diff --git a/harness/actions.go b/harness/actions.go
index 0dc9738..5742f31 100644
--- a/harness/actions.go
+++ b/harness/actions.go
@@ -127,6 +127,8 @@ func (a *App) authorize(ctx context.Context, req ActionRequest) (string, error)
 		return "", errors.New("invalid token")
 	}
 	switch {
+	case p.RejectedAt != nil:
+		return "", errors.New("proposal rejected")
 	case p.ApprovedAt == nil:
 		return "", errors.New("proposal not approved")
 	case p.ExecutedAt != nil:
diff --git a/harness/autoapprove.go b/harness/autoapprove.go
index 9ca84ac..e823692 100644
--- a/harness/autoapprove.go
+++ b/harness/autoapprove.go
@@ -9,16 +9,20 @@ import (
 // after the configured delay the harness approves the layer-2 proposal (actor "auto") and executes it exactly as the
 // HITL bot does after a button press (actor "ai-approved", MarkProposalExecuted). MTTA therefore equals the delay;
 // the verdict logic is unchanged (layer 2 from the ai-approved action). A proposal already decided, expired or
-// recorded-only is handled the same way as the API path.
+// recorded-only is handled the same way as the API path. The claim (approve + executed_at) is a single conditional
+// UPDATE, so a human approval/execution or a rejection that arrives first takes precedence (no double execution).
 func (a *App) autoApprove(token string, req ProposalRequest, after time.Duration) {
 	time.Sleep(after)
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Minute)
 	defer cancel()
 	p, err := a.store.GetProposal(ctx, token)
-	if err != nil || p.ApprovedAt != nil || time.Now().After(p.ExpiresAt) {
+	if err != nil {
 		return
 	}
-	if err := a.store.ApproveProposal(ctx, token, "auto"); err != nil {
+	// one conditional update claims approval + execution: a human/bot decision (approve, reject, execute) that lands
+	// first wins and this goroutine does nothing
+	claimed, err := a.store.ClaimProposal(ctx, token, "auto")
+	if err != nil || !claimed {
 		return
 	}
 	suffix := token[len(token)-6:]
@@ -31,7 +35,6 @@ func (a *App) autoApprove(token string, req ProposalRequest, after time.Duration
 	if !ce.Exec {
 		a.store.Event(ctx, p.RunID, p.ScenarioID, "action", "ai-approved", map[string]any{"action_id": p.ActionID, "target": req.Target, "recorded_only": true, "note": note, "layer": ce.Layer})
 		mActs.WithLabelValues(p.ActionID, "ai-approved", "recorded").Inc()
-		_ = a.store.MarkProposalExecuted(ctx, token)
 		return
 	}
 	res, err := a.executeAction(ctx, p.ActionID, req.Target, "ai-approved", p.RunID, p.ScenarioID)
@@ -41,5 +44,4 @@ func (a *App) autoApprove(token string, req ProposalRequest, after time.Duration
 	}
 	mActs.WithLabelValues(p.ActionID, "ai-approved", result).Inc()
 	a.store.Event(ctx, p.RunID, p.ScenarioID, "action", "ai-approved", map[string]any{"action_id": p.ActionID, "target": req.Target, "result": res, "error": errStr(err), "note": note})
-	_ = a.store.MarkProposalExecuted(ctx, token)
 }
diff --git a/harness/harness b/harness/harness
index f3a7c86..629e3b7 100755
Binary files a/harness/harness and b/harness/harness differ
diff --git a/harness/proposals.go b/harness/proposals.go
index 4810272..1e049ef 100644
--- a/harness/proposals.go
+++ b/harness/proposals.go
@@ -94,6 +94,9 @@ func (a *App) handleProposalApprove(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	switch {
+	case p.RejectedAt != nil:
+		writeErr(w, 409, errors.New("proposal rejected"))
+		return
 	case p.ApprovedAt != nil:
 		writeErr(w, 409, errors.New("proposal already approved"))
 		return
@@ -122,6 +125,7 @@ func (a *App) handleProposalReject(w http.ResponseWriter, r *http.Request) {
 		writeErr(w, 404, errors.New("unknown proposal"))
 		return
 	}
+	_ = a.store.RejectProposal(r.Context(), token) // blocks a later approval (button, API or auto-approval)
 	a.store.Event(r.Context(), p.RunID, p.ScenarioID, "note", req.RejectedBy, map[string]any{"rejected": true, "action_id": p.ActionID, "reason": req.Reason, "token_suffix": token[len(token)-6:]})
 	writeJSON(w, 200, map[string]any{"token": token, "rejected": true})
 }
diff --git a/harness/store.go b/harness/store.go
index f9e230d..259da93 100644
--- a/harness/store.go
+++ b/harness/store.go
@@ -26,6 +26,7 @@ CREATE TABLE IF NOT EXISTS runs (
 );
 ALTER TABLE runs ADD COLUMN IF NOT EXISTS model text;  -- LLM requested for the run's diagnoses (campaign-level model selection, 2026-10-14)
 ALTER TABLE runs ADD COLUMN IF NOT EXISTS auto_approve_after_s int;  -- measurement mode: the harness approves + executes layer-2 proposals after this delay (2026-10-15)
+ALTER TABLE proposals ADD COLUMN IF NOT EXISTS rejected_at timestamptz;  -- POST /proposals/{token}/reject (a rejected proposal can no longer be approved or executed)
 CREATE TABLE IF NOT EXISTS scenarios (
   scenario_id         text PRIMARY KEY,
   path                text NOT NULL,
@@ -287,13 +288,14 @@ type ProposalRow struct {
 	ApprovedAt *time.Time
 	ApprovedBy *string
 	ExecutedAt *time.Time
+	RejectedAt *time.Time
 	ExpiresAt  time.Time
 }
 
 func (s *Store) GetProposal(ctx context.Context, token string) (*ProposalRow, error) {
 	var p ProposalRow
-	err := s.pool.QueryRow(ctx, `SELECT token,run_id,scenario_id,action_id,approved_at,approved_by,executed_at,expires_at FROM proposals WHERE token=$1`, token).
-		Scan(&p.Token, &p.RunID, &p.ScenarioID, &p.ActionID, &p.ApprovedAt, &p.ApprovedBy, &p.ExecutedAt, &p.ExpiresAt)
+	err := s.pool.QueryRow(ctx, `SELECT token,run_id,scenario_id,action_id,approved_at,approved_by,executed_at,rejected_at,expires_at FROM proposals WHERE token=$1`, token).
+		Scan(&p.Token, &p.RunID, &p.ScenarioID, &p.ActionID, &p.ApprovedAt, &p.ApprovedBy, &p.ExecutedAt, &p.RejectedAt, &p.ExpiresAt)
 	if err != nil {
 		return nil, err
 	}
@@ -301,7 +303,23 @@ func (s *Store) GetProposal(ctx context.Context, token string) (*ProposalRow, er
 }
 
 func (s *Store) ApproveProposal(ctx context.Context, token, by string) error {
-	_, err := s.pool.Exec(ctx, `UPDATE proposals SET approved_at=now(), approved_by=$2 WHERE token=$1 AND approved_at IS NULL`, token, by)
+	_, err := s.pool.Exec(ctx, `UPDATE proposals SET approved_at=now(), approved_by=$2 WHERE token=$1 AND approved_at IS NULL AND rejected_at IS NULL`, token, by)
+	return err
+}
+
+// ClaimProposal approves and marks executed in one conditional update, so exactly one caller (auto-approval or a
+// human/bot execute racing with it) wins: it reports whether this call claimed the proposal.
+func (s *Store) ClaimProposal(ctx context.Context, token, by string) (bool, error) {
+	tag, err := s.pool.Exec(ctx, `UPDATE proposals SET approved_at=now(), approved_by=$2, executed_at=now()
+		WHERE token=$1 AND approved_at IS NULL AND executed_at IS NULL AND rejected_at IS NULL AND expires_at > now()`, token, by)
+	if err != nil {
+		return false, err
+	}
+	return tag.RowsAffected() == 1, nil
+}
+
+func (s *Store) RejectProposal(ctx context.Context, token string) error {
+	_, err := s.pool.Exec(ctx, `UPDATE proposals SET rejected_at=now() WHERE token=$1 AND approved_at IS NULL AND rejected_at IS NULL`, token)
 	return err
 }