diff --git a/agent/internal/grpc/client.go b/agent/internal/grpc/client.go
index fa6daff..7ad9878 100644
--- a/agent/internal/grpc/client.go
+++ b/agent/internal/grpc/client.go
@@ -80,7 +80,11 @@ func (c *Client) Register(serverID, preRegToken, hostname, ipAddress, osInfo str
 	return resp.AgentToken, nil
 }
 
-func (c *Client) SyncKeys(serverID, agentToken, version string) ([]string, error) {
+// SyncKeys returns the whole response rather than just the keys: the poll now
+// also carries CollectPackages, and a second RPC purely to learn one boolean
+// would be a message every 30 seconds for a value that changes at most when a
+// licence does.
+func (c *Client) SyncKeys(serverID, agentToken, version string) (*pb.SyncResponse, error) {
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()
 
@@ -92,7 +96,21 @@ func (c *Client) SyncKeys(serverID, agentToken, version string) ([]string, error
 	if err != nil {
 		return nil, err
 	}
-	return resp.PublicKeys, nil
+	return resp, nil
+}
+
+// ReportPackages sends a package report and returns whether the server wants
+// the full list. Given a longer deadline than the other unary calls because the
+// full body is ~150KB on a slow link.
+func (c *Client) ReportPackages(req *pb.ReportPackagesRequest) (bool, error) {
+	ctx, cancel := context.WithTimeout(context.Background(), 60*time.Second)
+	defer cancel()
+
+	resp, err := c.client.ReportPackages(ctx, req)
+	if err != nil {
+		return false, err
+	}
+	return resp.NeedFull, nil
 }
 
 func (c *Client) UploadGeneratedKey(serverID, agentToken, publicKey, privateKey, label string) (string, error) {
diff --git a/agent/internal/sync/packages.go b/agent/internal/sync/packages.go
new file mode 100644
index 0000000..6ebb316
--- /dev/null
+++ b/agent/internal/sync/packages.go
@@ -0,0 +1,90 @@
+package agentsync
+
+import (
+	"log"
+	"runtime"
+	"sync/atomic"
+
+	"gitea.hostxtra.co.uk/mrhid6/vantage/agent/internal/config"
+	grpcclient "gitea.hostxtra.co.uk/mrhid6/vantage/agent/internal/grpc"
+	"gitea.hostxtra.co.uk/mrhid6/vantage/agent/internal/grpc/pb"
+	"gitea.hostxtra.co.uk/mrhid6/vantage/agent/internal/packages"
+)
+
+// collectPackagesFlag is written by the 30s key poll and read by the hourly
+// package loop — two different goroutines, hence the atomic.
+//
+// It defaults to false, so an agent that has not yet completed a poll, or is
+// talking to a server too old to send the field, collects nothing. Off is the
+// safe default: collecting without a licence costs the customer storage they
+// are not paying for.
+var collectPackagesFlag atomic.Bool
+
+func collectPackagesEnabled() bool { return collectPackagesFlag.Load() }
+
+// reportPackages offers a hash of the installed package set and sends the full
+// list only if the server does not already hold it.
+//
+// It runs on the same hourly cadence as the update check because a package set
+// changes on roughly the same schedule, and reusing that loop means one timer
+// rather than two.
+func reportPackages(client *grpcclient.Client, cfg *config.Config) {
+	if runtime.GOOS != "linux" {
+		return
+	}
+	if !collectPackagesEnabled() {
+		return
+	}
+
+	osrel, pkgs, err := packages.Collect()
+	if err != nil {
+		log.Printf("package collection error: %v", err)
+		return
+	}
+
+	pbOS := pb.OSRelease{
+		Family:    osrel.Family,
+		VersionId: osrel.VersionID,
+		Arch:      osrel.Arch,
+	}
+	hash := packages.Hash(pkgs)
+
+	// The offer: hash only, no body. On an unchanged host this is the whole
+	// exchange, which is the point of the handshake.
+	needFull, err := client.ReportPackages(&pb.ReportPackagesRequest{
+		ServerId:   cfg.ServerID,
+		AgentToken: cfg.AgentToken,
+		Hash:       hash,
+		Os:         pbOS,
+	})
+	if err != nil {
+		log.Printf("ReportPackages offer error: %v", err)
+		return
+	}
+	if !needFull {
+		return
+	}
+
+	pbPkgs := make([]pb.InstalledPackage, len(pkgs))
+	for i, p := range pkgs {
+		pbPkgs[i] = pb.InstalledPackage{
+			Name:       p.Name,
+			Version:    p.Version,
+			Epoch:      int32(p.Epoch),
+			Arch:       p.Arch,
+			SourceName: p.SourceName,
+		}
+	}
+
+	if _, err := client.ReportPackages(&pb.ReportPackagesRequest{
+		ServerId:   cfg.ServerID,
+		AgentToken: cfg.AgentToken,
+		Hash:       hash,
+		Os:         pbOS,
+		Packages:   pbPkgs,
+	}); err != nil {
+		log.Printf("ReportPackages full error: %v", err)
+		return
+	}
+	log.Printf("reported %d installed packages", len(pkgs))
+}
diff --git a/agent/internal/sync/sync.go b/agent/internal/sync/sync.go
index 44b845f..538c008 100644
--- a/agent/internal/sync/sync.go
+++ b/agent/internal/sync/sync.go
@@ -92,11 +92,18 @@ func Run(ctx context.Context, cfg *config.Config, version string) error {
 }
 
 func poll(client *grpcclient.Client, cfg *config.Config, version string) error {
-	desired, err := client.SyncKeys(cfg.ServerID, cfg.AgentToken, version)
+	resp, err := client.SyncKeys(cfg.ServerID, cfg.AgentToken, version)
 	if err != nil {
 		return fmt.Errorf("SyncKeys: %w", err)
 	}
 
+	// Stored atomically: the hourly package loop reads this from another
+	// goroutine. Absent on the wire decodes as false, so an older server leaves
+	// collection off rather than on.
+	collectPackagesFlag.Store(resp.CollectPackages)
+
+	desired := resp.PublicKeys
+
 	if runtime.GOOS != "linux" {
 		return nil
 	}
@@ -395,6 +402,11 @@ func runUpdateCheck(ctx context.Context, cfg *config.Config) {
 			return
 		}
 		log.Printf("reported %d available OS updates", len(pkgs))
+
+		// Same hourly cadence, same connection. A package set changes on
+		// roughly the schedule available updates do, so this needs no timer of
+		// its own.
+		reportPackages(client, cfg)
 	}
 
 	doCheck()