diff --git a/arborist/cold_object.py b/arborist/cold_object.py
index f198499..a64a031 100644
--- a/arborist/cold_object.py
+++ b/arborist/cold_object.py
@@ -571,16 +571,18 @@ def stream_packs(
     state = _start_pack()
     try:
         # Pack format v2: ship the metadata table dumps as the FIRST tar
-        # members of the first pack. Add them up front so they're tarred
-        # before chunks; if the pack overflows the cap mid-chunk, the
-        # metadata is already safely in pack 1 and pack 2 onward holds
-        # only the chunk overflow.
+        # members of the first pack. Stream from disk — tarfile.addfile
+        # reads the file handle in chunks, never loading the whole thing
+        # into RAM. A 4 GB edges.jsonl thus uses ~tens of KB peak RAM
+        # instead of 4 GB per worker. TarInfo built by hand (NOT via
+        # gettarinfo) so mtime/uid/gid are zeroed — two writers' packs
+        # converge byte-for-byte regardless of filesystem timestamps.
         for member_name, source_path in pending_extras:
-            with open(source_path, "rb") as f:
-                source_bytes = f.read()
             info = tarfile.TarInfo(name=member_name)
-            info.size = len(source_bytes)
-            state["tar"].addfile(info, io.BytesIO(source_bytes))
+            info.size = source_path.stat().st_size
+            # mtime, uid, gid, mode default to 0 / "" — deterministic.
+            with open(source_path, "rb") as f:
+                state["tar"].addfile(info, f)
         # Force the compressor to flush so size accounting reflects the
         # metadata write before we start streaming chunks.
         if pending_extras: