diff --git a/hermes_cli/local_runtime/binaries.py b/hermes_cli/local_runtime/binaries.py
index d18985646c..c304f06ffa 100644
--- a/hermes_cli/local_runtime/binaries.py
+++ b/hermes_cli/local_runtime/binaries.py
@@ -9,6 +9,7 @@ import os
 import platform
 import shutil
 import subprocess
+import tempfile
 import urllib.request
 import zipfile
 from dataclasses import dataclass, field
@@ -185,19 +186,29 @@ def _download(url: str, dest: Path,
     """Stream url -> dest. ``progress(done_bytes, total_bytes)`` ticks per chunk (total 0 when
     the server sends no Content-Length) — a several-hundred-MB archive must never look hung."""
     logger.info("downloading %s", url)
-    tmp = dest.with_suffix(dest.suffix + ".part")
-    with urllib.request.urlopen(url, timeout=120) as r, open(tmp, "wb") as f:
-        total = int(r.headers.get("Content-Length") or 0)
-        done = 0
-        while True:
-            chunk = r.read(1 << 20)
-            if not chunk:
-                break
-            f.write(chunk)
-            done += len(chunk)
-            if progress is not None:
-                progress(done, total)
-    tmp.replace(dest)
+    staging = tempfile.NamedTemporaryFile(
+        mode="wb", dir=dest.parent, prefix=f"{dest.name}.", suffix=".part", delete=False)
+    tmp = Path(staging.name)
+    try:
+        with staging as f, urllib.request.urlopen(url, timeout=120) as r:
+            length = r.headers.get("Content-Length")
+            total = int(length) if length is not None else 0
+            done = 0
+            while True:
+                chunk = r.read(1 << 20)
+                if not chunk:
+                    break
+                f.write(chunk)
+                done += len(chunk)
+                if progress is not None:
+                    progress(done, total)
+            # Chunked reads can return EOF without raising IncompleteRead.
+            if length is not None and done != total:
+                raise BinaryResolutionError(
+                    f"incomplete download for {dest.name}: expected {total} bytes, got {done}")
+        tmp.replace(dest)
+    finally:
+        tmp.unlink(missing_ok=True)
 
 
 def _extract(archive: Path, dest: Path,
diff --git a/tests/hermes_cli/test_local_runtime_downloads.py b/tests/hermes_cli/test_local_runtime_downloads.py
new file mode 100644
index 0000000000..c0ac8558e4
--- /dev/null
+++ b/tests/hermes_cli/test_local_runtime_downloads.py
@@ -0,0 +1,96 @@
+"""Runtime downloads must not turn a transient transfer failure into a poisoned cache."""
+
+import threading
+from concurrent.futures import ThreadPoolExecutor
+from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
+from urllib.error import HTTPError
+
+import pytest
+
+from hermes_cli.local_runtime import binaries
+
+
+@pytest.fixture
+def asset_server():
+    responses = []
+    requests = []
+
+    class Handler(BaseHTTPRequestHandler):
+        def do_GET(self):
+            requests.append(self.path)
+            body, length, status = responses.pop(0)
+            self.send_response(status)
+            if length is not None:
+                self.send_header("Content-Length", str(length))
+            self.end_headers()
+            self.wfile.write(body)
+            self.close_connection = True
+
+        def log_message(self, *args):
+            pass
+
+    with ThreadingHTTPServer(("127.0.0.1", 0), Handler) as server:
+        thread = threading.Thread(target=server.serve_forever, daemon=True)
+        thread.start()
+        try:
+            yield f"http://127.0.0.1:{server.server_port}/{{asset}}", responses, requests
+        finally:
+            server.shutdown()
+            thread.join(timeout=5)
+
+
+@pytest.mark.parametrize("failure", ["short", "callback"])
+@pytest.mark.parametrize("known_length", [True, False])
+def test_download_only_publishes_complete_transfers(tmp_path, asset_server, failure, known_length):
+    url, responses, requests = asset_server
+    dest = tmp_path / "runtime.zip"
+    payload = b"runtime archive bytes"
+    responses.append((payload[:-1], len(payload), 200))
+
+    def progress(done, total):
+        if failure == "callback":
+            raise RuntimeError("cancelled")
+
+    error = RuntimeError if failure == "callback" else binaries.BinaryResolutionError
+    with pytest.raises(error):
+        binaries._download(url.format(asset=dest.name), dest, progress=progress)
+    assert not dest.exists()
+    assert not list(tmp_path.glob("*.part"))
+
+    # A retry works, including servers that omit Content-Length.
+    responses.append((payload, len(payload) if known_length else None, 200))
+    ticks = []
+    binaries._download(url.format(asset=dest.name), dest,
+                       progress=lambda done, total: ticks.append((done, total)))
+    assert dest.read_bytes() == payload
+    assert ticks[-1] == (len(payload), len(payload) if known_length else 0)
+    assert len(requests) == 2
+
+
+def test_failed_request_preserves_another_active_download(tmp_path, asset_server):
+    url, responses, requests = asset_server
+    dest = tmp_path / "runtime.zip"
+    payload = b"x" * (2 << 20)
+    responses.extend([(payload, len(payload), 200), (b"", 0, 503)])
+    started = threading.Event()
+    resume = threading.Event()
+
+    def pause_download(done, total):
+        started.set()
+        assert resume.wait(10), "active download was not resumed"
+
+    with ThreadPoolExecutor(max_workers=1) as pool:
+        active = pool.submit(binaries._download, url.format(asset=dest.name), dest,
+                             progress=pause_download)
+        try:
+            assert started.wait(10), "active download did not write its first chunk"
+            with pytest.raises(HTTPError) as failed:
+                binaries._download(url.format(asset=dest.name), dest)
+            assert failed.value.code == 503
+        finally:
+            resume.set()
+        active.result(timeout=10)
+
+    assert dest.read_bytes() == payload
+    assert not list(tmp_path.glob("*.part"))
+    assert len(requests) == 2