diff --git a/internal/domain/domain.go b/internal/domain/domain.go
index 7cc13ca..c6fccc3 100644
--- a/internal/domain/domain.go
+++ b/internal/domain/domain.go
@@ -17,6 +17,8 @@ var ErrConflict = errors.New("task version conflict")
 var ErrNotFound = errors.New("task not found")
 var ErrInvalid = errors.New("invalid event")
 
+const CurrentEventSchema = 1
+
 type TaskState string
 
 const (
@@ -53,13 +55,14 @@ type Task struct {
 }
 
 type Event struct {
-	Seq     uint64          `json:"seq"`
-	ID      string          `json:"id"`
-	Type    string          `json:"type"`
-	TaskID  string          `json:"task_id"`
-	Version int             `json:"version"`
-	At      time.Time       `json:"at"`
-	Payload json.RawMessage `json:"payload"`
+	SchemaVersion int             `json:"schema_version,omitempty"`
+	Seq           uint64          `json:"seq"`
+	ID            string          `json:"id"`
+	Type          string          `json:"type"`
+	TaskID        string          `json:"task_id"`
+	Version       int             `json:"version"`
+	At            time.Time       `json:"at"`
+	Payload       json.RawMessage `json:"payload"`
 }
 
 func Hash(v []byte) string { h := sha256.Sum256(v); return hex.EncodeToString(h[:]) }
@@ -76,7 +79,7 @@ func NewID() string {
 	return ulidEncoding.EncodeToString(b)
 }
 func ValidateEvent(e Event) error {
-	if e.Type == "" || e.TaskID == "" || len(e.Payload) == 0 || len(e.Payload) > 64*1024 {
+	if e.SchemaVersion > CurrentEventSchema || e.Type == "" || e.TaskID == "" || len(e.Payload) == 0 || len(e.Payload) > 64*1024 {
 		return ErrInvalid
 	}
 	allowed := map[string]bool{"TaskCreated": true, "TaskLeased": true, "TaskReleased": true, "TaskCompleted": true, "TaskFailed": true, "TaskBlocked": true, "ApprovalRequested": true, "ApprovalGranted": true, "ApprovalDenied": true, "TaskAmended": true, "QuotaReported": true, "StandupAdvisory": true}
@@ -87,6 +90,9 @@ func ValidateEvent(e Event) error {
 	if err := json.Unmarshal(e.Payload, &p); err != nil {
 		return fmt.Errorf("%w: payload is not JSON", ErrInvalid)
 	}
+	if p == nil {
+		return fmt.Errorf("%w: payload must be an object", ErrInvalid)
+	}
 	return ValidatePayload(e.Type, p)
 }
 func ValidateCreated(p map[string]any) error {
@@ -116,6 +122,9 @@ func ValidatePayload(typ string, p map[string]any) error {
 		if _, ok := p["until_ns"].(float64); !ok {
 			return fmt.Errorf("%w: until_ns required", ErrInvalid)
 		}
+		if v, ok := p["expected_version"].(float64); ok && v < 0 {
+			return fmt.Errorf("%w: expected_version invalid", ErrInvalid)
+		}
 	case "TaskReleased":
 		if err := requiredString("handoff_ref"); err != nil && p["reason"] == nil {
 			return err
diff --git a/internal/store/store.go b/internal/store/store.go
index 1d8d665..7a727c4 100644
--- a/internal/store/store.go
+++ b/internal/store/store.go
@@ -166,6 +166,9 @@ func (s *Store) Append(e domain.Event) error {
 	if e.Seq == 0 {
 		e.Seq = s.seq + 1
 	}
+	if e.SchemaVersion == 0 {
+		e.SchemaVersion = domain.CurrentEventSchema
+	}
 	if e.Type == "TaskCreated" {
 		var p map[string]any
 		if err := json.Unmarshal(e.Payload, &p); err != nil {
@@ -175,10 +178,22 @@ func (s *Store) Append(e domain.Event) error {
 			return nil
 		}
 	}
-	if t, ok := s.tasks[e.TaskID]; ok && e.Version != t.Version+1 {
+	t, taskExists := s.tasks[e.TaskID]
+	if taskExists && e.Version != t.Version+1 {
 		return domain.ErrConflict
 	}
-	if _, ok := s.tasks[e.TaskID]; !ok && e.Type != "TaskCreated" {
+	if e.Type == "TaskLeased" {
+		var p struct {
+			ExpectedVersion *int `json:"expected_version"`
+		}
+		if err := json.Unmarshal(e.Payload, &p); err != nil {
+			return err
+		}
+		if p.ExpectedVersion != nil && (t.Version != *p.ExpectedVersion) {
+			return domain.ErrConflict
+		}
+	}
+	if !taskExists && e.Type != "TaskCreated" {
 		return domain.ErrNotFound
 	}
 	if e.Type != "TaskCreated" && (e.Type == "TaskCompleted" || e.Type == "TaskBlocked" || e.Type == "TaskReleased") {
@@ -293,7 +308,7 @@ func (s *Store) Lease(id, harness string, ttl time.Duration) (domain.Event, erro
 	if t.State != domain.StateQueued {
 		return domain.Event{}, domain.ErrConflict
 	}
-	p, _ := json.Marshal(map[string]any{"harness_id": harness, "until_ns": time.Now().Add(ttl).UnixNano()})
+	p, _ := json.Marshal(map[string]any{"harness_id": harness, "until_ns": time.Now().Add(ttl).UnixNano(), "expected_version": t.Version})
 	e := domain.Event{ID: id, Type: "TaskLeased", TaskID: id, Version: t.Version + 1, Payload: p}
 	return e, s.Append(e)
 }
diff --git a/progress.md b/progress.md
index f3b7391..f521a7e 100644
--- a/progress.md
+++ b/progress.md
@@ -53,6 +53,8 @@ The latest pass now applies `AuthorizeEvent` to lifecycle and approval writes an
 
 The lifecycle API contract pass now requires callers to provide explicit release evidence (`reason` or `handoff_ref`), a block `blocker`, and a completion `report_ref`. The server no longer creates generated placeholder completion artifacts or converts malformed/empty lifecycle bodies into defaults. Regression coverage validates that release, completion, and block events reject missing evidence.
 
+Item 1 substrate hardening pass: newly appended events use schema envelope version 1; replay rejects unsupported versions and non-object payloads; and `TaskLeased` carries an `expected_version` guard that is checked before append. Legacy envelope events remain readable for tolerant replay.
+
 Recommended order:
 
 1. Add the orchestration coordinator: lease → worktree → harness session → bootstrap → lifecycle events.