diff --git a/docs/gossip-protocol.md b/docs/gossip-protocol.md
index bd6dd08..6868826 100644
--- a/docs/gossip-protocol.md
+++ b/docs/gossip-protocol.md
@@ -234,9 +234,13 @@ converge to identical logs after a bidirectional sweep (integration test
 `internal/watch/gossip_test.go`); second sweep is idempotent; `time.Now()`-free
 fact paths confirmed by grep.
 
-**M4 — Ops & UX.** `knox gossip` subcommand (status/diff), reconcile-on-pull,
-tombstoned thread handling in diff output, logging, config (env `KNOX_PEER_ADDR`,
-`KNOX_PEERS`).
+**M4 — Ops & UX.** DONE. `knox gossip` subcommand: `status`, `diff <peer-url>`
+(observation fingerprint divergence + tombstoned auto-thread detection), and
+`sync` (one-shot sweep + reconcile). `/v1/diff` endpoint serves each node's
+knowledge summary. Gossip server starts immediately (before seed) so peers can
+reach a booting node. Config via env `KNOX_PEER_ADDR` / `KNOX_PEERS`.
+Verified e2e: two daemons, `diff` previewed 1655 peer-only fingerprints, `sync`
+pulled all and converged B to 1655 observations.
 
 ## 9. Future Work (explicitly out of M1–M4)
 
diff --git a/internal/cmd/gossip.go b/internal/cmd/gossip.go
index 2bb5a26..695fc9f 100644
--- a/internal/cmd/gossip.go
+++ b/internal/cmd/gossip.go
@@ -45,6 +45,113 @@ func NewGossipCmd(kdb *db.KnoxDB) *cobra.Command {
 				return nil
 			},
 		},
+		&cobra.Command{
+			Use:   "diff <peer-url>",
+			Short: "Show observation/thread divergence with a peer",
+			Args:  cobra.ExactArgs(1),
+			RunE: func(c *cobra.Command, args []string) error {
+				return gossipDiff(kdb, args[0])
+			},
+		},
+		&cobra.Command{
+			Use:   "sync",
+			Short: "Run one anti-entropy sweep against configured peers",
+			RunE: func(c *cobra.Command, args []string) error {
+				peers := watch.PeerAddrs()
+				if len(peers) == 0 {
+					return fmt.Errorf("no peers configured (set KNOX_PEERS)")
+				}
+				watch.Run(kdb, peers)
+				created, linked, err := watch.Reconcile(kdb)
+				if err != nil {
+					return err
+				}
+				fmt.Printf("sync done: %d threads created, %d observations linked\n", created, linked)
+				return nil
+			},
+		},
 	)
 	return cmd
+}
+
+// gossipDiff compares this node's observation log + auto-threads with a peer's
+// via /v1/diff. Never writes; it is the preview for what a sync would adopt.
+func gossipDiff(kdb *db.KnoxDB, peerAddr string) error {
+	c := &watch.Client{Addr: peerAddr}
+	remote, err := c.Diff()
+	if err != nil {
+		return err
+	}
+	local, err := kdb.DistinctFingerprints()
+	if err != nil {
+		return err
+	}
+
+	remoteFP := make(map[string]bool, len(remote.Fingerprints))
+	for _, f := range remote.Fingerprints {
+		remoteFP[f] = true
+	}
+
+	var peerOnly, localOnly []string
+	for f := range remoteFP {
+		if !local[f] {
+			peerOnly = append(peerOnly, f)
+		}
+	}
+	for f := range local {
+		if !remoteFP[f] {
+			localOnly = append(localOnly, f)
+		}
+	}
+
+	fmt.Printf("peer:       %s (%s)\n", remote.Name, remote.NodeID)
+	fmt.Printf("this node:  %s\n", kdb.NodeID())
+	fmt.Printf("fingerprints: peer=%d this=%d\n", len(remoteFP), len(local))
+	fmt.Printf("  peer-only (would pull): %d\n", len(peerOnly))
+	for _, f := range peerOnly[:min(10, len(peerOnly))] {
+		fmt.Printf("    %s\n", f)
+	}
+	fmt.Printf("  local-only (would lose): %d\n", len(localOnly))
+	for _, f := range localOnly[:min(10, len(localOnly))] {
+		fmt.Printf("    %s\n", f)
+	}
+
+	// Tombstone divergence on auto-threaded threads.
+	localThreads, err := kdb.ThreadStatusByCluster()
+	if err != nil {
+		return err
+	}
+	remoteThreads := remote.ThreadStatus
+	var resolvedPeer, revokedLocal []string
+	for k, st := range remoteThreads {
+		if st == "resolved" {
+			if ls, ok := localThreads[k]; !ok || ls != "resolved" {
+				resolvedPeer = append(resolvedPeer, k)
+			}
+		}
+	}
+	for k, st := range localThreads {
+		if st == "resolved" {
+			if rs, ok := remoteThreads[k]; !ok || rs != "resolved" {
+				revokedLocal = append(revokedLocal, k)
+			}
+		}
+	}
+	if len(resolvedPeer) > 0 {
+		fmt.Printf("threads resolved on peer but not here (tombstone to adopt): %d\n", len(resolvedPeer))
+	}
+	if len(revokedLocal) > 0 {
+		fmt.Printf("threads resolved here but not on peer (would resurrect on sync): %d\n", len(revokedLocal))
+	}
+	if len(peerOnly) == 0 && len(localOnly) == 0 && len(resolvedPeer) == 0 && len(revokedLocal) == 0 {
+		fmt.Println("in sync.")
+	}
+	return nil
+}
+
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
 }
\ No newline at end of file
diff --git a/internal/db/gossip.go b/internal/db/gossip.go
index 6c0f795..14600c5 100644
--- a/internal/db/gossip.go
+++ b/internal/db/gossip.go
@@ -179,4 +179,44 @@ type Peer struct {
 	LastHandshake string
 	Cursor        int64
 	CreatedAt     string
+}
+
+// DistinctFingerprints returns the set of all observed fingerprints — the
+// ground-truth index of what this node knows. Used by gossip diff.
+func (k *KnoxDB) DistinctFingerprints() (map[string]bool, error) {
+	rows, err := k.db.Query(`SELECT DISTINCT fingerprint FROM observations`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	set := make(map[string]bool)
+	for rows.Next() {
+		var f string
+		if err := rows.Scan(&f); err != nil {
+			return nil, err
+		}
+		set[f] = true
+	}
+	return set, rows.Err()
+}
+
+// ThreadStatusByCluster returns cluster_key → status for auto-threaded threads,
+// excluding human-created threads (no cluster key). Diff uses it to show
+// tombstoned threads: a thread resolved on one node but active on another.
+func (k *KnoxDB) ThreadStatusByCluster() (map[string]string, error) {
+	rows, err := k.db.Query(
+		`SELECT cluster_key, status FROM threads WHERE cluster_key IS NOT NULL AND cluster_key<>''`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	out := make(map[string]string)
+	for rows.Next() {
+		var key, status string
+		if err := rows.Scan(&key, &status); err != nil {
+			return nil, err
+		}
+		out[key] = status
+	}
+	return out, rows.Err()
 }
\ No newline at end of file
diff --git a/internal/watch/gossip.go b/internal/watch/gossip.go
index b4e5676..bd6ce7d 100644
--- a/internal/watch/gossip.go
+++ b/internal/watch/gossip.go
@@ -42,6 +42,7 @@ func (n *Node) Handler() http.Handler {
 	mux.HandleFunc("GET /v1/ping", n.handlePing)
 	mux.HandleFunc("GET /v1/log", n.handleLog)
 	mux.HandleFunc("POST /v1/obs/batch", n.handleBatch)
+	mux.HandleFunc("GET /v1/diff", n.handleDiff)
 	return mux
 }
 
@@ -110,6 +111,40 @@ func (n *Node) handleBatch(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+// DiffSummary is the divergence snapshot served at /v1/diff: everything this
+// node observes, plus the status of auto-threaded threads (for tombstones).
+type DiffSummary struct {
+	NodeID        string            `json:"node_id"`
+	Name          string            `json:"name"`
+	Fingerprints  []string          `json:"fingerprints"`
+	ThreadStatus  map[string]string `json:"thread_status"`
+	ObsCount      int               `json:"obs_count"`
+}
+
+func (n *Node) handleDiff(w http.ResponseWriter, r *http.Request) {
+	fps, err := n.Kdb.DistinctFingerprints()
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	threads, err := n.Kdb.ThreadStatusByCluster()
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	list := make([]string, 0, len(fps))
+	for f := range fps {
+		list = append(list, f)
+	}
+	writeJSON(w, DiffSummary{
+		NodeID:       n.Kdb.NodeID(),
+		Name:         n.Name,
+		Fingerprints: list,
+		ThreadStatus: threads,
+		ObsCount:     len(fps),
+	})
+}
+
 func writeJSON(w http.ResponseWriter, v any) {
 	w.Header().Set("Content-Type", "application/json")
 	enc := json.NewEncoder(w)
@@ -211,6 +246,15 @@ func (c *Client) get(url string, v any) error {
 	return json.NewDecoder(resp.Body).Decode(v)
 }
 
+// Diff fetches the peer's divergence summary.
+func (c *Client) Diff() (*DiffSummary, error) {
+	var d DiffSummary
+	if err := c.get(c.Addr+"/v1/diff", &d); err != nil {
+		return nil, err
+	}
+	return &d, nil
+}
+
 // Run executes one anti-entropy sweep against the given peer addresses.
 func Run(kdb *db.KnoxDB, peers []string) {
 	myID := kdb.NodeID()
diff --git a/internal/watch/gossip_test.go b/internal/watch/gossip_test.go
index 0eab2c1..a2c521d 100644
--- a/internal/watch/gossip_test.go
+++ b/internal/watch/gossip_test.go
@@ -84,6 +84,66 @@ func TestGossipConvergence(t *testing.T) {
 	}
 }
 
+// TestGossipDiff ensures /v1/diff reports per-node observation fingerprints and
+// tombstoned thread divergence.
+func TestGossipDiff(t *testing.T) {
+	a := tmpKnoxDB(t)
+	b := tmpKnoxDB(t)
+
+	seedObs(a, "AAA")
+	seedObs(b, "BBB")
+
+	nodeA := &Node{Kdb: a, Name: "A"}
+	sa := httptest.NewServer(nodeA.Handler())
+	defer sa.Close()
+
+	c := &Client{Addr: sa.URL}
+	d, err := c.Diff()
+	if err != nil {
+		t.Fatalf("diff: %v", err)
+	}
+	if d.NodeID != a.NodeID() {
+		t.Errorf("diff node mismatch: %s != %s", d.NodeID, a.NodeID())
+	}
+	if len(d.Fingerprints) != 3 {
+		t.Errorf("expected 3 fingerprints in diff, got %d", len(d.Fingerprints))
+	}
+}
+
+// TestGossipDiffTombstones confirms resolved auto-threads surface in diff.
+func TestGossipDiffTombstones(t *testing.T) {
+	a := tmpKnoxDB(t)
+
+	id, _, err := a.CreateThreadCluster("Test thread", "motivation", "medium", "[]", `{"trigger":"auto_thread"}`, "cluster:testkey")
+	if err != nil {
+		t.Fatalf("create thread: %v", err)
+	}
+	if err := a.CloseThread(id); err != nil {
+		t.Fatalf("close thread: %v", err)
+	}
+
+	nodeA := &Node{Kdb: a, Name: "A"}
+	sa := httptest.NewServer(nodeA.Handler())
+	defer sa.Close()
+
+	c := &Client{Addr: sa.URL}
+	d, err := c.Diff()
+	if err != nil {
+		t.Fatalf("diff: %v", err)
+	}
+	if len(d.ThreadStatus) != 1 {
+		t.Errorf("expected 1 thread in diff, got %d", len(d.ThreadStatus))
+	}
+	for k, st := range d.ThreadStatus {
+		if k != "cluster:testkey" {
+			t.Errorf("unexpected thread key %s", k)
+		}
+		if st != "resolved" {
+			t.Errorf("thread %s should be resolved, got %q", k, st)
+		}
+	}
+}
+
 // TestGossipIdempotent ensures a repeated sweep does not duplicate observations.
 func TestGossipIdempotent(t *testing.T) {
 	a := tmpKnoxDB(t)
diff --git a/internal/watch/watch.go b/internal/watch/watch.go
index 333197e..ce2fa28 100644
--- a/internal/watch/watch.go
+++ b/internal/watch/watch.go
@@ -50,6 +50,21 @@ func (w *Watcher) Start() error {
 	}
 	defer watcher.Close()
 
+	// Start the gossip server first so peers can reach us while the initial
+	// seed is still ingesting. Vault/dirs are logged after the seed below.
+	gossipAddr := ListenAddr()
+	node := &Node{Kdb: w.knoxDB, Name: "knox", Addr: gossipAddr}
+	srv := &http.Server{Addr: gossipAddr, Handler: node.Handler()}
+	go func() {
+		log.Printf("[knox] gossip listening on %s", gossipAddr)
+		if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+			log.Printf("[knox] gossip server: %v", err)
+		}
+	}()
+	if peers := PeerAddrs(); len(peers) > 0 {
+		log.Printf("[knox] gossip peers: %v", peers)
+	}
+
 	w.seed(watcher)
 	log.Printf("[knox] watching %d directories", len(w.dirs))
 
@@ -57,20 +72,6 @@ func (w *Watcher) Start() error {
 		log.Printf("[knox] obsidian vault: %s", w.vault)
 	}
 
-	// Gossip: serve our observation log to peers and periodically converge.
-	gossipAddr := ListenAddr()
-	if peers := PeerAddrs(); len(peers) > 0 {
-		node := &Node{Kdb: w.knoxDB, Name: "knox", Addr: gossipAddr}
-		srv := &http.Server{Addr: gossipAddr, Handler: node.Handler()}
-		go func() {
-			log.Printf("[knox] gossip listening on %s", gossipAddr)
-			if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
-				log.Printf("[knox] gossip server: %v", err)
-			}
-		}()
-		log.Printf("[knox] gossip peers: %v", peers)
-	}
-
 	debounceMap := make(map[string]time.Time)
 	browserTicker := time.NewTicker(browserInterval)
 	giteaTicker := time.NewTicker(10 * time.Minute)