diff --git a/modules/agent-box.nix b/modules/agent-box.nix
index a9ce478b..0ae6a6c9 100644
--- a/modules/agent-box.nix
+++ b/modules/agent-box.nix
@@ -107,8 +107,13 @@ let
       silences it, so the watch spawns for it however many sessions are running — the
       ceiling below is the one thing left that can refuse the batch. Name that ref in
       your own `--include` when you pick such a run up.
-      Its prompt tells it to `agent-box-session rm NAME` when done — clean
-      stale `hook-*` sessions the same way. That cleanup is load-bearing: at most 4
+      A hook session is spawned `--ephemeral`, so it delists ITSELF: whatever parks
+      it — the agent quitting, or `agent-box-session stop` — the supervisor drops the
+      entry on its next tick, and the transcript stays on disk. Its prompt still asks
+      it to `agent-box-session rm NAME` when done, which is the same end reached
+      sooner. What is NOT reaped is a hook session that CRASHED: a non-zero exit is
+      never parked, so it stays listed and attachable for you to read — `rm` it once
+      you have. That cleanup is load-bearing: at most 4
       `hook-*` sessions may RUN at once, and once that ceiling is reached EVERY
       watch on the box is inert — a matching batch is refused and dropped, never
       queued. A stopped session frees its slot even before it is delisted. So before you conclude a repo has been quiet, run `agent-box-webhook
@@ -2216,7 +2221,8 @@ kill_session() {
 usage() {
   echo "usage: agent-box-session ls"
   echo "       agent-box-session add [NAME] [--agent AGENT] [--profile PROFILE] [--cwd DIR]"
-  echo "                             [--prompt TEXT] [--resume-prompt TEXT] [-- EXTRA_ARGS...]"
+  echo "                             [--prompt TEXT] [--resume-prompt TEXT] [--ephemeral]"
+  echo "                             [-- EXTRA_ARGS...]"
   echo "       agent-box-session rm NAME"
   echo "       agent-box-session stop NAME"
   echo "       agent-box-session restart NAME | --all"
@@ -2232,6 +2238,10 @@ usage() {
   echo "--agent and a '-- EXTRA_ARGS' tail override what the profile resolved."
   echo "--prompt kicks the session off with a task (first spawn only); a later"
   echo "respawn resumes the prior transcript instead of redoing it."
+  echo "--ephemeral marks a ONE-SHOT session: parking it (a clean agent exit, or"
+  echo "'stop') delists it outright, because nobody is going to resume it. A"
+  echo "CRASH still parks nothing and leaves the post-mortem shell attachable."
+  echo "The transcript is kept either way; only the registry entry goes."
   echo "Listed sessions are (re)started by the per-user supervisor within ~2s."
   echo "stop parks a session (no respawn; an agent quitting cleanly does the"
   echo "same) until 'restart NAME' revives it; rm delists it for good."
@@ -2394,7 +2404,7 @@ case "$cmd" in
       *) name="$1"; shift; valid_new_name "$name" || { usage >&2; exit 2; } ;;
     esac
     agent="$DEFAULT_AGENT"; cwd=""; prompt=""; rprompt=""; has_prompt=0; has_rprompt=0
-    profile=""; has_agent=0
+    profile=""; has_agent=0; ephemeral=0
     while [ $# -gt 0 ]; do
       case "$1" in
         --agent) agent="''${2:?--agent needs a value}"; has_agent=1; shift 2 ;;
@@ -2402,6 +2412,7 @@ case "$cmd" in
         --cwd) cwd="''${2:?--cwd needs a value}"; shift 2 ;;
         --prompt) prompt="''${2?--prompt needs a value}"; has_prompt=1; shift 2 ;;
         --resume-prompt) rprompt="''${2?--resume-prompt needs a value}"; has_rprompt=1; shift 2 ;;
+        --ephemeral) ephemeral=1; shift ;;
         --) shift; break ;;
         *) echo "unknown option: $1" >&2; usage >&2; exit 2 ;;
       esac
@@ -2478,8 +2489,8 @@ case "$cmd" in
     registry_edit --arg n "$name" --arg a "$agent" --arg c "$cwd" \
       --arg p "$prompt" --arg pp "$has_prompt" \
       --arg rp "$rprompt" --arg rpp "$has_rprompt" --arg bid "$bid" \
-      --arg prof "$profile" \
-      '.sessions[$n] = {agent: $a, skipPermissions: true, remoteControl: true,
+      --arg prof "$profile" --arg eph "$ephemeral" \
+      '.sessions[$n] = ({agent: $a, skipPermissions: true, remoteControl: true,
                         remoteControlName: null,
                         workingDirectory: (if $c == "" then null else $c end),
                         extraArgs: $ARGS.positional,
@@ -2487,7 +2498,10 @@ case "$cmd" in
                         initialPrompt: (if $pp == "1" then $p else null end),
                         resumePrompt: (if $rpp == "1" then $rp else null end),
                         boxSessionId: (if $bid == "" then null else $bid end),
-                        hasRun: false}' \
+                        hasRun: false}
+                       # Added only when set, so every session that is NOT
+                       # one-shot keeps the entry shape it has always had.
+                       + (if $eph == "1" then {ephemeral: true} else {} end))' \
       --args -- "''${sargs[@]}"
     registry_unlock
     # The mascot (issue #185) marks the closest thing this CLI has to
@@ -2519,7 +2533,9 @@ case "$cmd" in
     # Park a listed session (issue #167): flag it stopped FIRST so the
     # supervisor's post-spawn re-check catches a spawn racing this kill,
     # then take the live session down. The entry (agent, cwd, transcript
-    # id) stays listed for a later 'restart' to revive.
+    # id) stays listed for a later 'restart' to revive — unless the session
+    # is one-shot (--ephemeral), for which parked means finished and the
+    # supervisor reaps the entry on its next tick.
     name="''${1:-}"
     valid_name "$name" || { usage >&2; exit 2; }
     registry_ensure
@@ -2532,10 +2548,19 @@ case "$cmd" in
       echo "no such session: '$name' (see agent-box-session ls)" >&2
       exit 2
     fi
+    # Read before the write, under the same lock the write takes: what is
+    # printed below must describe the entry this command actually parked.
+    eph=0
+    "$JQ" -e --arg n "$name" '.sessions[$n].ephemeral == true' \
+      "$REGISTRY_FILE" >/dev/null 2>&1 && eph=1
     registry_edit --arg n "$name" '.sessions[$n].stopped = true'
     registry_unlock
     kill_session "$name" || exit 1
-    echo "session '$name' stopped — still listed, not respawned; 'agent-box-session restart $name' revives it"
+    if [ "$eph" = 1 ]; then
+      echo "session '$name' stopped — one-shot (--ephemeral), so the supervisor delists it within ~2s; its transcript is kept"
+    else
+      echo "session '$name' stopped — still listed, not respawned; 'agent-box-session restart $name' revives it"
+    fi
     ;;
   restart)
     # Clearing the stopped flag makes restart double as the revive verb
@@ -4665,12 +4690,20 @@ preamble="$(render_preamble "$topic" "$note" "$assignment" "$name" "$seeded")"
 # over a profile's own model.
 pflag=()
 [ -n "$hook_profile" ] && pflag=(--profile "$hook_profile")
+# --ephemeral: a hook session exists to work ONE event batch, and nobody
+# resumes it afterwards. Without it a hook agent that finishes and quits
+# cleanly is PARKED (mark-stopped records stopped=true, src/supervisor.sh),
+# so it stays listed for good — the preamble asks the agent to
+# `agent-box-session rm` itself, but that is a request to a model, not a
+# guarantee, and `/quit` reaches the same clean exit without it. The
+# post-mortem branch is unaffected: a hook agent that CRASHES is not parked,
+# so it stays attachable for inspection exactly as before.
 if [ "''${#extra[@]}" -gt 0 ]; then
-  exec agent-box-session add "$name" "''${pflag[@]+"''${pflag[@]}"}" --prompt "$preamble
+  exec agent-box-session add "$name" "''${pflag[@]+"''${pflag[@]}"}" --ephemeral --prompt "$preamble
 
 $PROMPT" -- "''${extra[@]}"
 fi
-exec agent-box-session add "$name" "''${pflag[@]+"''${pflag[@]}"}" --prompt "$preamble
+exec agent-box-session add "$name" "''${pflag[@]+"''${pflag[@]}"}" --ephemeral --prompt "$preamble
 
 $PROMPT"
   '');
@@ -6218,11 +6251,68 @@ $PROMPT"
       done
     }
 
+    # Delist ONE-SHOT sessions that have been parked (--ephemeral, issue #167's
+    # other half).
+    #
+    # Parking is the right end for a NAMED session: `stopped` keeps its
+    # boxSessionId, which is the only record mapping the name to a conversation,
+    # so `agent-box-session restart` can resume it. A hook-* session has no such
+    # future — it is spawned for one event batch and nobody ever resumes it — so
+    # for those the same flag means "finished", and the entry is litter that
+    # outlives the work. The preamble does ask the agent to `agent-box-session rm`
+    # itself, but that is a request to a MODEL: `/quit` and Ctrl+D reach the clean
+    # exit without it, and a session parked by `agent-box-session stop` never had
+    # an agent to ask.
+    #
+    # Only the parked ones. A CRASH takes the post-mortem branch and is never
+    # flagged, so a hook session that died stays listed and attachable for
+    # inspection, exactly as before.
+    #
+    # The transcript is untouched — it lives under the harness's own state dir and
+    # outlives every registry entry. What goes is the name->conversation mapping,
+    # which is what makes the name reusable; sweep_session_state (above) then
+    # reclaims the launch id on this same tick, so the next holder of the name
+    # cannot inherit this one's transcript.
+    reap_ephemeral() {
+      _cand="$($JQ -r '.sessions | to_entries[]
+                       | select(.value.ephemeral == true and .value.stopped == true)
+                       | .key' "$REGISTRY_FILE" 2>/dev/null)" || return 0
+      [ -n "$_cand" ] || return 0
+      _gone=""
+      registry_lock
+      while IFS= read -r _n; do
+        case "$_n" in (*[!A-Za-z0-9_-]*|"") continue ;; esac
+        # A live pane still owns the name: mark-stopped runs as the epilogue of a
+        # pane that is still exiting. Delisting under it would strand that pane as
+        # an unmanaged tmux session the reconcile loop tolerates forever. Waiting
+        # a tick costs nothing.
+        $TMUX has-session -t "=$_n" 2>/dev/null && continue
+        # Re-read under the lock: `restart` clears the flag, and a session revived
+        # between the candidate scan and here must not be delisted by a decision
+        # taken before it came back.
+        $JQ -e --arg s "$_n" \
+          '.sessions[$s].ephemeral == true and .sessions[$s].stopped == true' \
+          "$REGISTRY_FILE" >/dev/null 2>&1 || continue
+        registry_edit --arg s "$_n" 'del(.sessions[$s])'
+        _gone=1
+      done < <(printf '%s\n' "$_cand")
+      registry_unlock
+      # Startup-only otherwise, on the reasoning that both delete paths prune
+      # their own filter file. This is the third delete path and it cannot, so it
+      # pays for the sweep itself — only on a tick that actually delisted
+      # something, which is rare.
+      if [ -n "$_gone" ]; then
+        sweep_orphan_filters
+      fi
+    }
+
     # Reconcile forever; systemd stop tears the whole tree down (ExecStop
     # kill-server + cgroup kill), Restart=always revives a crashed loop.
     # Sessions flagged stopped (a clean agent exit, or agent-box-session
-    # stop) stay listed but are left down until a restart clears the flag.
+    # stop) stay listed but are left down until a restart clears the flag —
+    # except the one-shot ones, which reap_ephemeral delists instead.
     while true; do
+      reap_ephemeral
       sweep_session_state
       while IFS= read -r sname; do
         case "$sname" in
diff --git a/modules/src/default-agents-webhook.md b/modules/src/default-agents-webhook.md
index d27d98f2..25cf266c 100644
--- a/modules/src/default-agents-webhook.md
+++ b/modules/src/default-agents-webhook.md
@@ -67,8 +67,13 @@ by no session, because a red trunk has to reach somebody. No live session
 silences it, so the watch spawns for it however many sessions are running — the
 ceiling below is the one thing left that can refuse the batch. Name that ref in
 your own `--include` when you pick such a run up.
-Its prompt tells it to `agent-box-session rm NAME` when done — clean
-stale `hook-*` sessions the same way. That cleanup is load-bearing: at most 4
+A hook session is spawned `--ephemeral`, so it delists ITSELF: whatever parks
+it — the agent quitting, or `agent-box-session stop` — the supervisor drops the
+entry on its next tick, and the transcript stays on disk. Its prompt still asks
+it to `agent-box-session rm NAME` when done, which is the same end reached
+sooner. What is NOT reaped is a hook session that CRASHED: a non-zero exit is
+never parked, so it stays listed and attachable for you to read — `rm` it once
+you have. That cleanup is load-bearing: at most 4
 `hook-*` sessions may RUN at once, and once that ceiling is reached EVERY
 watch on the box is inert — a matching batch is refused and dropped, never
 queued. A stopped session frees its slot even before it is delisted. So before you conclude a repo has been quiet, run `agent-box-webhook
diff --git a/modules/src/session-cli.sh b/modules/src/session-cli.sh
index 06080005..8c535e0b 100644
--- a/modules/src/session-cli.sh
+++ b/modules/src/session-cli.sh
@@ -40,7 +40,8 @@ kill_session() {
 usage() {
   echo "usage: agent-box-session ls"
   echo "       agent-box-session add [NAME] [--agent AGENT] [--profile PROFILE] [--cwd DIR]"
-  echo "                             [--prompt TEXT] [--resume-prompt TEXT] [-- EXTRA_ARGS...]"
+  echo "                             [--prompt TEXT] [--resume-prompt TEXT] [--ephemeral]"
+  echo "                             [-- EXTRA_ARGS...]"
   echo "       agent-box-session rm NAME"
   echo "       agent-box-session stop NAME"
   echo "       agent-box-session restart NAME | --all"
@@ -56,6 +57,10 @@ usage() {
   echo "--agent and a '-- EXTRA_ARGS' tail override what the profile resolved."
   echo "--prompt kicks the session off with a task (first spawn only); a later"
   echo "respawn resumes the prior transcript instead of redoing it."
+  echo "--ephemeral marks a ONE-SHOT session: parking it (a clean agent exit, or"
+  echo "'stop') delists it outright, because nobody is going to resume it. A"
+  echo "CRASH still parks nothing and leaves the post-mortem shell attachable."
+  echo "The transcript is kept either way; only the registry entry goes."
   echo "Listed sessions are (re)started by the per-user supervisor within ~2s."
   echo "stop parks a session (no respawn; an agent quitting cleanly does the"
   echo "same) until 'restart NAME' revives it; rm delists it for good."
@@ -218,7 +223,7 @@ case "$cmd" in
       *) name="$1"; shift; valid_new_name "$name" || { usage >&2; exit 2; } ;;
     esac
     agent="$DEFAULT_AGENT"; cwd=""; prompt=""; rprompt=""; has_prompt=0; has_rprompt=0
-    profile=""; has_agent=0
+    profile=""; has_agent=0; ephemeral=0
     while [ $# -gt 0 ]; do
       case "$1" in
         --agent) agent="${2:?--agent needs a value}"; has_agent=1; shift 2 ;;
@@ -226,6 +231,7 @@ case "$cmd" in
         --cwd) cwd="${2:?--cwd needs a value}"; shift 2 ;;
         --prompt) prompt="${2?--prompt needs a value}"; has_prompt=1; shift 2 ;;
         --resume-prompt) rprompt="${2?--resume-prompt needs a value}"; has_rprompt=1; shift 2 ;;
+        --ephemeral) ephemeral=1; shift ;;
         --) shift; break ;;
         *) echo "unknown option: $1" >&2; usage >&2; exit 2 ;;
       esac
@@ -302,8 +308,8 @@ case "$cmd" in
     registry_edit --arg n "$name" --arg a "$agent" --arg c "$cwd" \
       --arg p "$prompt" --arg pp "$has_prompt" \
       --arg rp "$rprompt" --arg rpp "$has_rprompt" --arg bid "$bid" \
-      --arg prof "$profile" \
-      '.sessions[$n] = {agent: $a, skipPermissions: true, remoteControl: true,
+      --arg prof "$profile" --arg eph "$ephemeral" \
+      '.sessions[$n] = ({agent: $a, skipPermissions: true, remoteControl: true,
                         remoteControlName: null,
                         workingDirectory: (if $c == "" then null else $c end),
                         extraArgs: $ARGS.positional,
@@ -311,7 +317,10 @@ case "$cmd" in
                         initialPrompt: (if $pp == "1" then $p else null end),
                         resumePrompt: (if $rpp == "1" then $rp else null end),
                         boxSessionId: (if $bid == "" then null else $bid end),
-                        hasRun: false}' \
+                        hasRun: false}
+                       # Added only when set, so every session that is NOT
+                       # one-shot keeps the entry shape it has always had.
+                       + (if $eph == "1" then {ephemeral: true} else {} end))' \
       --args -- "${sargs[@]}"
     registry_unlock
     # The mascot (issue #185) marks the closest thing this CLI has to
@@ -343,7 +352,9 @@ case "$cmd" in
     # Park a listed session (issue #167): flag it stopped FIRST so the
     # supervisor's post-spawn re-check catches a spawn racing this kill,
     # then take the live session down. The entry (agent, cwd, transcript
-    # id) stays listed for a later 'restart' to revive.
+    # id) stays listed for a later 'restart' to revive — unless the session
+    # is one-shot (--ephemeral), for which parked means finished and the
+    # supervisor reaps the entry on its next tick.
     name="${1:-}"
     valid_name "$name" || { usage >&2; exit 2; }
     registry_ensure
@@ -356,10 +367,19 @@ case "$cmd" in
       echo "no such session: '$name' (see agent-box-session ls)" >&2
       exit 2
     fi
+    # Read before the write, under the same lock the write takes: what is
+    # printed below must describe the entry this command actually parked.
+    eph=0
+    "$JQ" -e --arg n "$name" '.sessions[$n].ephemeral == true' \
+      "$REGISTRY_FILE" >/dev/null 2>&1 && eph=1
     registry_edit --arg n "$name" '.sessions[$n].stopped = true'
     registry_unlock
     kill_session "$name" || exit 1
-    echo "session '$name' stopped — still listed, not respawned; 'agent-box-session restart $name' revives it"
+    if [ "$eph" = 1 ]; then
+      echo "session '$name' stopped — one-shot (--ephemeral), so the supervisor delists it within ~2s; its transcript is kept"
+    else
+      echo "session '$name' stopped — still listed, not respawned; 'agent-box-session restart $name' revives it"
+    fi
     ;;
   restart)
     # Clearing the stopped flag makes restart double as the revive verb
diff --git a/modules/src/supervisor.sh b/modules/src/supervisor.sh
index abc313c6..5867df24 100644
--- a/modules/src/supervisor.sh
+++ b/modules/src/supervisor.sh
@@ -901,11 +901,68 @@ sweep_session_state() {
   done
 }
 
+# Delist ONE-SHOT sessions that have been parked (--ephemeral, issue #167's
+# other half).
+#
+# Parking is the right end for a NAMED session: `stopped` keeps its
+# boxSessionId, which is the only record mapping the name to a conversation,
+# so `agent-box-session restart` can resume it. A hook-* session has no such
+# future — it is spawned for one event batch and nobody ever resumes it — so
+# for those the same flag means "finished", and the entry is litter that
+# outlives the work. The preamble does ask the agent to `agent-box-session rm`
+# itself, but that is a request to a MODEL: `/quit` and Ctrl+D reach the clean
+# exit without it, and a session parked by `agent-box-session stop` never had
+# an agent to ask.
+#
+# Only the parked ones. A CRASH takes the post-mortem branch and is never
+# flagged, so a hook session that died stays listed and attachable for
+# inspection, exactly as before.
+#
+# The transcript is untouched — it lives under the harness's own state dir and
+# outlives every registry entry. What goes is the name->conversation mapping,
+# which is what makes the name reusable; sweep_session_state (above) then
+# reclaims the launch id on this same tick, so the next holder of the name
+# cannot inherit this one's transcript.
+reap_ephemeral() {
+  _cand="$($JQ -r '.sessions | to_entries[]
+                   | select(.value.ephemeral == true and .value.stopped == true)
+                   | .key' "$REGISTRY_FILE" 2>/dev/null)" || return 0
+  [ -n "$_cand" ] || return 0
+  _gone=""
+  registry_lock
+  while IFS= read -r _n; do
+    case "$_n" in (*[!A-Za-z0-9_-]*|"") continue ;; esac
+    # A live pane still owns the name: mark-stopped runs as the epilogue of a
+    # pane that is still exiting. Delisting under it would strand that pane as
+    # an unmanaged tmux session the reconcile loop tolerates forever. Waiting
+    # a tick costs nothing.
+    $TMUX has-session -t "=$_n" 2>/dev/null && continue
+    # Re-read under the lock: `restart` clears the flag, and a session revived
+    # between the candidate scan and here must not be delisted by a decision
+    # taken before it came back.
+    $JQ -e --arg s "$_n" \
+      '.sessions[$s].ephemeral == true and .sessions[$s].stopped == true' \
+      "$REGISTRY_FILE" >/dev/null 2>&1 || continue
+    registry_edit --arg s "$_n" 'del(.sessions[$s])'
+    _gone=1
+  done < <(printf '%s\n' "$_cand")
+  registry_unlock
+  # Startup-only otherwise, on the reasoning that both delete paths prune
+  # their own filter file. This is the third delete path and it cannot, so it
+  # pays for the sweep itself — only on a tick that actually delisted
+  # something, which is rare.
+  if [ -n "$_gone" ]; then
+    sweep_orphan_filters
+  fi
+}
+
 # Reconcile forever; systemd stop tears the whole tree down (ExecStop
 # kill-server + cgroup kill), Restart=always revives a crashed loop.
 # Sessions flagged stopped (a clean agent exit, or agent-box-session
-# stop) stay listed but are left down until a restart clears the flag.
+# stop) stay listed but are left down until a restart clears the flag —
+# except the one-shot ones, which reap_ephemeral delists instead.
 while true; do
+  reap_ephemeral
   sweep_session_state
   while IFS= read -r sname; do
     case "$sname" in
diff --git a/modules/src/webhook-spawn.sh b/modules/src/webhook-spawn.sh
index 29d75508..1bcb76b4 100644
--- a/modules/src/webhook-spawn.sh
+++ b/modules/src/webhook-spawn.sh
@@ -606,11 +606,19 @@ preamble="$(render_preamble "$topic" "$note" "$assignment" "$name" "$seeded")"
 # over a profile's own model.
 pflag=()
 [ -n "$hook_profile" ] && pflag=(--profile "$hook_profile")
+# --ephemeral: a hook session exists to work ONE event batch, and nobody
+# resumes it afterwards. Without it a hook agent that finishes and quits
+# cleanly is PARKED (mark-stopped records stopped=true, src/supervisor.sh),
+# so it stays listed for good — the preamble asks the agent to
+# `agent-box-session rm` itself, but that is a request to a model, not a
+# guarantee, and `/quit` reaches the same clean exit without it. The
+# post-mortem branch is unaffected: a hook agent that CRASHES is not parked,
+# so it stays attachable for inspection exactly as before.
 if [ "${#extra[@]}" -gt 0 ]; then
-  exec agent-box-session add "$name" "${pflag[@]+"${pflag[@]}"}" --prompt "$preamble
+  exec agent-box-session add "$name" "${pflag[@]+"${pflag[@]}"}" --ephemeral --prompt "$preamble
 
 $PROMPT" -- "${extra[@]}"
 fi
-exec agent-box-session add "$name" "${pflag[@]+"${pflag[@]}"}" --prompt "$preamble
+exec agent-box-session add "$name" "${pflag[@]+"${pflag[@]}"}" --ephemeral --prompt "$preamble
 
 $PROMPT"
diff --git a/tests/golden/vm/payloads/agent-box-session/bin/agent-box-session b/tests/golden/vm/payloads/agent-box-session/bin/agent-box-session
index 263ebf59..9dd25e53 100644
--- a/tests/golden/vm/payloads/agent-box-session/bin/agent-box-session
+++ b/tests/golden/vm/payloads/agent-box-session/bin/agent-box-session
@@ -272,7 +272,8 @@ kill_session() {
 usage() {
   echo "usage: agent-box-session ls"
   echo "       agent-box-session add [NAME] [--agent AGENT] [--profile PROFILE] [--cwd DIR]"
-  echo "                             [--prompt TEXT] [--resume-prompt TEXT] [-- EXTRA_ARGS...]"
+  echo "                             [--prompt TEXT] [--resume-prompt TEXT] [--ephemeral]"
+  echo "                             [-- EXTRA_ARGS...]"
   echo "       agent-box-session rm NAME"
   echo "       agent-box-session stop NAME"
   echo "       agent-box-session restart NAME | --all"
@@ -288,6 +289,10 @@ usage() {
   echo "--agent and a '-- EXTRA_ARGS' tail override what the profile resolved."
   echo "--prompt kicks the session off with a task (first spawn only); a later"
   echo "respawn resumes the prior transcript instead of redoing it."
+  echo "--ephemeral marks a ONE-SHOT session: parking it (a clean agent exit, or"
+  echo "'stop') delists it outright, because nobody is going to resume it. A"
+  echo "CRASH still parks nothing and leaves the post-mortem shell attachable."
+  echo "The transcript is kept either way; only the registry entry goes."
   echo "Listed sessions are (re)started by the per-user supervisor within ~2s."
   echo "stop parks a session (no respawn; an agent quitting cleanly does the"
   echo "same) until 'restart NAME' revives it; rm delists it for good."
@@ -450,7 +455,7 @@ case "$cmd" in
       *) name="$1"; shift; valid_new_name "$name" || { usage >&2; exit 2; } ;;
     esac
     agent="$DEFAULT_AGENT"; cwd=""; prompt=""; rprompt=""; has_prompt=0; has_rprompt=0
-    profile=""; has_agent=0
+    profile=""; has_agent=0; ephemeral=0
     while [ $# -gt 0 ]; do
       case "$1" in
         --agent) agent="${2:?--agent needs a value}"; has_agent=1; shift 2 ;;
@@ -458,6 +463,7 @@ case "$cmd" in
         --cwd) cwd="${2:?--cwd needs a value}"; shift 2 ;;
         --prompt) prompt="${2?--prompt needs a value}"; has_prompt=1; shift 2 ;;
         --resume-prompt) rprompt="${2?--resume-prompt needs a value}"; has_rprompt=1; shift 2 ;;
+        --ephemeral) ephemeral=1; shift ;;
         --) shift; break ;;
         *) echo "unknown option: $1" >&2; usage >&2; exit 2 ;;
       esac
@@ -534,8 +540,8 @@ case "$cmd" in
     registry_edit --arg n "$name" --arg a "$agent" --arg c "$cwd" \
       --arg p "$prompt" --arg pp "$has_prompt" \
       --arg rp "$rprompt" --arg rpp "$has_rprompt" --arg bid "$bid" \
-      --arg prof "$profile" \
-      '.sessions[$n] = {agent: $a, skipPermissions: true, remoteControl: true,
+      --arg prof "$profile" --arg eph "$ephemeral" \
+      '.sessions[$n] = ({agent: $a, skipPermissions: true, remoteControl: true,
                         remoteControlName: null,
                         workingDirectory: (if $c == "" then null else $c end),
                         extraArgs: $ARGS.positional,
@@ -543,7 +549,10 @@ case "$cmd" in
                         initialPrompt: (if $pp == "1" then $p else null end),
                         resumePrompt: (if $rpp == "1" then $rp else null end),
                         boxSessionId: (if $bid == "" then null else $bid end),
-                        hasRun: false}' \
+                        hasRun: false}
+                       # Added only when set, so every session that is NOT
+                       # one-shot keeps the entry shape it has always had.
+                       + (if $eph == "1" then {ephemeral: true} else {} end))' \
       --args -- "${sargs[@]}"
     registry_unlock
     # The mascot (issue #185) marks the closest thing this CLI has to
@@ -575,7 +584,9 @@ case "$cmd" in
     # Park a listed session (issue #167): flag it stopped FIRST so the
     # supervisor's post-spawn re-check catches a spawn racing this kill,
     # then take the live session down. The entry (agent, cwd, transcript
-    # id) stays listed for a later 'restart' to revive.
+    # id) stays listed for a later 'restart' to revive — unless the session
+    # is one-shot (--ephemeral), for which parked means finished and the
+    # supervisor reaps the entry on its next tick.
     name="${1:-}"
     valid_name "$name" || { usage >&2; exit 2; }
     registry_ensure
@@ -588,10 +599,19 @@ case "$cmd" in
       echo "no such session: '$name' (see agent-box-session ls)" >&2
       exit 2
     fi
+    # Read before the write, under the same lock the write takes: what is
+    # printed below must describe the entry this command actually parked.
+    eph=0
+    "$JQ" -e --arg n "$name" '.sessions[$n].ephemeral == true' \
+      "$REGISTRY_FILE" >/dev/null 2>&1 && eph=1
     registry_edit --arg n "$name" '.sessions[$n].stopped = true'
     registry_unlock
     kill_session "$name" || exit 1
-    echo "session '$name' stopped — still listed, not respawned; 'agent-box-session restart $name' revives it"
+    if [ "$eph" = 1 ]; then
+      echo "session '$name' stopped — one-shot (--ephemeral), so the supervisor delists it within ~2s; its transcript is kept"
+    else
+      echo "session '$name' stopped — still listed, not respawned; 'agent-box-session restart $name' revives it"
+    fi
     ;;
   restart)
     # Clearing the stopped flag makes restart double as the revive verb
diff --git a/tests/golden/vm/payloads/agent-box-supervisor/bin/agent-box-supervisor b/tests/golden/vm/payloads/agent-box-supervisor/bin/agent-box-supervisor
index 618d3b39..319bf037 100644
--- a/tests/golden/vm/payloads/agent-box-supervisor/bin/agent-box-supervisor
+++ b/tests/golden/vm/payloads/agent-box-supervisor/bin/agent-box-supervisor
@@ -1114,11 +1114,68 @@ sweep_session_state() {
   done
 }
 
+# Delist ONE-SHOT sessions that have been parked (--ephemeral, issue #167's
+# other half).
+#
+# Parking is the right end for a NAMED session: `stopped` keeps its
+# boxSessionId, which is the only record mapping the name to a conversation,
+# so `agent-box-session restart` can resume it. A hook-* session has no such
+# future — it is spawned for one event batch and nobody ever resumes it — so
+# for those the same flag means "finished", and the entry is litter that
+# outlives the work. The preamble does ask the agent to `agent-box-session rm`
+# itself, but that is a request to a MODEL: `/quit` and Ctrl+D reach the clean
+# exit without it, and a session parked by `agent-box-session stop` never had
+# an agent to ask.
+#
+# Only the parked ones. A CRASH takes the post-mortem branch and is never
+# flagged, so a hook session that died stays listed and attachable for
+# inspection, exactly as before.
+#
+# The transcript is untouched — it lives under the harness's own state dir and
+# outlives every registry entry. What goes is the name->conversation mapping,
+# which is what makes the name reusable; sweep_session_state (above) then
+# reclaims the launch id on this same tick, so the next holder of the name
+# cannot inherit this one's transcript.
+reap_ephemeral() {
+  _cand="$($JQ -r '.sessions | to_entries[]
+                   | select(.value.ephemeral == true and .value.stopped == true)
+                   | .key' "$REGISTRY_FILE" 2>/dev/null)" || return 0
+  [ -n "$_cand" ] || return 0
+  _gone=""
+  registry_lock
+  while IFS= read -r _n; do
+    case "$_n" in (*[!A-Za-z0-9_-]*|"") continue ;; esac
+    # A live pane still owns the name: mark-stopped runs as the epilogue of a
+    # pane that is still exiting. Delisting under it would strand that pane as
+    # an unmanaged tmux session the reconcile loop tolerates forever. Waiting
+    # a tick costs nothing.
+    $TMUX has-session -t "=$_n" 2>/dev/null && continue
+    # Re-read under the lock: `restart` clears the flag, and a session revived
+    # between the candidate scan and here must not be delisted by a decision
+    # taken before it came back.
+    $JQ -e --arg s "$_n" \
+      '.sessions[$s].ephemeral == true and .sessions[$s].stopped == true' \
+      "$REGISTRY_FILE" >/dev/null 2>&1 || continue
+    registry_edit --arg s "$_n" 'del(.sessions[$s])'
+    _gone=1
+  done < <(printf '%s\n' "$_cand")
+  registry_unlock
+  # Startup-only otherwise, on the reasoning that both delete paths prune
+  # their own filter file. This is the third delete path and it cannot, so it
+  # pays for the sweep itself — only on a tick that actually delisted
+  # something, which is rare.
+  if [ -n "$_gone" ]; then
+    sweep_orphan_filters
+  fi
+}
+
 # Reconcile forever; systemd stop tears the whole tree down (ExecStop
 # kill-server + cgroup kill), Restart=always revives a crashed loop.
 # Sessions flagged stopped (a clean agent exit, or agent-box-session
-# stop) stay listed but are left down until a restart clears the flag.
+# stop) stay listed but are left down until a restart clears the flag —
+# except the one-shot ones, which reap_ephemeral delists instead.
 while true; do
+  reap_ephemeral
   sweep_session_state
   while IFS= read -r sname; do
     case "$sname" in
diff --git a/tests/golden/web/etc/agent-box-guides/AGENTS.agent.md b/tests/golden/web/etc/agent-box-guides/AGENTS.agent.md
index fe95d2af..e5fd23ee 100644
--- a/tests/golden/web/etc/agent-box-guides/AGENTS.agent.md
+++ b/tests/golden/web/etc/agent-box-guides/AGENTS.agent.md
@@ -185,8 +185,13 @@ by no session, because a red trunk has to reach somebody. No live session
 silences it, so the watch spawns for it however many sessions are running — the
 ceiling below is the one thing left that can refuse the batch. Name that ref in
 your own `--include` when you pick such a run up.
-Its prompt tells it to `agent-box-session rm NAME` when done — clean
-stale `hook-*` sessions the same way. That cleanup is load-bearing: at most 4
+A hook session is spawned `--ephemeral`, so it delists ITSELF: whatever parks
+it — the agent quitting, or `agent-box-session stop` — the supervisor drops the
+entry on its next tick, and the transcript stays on disk. Its prompt still asks
+it to `agent-box-session rm NAME` when done, which is the same end reached
+sooner. What is NOT reaped is a hook session that CRASHED: a non-zero exit is
+never parked, so it stays listed and attachable for you to read — `rm` it once
+you have. That cleanup is load-bearing: at most 4
 `hook-*` sessions may RUN at once, and once that ceiling is reached EVERY
 watch on the box is inert — a matching batch is refused and dropped, never
 queued. A stopped session frees its slot even before it is delisted. So before you conclude a repo has been quiet, run `agent-box-webhook
diff --git a/tests/golden/web/payloads/agent-box-webhook-spawn/bin/agent-box-webhook-spawn b/tests/golden/web/payloads/agent-box-webhook-spawn/bin/agent-box-webhook-spawn
index f24e51c4..c1593803 100644
--- a/tests/golden/web/payloads/agent-box-webhook-spawn/bin/agent-box-webhook-spawn
+++ b/tests/golden/web/payloads/agent-box-webhook-spawn/bin/agent-box-webhook-spawn
@@ -833,12 +833,20 @@ preamble="$(render_preamble "$topic" "$note" "$assignment" "$name" "$seeded")"
 # over a profile's own model.
 pflag=()
 [ -n "$hook_profile" ] && pflag=(--profile "$hook_profile")
+# --ephemeral: a hook session exists to work ONE event batch, and nobody
+# resumes it afterwards. Without it a hook agent that finishes and quits
+# cleanly is PARKED (mark-stopped records stopped=true, src/supervisor.sh),
+# so it stays listed for good — the preamble asks the agent to
+# `agent-box-session rm` itself, but that is a request to a model, not a
+# guarantee, and `/quit` reaches the same clean exit without it. The
+# post-mortem branch is unaffected: a hook agent that CRASHES is not parked,
+# so it stays attachable for inspection exactly as before.
 if [ "${#extra[@]}" -gt 0 ]; then
-  exec agent-box-session add "$name" "${pflag[@]+"${pflag[@]}"}" --prompt "$preamble
+  exec agent-box-session add "$name" "${pflag[@]+"${pflag[@]}"}" --ephemeral --prompt "$preamble
 
 $PROMPT" -- "${extra[@]}"
 fi
-exec agent-box-session add "$name" "${pflag[@]+"${pflag[@]}"}" --prompt "$preamble
+exec agent-box-session add "$name" "${pflag[@]+"${pflag[@]}"}" --ephemeral --prompt "$preamble
 
 $PROMPT"
 
diff --git a/tests/native/expected/etc/agent-box-guides/AGENTS.agent.md b/tests/native/expected/etc/agent-box-guides/AGENTS.agent.md
index 6b323bd4..b60afea0 100644
--- a/tests/native/expected/etc/agent-box-guides/AGENTS.agent.md
+++ b/tests/native/expected/etc/agent-box-guides/AGENTS.agent.md
@@ -185,8 +185,13 @@ by no session, because a red trunk has to reach somebody. No live session
 silences it, so the watch spawns for it however many sessions are running — the
 ceiling below is the one thing left that can refuse the batch. Name that ref in
 your own `--include` when you pick such a run up.
-Its prompt tells it to `agent-box-session rm NAME` when done — clean
-stale `hook-*` sessions the same way. That cleanup is load-bearing: at most 4
+A hook session is spawned `--ephemeral`, so it delists ITSELF: whatever parks
+it — the agent quitting, or `agent-box-session stop` — the supervisor drops the
+entry on its next tick, and the transcript stays on disk. Its prompt still asks
+it to `agent-box-session rm NAME` when done, which is the same end reached
+sooner. What is NOT reaped is a hook session that CRASHED: a non-zero exit is
+never parked, so it stays listed and attachable for you to read — `rm` it once
+you have. That cleanup is load-bearing: at most 4
 `hook-*` sessions may RUN at once, and once that ceiling is reached EVERY
 watch on the box is inert — a matching batch is refused and dropped, never
 queued. A stopped session frees its slot even before it is delisted. So before you conclude a repo has been quiet, run `agent-box-webhook
diff --git a/tests/sessions.nix b/tests/sessions.nix
index cea61a24..8f8ccfdb 100644
--- a/tests/sessions.nix
+++ b/tests/sessions.nix
@@ -1178,6 +1178,56 @@ in
         assert re.search(r"^quitter\s+claude\s+stopped", ls_out, re.M), ls_out
         machine.succeed(as_agent("agent-box-session rm quitter"))
 
+    with subtest("a one-shot session is delisted, not parked"):
+        # A hook-* session's shape: spawned --ephemeral because nobody will
+        # ever resume it. Both ways of parking one must end in a DELIST, so
+        # the entry cannot outlive the work (before this, a hook agent that
+        # answered its event and quit stayed listed for good — the preamble
+        # asking it to `rm` itself is a request to a model, not a guarantee).
+        #
+        # First the clean-exit path, the one a finished hook agent takes.
+        # `claude --help` exits 0, exactly as /quit does minus the TUI. No
+        # assertion on the intermediate stopped=true: the reap is meant to
+        # follow it within one 2s tick, so only the END state is stable.
+        machine.succeed(as_agent("agent-box-session add gone --ephemeral -- --help"))
+        machine.wait_until_succeeds(
+            f"jq -e '.sessions | has(\"gone\") | not' {sfile}", timeout=120
+        )
+        settle()
+        machine.fail(tmux("has-session -t =gone"))
+        ls_out = machine.succeed(as_agent("agent-box-session ls"))
+        assert not re.search(r"^gone\s", ls_out, re.M), ls_out
+        # The supervisor's per-session state went with it (issue #282), so the
+        # next holder of this name cannot inherit its launch id, and with it
+        # its transcript.
+        machine.succeed(
+            as_agent("test ! -e ~/.local/state/agent-box/session/gone.json")
+        )
+
+        # Then the `stop` path, on a session that is up rather than exiting —
+        # which also pins the flag itself, racelessly.
+        machine.succeed(as_agent("agent-box-session add oneshot --ephemeral"))
+        machine.wait_until_succeeds(tmux("has-session -t =oneshot"), timeout=60)
+        machine.succeed(f"jq -e '.sessions.oneshot.ephemeral == true' {sfile}")
+        stop_out = machine.succeed(as_agent("agent-box-session stop oneshot"))
+        # The message must not promise a `restart` that will never come.
+        assert "one-shot" in stop_out, stop_out
+        machine.wait_until_succeeds(
+            f"jq -e '.sessions | has(\"oneshot\") | not' {sfile}", timeout=120
+        )
+
+    with subtest("a plain session is still parked, not reaped"):
+        # The reap must key on --ephemeral alone. A named session parked by
+        # the very same flag keeps its entry, because its boxSessionId is the
+        # only record mapping the name to a conversation `restart` can resume.
+        machine.succeed(as_agent("agent-box-session add keeper -- --help"))
+        machine.wait_until_succeeds(
+            f"jq -e '.sessions.keeper.stopped == true' {sfile}", timeout=120
+        )
+        settle()
+        machine.succeed(f"jq -e '.sessions | has(\"keeper\")' {sfile}")
+        machine.succeed(as_agent("agent-box-session rm keeper"))
+
     with subtest("stop parks a listed session; restart revives it"):
         machine.succeed(as_agent("agent-box-session stop main"))
         machine.succeed(f"jq -e '.sessions.main.stopped == true' {sfile}")
diff --git a/tests/webhook.nix b/tests/webhook.nix
index cdf0d2bc..819e80dd 100644
--- a/tests/webhook.nix
+++ b/tests/webhook.nix
@@ -2014,17 +2014,29 @@
         f" {tmux_bin} -L agent-box list-sessions -F '#S'"
         f" | grep -x {busy} >/dev/null"
     )
-    # The second one is stopped only once it has really started, so "finished
-    # entry with no pane" is the state under test and not a spawn still in
-    # flight.
-    cap_spawn("capdone", 2)
-    done = cap_session("capdone")
+    # The second one is a hook-* KEY with no pane — the divergence between the
+    # key count and what is actually running, which is the whole reason the
+    # fallback below exists. Stopped only once it has really started, so the
+    # state under test is "finished entry", not a spawn still in flight.
+    #
+    # Added DIRECTLY rather than through the spawn wrapper: a wrapper-spawned
+    # session is --ephemeral, so parking it delists it within a tick and there
+    # would be no lingering key left to count. The divergence itself is not
+    # gone with it — a hook-* entry outlives its pane whenever it is listed
+    # while the supervisor is down, caught between respawns, or predates
+    # --ephemeral — so the fallback still has to be conservative, and this is
+    # the deterministic way to put it in that state.
+    done = "hook-defangdevs-capdone-0000"
+    machine.succeed(
+        f"sudo -u agent env HOME=/home/agent agent-box-session add {done}"
+    )
     machine.wait_until_succeeds(f"{hook_ls} | grep -x {done} >/dev/null", timeout=60)
     machine.succeed(
         f"sudo -u agent env HOME=/home/agent agent-box-session stop {done}"
     )
     machine.succeed(
-        f"jq -e '.sessions[\"{done}\"].stopped == true'"
+        f"jq -e '.sessions[\"{done}\"]"
+        " | .stopped == true and (has(\"ephemeral\") | not)'"
         " /home/agent/.config/agent-box/sessions.json"
     )
     machine.wait_until_fails(f"{hook_ls} | grep -x {done} >/dev/null", timeout=60)