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diff --git a/packages/meshbay-hub/tests/test_zip_size_limit.py b/packages/meshbay-hub/tests/test_zip_size_limit.py
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+++ b/packages/meshbay-hub/tests/test_zip_size_limit.py
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+"""
+The arbitrary ceiling on a directory zip.
+
+`downloadDirectory` is the one implementation behind every "download this
+folder as a zip" button — Files' single folder, Files' multi-folder selection,
+and the Photos album button (docs/photos.md §3) — so the limit is checked
+once, there, and holds for all of them.
+
+Two things are worth pinning. That an oversized folder is refused *before*
+`_openDownloadTarget`, because a save dialog for an archive that will never be
+written is worse than no dialog at all. And that a folder at exactly the limit
+still goes through, since an off-by-one here silently costs a whole megabyte
+of allowance and nobody would ever notice.
+"""
+
+import json
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+STATIC = Path(__file__).resolve().parents[1] / "src" / "meshbay_hub" / "static"
+FILE_UTILS = STATIC / "file-utils.js"
+
+pytestmark = pytest.mark.skipif(
+    shutil.which("node") is None or not FILE_UTILS.exists(),
+    reason="node or the SPA sources are not available")
+
+MIB = 1024 * 1024
+
+
+def _run(total_bytes, tmp_path):
+    """
+    Call downloadDirectory over one folder holding `total_bytes`, and report
+    what it did: the errors it set, how many times it put a question to the
+    person, and how many transfers it started.
+    """
+    sandbox = tmp_path / "static"
+    sandbox.mkdir()
+    for src in STATIC.glob("*.js"):
+        (sandbox / src.name).write_text(src.read_text(encoding="utf-8"),
+                                        encoding="utf-8")
+    (tmp_path / "package.json").write_text('{"type":"module"}')
+
+    script = tmp_path / "case.mjs"
+    script.write_text(f"""
+const store = new Map();
+globalThis.localStorage = {{
+  getItem: k => (store.has(k) ? store.get(k) : null),
+  setItem: (k, v) => store.set(k, String(v)),
+  removeItem: k => store.delete(k),
+}};
+// Node 22 defines `navigator` itself, so it is left alone; `window` is what
+// platform.js reaches for to decide it is not running in the desktop app.
+globalThis.window = globalThis;
+const out = {{ errors: [], started: 0, asked: 0 }};
+// Reached only once the size check has passed: with no File System Access API
+// under Node, downloadDirectory falls through to its build-in-memory path and
+// asks first. Answering yes is what lets the at-the-limit case get as far as
+// starting a transfer, and `asked` is how the refusal proves it never did.
+globalThis.confirm = () => {{ out.asked += 1; return true; }};
+
+const M = await import('{(sandbox / "file-utils.js").as_posix()}');
+
+const transfers = {{ start: () => {{ out.started += 1; }} }};
+const transport = {{ connected: true }};
+// One file, in the folder itself — entriesUnder keys on `path`.
+const entries = [{{ id: 'f1', name: 'big.bin', path: 'album',
+                   size: {total_bytes}, added_at: 0 }}];
+
+await M.downloadDirectory(transfers, transport, null, entries, 'album', {{
+  setError: (m) => out.errors.push(m),
+}});
+
+out.limit = M.ZIP_MAX_BYTES;
+console.log(JSON.stringify(out));
+""", encoding="utf-8")
+
+    proc = subprocess.run(["node", str(script)], capture_output=True, text=True)
+    assert proc.returncode == 0, proc.stderr
+    return json.loads(proc.stdout)
+
+
+def test_the_limit_is_512_mib(tmp_path):
+    """The number the refusal quotes is the number the module enforces."""
+    assert _run(0, tmp_path)["limit"] == 512 * MIB
+
+
+def test_an_oversized_folder_is_refused_before_anything_opens(tmp_path):
+    """
+    One byte over. Nothing is started, and the person is told why — a silent
+    return would read as a dead button.
+    """
+    result = _run(512 * MIB + 1, tmp_path)
+    assert result["started"] == 0, "no transfer may begin"
+    assert result["asked"] == 0, "nor may a save dialog have been put up first"
+    assert result["errors"] == ["group.zip_too_large"], (
+        "the refusal must name its own key; t() falls back to the key with no "
+        "catalogue loaded, and test_locales.py holds the ten translations of it")
+
+
+def test_a_folder_exactly_at_the_limit_still_downloads(tmp_path):
+    """The bound is inclusive: `> ZIP_MAX_BYTES`, not `>=`."""
+    result = _run(512 * MIB, tmp_path)
+    assert result["errors"] == []
+    assert result["started"] == 1