount = (args, send) => {
   // which is never sent to the server.
   const ideLink = () => {
     const codeFiles = readCodeFiles()
-    if (!lzString || !codeFiles || codeFiles.length === 0) return null
+    if (!linkReady() || !codeFiles || codeFiles.length === 0) return null
     const program = programOf(codeFiles, [])
     const libraries = libraryUrls
       .map((url) => ({ path: moduleFileName(fileNameOfUrl(url), "bibliothek.py"), url }))
@@ -400,12 +424,15 @@ window.__glpythonMount = (args, send) => {
       ...codeFiles.map(({ path, source }) => [path, { path, label: path, kind: /\.py$/i.test(path) ? "python" : "text", code: source }]),
     ])
     const heading = frame?.closest(".lia-slide__content")?.querySelector("h1, h2, h3, h4, h5, h6")?.textContent?.trim()
-    const encoded = lzString.compressToEncodedURIComponent(JSON.stringify({ v: libraries.length > 0 ? 2 : 1, name: heading || document.title || strings.ideProject, mainFile: program.mainFile, files }))
+    const json = JSON.stringify({ v: libraries.length > 0 ? 2 : 1, name: heading || document.title || strings.ideProject, mainFile: program.mainFile, files })
+    const [param, encoded] = fflate
+      ? ["p", base64Url(fflate.deflateSync(new TextEncoder().encode(json), { level: 9 }))]
+      : ["project", lzString.compressToEncodedURIComponent(json)]
     const url = new URL(ideUrl)
-    url.searchParams.set("project", encoded)
+    url.searchParams.set(param, encoded)
     if (url.pathname.length + url.search.length <= 8192) return url.toString()
     url.search = ""
-    url.hash = new URLSearchParams({ project: encoded }).toString()
+    url.hash = new URLSearchParams({ [param]: encoded }).toString()
     return url.toString()
   }
 
@@ -461,7 +488,7 @@ window.__glpythonMount = (args, send) => {
     const menu = document.getElementById(`glpython-ide-list-${sessionId}`)
     if (!split || !copyButton || !menuButton || !menu) return
     setShown(split, mode !== "visual" && hasFeature("ide"), "inline-flex")
-    const disabled = lzString === null
+    const disabled = !linkReady()
     if (copyButton.disabled !== disabled) copyButton.disabled = disabled
     if (menuButton.disabled !== disabled) menuButton.disabled = disabled
     if (copyButton.textContent !== strings.copyIdeLink) copyButton.textContent = strings.copyIdeLink
@@ -1109,16 +1136,20 @@ link in a new tab; its tooltip says that changes there don't come back to the
 course. The link is an IDE
 share link: every file of the block and the file that runs, and the block's
 libraries by their URL - the IDE fetches them, so a block with a library keeps
-a short link (an Etappe 4 block with `parser.py`: about 1,300 characters).
+a short link (an Etappe 4 hand-in measured 1,980 characters in the IDE's old
+form and 1,185 in the compact one).
 Libraries are read-only in the IDE (a lock, no editing, renaming or deleting),
 as they are here. A link longer than 2,083 characters - OneNote, for one, cuts
 links there - is still copied, with a note to choose «In IDE öffnen» and
 download the project there for a hand-in.
 A link longer than GitHub Pages accepts (8192 characters of path and query, a
 program of roughly 5,000 to 30,000 characters) carries the project after `#`
-instead of `?`. The macro loads [lz-string](https://github.com/pieroxy/lz-string)
-1.5.0 from jsDelivr for it, the IDE's compression; until that has loaded, the
-button stays disabled.
+instead of `?`. The link is the IDE's compact form (`?p=`: raw deflate in
+base64url, about 40 % shorter than before), for which the macro loads
+[fflate](https://github.com/101arrowz/fflate) 0.8.3 from jsDelivr; where that
+can't load, it writes the IDE's older form (`?project=`, with
+[lz-string](https://github.com/pieroxy/lz-string) 1.5.0), which the IDE reads
+as well. Until one of them has loaded, the button stays disabled.
 
 `features=` in the macro's argument shows only the listed parts: `run`, `tests`,
 `visualize`, `reset`, `repl` (the input line below the output), `export` (the
@@ -1235,6 +1266,7 @@ header to the origin printed by `npm run dev:embed` in `glpython-editor`.
 ## Lizenz / License
 
 MIT – see [LICENSE](https://github.com/glpython-edu/glpython-liascript/blob/main/LICENSE).
-The macro includes no third-party code; it loads the glpython embed (MIT) and
-[lz-string](https://github.com/pieroxy/lz-string) (MIT, for the IDE links)
-when a course runs.
+The macro includes no third-party code; it loads the glpython embed (MIT),
+[fflate](https://github.com/101arrowz/fflate) and
+[lz-string](https://github.com/pieroxy/lz-string) (both MIT, for the IDE
+links) when a course runs. The checks install both as devDependencies.
diff --git a/package-lock.json b/package-lock.json
index a13268f..00dcdf8 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -6,9 +6,28 @@
     "": {
       "name": "glpython-liascript",
       "devDependencies": {
+        "fflate": "0.8.3",
+        "lz-string": "1.5.0",
         "playwright-core": "^1.48.2"
       }
     },
+    "node_modules/fflate": {
+      "version": "0.8.3",
+      "resolved": "https://registry.npmjs.org/fflate/-/fflate-0.8.3.tgz",
+      "integrity": "sha512-tbZNuJrLwGUp3zshBtdy4W+ORxZuIh8a5ilyIEQDC5rY1f3U20JMry0Ll3WBzU58EZKsEuJFXhb5gwv8CsPvgA==",
+      "dev": true,
+      "license": "MIT"
+    },
+    "node_modules/lz-string": {
+      "version": "1.5.0",
+      "resolved": "https://registry.npmjs.org/lz-string/-/lz-string-1.5.0.tgz",
+      "integrity": "sha512-h5bgJWpxJNswbU7qCrV0tIKQCaS3blPDrqKWx+QxzuzL1zGUzij9XCWLrSLsJPu5t+eWA/ycetzYAO5IOMcWAQ==",
+      "dev": true,
+      "license": "MIT",
+      "bin": {
+        "lz-string": "bin/bin.js"
+      }
+    },
     "node_modules/playwright-core": {
       "version": "1.65.0",
       "resolved": "https://registry.npmjs.org/playwright-core/-/playwright-core-1.65.0.tgz",
diff --git a/package.json b/package.json
index b37a356..9384e6f 100644
--- a/package.json
+++ b/package.json
@@ -8,6 +8,8 @@
     "check:player": "node scripts/check-player.mjs"
   },
   "devDependencies": {
+    "fflate": "0.8.3",
+    "lz-string": "1.5.0",
     "playwright-core": "^1.48.2"
   }
 }
diff --git a/scripts/check-ready.mjs b/scripts/check-ready.mjs
index 94cd864..f04562c 100644
--- a/scripts/check-ready.mjs
+++ b/scripts/check-ready.mjs
@@ -11,6 +11,8 @@
 //   npm run check:ready      (needs Chrome installed, as on GitHub's runners)
 import assert from 'node:assert/strict'
 import { readFileSync } from 'node:fs'
+import { inflateRawSync } from 'node:zlib'
+import lz from 'lz-string'
 import { chromium } from 'playwright-core'
 
 const readme = readFileSync(new URL('../README.md', import.meta.url), 'utf8')
@@ -162,6 +164,44 @@ try {
     const messages = await received('click')
     assert.deepEqual(messages.map((message) => message.type), ['glpython:run', 'glpython:test'])
   })
+
+  // «IDE-Link kopieren»: the IDE's compact link (?p=, raw deflate in
+  // base64url), read here with Node's zlib; where fflate doesn't load, the
+  // old ?project= (lz-string). Both libraries come from node_modules, the
+  // same bytes as on jsDelivr (their integrity hashes hold).
+  const library = (name) => readFileSync(new URL(`../node_modules/${name}`, import.meta.url))
+  for (const fflateLoads of [true, false]) {
+    const label = fflateLoads ? '«IDE-Link kopieren» writes ?p=' : '«IDE-Link kopieren» without fflate writes ?project='
+    const linkContext = await browser.newContext({ viewport: { width: 1000, height: 4000 }, permissions: ['clipboard-read', 'clipboard-write'] })
+    await linkContext.route('**/*', (route) => {
+      const url = new URL(route.request().url())
+      const headers = { 'Access-Control-Allow-Origin': '*' }
+      if (url.origin === courseOrigin) return route.fulfill({ contentType: 'text/html', body: page })
+      if (url.origin === embedOrigin) return route.fulfill({ contentType: 'text/html', body: stub })
+      if (url.href.endsWith('/lz-string@1.5.0/libs/lz-string.min.js')) return route.fulfill({ contentType: 'text/javascript', headers, body: library('lz-string/libs/lz-string.min.js') })
+      if (fflateLoads && url.href.endsWith('/fflate@0.8.3/umd/index.js')) return route.fulfill({ contentType: 'text/javascript', headers, body: library('fflate/umd/index.js') })
+      return route.abort()
+    })
+    const linkTab = await linkContext.newPage()
+    linkTab.on('pageerror', (error) => failures.push(`page error: ${error.message}`))
+    await linkTab.goto(`${courseOrigin}/`)
+    await check(label, async () => {
+      const button = linkTab.locator('#glpython-ide-files')
+      await linkTab.waitForFunction(() => document.querySelector('#glpython-ide-files')?.disabled === false, null, { timeout: 10000 })
+      await button.click()
+      await linkTab.waitForTimeout(300)
+      const link = new URL(await linkTab.evaluate(() => navigator.clipboard.readText()))
+      assert.equal(link.origin + link.pathname, `${embedOrigin}/`)
+      const compact = link.searchParams.get('p')
+      const old = link.searchParams.get('project')
+      assert.equal(compact !== null, fflateLoads, `the link: ${link.search.slice(0, 12)}…`)
+      if (compact !== null) assert.match(compact, /^[A-Za-z0-9_-]+$/, 'base64url without padding')
+      const payload = JSON.parse(compact !== null ? inflateRawSync(Buffer.from(compact, 'base64url')).toString('utf8') : lz.decompressFromEncodedURIComponent(old))
+      assert.equal(payload.mainFile, 'datei2.py')
+      assert.deepEqual(Object.values(payload.files).map((file) => [file.path, file.code]), [['datei1.py', ''], ['datei2.py', 'import datei1\nz = 3']])
+    })
+    await linkContext.close()
+  }
 } finally {
   await browser.close()
 }