diff --git a/bin/gstack-brain-sync b/bin/gstack-brain-sync
index 9e9489512..f77639514 100755
--- a/bin/gstack-brain-sync
+++ b/bin/gstack-brain-sync
@@ -56,6 +56,9 @@
 #   stale-push   trigger: no push for 24 hours while the drain's drainable count is above zero
 #                line:    ARTIFACTS_SYNC: attention: stale-push. Files are ready to sync but nothing has been pushed for over 24 hours. See why: <bin> --status
 #                fix:     the waiting reason <bin> --status names (a coupled group waiting for a consistent generation, a git add failure)
+#   case-collision trigger: case_names is set: artifact paths differ only by letter case, so the drain holds queued files under them
+#                line:    ARTIFACTS_SYNC: attention: case-collision held=<N> names=<names>. Artifact paths differ only by letter case, which a Mac or Windows disk stores as one file; sync holds <N> queued file(s) under them and syncs everything else. Repair: https://github.com/garrytan/gstack/blob/main/docs/troubleshooting.md#artifact-case-collision. See the paths: <bin> --status
+#                fix:     keep one spelling: the manual repair in docs/troubleshooting.md#artifact-case-collision
 #   stale-drain  trigger: last_drain_at older than 24 hours while the queue directory holds records
 #                line:    ARTIFACTS_SYNC: attention: stale-drain. The sync has not run for over 24 hours while files wait in the queue. See why: <bin> --status
 #                fix:     if no gstack process is running, remove ~/.gstack/.brain-sync.lock.d, then run <bin> --once
@@ -115,6 +118,8 @@ REMOVAL_VALVE=20
 ST_HELD='[]'
 ST_DRAINABLE=0
 ST_NOTE=""
+ST_CASE_HELD=0
+ST_CASE_NAMES=""
 
 # origin host for receipt records (github.com etc).
 remote_host() {
@@ -136,8 +141,13 @@ remote_auth_hint() {
 }
 
 # Status file schema (one JSON object; readers: --status, --attention, doctor):
-#   status        ok | idle | held | blocked | push_failed | error
+#   status        ok | idle | held | collision | blocked | push_failed | error
+#                 (collision: files were held only for case-colliding paths)
 #   held_count    files the secret scan held back this drain (hits + dependents)
+#   case_held     queued files held because their path differs only by letter
+#                 case from another path in the repo or the batch
+#   case_names    the colliding path component names, comma-separated and
+#                 clamped to [A-Za-z0-9._?-]; empty when the tree has none
 #   drainable     queued files a drain could publish now: excludes scanner-held
 #                 and privacy-held records
 #   last_drain_at UTC time this drain ran (every write happens inside a drain)
@@ -152,9 +162,9 @@ write_status() {
   local code="$1" msg="$2${ST_NOTE}" ts last_push=""
   ts=$(date -u +%Y-%m-%dT%H:%M:%SZ 2>/dev/null || echo "")
   [ -f "$LAST_PUSH_FILE" ] && last_push=$(head -1 "$LAST_PUSH_FILE" 2>/dev/null || echo "")
-  python3 - "$STATUS_FILE" "$code" "$msg" "$ts" "$last_push" "$ST_DRAINABLE" "$ST_HELD" <<'PYEOF' 2>/dev/null || true
+  python3 - "$STATUS_FILE" "$code" "$msg" "$ts" "$last_push" "$ST_DRAINABLE" "$ST_HELD" "$ST_CASE_HELD" "$ST_CASE_NAMES" <<'PYEOF' 2>/dev/null || true
 import json, re, sys
-path, code, msg, ts, last_push, drainable, held = sys.argv[1:8]
+path, code, msg, ts, last_push, drainable, held, case_held, case_names = sys.argv[1:10]
 try:
     held = [h for h in json.loads(held) if isinstance(h, dict)]
 except Exception:
@@ -171,6 +181,8 @@ data = {
     "last_drain_at": ts,
     "last_push_at": last_push if re.fullmatch(r"\d{4}-\d\d-\d\dT\d\d:\d\d:\d\dZ", last_push) else None,
     "ts": ts,
+    "case_held": int(case_held) if case_held.isdigit() else 0,
+    "case_names": case_names if re.fullmatch(r"[A-Za-z0-9._?,-]{0,300}", case_names) else "",
     "held": held,
     "message": msg,
 }
@@ -769,13 +781,48 @@ stage_paths() {
 
 # Check outgoing trees too: a retry merge can combine two individually safe
 # spellings, and a retained local commit can be pushed with an empty queue.
+# Also records the tree's case-colliding names for skill start. Without
+# python3 no drain can stage anything, so there is no new tree to check. A
+# passing verdict is cached by HEAD's commit id, so the empty-queue drain at
+# every skill boundary skips the tree walk until HEAD moves. A cached HEAD
+# that falls behind origin is only pushed after a merge, which moves HEAD.
 validate_outgoing_paths() {
-  local path_error
-  if ! path_error=$(python3 "$SCRIPT_DIR/gstack-brain-sync-index.py" --outgoing "$GSTACK_HOME" 2>&1); then
-    write_status "error" "outgoing artifact path validation refused; local commits and queue preserved; inspect path spellings"
-    printf '%s\n' "$path_error" >&2
+  local out err key cached cache="$GSTACK_HOME/.brain-case-check"
+  command -v python3 >/dev/null 2>&1 || return 0
+  key=$(git -C "$GSTACK_HOME" rev-parse --verify -q HEAD 2>/dev/null)
+  cached=$(head -1 "$cache" 2>/dev/null || true)
+  if [ "${cached%%	*}" = "$key" ]; then
+    [ -n "$ST_CASE_NAMES" ] || ST_CASE_NAMES=${cached#*	}
+    return 0
+  fi
+  err=$(mktemp "${TMPDIR:-/tmp}/brain-sync-outgoing.XXXXXX") || return 0
+  if ! out=$(python3 "$SCRIPT_DIR/gstack-brain-sync-index.py" --outgoing "$GSTACK_HOME" 2>"$err"); then
+    ST_CASE_NAMES=$out
+    write_status "error" "outgoing artifact path validation refused: a local commit adds a path that differs only by letter case from a path on the remote; local commits and queue preserved. Repair: https://github.com/garrytan/gstack/blob/main/docs/troubleshooting.md#artifact-case-collision"
+    cat "$err" >&2
+    rm -f "$err"
     return 1
   fi
+  rm -f "$err"
+  printf '%s\t%s\n' "$key" "$out" > "$cache" 2>/dev/null || true
+  [ -n "$ST_CASE_NAMES" ] || ST_CASE_NAMES=$out
+}
+
+# Hold the queued paths in $1 whose spelling collides by letter case with a
+# path in the index or the batch (see gstack-brain-sync-index.py hold). They
+# leave $1 before staging, so their records stay queued. Sets ST_CASE_HELD.
+hold_case_collisions() {
+  local paths_file="$1" out
+  tr -d '\r' < "$paths_file" > "$paths_file.lf" && mv -f "$paths_file.lf" "$paths_file" || return 1
+  out=$(python3 "$SCRIPT_DIR/gstack-brain-sync-index.py" --hold "$GSTACK_HOME" "$paths_file" "$paths_file.case") || return 1
+  ST_CASE_HELD=$(grep -c . "$paths_file.case" 2>/dev/null || true)
+  ST_CASE_HELD=${ST_CASE_HELD:-0}
+  [ "$ST_CASE_HELD" -gt 0 ] || return 0
+  ST_CASE_NAMES=$out
+  { grep -vxF -f "$paths_file.case" "$paths_file" || true; } > "$paths_file.kept"
+  mv -f "$paths_file.kept" "$paths_file"
+  ST_NOTE="$ST_NOTE; $ST_CASE_HELD file(s) held because their path differs only by letter case from another path ($out), NOT synced and still queued: $(head -3 "$paths_file.case" | tr '\n' ' ')(repair: https://github.com/garrytan/gstack/blob/main/docs/troubleshooting.md#artifact-case-collision)"
+  echo "BRAIN_SYNC: holding $ST_CASE_HELD file(s) whose path differs only by letter case ($out); see $SELF_BIN --status" >&2
 }
 
 # Plan the holds for one drain (one Python pass):
@@ -1196,13 +1243,18 @@ subcmd_once() {
     write_status "error" "classification failed; queue preserved (next run retries)"
     exit 0
   fi
+  if [ -s "$paths_file" ] && ! hold_case_collisions "$paths_file"; then
+    write_status "error" "could not check queued paths for letter-case collisions; queue preserved (next run retries)"
+    exit 0
+  fi
   if [ ! -s "$paths_file" ]; then
     # Nothing stageable. Finalize the snapshot (retained entries survive;
     # classified drops removed; records created after the snapshot untouched).
     finalize_queue "$snapshot_file" "$class_file" "$paths_file"
-    local summary
+    local summary idle_code=idle
     summary=$(queue_summary "$class_file")
-    write_status "idle" "no stageable changes${summary:+ ($summary)}"
+    [ "$ST_CASE_HELD" -gt 0 ] && idle_code=collision
+    write_status "$idle_code" "no stageable changes${summary:+ ($summary)}"
     exit 0
   fi
   local n_stageable
@@ -1246,6 +1298,8 @@ subcmd_once() {
     ok_code="held"
     ST_NOTE="$ST_NOTE; the secret scan held back $held_n flagged file(s) (and any coupled files), NOT synced; see held[] for each file's fixes"
     echo "BRAIN_SYNC: held $held_n flagged file(s); see $SELF_BIN --status" >&2
+  elif [ "$ST_CASE_HELD" -gt 0 ]; then
+    ok_code="collision"
   fi
 
   # A verified empty staged diff is the only "nothing to commit"; anything
@@ -1254,8 +1308,8 @@ subcmd_once() {
   git -C "$GSTACK_HOME" diff --cached --quiet >/dev/null 2>&1 || cached_rc=$?
   if [ "$cached_rc" -eq 0 ]; then
     finalize_queue "$snapshot_file" "$class_file" "$paths_file"
-    if [ "$ok_code" = "held" ]; then
-      write_status "held" "nothing else to sync"
+    if [ "$ok_code" != "ok" ]; then
+      write_status "$ok_code" "nothing else to sync"
     else
       write_status "idle" "queue drained but no new changes to commit"
     fi
@@ -1413,11 +1467,12 @@ subcmd_unskip_file() {
 # Skill start's attention lines: one per row of the sync status table in this
 # script's header that applies. Every value read from the status file is
 # clamped (status code to the table's closed set, counts to digits,
-# timestamps to their shape); the message, held paths and scanner output never
-# reach this output, only fixed text and this script's absolute path.
+# timestamps to their shape, case-colliding names to [A-Za-z0-9._?,-]); the
+# message, held paths and scanner output never reach this output, only fixed
+# text, those clamped values and this script's absolute path.
 subcmd_attention() {
   sync_active || exit 0
-  local line="" prefix code="" held="" drainable="" last_drain="" last_push="" bin="$SELF_BIN"
+  local line="" prefix code="" held="" drainable="" last_drain="" last_push="" case_held="" case_names="" bin="$SELF_BIN"
   if [ -f "$STATUS_FILE" ]; then
     line=$(head -c 65536 "$STATUS_FILE" 2>/dev/null | tr -d '\r\n')
     # The counts and timestamps precede every free-text field.
@@ -1427,10 +1482,13 @@ subcmd_attention() {
     drainable=$(printf '%s' "$prefix" | sed -n 's/.*"drainable": *\([0-9][0-9]*\).*/\1/p')
     last_drain=$(printf '%s' "$prefix" | sed -n 's/.*"last_drain_at": *"\([^"]*\)".*/\1/p')
     last_push=$(printf '%s' "$prefix" | sed -n 's/.*"last_push_at": *"\([^"]*\)".*/\1/p')
-    case "$code" in ok|idle|held|blocked|push_failed|error) ;; *) code="unknown" ;; esac
+    case_held=$(printf '%s' "$prefix" | sed -n 's/.*"case_held": *\([0-9][0-9]*\).*/\1/p')
+    case_names=$(printf '%s' "$prefix" | sed -n 's/.*"case_names": *"\([A-Za-z0-9._?,-]*\)".*/\1/p' | head -c 300)
+    case "$code" in ok|idle|held|collision|blocked|push_failed|error) ;; *) code="unknown" ;; esac
   fi
   case "$held" in ''|*[!0-9]*) held=0 ;; esac
   case "$drainable" in ''|*[!0-9]*) drainable=0 ;; esac
+  case "$case_held" in ''|*[!0-9]*) case_held=0 ;; esac
   local iso='[0-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9]T[0-9][0-9]:[0-9][0-9]:[0-9][0-9]Z'
   case "$last_drain" in $iso) ;; *) last_drain="" ;; esac
   case "$last_push" in $iso) ;; *) last_push="" ;; esac
@@ -1442,6 +1500,9 @@ subcmd_attention() {
     error) echo "ARTIFACTS_SYNC: attention: status=error. The last sync stopped before committing; the queue is kept and retried. See the cause: $bin --status" ;;
     unknown) echo "ARTIFACTS_SYNC: attention: status=unknown. The sync status file is unreadable. See it: $bin --status" ;;
   esac
+  if [ -n "$case_names" ] || [ "$case_held" -gt 0 ]; then
+    echo "ARTIFACTS_SYNC: attention: case-collision held=$case_held names=$case_names. Artifact paths differ only by letter case, which a Mac or Windows disk stores as one file; sync holds $case_held queued file(s) under them and syncs everything else. Repair: https://github.com/garrytan/gstack/blob/main/docs/troubleshooting.md#artifact-case-collision. See the paths: $bin --status"
+  fi
 
   local now cutoff oldest="" oldest_epoch
   now=$(date +%s 2>/dev/null || echo "")
diff --git a/bin/gstack-brain-sync-index.py b/bin/gstack-brain-sync-index.py
index db0df69a4..7b6ac53d9 100644
--- a/bin/gstack-brain-sync-index.py
+++ b/bin/gstack-brain-sync-index.py
@@ -3,6 +3,7 @@
 import json
 import os
 from pathlib import Path
+import re
 import stat
 import subprocess
 import sys
@@ -10,7 +11,9 @@ import unicodedata
 
 
 class Refusal(Exception):
-    pass
+    def __init__(self, message, spellings=()):
+        super().__init__(message)
+        self.spellings = spellings
 
 
 def git(root, *args, data=None, allow_missing=False):
@@ -38,15 +41,57 @@ def prefixes(paths):
     return groups
 
 
+def names(spellings):
+    # Only the colliding component names, clamped to a slug's charset: skill
+    # start prints them, and the artifacts repo's paths are not trusted text.
+    leaves = sorted({re.sub(r'[^A-Za-z0-9._-]', '?', s.rsplit('/', 1)[-1])[:64] for s in spellings})
+    return ','.join(leaves[:4])
+
+
+def first_collision(groups):
+    found = sorted((key.count('/'), key) for key, spellings in groups.items() if len(spellings) > 1)
+    return groups[found[0][1]] if found else ()
+
+
+def tree_paths(root, *args):
+    return {p.decode('utf-8') for p in git(root, *args).split(b'\0') if p}
+
+
 def refuse_new_collisions(root, paths, baseline='HEAD'):
     # Compare complete component prefixes: Foo/a and foo/b collide too.
     # Existing aliases are historical data; this check does not migrate them
     # or block unrelated writes, but it refuses any additional spelling.
     head = git(root, 'ls-tree', '-rz', '--name-only', baseline) if baseline else b''
     previous = prefixes(p.decode('utf-8') for p in head.split(b'\0') if p)
-    for key, spellings in prefixes(paths).items():
+    groups = prefixes(paths)
+    for key, spellings in sorted(groups.items(), key=lambda item: (item[0].count('/'), item[0])):
         if len(spellings) > 1 and not spellings <= previous.get(key, set()):
-            raise Refusal('new casefold path collision; inspect path spellings before retrying')
+            raise Refusal('new casefold path collision; inspect path spellings before retrying', spellings)
+    return first_collision(groups)
+
+
+def hold(root, requested_file, held_file):
+    # A requested path is held when one of its prefixes has another spelling
+    # in the index or in this batch, unless its own spelling is the only one
+    # the index already tracks (an established project keeps syncing; the new
+    # alias waits). Held paths are never staged, so git cannot record them
+    # under the other spelling, and their queue records stay.
+    requested = {p for p in Path(requested_file).read_text(encoding='utf-8').replace('\r', '').split('\n') if p}
+    tracked_paths = tree_paths(root, 'ls-files', '-z')
+    tracked = prefixes(tracked_paths)
+    groups = prefixes(tracked_paths | requested)
+    held, colliding = set(), []
+    for path in requested:
+        parts = path.split('/')
+        for i in range(1, len(parts) + 1):
+            spelling = '/'.join(parts[:i])
+            key = unicodedata.normalize('NFC', spelling).casefold()
+            if len(groups[key]) > 1 and tracked.get(key) != {spelling}:
+                held.add(path)
+                colliding.append((key.count('/'), key))
+                break
+    Path(held_file).write_text(''.join(p + '\n' for p in sorted(held)), encoding='utf-8')
+    return groups[min(colliding)[1]] if colliding else ()
 
 
 def verify(root, requested_file, names_file):
@@ -90,19 +135,23 @@ def verify(root, requested_file, names_file):
 
 
 def outgoing(root):
+    if not git(root, 'rev-parse', '--verify', '--quiet', 'HEAD', allow_missing=True):
+        return ()
     branch = git(root, 'rev-parse', '--abbrev-ref', 'HEAD').decode('utf-8').strip()
     remote = 'refs/remotes/origin/' + branch
     baseline = remote if git(root, 'rev-parse', '--verify', '--quiet', remote, allow_missing=True) else None
-    head = git(root, 'ls-tree', '-rz', '--name-only', 'HEAD')
-    refuse_new_collisions(root, (p.decode('utf-8') for p in head.split(b'\0') if p), baseline)
+    return refuse_new_collisions(root, tree_paths(root, 'ls-tree', '-rz', '--name-only', 'HEAD'), baseline)
 
 
 if __name__ == '__main__':
     try:
         if sys.argv[1] == '--outgoing':
-            outgoing(Path(sys.argv[2]))
+            print(names(outgoing(Path(sys.argv[2]))))
+        elif sys.argv[1] == '--hold':
+            print(names(hold(Path(sys.argv[2]), sys.argv[3], sys.argv[4])))
         else:
             verify(Path(sys.argv[1]), sys.argv[2], sys.argv[3])
     except (Refusal, OSError, UnicodeError, ValueError, IndexError) as error:
+        print(names(getattr(error, 'spellings', ())))
         print('BRAIN_SYNC: staged artifact validation refused: ' + str(error), file=sys.stderr)
         sys.exit(1)
diff --git a/bin/gstack-brain-uninstall b/bin/gstack-brain-uninstall
index cb2a289c9..7a12b4e20 100755
--- a/bin/gstack-brain-uninstall
+++ b/bin/gstack-brain-uninstall
@@ -27,6 +27,7 @@
 #   .brain-skip.txt            — user-maintained skip list
 #   .brain-sync.lock.d/        — lock dir (if present)
 #   .brain-sync-status.json    — health status
+#   .brain-case-check          — cached case-collision verdict for HEAD
 #   consumers.json             — consumer/reader registry
 #
 # What it clears (via gstack-config):
@@ -131,6 +132,7 @@ rm -f "$GSTACK_HOME/.brain-last-pull" 2>/dev/null || true
 rm -f "$GSTACK_HOME/.brain-worktree-last-advance" 2>/dev/null || true
 rm -f "$GSTACK_HOME/.brain-skip.txt" 2>/dev/null || true
 rm -f "$GSTACK_HOME/.brain-sync-status.json" 2>/dev/null || true
+rm -f "$GSTACK_HOME/.brain-case-check" 2>/dev/null || true
 rm -rf "$GSTACK_HOME/.brain-sync.lock.d" 2>/dev/null || true
 
 # ---- unregister gbrain federated source + remove worktree (best-effort) ----
diff --git a/bin/gstack-doctor b/bin/gstack-doctor
index 75be5e044..28ec0893a 100755
--- a/bin/gstack-doctor
+++ b/bin/gstack-doctor
@@ -267,7 +267,7 @@ check_codex_probe() {
 }
 
 check_sync() {
-  local mode file json status held drainable drain push queue=0 now f detail fix="$BIN/gstack-brain-sync --status"
+  local mode file json status held drainable drain push queue=0 now f detail case_names fix="$BIN/gstack-brain-sync --status"
   mode=$("$BIN/gstack-config" get artifacts_sync_mode 2>/dev/null || echo off)
   if [ ! -d "$STATE_ROOT/.git" ]; then
     row "not configured" "artifacts sync" "off; cross-machine artifacts sync is unavailable" "$BIN/gstack-artifacts-init"
@@ -289,8 +289,9 @@ check_sync() {
   fi
   json=$(head -c 16384 "$file" 2>/dev/null | tr '\n\r' '  ')
   status=$(json_field status "$json")
-  case "$status" in ok|idle|held|blocked|push_failed|error) ;; *) status=unknown ;; esac
+  case "$status" in ok|idle|held|collision|blocked|push_failed|error) ;; *) status=unknown ;; esac
   held=$(json_field held_count "$json" | tr -cd '0-9')
+  case_names=$(json_field case_names "$json" | tr -cd 'A-Za-z0-9._?,-' | head -c 300)
   drainable=$(json_field drainable "$json" | tr -cd '0-9')
   drain=$(epoch_of "$(json_field last_drain_at "$json")")
   push=$(epoch_of "$(json_field last_push_at "$json")")
@@ -301,6 +302,10 @@ check_sync() {
     blocked) row warn "artifacts sync" "status blocked (a re-scan still flags staged content)" "$fix"; return ;;
     push_failed|error|unknown) row warn "artifacts sync" "status $status" "$fix"; return ;;
   esac
+  if [ -n "$case_names" ]; then
+    row warn "artifacts sync" "status $status; artifact paths differ only by letter case ($case_names), see docs/troubleshooting.md#artifact-case-collision" "$fix"
+    return
+  fi
   if [ "${drainable:-0}" -gt 0 ] && { [ -z "$push" ] || [ $((now - push)) -gt 86400 ]; }; then
     row warn "artifacts sync" "status $status; no push for 24h while $drainable record(s) wait" "$fix"
   elif [ "$queue" -gt 0 ] && [ -n "$drain" ] && [ $((now - drain)) -gt 86400 ]; then
diff --git a/docs/gbrain-sync-errors.md b/docs/gbrain-sync-errors.md
index 8c4152326..1d6f47990 100644
--- a/docs/gbrain-sync-errors.md
+++ b/docs/gbrain-sync-errors.md
@@ -44,6 +44,7 @@ cause, the held files and the fix.
 | `error` | The drain stopped before committing and kept the queue | `status=error. The last sync stopped before committing; the queue is kept and retried. See the cause: <bin> --status` | The cause `--status` names: a git index lock, a commit hook, a `git add` failure |
 | `unknown` | The status file holds a status code outside this table | `status=unknown. The sync status file is unreadable. See it: <bin> --status` | Run `<bin> --once` to rewrite it |
 | stale-push | No push for 24 hours while the drain's `drainable` count is above zero | `stale-push. Files are ready to sync but nothing has been pushed for over 24 hours. See why: <bin> --status` | The waiting reason `--status` names (a coupled group waiting for a consistent generation, a `git add` failure) |
+| case-collision | `case_names` is set: artifact paths differ only by letter case, so the drain holds queued files under them | `case-collision held=<N> names=<names>. Artifact paths differ only by letter case, which a Mac or Windows disk stores as one file; sync holds <N> queued file(s) under them and syncs everything else. Repair: https://github.com/garrytan/gstack/blob/main/docs/troubleshooting.md#artifact-case-collision. See the paths: <bin> --status` | Keep one spelling: the manual repair in [troubleshooting](troubleshooting.md#artifact-case-collision) |
 | stale-drain | `last_drain_at` older than 24 hours while the queue holds records | `stale-drain. The sync has not run for over 24 hours while files wait in the queue. See why: <bin> --status` | If no gstack process is running, remove `~/.gstack/.brain-sync.lock.d`, then run `<bin> --once` |
 
 `<bin>` is the absolute path, for example
diff --git a/docs/gbrain-sync.md b/docs/gbrain-sync.md
index 3e4348784..d53b57b7b 100644
--- a/docs/gbrain-sync.md
+++ b/docs/gbrain-sync.md
@@ -90,8 +90,11 @@ On machine B:
 gstack-brain-sync --status
 ```
 
-Shows: the status (`ok`, `idle`, `held`, `blocked`, `push_failed`,
-`error`), files held back by the secret scan with their fixes, the
+Shows: the status (`ok`, `idle`, `held`, `collision`, `blocked`,
+`push_failed`, `error`), files held back by the secret scan with their
+fixes, files held because their path differs only by letter case from
+another path (`case_held`, `case_names`; see
+[troubleshooting](troubleshooting.md#artifact-case-collision)), the
 `drainable` count, the last sync and push times, pending queue depth, and
 the current privacy mode.
 
diff --git a/docs/troubleshooting.md b/docs/troubleshooting.md
index 380a19a2c..589de0dbb 100644
--- a/docs/troubleshooting.md
+++ b/docs/troubleshooting.md
@@ -630,6 +630,64 @@ has nothing to calibrate from. It no longer says "calibrated".
 
 **Fix.** Answer more registered questions, then `gstack-developer-profile --derive`.
 
+<a id="artifact-case-collision"></a>
+### `ARTIFACTS_SYNC: attention: case-collision held=<N> names=<names>. Artifact paths differ only by letter case, ...`
+
+**Meaning.** The artifacts repo holds paths that differ only by letter case,
+such as `projects/MyApp/` and `projects/myapp/`. A Mac or Windows disk stores
+them as one file, so git there would stage a write under the other spelling,
+and sync would report success while one spelling stays modified and the other
+machines read stale data (#3220). Sync now holds every queued file under such a
+path instead of staging it: the file stays queued, everything else keeps
+syncing, and the status is `collision` rather than `ok`. When only one
+spelling is tracked, that spelling keeps syncing and only the new alias is
+held. `<names>` are the colliding folder or file names; `gstack-brain-sync
+--status` lists the held paths in `message`. A local commit that would push a
+new alias to the remote (two machines created both spellings at once) is
+refused with status `error` and kept locally.
+
+Nothing is repaired automatically. Slug resolution and skill start are
+unaffected; only the sync holds files.
+
+**Fix.** Keep one spelling, by hand, on one machine:
+
+1. Choose the spelling to keep: the one `gstack-slug` prints in that project
+   today. If the project resolves to different spellings (a remote URL typed
+   two ways, a folder reached as `~/code/MyApp` and `~/code/myapp`), make it
+   stable first: fix the remote URL's case, or set `GSTACK_PROJECT_SLUG`.
+2. Fold the other spelling's tracked files into the kept one. Read both
+   spellings from git, because a Mac's working tree shows only one of them,
+   and the working files too, because they hold the held, unsynced writes.
+   For JSONL logs, concatenate and drop duplicate lines:
+
+   ```bash
+   cd ~/.gstack
+   keep=projects/MyApp; drop=projects/myapp
+   git -c core.ignorecase=false ls-files -- "$drop" | while IFS= read -r f; do
+     rel=${f#"$drop"/}
+     { git show ":$keep/$rel"; git show ":$f"; cat "$keep/$rel" "$f"; } 2>/dev/null | awk '!seen[$0]++' > "$HOME/.gstack-fold.tmp"
+     mkdir -p "$keep/$(dirname "$rel")" && mv "$HOME/.gstack-fold.tmp" "$keep/$rel"
+     git -c core.ignorecase=false add -f -- "$keep/$rel"
+     git -c core.ignorecase=false rm -q --cached -- "$f"
+   done
+   git -c core.ignorecase=false commit -m "repair: fold $drop into $keep"
+   git push origin HEAD
+   ```
+
+   For Markdown and other non-log files that exist under both spellings,
+   compare `git show ":$keep/<file>"` with `git show ":$drop/<file>"` and keep
+   the right content instead of concatenating. On Linux the two folders are
+   separate on disk: remove the dropped folder after the commit
+   (`rm -r "$drop"`). On a Mac or Windows they are the same folder, so do not
+   remove anything.
+3. Clear the held records of the dropped spelling and queue the kept files:
+   `gstack-brain-sync --skip-file <dropped path>` for each held path that
+   `--status` lists, then `gstack-brain-enqueue <kept path>` and
+   `gstack-brain-sync --once`.
+4. On every other machine, let the next skill start pull the repair (or run
+   `git -C ~/.gstack pull`). The line disappears once no tracked path
+   differs only by case.
+
 <a id="skill-start-former-bucket"></a>
 ### `gstack: earlier data for this project is in projects/<slug> (a worktree's old bucket). Merge it: ...`
 
diff --git a/test/brain-sync.test.ts b/test/brain-sync.test.ts
index c771ca352..cb0aad222 100644
--- a/test/brain-sync.test.ts
+++ b/test/brain-sync.test.ts
@@ -678,7 +678,7 @@ describe('gstack-brain-sync per-file hold', () => {
     ['projects/p/ceo-plans/Foo/old.md', 'projects/p/ceo-plans/foo/new.md'],
     ['projects/Straße/ceo-plans/old.md', 'projects/STRASSE/ceo-plans/new.md'],
   ]) {
-    test(`new casefold prefix collision is refused: ${oldPath}`, () => {
+    test(`a new casefold prefix alias is held, never staged: ${oldPath}`, () => {
       init();
       // Create the historical tree directly so this works on either a case-
       // sensitive or insensitive host, without materializing conflicting dirs.
@@ -687,11 +687,13 @@ describe('gstack-brain-sync per-file hold', () => {
       write(newPath, '# queued plan\n');
       const before = git(['rev-parse', 'HEAD']).stdout;
       expect(drain().status).toBe(0);
-      expect(statusJson().status).toBe('error');
+      const s = statusJson();
+      expect(s.status).toBe('collision');
+      expect(s.case_held).toBe(1);
       expect(spoolText()).toContain(newPath);
       expect(git(['rev-parse', 'HEAD']).stdout).toBe(before);
       expect(remoteFiles()).not.toContain(newPath);
-      expect(staged()).toContain(newPath);
+      expect(staged()).toBe('');
     });
   }
 
@@ -701,10 +703,72 @@ describe('gstack-brain-sync per-file hold', () => {
     write(CLEAN, '# independent plan\n');
     expect(drain().status).toBe(0);
     expect(statusJson().status).toBe('ok');
+    expect(statusJson().case_held).toBe(0);
+    expect(statusJson().case_names).toBe('OLD,old');
     expect(remoteFiles()).toContain(CLEAN);
     expect(spoolFiles()).toHaveLength(0);
   });
 
+  // #3220: two spellings of one project are both tracked (a case-sensitive
+  // producer committed both). A write to either is held instead of staged, so
+  // a case-insensitive checkout cannot record it under the other spelling and
+  // report ok; every other file still syncs.
+  const WS_UPPER = 'projects/Example_Workspace/learnings.jsonl';
+  const WS_LOWER = 'projects/Example_workspace/learnings.jsonl';
+  test('#3220: a write into a tracked case-colliding project is held; the rest syncs and nothing reports success', () => {
+    init();
+    seedIndexedPaths([WS_UPPER, WS_LOWER]);
+    write(WS_LOWER, '{"key":"c","insight":"new on mac"}\n');
+    write(CLEAN, '# independent plan\n');
+    const result = drain();
+    expect(result.status).toBe(0);
+    expect(result.stderr).toContain('differs only by letter case (Example_Workspace,Example_workspace)');
+    expect(remoteFiles()).toContain(CLEAN);
+    expect(remoteShow(WS_LOWER).stdout).toBe('# historical artifact\n');
+    expect(staged()).toBe('');
+    expect(spoolText()).toContain(WS_LOWER);
+    expect(spoolText()).not.toContain(CLEAN);
+    const s = statusJson();
+    expect(s.status).toBe('collision');
+    expect(s.case_held).toBe(1);
+    expect(s.case_names).toBe('Example_Workspace,Example_workspace');
+    expect(s.message).toContain(WS_LOWER);
+    expect(s.message).toContain('docs/troubleshooting.md#artifact-case-collision');
+    expect(run(['gstack-brain-sync', '--attention']).stdout.trim()).toBe(
+      `ARTIFACTS_SYNC: attention: case-collision held=1 names=Example_Workspace,Example_workspace. Artifact paths differ only by letter case, which a Mac or Windows disk stores as one file; sync holds 1 queued file(s) under them and syncs everything else. Repair: https://github.com/garrytan/gstack/blob/main/docs/troubleshooting.md#artifact-case-collision. See the paths: ${SELF} --status`,
+    );
+    // Held, not dropped: the next drain holds it again and still never says ok.
+    expect(drain().status).toBe(0);
+    expect(statusJson().status).toBe('collision');
+    expect(spoolText()).toContain(WS_LOWER);
+  });
+
+  test('a tracked spelling keeps syncing while only the new case alias is held', () => {
+    init();
+    seedIndexedPaths(['projects/MyApp/ceo-plans/a.md']);
+    write('projects/MyApp/ceo-plans/a.md', '# updated\n');
+    write('projects/myapp/ceo-plans/b.md', '# other project\n');
+    expect(drain().status).toBe(0);
+    expect(remoteShow('projects/MyApp/ceo-plans/a.md').stdout).toBe('# updated\n');
+    expect(remoteFiles()).not.toContain('projects/myapp/ceo-plans/b.md');
+    const s = statusJson();
+    expect(s.status).toBe('collision');
+    expect(s.case_held).toBe(1);
+    expect(s.case_names).toBe('MyApp,myapp');
+    expect(spoolText()).toContain('projects/myapp/ceo-plans/b.md');
+  });
+
+  test('without a case collision the status carries no case fields and skill start no case line', () => {
+    init();
+    write(CLEAN, '# a plan\n');
+    expect(drain().status).toBe(0);
+    const s = statusJson();
+    expect(s.status).toBe('ok');
+    expect(s.case_held).toBe(0);
+    expect(s.case_names).toBe('');
+    expect(run(['gstack-brain-sync', '--attention']).stdout).toBe('');
+  });
+
   test('a retry merge introducing casefold aliases stays local and keeps the queue', () => {
     init();
     const peer = fs.mkdtempSync(path.join(tmpHome, 'peer-'));
@@ -719,6 +783,7 @@ describe('gstack-brain-sync per-file hold', () => {
     write(CLEAN, '# local plan\n');
     expect(drain().status).toBe(0);
     expect(statusJson().status).toBe('error');
+    expect(statusJson().case_names).toBe('P,p');
     expect(spoolText()).toContain(CLEAN);
     expect(remoteFiles()).toContain(peerPath);
     expect(remoteFiles()).not.toContain(CLEAN);
diff --git a/test/gstack-doctor.test.ts b/test/gstack-doctor.test.ts
index f8852f7c4..cc114b498 100644
--- a/test/gstack-doctor.test.ts
+++ b/test/gstack-doctor.test.ts
@@ -263,6 +263,20 @@ describe('gstack-doctor', () => {
     expect(doctor(f).row('artifacts sync').state).toBe('ok');
   });
 
+  test('artifacts sync: case-colliding paths warn with the clamped names, held or not', () => {
+    const f = makeFixture();
+    syncOn(f, { status: 'collision', message: 'held projects/myapp/x.md', case_held: 1, case_names: 'MyApp,myapp', drainable: 0, last_drain_at: iso(60), last_push_at: iso(60) });
+    const held = doctor(f);
+    expect(held.row('artifacts sync').state).toBe('warn');
+    expect(held.row('artifacts sync').detail).toBe('status collision; artifact paths differ only by letter case (MyApp,myapp), see docs/troubleshooting.md#artifact-case-collision');
+    expect(held.out).not.toContain('projects/myapp');
+
+    syncOn(f, { status: 'ok', message: 'pushed 1 file(s)', case_held: 0, case_names: 'OLD,old', drainable: 0, last_drain_at: iso(60), last_push_at: iso(60) });
+    expect(doctor(f).row('artifacts sync').state).toBe('warn');
+    syncOn(f, { status: 'ok', message: 'pushed 1 file(s)', case_held: 0, case_names: '', drainable: 0, last_drain_at: iso(60), last_push_at: iso(60) });
+    expect(doctor(f).row('artifacts sync').state).toBe('ok');
+  });
+
   test('artifacts sync: stale push and stale drain warn, a fresh push clears them', () => {
     const f = makeFixture();
     syncOn(f, { status: 'ok', message: 'x', drainable: 4, last_drain_at: iso(60), last_push_at: iso(90_000) });
diff --git a/test/gstack-skill-start.test.ts b/test/gstack-skill-start.test.ts
index 1bad23af9..4c87ba9aa 100644
--- a/test/gstack-skill-start.test.ts
+++ b/test/gstack-skill-start.test.ts
@@ -611,8 +611,9 @@ describe('ARTIFACTS_SYNC attention lines (sync status table)', () => {
     status: code, held_count: 0, drainable: 0, last_drain_at: NOW(), last_push_at: NOW(), ts: NOW(), held: [], message: 'm', ...extra,
   });
 
-  const FIXTURES: Record<string, { status: () => Record<string, unknown>; queued?: number; n?: number }> = {
+  const FIXTURES: Record<string, { status: () => Record<string, unknown>; queued?: number; n?: number; names?: string }> = {
     held: { status: () => status('held', { held_count: 2 }), n: 2 },
+    'case-collision': { status: () => status('collision', { case_held: 2, case_names: 'MyApp,myapp' }), n: 2, names: 'MyApp,myapp' },
     blocked: { status: () => status('blocked') },
     push_failed: { status: () => status('push_failed') },
     error: { status: () => status('error') },
@@ -640,7 +641,7 @@ describe('ARTIFACTS_SYNC attention lines (sync status table)', () => {
       expect(fx).toBeDefined();
       const gh = syncHome(fx.status(), fx.queued ?? 0);
       try {
-        const expected = row.line.replaceAll('<bin>', SYNC).replaceAll('<N>', String(fx.n ?? 0));
+        const expected = row.line.replaceAll('<bin>', SYNC).replaceAll('<N>', String(fx.n ?? 0)).replaceAll('<names>', fx.names ?? '');
         expect(attention(runStart([], { GSTACK_HOME: gh }))).toEqual([expected]);
       } finally {
         fs.rmSync(gh, { recursive: true, force: true });