diff --git a/.agents/skills/lib.sh b/.agents/skills/lib.sh
index acb8878..ca62ae1 100644
--- a/.agents/skills/lib.sh
+++ b/.agents/skills/lib.sh
@@ -450,13 +450,40 @@ except Exception:
             exit 1
           fi
           buf="$(echo "$2" | tr -cd 'A-Za-z0-9_.-')"
-          if [ -z "$buf" ]; then buf="tmp_buffer"; else buf="buf_$buf"; fi
+          if [ -z "$buf" ]; then
+            echo "Error: -b name contains no usable characters" >&2
+            exit 1
+          fi
+          buf="buf_$buf"
           shift 2
           ;;
         *) text="$1"; shift ;;
       esac
     done
-    echo -n "$text" > "$wrapper_dir/$buf"
+    # A-3 GC: unique buffer names mean an abandoned buffer (SIGINT/SIGTERM/
+    # timeout between set-buffer and delete-buffer) is never overwritten, so
+    # it would leak forever. Sweep here -- at creation time, the moment new
+    # garbage can appear -- rather than from a separate reaper.
+    #
+    # SAFETY: the age threshold must comfortably exceed the lifetime of a LIVE
+    # buffer, or the sweep would delete a buffer awaiting its paste and
+    # re-create the silent loss A-3 exists to remove. A live buffer lives from
+    # set-buffer to delete-buffer (sub-second in practice, bounded by the
+    # blocking `agent send`), so 60 minutes is ~4 orders of magnitude of slack.
+    # Failures are ignored: garbage collection must never break an injection.
+    #
+    # The '.buf_sks_*' arm is not redundant: F2's atomic write stages through
+    # a DOT-prefixed temp ('.buf_sks_....tmp'), which 'buf_sks_*' cannot match.
+    # Without it a temp orphaned by SIGKILL between write and rename would leak
+    # forever, exactly like the buffers this sweep exists to reclaim.
+    find "$wrapper_dir" \( -name 'buf_sks_*' -o -name '.buf_sks_*' \) \
+      -mmin +${MAM_BUFFER_GC_MINUTES:-60} -delete 2>/dev/null || true
+    _tmp="$wrapper_dir/.$buf.$$.tmp"
+    if ! { echo -n "$text" > "$_tmp" && mv -f "$_tmp" "$wrapper_dir/$buf"; }; then
+      rm -f "$_tmp"
+      echo "Error: failed to write buffer $buf" >&2
+      exit 1
+    fi
     ;;
   paste-buffer)
     buf="tmp_buffer"
@@ -465,7 +492,11 @@ except Exception:
       case "$1" in
         -b)
           buf="$(echo "$2" | tr -cd 'A-Za-z0-9_.-')"
-          if [ -z "$buf" ]; then buf="tmp_buffer"; else buf="buf_$buf"; fi
+          if [ -z "$buf" ]; then
+            echo "Error: -b name contains no usable characters" >&2
+            exit 1
+          fi
+          buf="buf_$buf"
           shift 2
           ;;
         -t)
@@ -492,7 +523,11 @@ except Exception:
       case "$1" in
         -b)
           buf="$(echo "$2" | tr -cd 'A-Za-z0-9_.-')"
-          if [ -z "$buf" ]; then buf="tmp_buffer"; else buf="buf_$buf"; fi
+          if [ -z "$buf" ]; then
+            echo "Error: -b name contains no usable characters" >&2
+            exit 1
+          fi
+          buf="buf_$buf"
           shift 2
           ;;
         *) shift ;;
@@ -1706,9 +1741,10 @@ send_keys_safe() {
     sleep 2
   done
 
-  _sks_herdr set-buffer -b "sks_$job_id" "$text"
-  _sks_herdr paste-buffer -b "sks_$job_id" -t "$sess"
-  _sks_herdr delete-buffer -b "sks_$job_id" 2>/dev/null || true
+  local sks_buf="sks_${sess}_${job_id}_$$_${RANDOM}_$(date +%s%N 2>/dev/null || date +%s)"
+  _sks_herdr set-buffer -b "$sks_buf" "$text"
+  _sks_herdr paste-buffer -b "$sks_buf" -t "$sess"
+  _sks_herdr delete-buffer -b "$sks_buf" 2>/dev/null || true
   if [[ "$sess" =~ "agy" ]]; then
     _sks_herdr send-keys -t "$sess" C-m
     return 0
diff --git a/IMPROVEMENTS.md b/IMPROVEMENTS.md
index bf0ff05..f4d60d2 100644
--- a/IMPROVEMENTS.md
+++ b/IMPROVEMENTS.md
@@ -1,9 +1,9 @@
 # 🛠️ Multi-Agent Mux 종합 개선 및 미해결 과제 백로그 (`IMPROVEMENTS.md`)
 
-- **최종 갱신일**: 2026-08-06 (B-3 herdr real binary preflight verification 완료 반영)
+- **최종 갱신일**: 2026-08-06 (A-3 call-unique buffer isolation & automatic GC 완료 반영)
 - **통합 관리 대상**: 기존 `CODEBASE_REVIEW_REPORT.md` + `OPTIMIZATION.md`
-- **총 추적 미해결 과제**: **15건** (아키텍처 2건, 엣지케이스 7건, 오케스트레이션 2건, 레거시 잔재 4건)
-- **완료된 과제**: **6건** (A-1, A-5, B-1, B-3, C-1, O-1)
+- **총 추적 미해결 과제**: **14건** (아키텍처 1건, 엣지케이스 7건, 오케스트레이션 2건, 레거시 잔재 4건)
+- **완료된 과제**: **7건** (A-1, A-3, A-5, B-1, B-3, C-1, O-1)
 
 ---
 
@@ -13,16 +13,12 @@
 
 ---
 
-## 1. 🔴 아키텍처 결함 (Architecture Flaws — 2건)
+## 1. 🔴 아키텍처 결함 (Architecture Flaws — 1건)
 
 ### **A-2: 공개 브로커 + HMAC 인증 Off + 와일드카드 전파**
 - **현상**: `mqtt_common.py`의 기본 브로커가 공개 서버(`broker.hivemq.com`), HMAC 무조건 True 반환으로 설정되어 있습니다.
 - **파급 효과**: 외부에서 유입되는 malicious `error` 이벤트 수신 시 `reconcile.sh`가 라이브 에이전트 pane을 `kill-session`으로 강제 파괴하는 치명적 보안/안정성 위험이 존재합니다.
 
-### **A-3: 시프트 버퍼 단일 파일 공유 및 동시 주입 오염**
-- **현상**: `send_keys_safe` 시프트의 `set/paste/delete-buffer`가 `-b` 세션 버퍼 이름을 무시하고 단일 `.mam/shim/tmp_buffer` 파일 하나만을 공유합니다.
-- **파급 효과**: 다중 에이전트 동시 주입 시 대화 텍스트 교차 오염 및 무음 유실(t5 silent loss)이 발생합니다.
-
 ---
 
 ## 2. 🟠 엣지 케이스 및 런타임 버그 (Edge-case Bugs — 7건)
@@ -84,12 +80,18 @@
 
 ---
 
-## 5. 🎉 완료된 과제 (Completed Tasks — 6건)
+## 5. 🎉 완료된 과제 (Completed Tasks — 7건)
 
 ### **A-1: 워크스페이스 세션 격리 & drift-B 오등록 방지** — ✅ 완료
 - `derive_workspace_slug` 헬퍼 함수를 추가하여 워크스페이스 경로 기반 단일 소켓 슬러그(`mam-<parent>-<work>`) 도출 체계를 구축했습니다.
 - `reconcile.sh` drift-B 자동 등록 시 foreign cwd 차단 게이트를 구축하여 세션 오등록을 방지했습니다.
 
+### **A-3: 시프트 버퍼 동시 주입 오염 & 자동 GC 체계 구축** — ✅ 완료
+- `lib.sh` 내 `send_keys_safe` 시프트 버퍼 명령의 임시 파일명을 `sks_${sess}_${job_id}_$$_${RANDOM}_$(date +%s%N)` 식 호출 단위 독립 토큰으로 변환하여 다중 에이전트 동시 주입 시 대화 교차 오염 및 무음 유실(T5)을 원천 차단했습니다.
+- `set-buffer` 시 원자적 임시 쓰기(`.$buf.$$.tmp`) 및 rename(`mv -f`) 구조를 구현하여 파티셜 레코드 관측을 방지했습니다.
+- 인터럽트/예외 종료 시 남는 stale 버퍼 파일을 자동으로 정리하는 60분 내장 GC(`find -mmin +60 -delete`)를 내장했습니다.
+- 전용 회귀 테스트 스위트 `tests/test_a3_buffer_isolation.py` (12/12 PASS)를 작성하여 입증했습니다.
+
 ### **A-5: `HERDR_SESSION_NAME` 네이티브 전환** — ✅ 완료
 - 기존 `HERDR_SERVER_NAME` 환경변수를 herdr 시프트가 직접 읽는 네이티브 **`HERDR_SESSION_NAME`** 및 YAML 레지스트리 키 **`herdr_session`**으로 전수 전환 단일화했습니다.
 
diff --git a/tests/test_a3_buffer_isolation.py b/tests/test_a3_buffer_isolation.py
new file mode 100644
index 0000000..48a9d8e
--- /dev/null
+++ b/tests/test_a3_buffer_isolation.py
@@ -0,0 +1,276 @@
+"""A-3 — shift buffer isolation & garbage collection.
+
+Defects addressed:
+  ① Colliding buffer names (e.g. constant "sks_adhoc" or "sks_onboard") caused
+     cross-agent prompt contamination and silent buffer deletion loss.
+  ② Call-unique buffer names (sks_<job>_<pid>_<rand>) fixed ① but introduced a
+     permanent leak of abandoned buffers (SIGINT/SIGTERM/timeout between
+     set-buffer and delete-buffer). Fixed with automatic GC (-mmin +60).
+  ③ F2's atomic write created DOT-prefixed temp files ('.buf_sks_....tmp') that
+     F4's initial GC pattern ('buf_sks_*') missed. Fixed by widening the GC
+     pattern to include '.buf_sks_*' and cleaning up on write/rename failure.
+"""
+import os
+import subprocess
+import time
+
+import pytest
+
+BARE_PATH = "/usr/bin:/bin:/usr/sbin:/sbin"
+
+
+def _bash(sandbox, snippet, extra_env=None):
+    env = dict(os.environ)
+    env["PATH"] = BARE_PATH
+    env["WORKSPACE_ROOT"] = str(sandbox)
+    if extra_env:
+        env.update(extra_env)
+    script = f'source "{sandbox}/.agents/skills/lib.sh" >/dev/null 2>&1\n{snippet}'
+    return subprocess.run(["bash", "-c", script], capture_output=True,
+                          text=True, cwd=str(sandbox), env=env)
+
+
+# Harness to invoke send_keys_safe without a running herdr daemon or tmux
+def _run_send_keys_safe(sandbox, sess, text, job_id=None):
+    buflog = sandbox / "buflog.txt"
+    job_arg = f' "{job_id}"' if job_id is not None else ""
+    snippet = f"""
+_pane_quiescent()   {{ return 0; }}
+_pane_dialog_open() {{ return 1; }}
+_pane_capture()     {{ printf '%s' "$SKS_FAKE_PANE"; }}
+_sks_herdr() {{
+  case "$1" in
+    set-buffer|paste-buffer|delete-buffer)
+      [ "$2" = "-b" ] && printf '%s %s\\n' "$1" "$3" >> "{buflog}" ;;
+    *)
+      return 0 ;;
+  esac
+  return 0
+}}
+export SKS_FAKE_PANE="● $text"
+send_keys_safe "{sess}" "{text}"{job_arg}
+"""
+    res = _bash(sandbox, snippet)
+    log_lines = buflog.read_text().splitlines() if buflog.exists() else []
+    return res, log_lines
+
+
+# X-1 — Two calls with the same job_id (e.g. "onboard") get DIFFERENT buffer names
+def test_a3_same_job_id_gets_different_buffer_names(mam_sandbox):
+    res1, lines1 = _run_send_keys_safe(mam_sandbox, "claude_sess1", "hello", "onboard")
+    res2, lines2 = _run_send_keys_safe(mam_sandbox, "claude_sess2", "world", "onboard")
+
+    assert res1.returncode == 0 and res2.returncode == 0
+    buf1 = lines1[0].split()[1]
+    buf2 = lines2[-3].split()[1]
+    assert buf1 != buf2, f"buffer names collided: {buf1} == {buf2}"
+
+
+# X-2 — Within one call, set-buffer, paste-buffer, and delete-buffer use the SAME name
+def test_a3_triple_uses_same_buffer_name(mam_sandbox):
+    res, lines = _run_send_keys_safe(mam_sandbox, "claude_sess1", "test payload", "job1")
+    assert res.returncode == 0
+    assert len(lines) == 3
+    cmd0, name0 = lines[0].split()
+    cmd1, name1 = lines[1].split()
+    cmd2, name2 = lines[2].split()
+
+    assert cmd0 == "set-buffer" and cmd1 == "paste-buffer" and cmd2 == "delete-buffer"
+    assert name0 == name1 == name2
+
+
+# X-3 — Different buffer names map to different files; delete does not touch another's file
+def test_a3_different_names_do_not_interfere(mam_sandbox):
+    res = _bash(mam_sandbox, """
+SHIM="$WORKSPACE_ROOT/.mam/shim/herdr"
+DIR="$WORKSPACE_ROOT/.mam/shim"
+"$SHIM" set-buffer -b "buf_A" "PAYLOAD_A"
+"$SHIM" set-buffer -b "buf_B" "PAYLOAD_B"
+
+assert_file() { [ -f "$DIR/$1" ] && echo "$1:EXISTS" || echo "$1:MISSING"; }
+assert_file "buf_buf_A"
+assert_file "buf_buf_B"
+
+"$SHIM" delete-buffer -b "buf_A"
+assert_file "buf_buf_A"
+assert_file "buf_buf_B"
+""")
+    assert "buf_buf_A:EXISTS" in res.stdout
+    assert "buf_buf_B:EXISTS" in res.stdout
+    assert "buf_buf_A:MISSING" in res.stdout
+    assert "buf_buf_B:EXISTS" in res.stdout
+
+
+# X-4 — Invalid/empty -b names are rejected explicitly, not falling back to tmp_buffer
+def test_a3_unusable_buffer_name_is_rejected(mam_sandbox):
+    res = _bash(mam_sandbox, """
+SHIM="$WORKSPACE_ROOT/.mam/shim/herdr"
+"$SHIM" set-buffer -b "!!!" "PAYLOAD" 2>&1
+echo "RC:$?"
+""")
+    assert "RC:1" in res.stdout
+    assert "contains no usable characters" in res.stdout + res.stderr
+
+
+# X-5 — Parallel execution test: 12 callers get isolated buffer text
+def test_a3_parallel_calls_are_isolated(mam_sandbox):
+    res = _bash(mam_sandbox, """
+SHIM="$WORKSPACE_ROOT/.mam/shim/herdr"
+DIR="$WORKSPACE_ROOT/.mam/shim"
+
+for i in $(seq 1 12); do
+  "$SHIM" set-buffer -b "par_$i" "DATA_$i" &
+done
+wait
+
+for i in $(seq 1 12); do
+  content=$(cat "$DIR/buf_par_$i" 2>/dev/null)
+  if [ "$content" != "DATA_$i" ]; then
+    echo "CORRUPTION in par_$i: got '$content'"
+  fi
+done
+echo "PARALLEL:OK"
+""")
+    assert "PARALLEL:OK" in res.stdout
+    assert "CORRUPTION" not in res.stdout
+
+
+# X-6 — Atomic write does not leave .tmp files behind on success
+def test_a3_atomic_write_leaves_no_tmp_on_success(mam_sandbox):
+    res = _bash(mam_sandbox, """
+SHIM="$WORKSPACE_ROOT/.mam/shim/herdr"
+DIR="$WORKSPACE_ROOT/.mam/shim"
+"$SHIM" set-buffer -b "atomic_test" "ATOMIC_PAYLOAD"
+ls -a "$DIR"
+""")
+    assert "buf_atomic_test" in res.stdout
+    assert not any(f.endswith(".tmp") for f in res.stdout.split())
+
+
+# X-7 — Abandoned buffers (older than GC threshold) are reclaimed
+def test_a3_abandoned_buffers_are_collected(mam_sandbox):
+    res = _bash(mam_sandbox, """
+DIR="$WORKSPACE_ROOT/.mam/shim"
+mkdir -p "$DIR"
+ABANDONED="$DIR/buf_sks_OLD_1_1"
+: > "$ABANDONED"
+touch -t 202501010000 "$ABANDONED"
+
+SHIM="$DIR/herdr"
+"$SHIM" set-buffer -b "trigger_gc" "NEW_DATA"
+
+if [ -f "$ABANDONED" ]; then
+  echo "GC:LEAK"
+else
+  echo "GC:SWEPT"
+fi
+""")
+    assert "GC:SWEPT" in res.stdout
+
+
+# X-8 — Live buffers (newer than GC threshold) are preserved
+def test_a3_live_buffers_are_preserved(mam_sandbox):
+    res = _bash(mam_sandbox, """
+DIR="$WORKSPACE_ROOT/.mam/shim"
+mkdir -p "$DIR"
+LIVE="$DIR/buf_sks_LIVE_2_2"
+echo "LIVE_CONTENT" > "$LIVE"
+
+SHIM="$DIR/herdr"
+"$SHIM" set-buffer -b "trigger_gc" "NEW_DATA"
+
+if [ -f "$LIVE" ]; then
+  echo "LIVE:PRESERVED"
+else
+  echo "LIVE:DELETED"
+fi
+""")
+    assert "LIVE:PRESERVED" in res.stdout
+
+
+# X-9 — Sweep is scoped to buf_sks_* and does not touch non-matching files
+def test_a3_sweep_is_scoped(mam_sandbox):
+    res = _bash(mam_sandbox, """
+DIR="$WORKSPACE_ROOT/.mam/shim"
+mkdir -p "$DIR"
+OTHER="$DIR/other_old_file"
+: > "$OTHER"
+touch -t 202501010000 "$OTHER"
+
+SHIM="$DIR/herdr"
+"$SHIM" set-buffer -b "trigger_gc" "NEW_DATA"
+
+if [ -f "$OTHER" ]; then
+  echo "OTHER:PRESERVED"
+else
+  echo "OTHER:DELETED"
+fi
+""")
+    assert "OTHER:PRESERVED" in res.stdout
+
+
+# X-10 — Abandoned atomic write DOT-temp files (.buf_sks_*.tmp) are collected
+def test_a3_abandoned_dot_temps_are_collected(mam_sandbox):
+    res = _bash(mam_sandbox, """
+DIR="$WORKSPACE_ROOT/.mam/shim"
+mkdir -p "$DIR"
+DOT_TEMP="$DIR/.buf_sks_ORPHAN_1_1.99.tmp"
+: > "$DOT_TEMP"
+touch -t 202501010000 "$DOT_TEMP"
+
+SHIM="$DIR/herdr"
+"$SHIM" set-buffer -b "trigger_gc" "NEW_DATA"
+
+if [ -f "$DOT_TEMP" ]; then
+  echo "DOT_TEMP:LEAK"
+else
+  echo "DOT_TEMP:SWEPT"
+fi
+""")
+    assert "DOT_TEMP:SWEPT" in res.stdout
+
+
+# X-11 — Failed rename cleans up temp file immediately and fails with exit code 1
+def test_a3_failed_rename_cleans_up_and_fails(mam_sandbox):
+    res = _bash(mam_sandbox, """
+DIR="$WORKSPACE_ROOT/.mam/shim"
+mkdir -p "$DIR"
+# Block creation of buf_sks_blocked by placing a read-only directory in its place
+BLOCK_DIR="$DIR/buf_sks_blocked"
+mkdir -p "$BLOCK_DIR"
+chmod 500 "$BLOCK_DIR"
+
+SHIM="$DIR/herdr"
+"$SHIM" set-buffer -b "sks_blocked" "PAYLOAD" 2>&1
+RC=$?
+echo "RC:$RC"
+
+# Clean up permissions so test sandbox can clean up
+chmod 700 "$BLOCK_DIR"
+
+# Check if any .tmp files were left behind
+ls -a "$DIR" | grep -q '\.tmp$' && echo "TEMP:LEAKED" || echo "TEMP:CLEAN"
+""")
+    assert "RC:1" in res.stdout
+    assert "TEMP:CLEAN" in res.stdout
+
+
+# X-12 — Scoped GC preserves unrelated dot-files
+def test_a3_unrelated_dot_files_preserved(mam_sandbox):
+    res = _bash(mam_sandbox, """
+DIR="$WORKSPACE_ROOT/.mam/shim"
+mkdir -p "$DIR"
+DOT_OTHER="$DIR/.some_other_dot_file"
+: > "$DOT_OTHER"
+touch -t 202501010000 "$DOT_OTHER"
+
+SHIM="$DIR/herdr"
+"$SHIM" set-buffer -b "trigger_gc" "NEW_DATA"
+
+if [ -f "$DOT_OTHER" ]; then
+  echo "DOT_OTHER:PRESERVED"
+else
+  echo "DOT_OTHER:DELETED"
+fi
+""")
+    assert "DOT_OTHER:PRESERVED" in res.stdout