diff --git a/internal/client/agent/ack.go b/internal/client/agent/ack.go
new file mode 100644
index 0000000..be44629
--- /dev/null
+++ b/internal/client/agent/ack.go
@@ -0,0 +1,29 @@
+package agent
+
+import (
+	"context"
+	"errors"
+
+	rvboxv1 "github.com/rvbox/rvbox/gen/go/rvbox/v1"
+	"github.com/rvbox/rvbox/internal/domain"
+)
+
+var ErrInvalidEventAck = errors.New("invalid event acknowledgement")
+
+type EventAckStore interface {
+	Ack(context.Context, domain.UUID, uint64) error
+}
+
+// ApplyEventAck releases only the cumulative, server-confirmed event prefix.
+// The spool remains the source of truth if the process stops before this call
+// returns; a repeated acknowledgement is intentionally harmless.
+func ApplyEventAck(ctx context.Context, store EventAckStore, ack *rvboxv1.EventAck) error {
+	if store == nil || ack == nil || ack.GetThroughEventSeq() == 0 {
+		return ErrInvalidEventAck
+	}
+	issue, err := domain.ParseUUIDv7(ack.GetIssueUuid())
+	if err != nil {
+		return ErrInvalidEventAck
+	}
+	return store.Ack(ctx, issue, ack.GetThroughEventSeq())
+}
diff --git a/internal/client/agent/handshake_test.go b/internal/client/agent/handshake_test.go
index 4b91f47..3fa0838 100644
--- a/internal/client/agent/handshake_test.go
+++ b/internal/client/agent/handshake_test.go
@@ -118,6 +118,17 @@ func TestSendCommandEventCanonicalDigest_HP_EVENT_02(t *testing.T) {
 	}
 }
 
+func TestApplyEventAckValidatesAndReleasesPrefix_HP_EVENT_03(t *testing.T) {
+	store := &ackStore{}
+	ack := &rvboxv1.EventAck{IssueUuid: "019c46f1-1d02-7000-8000-000000000066", ThroughEventSeq: 4}
+	if err := ApplyEventAck(context.Background(), store, ack); err != nil || store.issue.String() != ack.IssueUuid || store.sequence != 4 {
+		t.Fatalf("ApplyEventAck = issue=%s sequence=%d err=%v", store.issue, store.sequence, err)
+	}
+	if err := ApplyEventAck(context.Background(), store, &rvboxv1.EventAck{IssueUuid: "bad", ThroughEventSeq: 1}); !errors.Is(err, ErrInvalidEventAck) {
+		t.Fatalf("invalid ack error = %v", err)
+	}
+}
+
 type fakeTransport struct {
 	written []byte
 	read    []byte
@@ -130,6 +141,16 @@ func (store *reconcileStore) DiscardTerminal(_ context.Context, issue domain.UUI
 	return nil
 }
 
+type ackStore struct {
+	issue    domain.UUID
+	sequence uint64
+}
+
+func (store *ackStore) Ack(_ context.Context, issue domain.UUID, sequence uint64) error {
+	store.issue, store.sequence = issue, sequence
+	return nil
+}
+
 func (transport *fakeTransport) Write(_ context.Context, value []byte) error {
 	transport.written = append([]byte(nil), value...)
 	return nil
diff --git a/test/coverage.toml b/test/coverage.toml
index d43ff6d..0d80231 100644
--- a/test/coverage.toml
+++ b/test/coverage.toml
@@ -230,6 +230,12 @@ layer = "unit"
 status = "implemented"
 tests = ["internal/client/agent/handshake_test.go:TestSendCommandEventCanonicalDigest_HP_EVENT_02"]
 
+[[requirements]]
+id = "HP-EVENT-03"
+layer = "unit"
+status = "implemented"
+tests = ["internal/client/agent/handshake_test.go:TestApplyEventAckValidatesAndReleasesPrefix_HP_EVENT_03"]
+
 [[requirements]]
 id = "HP-SES-05"
 layer = "integration"