diff --git a/datasets/add_months.sh b/datasets/add_months.sh
index 3da60a6..c046235 100755
--- a/datasets/add_months.sh
+++ b/datasets/add_months.sh
@@ -1,8 +1,8 @@
 #!/usr/bin/env bash
 #
 # Add one or more new months to the existing parquet datasets using a
-# layered append. Part 1 runs on a compute node; Part 2 must be launched
-# from a login node (it calls salloc via start_spark_and_run.sh).
+# layered append. The live datasets are never touched until the final
+# copy step, so they remain safe and queryable throughout.
 #
 # Usage:
 #   add_months.sh YYYY-MM [YYYY-MM ...]
@@ -21,15 +21,19 @@
 #   SUBMISSIONS_DUMPDIR=/path/to/new/submissions \
 #   ./add_months.sh 2025-01 2025-02
 #
-# How layering works: Part 2 appends a new set of sorted partition files
-# alongside the existing ones. Spark and DuckDB read all layers together
-# transparently. Run merge_layers.sh to collapse layers into one when n
-# gets large. Run build_from_scratch.sh to rebuild everything from raw dumps.
+# Workflow:
+#   Part 1  — parse new .zst files into per-month parquets (compute node)
+#   Part 2  — sort into staging directories, not the live datasets (fat node)
+#   Verify  — inspect staging before committing (manual step, not scripted)
+#   Copy    — move staging files into live datasets (run manually after verify)
+#   Cleanup — remove temp and staging dirs (run manually after copy)
+#
+# Every command below is independently runnable for debugging. The copy
+# and cleanup steps are intentionally left as separate commands so you can
+# verify the staging output before touching the live datasets.
 #
 # NOTE: This script and its workflow are written but not yet tested.
 # Remove this notice after a successful end-to-end run.
-#
-# Every command below is independently runnable for debugging.
 
 set -e
 cd "$(dirname "$0")"
@@ -42,37 +46,79 @@ fi
 COMMENTS_DUMPDIR="${COMMENTS_DUMPDIR:-/gscratch/comdata/raw_data/reddit_dumps/comments}"
 SUBMISSIONS_DUMPDIR="${SUBMISSIONS_DUMPDIR:-/gscratch/comdata/raw_data/reddit_dumps/submissions}"
 
+# Part 1 temp dirs (per-month parquets, parsed from .zst)
 TEMP_COMMENTS="/gscratch/comdata/output/temp/add_months_comments.parquet"
 TEMP_SUBMISSIONS="/gscratch/comdata/output/temp/add_months_submissions.parquet"
 
-# --- Part 1: parse new months in parallel -----------------------------------
+# Staging dirs (sorted new layer; inspected before copying to live)
+STAGING_COMMENTS_SUB="/gscratch/comdata/output/temp/new_layer_comments_by_subreddit.parquet"
+STAGING_COMMENTS_AUTH="/gscratch/comdata/output/temp/new_layer_comments_by_author.parquet"
+STAGING_SUBMISSIONS_SUB="/gscratch/comdata/output/temp/new_layer_submissions_by_subreddit.parquet"
+STAGING_SUBMISSIONS_AUTH="/gscratch/comdata/output/temp/new_layer_submissions_by_author.parquet"
+
+# Live dataset dirs
+LIVE_COMMENTS_SUB="/gscratch/comdata/output/reddit_comments_by_subreddit.parquet"
+LIVE_COMMENTS_AUTH="/gscratch/comdata/output/reddit_comments_by_author.parquet"
+LIVE_SUBMISSIONS_SUB="/gscratch/comdata/output/reddit_submissions_by_subreddit.parquet"
+LIVE_SUBMISSIONS_AUTH="/gscratch/comdata/output/reddit_submissions_by_author.parquet"
+
+# --- Part 1: parse new months in parallel (run on a compute node) -----------
 
-# build task lists for the specified months
 printf "python3 comments_part1.py parse_dump RC_%s.zst --dumpdir=\"$COMMENTS_DUMPDIR\" --outdir=\"$TEMP_COMMENTS\"\n" "$@" \
     > add_months_comments_tasks.txt
 
 printf "python3 submissions_part1.py parse_dump RS_%s.zst --dumpdir=\"$SUBMISSIONS_DUMPDIR\" --outdir=\"$TEMP_SUBMISSIONS\"\n" "$@" \
     > add_months_submissions_tasks.txt
 
-# parse all new comment months in parallel
 parallel --joblog add_months_comments_joblog.txt --results add_months_comments_logs \
     < add_months_comments_tasks.txt
 
-# parse all new submission months in parallel
 parallel --joblog add_months_submissions_joblog.txt --results add_months_submissions_logs \
     < add_months_submissions_tasks.txt
 
-# --- Part 2: sort and append new layer (run from a login node) --------------
+# --- Part 2: sort new months into staging (not the live datasets) -----------
 #
-# start_spark_and_run.sh calls salloc; run these two lines from a login node,
-# not from within an existing compute allocation.
+# start_spark_and_run.sh calls salloc — run from a login node, or replace
+# with start_spark_cluster.sh + spark-submit if already on a suitable node.
 
-# append new comment layer to reddit_comments_by_{subreddit,author}.parquet
-start_spark_and_run.sh 1 comments_part2.py --indir="$TEMP_COMMENTS" --mode=append
+start_spark_and_run.sh 1 comments_part2.py \
+    --indir="$TEMP_COMMENTS" \
+    --out_by_subreddit="$STAGING_COMMENTS_SUB" \
+    --out_by_author="$STAGING_COMMENTS_AUTH"
 
-# append new submission layer to reddit_submissions_by_{subreddit,author}.parquet
-start_spark_and_run.sh 1 submissions_part2.py --indir="$TEMP_SUBMISSIONS" --mode=append
+start_spark_and_run.sh 1 submissions_part2.py \
+    --indir="$TEMP_SUBMISSIONS" \
+    --out_by_subreddit="$STAGING_SUBMISSIONS_SUB" \
+    --out_by_author="$STAGING_SUBMISSIONS_AUTH"
 
-# --- cleanup: remove temporary Part 1 files ---------------------------------
+# --- Verify: inspect staging before copying to live -------------------------
+#
+# Stop here and check that the staging output looks right before running
+# the copy step. The live datasets are untouched at this point. Example:
+#
+#   ls -lah "$STAGING_COMMENTS_SUB" | head
+#   python3 -c "
+#   import pyarrow.parquet as pq, os
+#   f = sorted(os.listdir('$STAGING_COMMENTS_SUB'))[0]
+#   t = pq.read_table('$STAGING_COMMENTS_SUB/' + f, columns=['created_utc'])
+#   print(t.column('created_utc')[0].as_py(), t.column('created_utc')[-1].as_py())
+#   "
+
+# --- Copy: add staging files into live datasets -----------------------------
+#
+# Run these lines manually after verifying staging. This is the only step
+# that touches the live datasets. It only adds new files — existing files
+# are never deleted or overwritten.
+
+find "$STAGING_COMMENTS_SUB"  -maxdepth 1 -type f -exec cp {} "$LIVE_COMMENTS_SUB"/  \;
+find "$STAGING_COMMENTS_AUTH" -maxdepth 1 -type f -exec cp {} "$LIVE_COMMENTS_AUTH"/ \;
+find "$STAGING_SUBMISSIONS_SUB"  -maxdepth 1 -type f -exec cp {} "$LIVE_SUBMISSIONS_SUB"/  \;
+find "$STAGING_SUBMISSIONS_AUTH" -maxdepth 1 -type f -exec cp {} "$LIVE_SUBMISSIONS_AUTH"/ \;
+
+# --- Cleanup: remove temp and staging dirs ----------------------------------
+#
+# Run after confirming the copy succeeded and the live datasets look right.
 
 rm -rf "$TEMP_COMMENTS" "$TEMP_SUBMISSIONS"
+rm -rf "$STAGING_COMMENTS_SUB" "$STAGING_COMMENTS_AUTH"
+rm -rf "$STAGING_SUBMISSIONS_SUB" "$STAGING_SUBMISSIONS_AUTH"
diff --git a/datasets/comments_part2.py b/datasets/comments_part2.py
index bb98ac0..f767b5f 100755
--- a/datasets/comments_part2.py
+++ b/datasets/comments_part2.py
@@ -6,8 +6,8 @@ Must be launched from a login node via the Hyak-provided wrapper:
   start_spark_and_run.sh 1 comments_part2.py --indir=/path/to/parquets --mode=append
 
 --indir defaults to the temp comments dir in dumps_helper.py.
---mode defaults to 'overwrite'; use 'append' to add a new layer without
-touching existing partition files (see add_months.sh).
+--out_by_subreddit and --out_by_author default to the live dataset paths;
+override them to write to staging directories first (see add_months.sh).
 """
 
 import fire
@@ -15,4 +15,7 @@ from dumps_helper import COMMENTS, sort_and_write
 
 
 if __name__ == "__main__":
-    fire.Fire(lambda indir=None, mode='overwrite': sort_and_write(COMMENTS, indir=indir, mode=mode))
+    fire.Fire(lambda indir=None, out_by_subreddit=None, out_by_author=None:
+              sort_and_write(COMMENTS, indir=indir,
+                             out_by_subreddit=out_by_subreddit,
+                             out_by_author=out_by_author))
diff --git a/datasets/dumps_helper.py b/datasets/dumps_helper.py
index 1d85420..bfd6ae6 100644
--- a/datasets/dumps_helper.py
+++ b/datasets/dumps_helper.py
@@ -256,13 +256,14 @@ def gen_task_list(config, script_name, dumpdir=None, tasklist=None):
 
 # --- Part 2: spark sort + repartition --------------------------------------
 
-def sort_and_write(config, indir=None, mode='overwrite'):
+def sort_and_write(config, indir=None, out_by_subreddit=None, out_by_author=None):
     """Read a directory of per-source parquets, sort and repartition twice
-    (once by subreddit, once by author), and write the two final datasets.
+    (once by subreddit, once by author), and write the two output datasets.
 
-    indir defaults to config['outdir']. mode is passed to parquet write and
-    may be 'overwrite' (default, used by build_from_scratch) or 'append'
-    (used by add_months to layer new data alongside existing files).
+    indir defaults to config['outdir'].
+    out_by_subreddit and out_by_author default to config['output_by_subreddit']
+    and config['output_by_author']. Override them to write to staging directories
+    instead of the live datasets (see add_months.sh).
 
     Pyspark is imported lazily so Part 1 callers don't pay the Spark startup
     cost.
@@ -270,6 +271,9 @@ def sort_and_write(config, indir=None, mode='overwrite'):
     from pyspark.sql import SparkSession, functions as f
 
     indir = indir or config['outdir']
+    out_by_subreddit = out_by_subreddit or config['output_by_subreddit']
+    out_by_author = out_by_author or config['output_by_author']
+
     spark = SparkSession.builder.appName(config['app_name']).getOrCreate()
 
     df = spark.read.parquet(indir, compression='snappy')
@@ -286,12 +290,12 @@ def sort_and_write(config, indir=None, mode='overwrite'):
     sub_keys = config['subreddit_sort_keys']
     df_sub = df.repartition('subreddit').sort(sub_keys, ascending=True)
     df_sub = df_sub.sortWithinPartitions(sub_keys, ascending=True)
-    df_sub.write.parquet(config['output_by_subreddit'], mode=mode, compression='snappy')
+    df_sub.write.parquet(out_by_subreddit, mode='overwrite', compression='snappy')
 
     auth_keys = config['author_sort_keys']
     df_auth = df.repartition('author').sort(auth_keys, ascending=True)
     df_auth = df_auth.sortWithinPartitions(auth_keys, ascending=True)
-    df_auth.write.parquet(config['output_by_author'], mode=mode, compression='snappy')
+    df_auth.write.parquet(out_by_author, mode='overwrite', compression='snappy')
 
 
 def merge_layers(config):
diff --git a/datasets/submissions_part2.py b/datasets/submissions_part2.py
index 7a45bb9..d585b8d 100755
--- a/datasets/submissions_part2.py
+++ b/datasets/submissions_part2.py
@@ -6,8 +6,8 @@ Must be launched from a login node via the Hyak-provided wrapper:
   start_spark_and_run.sh 1 submissions_part2.py --indir=/path/to/parquets --mode=append
 
 --indir defaults to the temp submissions dir in dumps_helper.py.
---mode defaults to 'overwrite'; use 'append' to add a new layer without
-touching existing partition files (see add_months.sh).
+--out_by_subreddit and --out_by_author default to the live dataset paths;
+override them to write to staging directories first (see add_months.sh).
 """
 
 import fire
@@ -15,4 +15,7 @@ from dumps_helper import SUBMISSIONS, sort_and_write
 
 
 if __name__ == "__main__":
-    fire.Fire(lambda indir=None, mode='overwrite': sort_and_write(SUBMISSIONS, indir=indir, mode=mode))
+    fire.Fire(lambda indir=None, out_by_subreddit=None, out_by_author=None:
+              sort_and_write(SUBMISSIONS, indir=indir,
+                             out_by_subreddit=out_by_subreddit,
+                             out_by_author=out_by_author))