diff --git a/contracts/work/lease.go b/contracts/work/lease.go
index 1406898..756df90 100644
--- a/contracts/work/lease.go
+++ b/contracts/work/lease.go
@@ -20,3 +20,21 @@ type AgentLease struct {
 	AcquiredAt   time.Time
 	ExpiresAt    time.Time
 }
+
+type AgentCallbackKind string
+
+const (
+	AgentCallbackHeartbeat          AgentCallbackKind = "HEARTBEAT"
+	AgentCallbackCheckpoint         AgentCallbackKind = "CHECKPOINT"
+	AgentCallbackResult             AgentCallbackKind = "RESULT"
+	AgentCallbackCancelAcknowledged AgentCallbackKind = "CANCEL_ACKNOWLEDGED"
+)
+
+type AgentCallback struct {
+	RunID        AgentRunID
+	WorkerID     string
+	FencingToken AgentFencingToken
+	Sequence     uint64
+	Kind         AgentCallbackKind
+	ObservedAt   time.Time
+}
diff --git a/dev/ACTIVE_TASK.json b/dev/ACTIVE_TASK.json
index aad79b9..a51ac48 100644
--- a/dev/ACTIVE_TASK.json
+++ b/dev/ACTIVE_TASK.json
@@ -1,5 +1,5 @@
 {
-  "task_file": "dev/tasks/DEV-000116-agent-lease-fencing.json",
-  "task_id": "DEV-000116",
+  "task_file": "dev/tasks/DEV-000117-agent-callback-envelope.json",
+  "task_id": "DEV-000117",
   "task_revision": 1
 }
diff --git a/dev/tasks/DEV-000117-agent-callback-envelope.json b/dev/tasks/DEV-000117-agent-callback-envelope.json
new file mode 100644
index 0000000..41bb81e
--- /dev/null
+++ b/dev/tasks/DEV-000117-agent-callback-envelope.json
@@ -0,0 +1,23 @@
+{
+  "task_id": "DEV-000117",
+  "task_revision": 1,
+  "architecture_revision": "ADR-0018",
+  "base_repo_revision": "9766e53",
+  "risk_tier": "E",
+  "intent": "Add typed callback envelope validation for authenticated worker identity, fencing token, monotonic sequence and lease freshness.",
+  "requirement_ids": ["WP-301", "Callback auth binding", "Forged/out-of-order result rejection"],
+  "allowed_change_scope": {"modules": ["work-agent-execution"], "paths": ["contracts/work/**", "modules/work/domain/**", "tests/agent_execution/**", "dev/ACTIVE_TASK.json", "dev/tasks/DEV-000117-agent-callback-envelope.json"]},
+  "forbidden_change_scope": {"paths": ["architecture/**", "database/**", "migrations/**", "deploy/**", "security/**", "cmd/**", "runtime/**", "adapters/**", ".gitea/**"]},
+  "allowed_architecture_delta": {"new_cross_module_dependencies": 0, "new_canonical_owners": 0, "new_privileges": 0, "new_third_party_dependencies": 0, "new_database_owner_roles": 0},
+  "contracts": {"consume": ["AgentLease/fencing contract from DEV-000116", "ADR-0020 callback ordering boundary"], "may_change": ["Agent callback envelope contract"]},
+  "semantic_owner": "work-agent-execution",
+  "canonical_state": [],
+  "state_transitions": ["lease + callback envelope -> accepted callback", "wrong identity/token/sequence/expiry -> rejected callback"],
+  "side_effects_and_privileges": ["Pure validation; no transport, persistence, result mutation or external mutation"],
+  "persistence_and_migrations": ["No schema or migration changes"],
+  "failure_retry_restart": ["Duplicate and late callbacks are rejected by sequence/fencing; accepted sequence is persisted later by adapter"],
+  "assumptions": ["Transport authenticates worker identity before this validation", "Sequence is monotonic within a run and lease token", "Result verification remains a separate step"],
+  "acceptance_scenarios": ["Matching heartbeat accepted", "Wrong worker/token rejected", "Duplicate/out-of-order sequence rejected", "Expired lease rejected", "Callback run mismatch rejected"],
+  "required_tests": ["go test ./...", "go vet ./...", "go run ./cmd/devctl preflight -active dev/ACTIVE_TASK.json", "go run ./cmd/devctl check-diff -active dev/ACTIVE_TASK.json", "go run ./cmd/devctl check-architecture", "go run ./cmd/devctl/independent", "git diff --check", "agent callback tests"],
+  "terminal_gap_states": ["BLOCKED_BY_MISSING_CAPABILITY", "BLOCKED_BY_SPEC_GAP", "BLOCKED_BY_CONFLICT"]
+}
diff --git a/modules/work/domain/lease.go b/modules/work/domain/lease.go
index f1f34bb..2964af8 100644
--- a/modules/work/domain/lease.go
+++ b/modules/work/domain/lease.go
@@ -9,6 +9,8 @@ import (
 
 var ErrLeaseRejected = errors.New("agent lease rejected")
 
+var ErrCallbackRejected = errors.New("agent callback rejected")
+
 func AcquireAgentLease(runID work.AgentRunID, workerID string, token work.AgentFencingToken, now time.Time, ttl time.Duration) (work.AgentLease, error) {
 	if runID == "" || workerID == "" || token == 0 || now.IsZero() || ttl <= 0 {
 		return work.AgentLease{}, ErrLeaseRejected
@@ -23,3 +25,10 @@ func RenewAgentLease(lease work.AgentLease, workerID string, token work.AgentFen
 	lease.ExpiresAt = now.Add(ttl)
 	return lease, nil
 }
+
+func ValidateAgentCallback(callback work.AgentCallback, lease work.AgentLease, lastSequence uint64) error {
+	if lease.State != work.AgentLeaseActive || callback.RunID == "" || callback.RunID != lease.RunID || callback.WorkerID == "" || callback.WorkerID != lease.WorkerID || callback.FencingToken == 0 || callback.FencingToken != lease.FencingToken || callback.Sequence <= lastSequence || callback.Kind == "" || callback.ObservedAt.IsZero() || !callback.ObservedAt.Before(lease.ExpiresAt) {
+		return ErrCallbackRejected
+	}
+	return nil
+}
diff --git a/tests/agent_execution/lease_test.go b/tests/agent_execution/lease_test.go
index eb9a74d..ea54280 100644
--- a/tests/agent_execution/lease_test.go
+++ b/tests/agent_execution/lease_test.go
@@ -32,3 +32,19 @@ func TestExpiredLeaseCannotRenew(t *testing.T) {
 		t.Fatal("expected expired lease rejection")
 	}
 }
+
+func TestAgentCallbackRejectsReplayAndWrongFencing(t *testing.T) {
+	now := time.Unix(100, 0)
+	lease, _ := workdomain.AcquireAgentLease("run-1", "worker-a", 1, now, time.Minute)
+	callback := work.AgentCallback{RunID: "run-1", WorkerID: "worker-a", FencingToken: 1, Sequence: 2, Kind: work.AgentCallbackHeartbeat, ObservedAt: now.Add(time.Second)}
+	if err := workdomain.ValidateAgentCallback(callback, lease, 1); err != nil {
+		t.Fatal(err)
+	}
+	if err := workdomain.ValidateAgentCallback(callback, lease, 2); err == nil {
+		t.Fatal("expected replay rejection")
+	}
+	callback.FencingToken = 2
+	if err := workdomain.ValidateAgentCallback(callback, lease, 1); err == nil {
+		t.Fatal("expected fencing rejection")
+	}
+}