diff --git a/static/js/cookbookProgressSignal.js b/static/js/cookbookProgressSignal.js
new file mode 100644
index 0000000..3346b4e
--- /dev/null
+++ b/static/js/cookbookProgressSignal.js
@@ -0,0 +1,29 @@
+// static/js/cookbookProgressSignal.js
+/**
+ * Liveness signal for a running cookbook download/install. The watchdog treats a
+ * task as stalled when this signal stays unchanged for too long, so it must move
+ * whenever the task is genuinely making progress.
+ *
+ * During a model DOWNLOAD the honest signal is the downloaded-byte counter
+ * ("1.81G" from "1.81G/2.49G"): it climbs while transferring and freezes when
+ * stuck — and unlike a % bar or speed/ETA it doesn't keep animating on a frozen
+ * frame. That path is kept exactly as-is.
+ *
+ * But a dependency install (e.g. vllm) spends long stretches with NO byte
+ * counter — pip dependency resolution and the native CUDA build/compile. A
+ * byte-only signal freezes there, so the watchdog falsely declares the install
+ * stale and restarts it mid-build, looping forever (#1568). When there's no byte
+ * counter, fall back to a fingerprint of the output tail: resolver/compile lines
+ * keep changing while the process is alive, and only a truly hung process leaves
+ * the tail frozen.
+ *
+ * Pure (string in, string out) so it's unit-testable; cookbookRunning.js pulls
+ * in browser-only modules and can't load under node.
+ */
+export function computeProgressSignal(bytes, dlAgg, lastPct, snapshot) {
+  if (bytes) return bytes;
+  const base = dlAgg != null ? String(dlAgg) : (lastPct || '0');
+  // No byte counter → use the output tail so a build/resolve phase that emits new
+  // lines counts as progress instead of a false stall (#1568).
+  return base + '|' + String(snapshot || '').slice(-300);
+}
diff --git a/static/js/cookbookRunning.js b/static/js/cookbookRunning.js
index 3395053..fb1d810 100644
--- a/static/js/cookbookRunning.js
+++ b/static/js/cookbookRunning.js
@@ -7,6 +7,7 @@
 import uiModule from './ui.js';
 import { _diagnose, _showDiagnosis, _clearDiagnosis } from './cookbook-diagnosis.js';
 import { registerMenuDismiss } from './escMenuStack.js';
+import { computeProgressSignal } from './cookbookProgressSignal.js';
 
 // Human-friendly badge label for a task's internal status. Avoids surfacing
 // the word "error" in the sidebar — a server the user stopped or one that
@@ -2443,7 +2444,11 @@ async function _reconnectTask(el, task) {
             // back to %/aggregate only when no byte counter is present.
             const _byteMatches = [...snapshot.matchAll(/([\d.]+\s?[KMGT])B?\s*\/\s*[\d.]+\s?[KMGT]B?/gi)];
             const _bytes = _byteMatches.length ? _byteMatches[_byteMatches.length - 1][1].replace(/\s/g, '') : null;
-            const curProgress = _bytes || (_dlAgg != null ? String(_dlAgg) : (lastPct || '0'));
+            // When there's no byte counter (pip resolve / native build phase of a
+            // dependency install), key off the output tail so new build lines count
+            // as progress — otherwise a long quiet build is falsely declared stale
+            // and restarted mid-build, looping forever (#1568).
+            const curProgress = computeProgressSignal(_bytes, _dlAgg, lastPct, snapshot);
             const _fetchPctMatches = [...snapshot.matchAll(/Fetching\s+\d+\s+files:\s*(\d+)%/g)];
             const _fetchPct = _fetchPctMatches.length ? parseInt(_fetchPctMatches[_fetchPctMatches.length - 1][1]) : null;
             const _startupStalled = !_bytes && ((_dlAgg === 0) || (_fetchPct === 0)) && curProgress === '0';
diff --git a/tests/test_cookbook_progress_signal_js.py b/tests/test_cookbook_progress_signal_js.py
new file mode 100644
index 0000000..4067f70
--- /dev/null
+++ b/tests/test_cookbook_progress_signal_js.py
@@ -0,0 +1,85 @@
+"""Regression for issue #1568 — installing a heavy dependency (vllm) in the
+Cookbook crashes in a "stale — restarting" loop.
+
+The download/install watchdog (static/js/cookbookRunning.js) decides a task is
+stalled when its progress signal stays unchanged for STALE_PROGRESS_MS. That
+signal used to be the downloaded-byte counter only, which freezes during the long
+no-byte-counter phases of a dependency install — pip dependency resolution and
+the native CUDA build — so the watchdog falsely declared the install stale and
+restarted it mid-build, looping forever.
+
+computeProgressSignal (cookbookProgressSignal.js) keeps the byte signal for the
+download phase (so a genuinely stuck download is still caught) and falls back to
+the output tail when there's no byte counter, so build/resolver output counts as
+progress. Pure function → executed under node here (cookbookRunning.js pulls in
+browser-only modules and can't load).
+"""
+
+import json
+import shutil
+import subprocess
+import textwrap
+from pathlib import Path
+
+import pytest
+
+_REPO = Path(__file__).resolve().parent.parent
+_HAS_NODE = shutil.which("node") is not None
+
+
+@pytest.fixture(scope="module")
+def node_available():
+    if not _HAS_NODE:
+        pytest.skip("node binary not on PATH")
+
+
+def _run_node(script: str) -> dict:
+    res = subprocess.run(
+        ["node", "--input-type=module", "-e", script],
+        cwd=_REPO, capture_output=True, timeout=15, text=True,
+    )
+    if res.returncode != 0:
+        raise AssertionError(f"node failed:\n{res.stderr}")
+    out = [ln for ln in res.stdout.splitlines() if ln.strip()]
+    if not out:
+        raise AssertionError("node produced no stdout")
+    return json.loads(out[-1])
+
+
+def test_download_phase_uses_byte_counter_and_ignores_animated_tail(node_available):
+    """During a download the byte counter is the signal; a stuck download whose
+    only the ETA/spinner keeps animating must yield the SAME signal (so a real
+    download stall is still detected)."""
+    script = textwrap.dedent("""
+        const { computeProgressSignal } = await import('./static/js/cookbookProgressSignal.js');
+        // Same downloaded bytes, different animated ETA/spinner in the tail.
+        const a = computeProgressSignal('1.81G', null, '73', 'Downloading 73%| 1.81G/2.49G [eta 0:05:11]');
+        const b = computeProgressSignal('1.81G', null, '73', 'Downloading 73%| 1.81G/2.49G [eta 0:09:42] -');
+        // Bytes climb -> different.
+        const c = computeProgressSignal('2.10G', null, '84', 'Downloading 84%| 2.10G/2.49G');
+        console.log(JSON.stringify({ a, b, stuck_same: a === b, climbed_diff: a !== c }));
+    """)
+    out = _run_node(script)
+    assert out["a"] == "1.81G"
+    assert out["stuck_same"] is True, "a stuck download (only ETA animating) must stay the same signal"
+    assert out["climbed_diff"] is True, "climbing bytes must change the signal"
+
+
+def test_build_phase_progresses_on_new_output(node_available):
+    """The #1568 case: no byte counter (pip resolve / CUDA build). New build
+    output must change the signal so it isn't falsely declared stale — whereas a
+    byte-only signal would read '0' for both and trip the stall timer."""
+    script = textwrap.dedent("""
+        const { computeProgressSignal } = await import('./static/js/cookbookProgressSignal.js');
+        const s1 = computeProgressSignal(null, null, null, 'Building wheel for vllm ... compiling csrc/attention.cu');
+        const s2 = computeProgressSignal(null, null, null, 'Building wheel for vllm ... compiling csrc/cache_kernels.cu');
+        const hung1 = computeProgressSignal(null, null, null, 'Building wheel for vllm ... (no output)');
+        const hung2 = computeProgressSignal(null, null, null, 'Building wheel for vllm ... (no output)');
+        console.log(JSON.stringify({
+          build_progresses: s1 !== s2,
+          true_hang_stays: hung1 === hung2,
+        }));
+    """)
+    out = _run_node(script)
+    assert out["build_progresses"] is True, "new build output must count as progress (#1568)"
+    assert out["true_hang_stays"] is True, "a genuinely frozen tail must still read as stalled"