feat: EXT-007 P2P message history gossip

After hello verification, the connecting peer sends history_request to
the first peer it meets (one per room, no fan-out). The responder queries
SQLite and replies with a history_chunk. Received history is stored via
INSERT OR IGNORE (mid dedup) and emitted as history_loaded IPC events.

- proto: MsgHistoryRequest/Chunk types, HistoryEntry, EvtHistoryLoaded,
  ComputeMsgID (sha256 content-addressed ID), MsgID field on ChatMessage
- store: ALTER TABLE ADD COLUMN msg_id + unique index migration (idempotent);
  RecentMessagesSince query (msg_id IS NOT NULL filter); msg_id persisted on save
- mesh: RequestHistoryFrom, HandleHistoryRequest, HandleHistoryChunk methods;
  historyRequested/historyFirstPeer state to ensure single-peer requests
- peer: dispatch history_request/history_chunk; RequestHistoryFrom after hello;
  stamp MsgID on incoming chat messages
- ipc: stamp MsgID on outgoing group chat messages
- EXTENSIONS.md: EXT-007 documented

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

EXTENSIONS.md

@@ -224,3 +224,83 @@ Each in-progress `.tmp` file has a corresponding `.tmp.meta` JSON sidecar:
 
 The sidecar is written when the transfer starts and removed on completion or
 corruption. Interrupted transfers keep the sidecar indefinitely.
+
+---
+
+## EXT-007 — P2P Message History Gossip
+
+**Status:** implemented (daemon mode)
+**Affects:** peer-to-peer wire (two new message types); IPC (new event)
+
+### Motivation
+
+When a peer joins a network for the first time (or reconnects after an
+absence), they have no history. This extension lets them request recent
+messages from an existing peer over the already-established encrypted
+DataChannel, without involving the anchor.
+
+### Wire messages
+
+#### `history_request`
+
+Sent by the newly-connected peer to the first peer whose hello is verified.
+One request per room.
+
+```json
+{
+  "type": "history_request",
+  "room":  "general",
+  "since": 1700000000000,
+  "limit": 200
+}
+```
+
+| Field   | Type          | Description |
+|---------|---------------|-------------|
+| `room`  | string        | Room to request history for. |
+| `since` | int64 (ms)    | Only return messages with `ts > since`. 0 = return up to `limit` most recent. |
+| `limit` | int (max 500) | Maximum messages to return. Responder may return fewer. |
+
+#### `history_chunk`
+
+```json
+{
+  "type": "history_chunk",
+  "room": "general",
+  "history": [
+    { "mid": "...", "from": "<peer-id>", "from_alias": "alice", "text": "hello", "ts": 1700000001000 }
+  ],
+  "history_done": true
+}
+```
+
+| Field          | Type   | Description |
+|----------------|--------|-------------|
+| `history`      | array  | Messages, oldest-first. |
+| `history_done` | bool   | Always `true` (single-chunk response). |
+
+### Deduplication
+
+`mid` is the deduplication key. The store uses `INSERT OR IGNORE` on `mid`,
+so receiving a message twice (live or via gossip) is a no-op. Messages
+without a `mid` are assigned one at receive time and are not gossipped.
+
+### Behaviour
+
+- The **receiver** sends one `history_request` per known room immediately
+  after hello verification with the **first** peer it connects to. Requesting
+  only the first peer avoids fan-out amplification.
+- The **responder** queries its SQLite store and replies with a single
+  `history_chunk`. `limit` is capped at 500 server-side. Rate-limited to one
+  request per (peer, room) per 60 seconds.
+- Received history messages are saved to the local store (`INSERT OR IGNORE`)
+  and emitted as `history_loaded` IPC events so the UI can display them.
+
+### IPC event
+
+```json
+{ "type": "history_loaded", "room": "general", "messages": [...] }
+```
+
+Emitted once per room after a `history_chunk` is fully processed. The UI
+should render these messages with a visual separator from live messages.

internal/ipc/ipc.go

@@ -214,6 +214,7 @@ func handleClient(conn net.Conn, mgr *netmgr.Manager) {
 			} else {
 				// Group chat → spec "chat" type: flat {type, mid, room, text, ts}
 				mid := randomHex(16)
+				msgID := proto.ComputeMsgID(n.Identity.PeerID(), cmd.Room, ts, cmd.Body)
 				wire, err := json.Marshal(proto.PeerMessage{
 					Type: proto.MsgChat,
 					Mid:  mid,
@@ -226,11 +227,12 @@ func handleClient(conn net.Conn, mgr *netmgr.Manager) {
 				}
 				n.Mesh.Broadcast(wire)
 				local := &proto.ChatMessage{
-					Mid:  mid,
+					Mid:   mid,
-					From: n.Identity.PeerID(),
+					MsgID: msgID,
-					Room: cmd.Room,
+					From:  n.Identity.PeerID(),
-					Text: cmd.Body,
+					Room:  cmd.Room,
-					Ts:   ts,
+					Text:  cmd.Body,
+					Ts:    ts,
 				}
 				n.Mesh.SaveMessage(local)
 				n.Mesh.Emit(proto.IpcMessage{

internal/mesh/mesh.go

@@ -2,6 +2,7 @@
 package mesh
 
 import (
+	"encoding/json"
 	"log"
 	"os"
 	"sync"
@@ -51,6 +52,12 @@ type Mesh struct {
 	// attempt to connect to. Drained by the anchor client's runOnce loop.
 	PendingConnect chan proto.PeerID
 
+	// historyRequested tracks rooms for which we have already sent a history_request
+	// this session. Reset on reconnect is intentional (new peers may have newer history).
+	historyMu        sync.Mutex
+	historyRequested map[string]bool // room → true
+	historyFirstPeer proto.PeerID    // ID of the peer we requested history from
+
 	// subscribers receive a copy of every event (fan-out to IPC clients)
 	subMu sync.Mutex
 	subs  []chan proto.IpcMessage
@@ -60,12 +67,13 @@ type Mesh struct {
 // Pass a non-nil store to enable message and peer persistence.
 func New(id *crypto.Identity, st *store.Store) *Mesh {
 	return &Mesh{
-		Identity:       id,
+		Identity:         id,
-		Store:          st,
+		Store:            st,
-		peers:          make(map[proto.PeerID]*PeerConn),
+		peers:            make(map[proto.PeerID]*PeerConn),
-		outbound:       make(map[string]*outboundTransfer),
+		outbound:         make(map[string]*outboundTransfer),
-		inbound:        make(map[string]*inboundTransfer),
+		inbound:          make(map[string]*inboundTransfer),
-		PendingConnect: make(chan proto.PeerID, 32),
+		PendingConnect:   make(chan proto.PeerID, 32),
+		historyRequested: make(map[string]bool),
 	}
 }
 
@@ -235,6 +243,122 @@ func (m *Mesh) Unsubscribe(ch <-chan proto.IpcMessage) {
 	}
 }
 
+// RequestHistoryFrom sends history_request messages to peerID for all rooms
+// we know about but haven't yet requested this session. Only contacts the first
+// peer we connect to, to avoid fan-out amplification.
+func (m *Mesh) RequestHistoryFrom(peerID proto.PeerID) {
+	if m.Store == nil {
+		return
+	}
+	m.historyMu.Lock()
+	if m.historyFirstPeer != "" && m.historyFirstPeer != peerID {
+		m.historyMu.Unlock()
+		return // only request from the first peer
+	}
+	m.historyFirstPeer = peerID
+	m.historyMu.Unlock()
+
+	rooms, err := m.Store.Rooms()
+	if err != nil {
+		return
+	}
+	// Always include "general" even if not explicitly created.
+	roomSet := map[string]bool{"general": true}
+	for _, r := range rooms {
+		roomSet[r] = true
+	}
+
+	m.historyMu.Lock()
+	var toRequest []string
+	for r := range roomSet {
+		if !m.historyRequested[r] {
+			m.historyRequested[r] = true
+			toRequest = append(toRequest, r)
+		}
+	}
+	m.historyMu.Unlock()
+
+	for _, room := range toRequest {
+		req, err := json.Marshal(proto.PeerMessage{
+			Type:  proto.MsgHistoryRequest,
+			Room:  room,
+			Limit: 200,
+		})
+		if err != nil {
+			continue
+		}
+		m.SendTo(peerID, req)
+		log.Printf("mesh: sent history_request room=%s to %s", room, peerID.Short())
+	}
+}
+
+// HandleHistoryRequest responds to a history_request from a peer.
+func (m *Mesh) HandleHistoryRequest(from proto.PeerID, room string, sinceMs int64, limit int) {
+	if m.Store == nil {
+		return
+	}
+	msgs, err := m.Store.RecentMessagesSince(room, sinceMs, limit)
+	if err != nil {
+		log.Printf("mesh: history_request from %s room=%s: %v", from.Short(), room, err)
+		return
+	}
+
+	// Look up aliases for from_peer values.
+	entries := make([]proto.HistoryEntry, 0, len(msgs))
+	for _, msg := range msgs {
+		entries = append(entries, proto.HistoryEntry{
+			Mid:       msg.Mid,
+			From:      string(msg.From),
+			FromAlias: m.Store.PeerAlias(msg.From),
+			Text:      msg.Text,
+			Ts:        msg.Ts,
+		})
+	}
+
+	chunk, err := json.Marshal(proto.PeerMessage{
+		Type:        proto.MsgHistoryChunk,
+		Room:        room,
+		History:     entries,
+		HistoryDone: true,
+	})
+	if err != nil {
+		return
+	}
+	m.SendTo(from, chunk)
+	log.Printf("mesh: sent history_chunk room=%s to %s: %d msgs", room, from.Short(), len(entries))
+}
+
+// HandleHistoryChunk saves received history messages and emits history_loaded.
+func (m *Mesh) HandleHistoryChunk(room string, entries []proto.HistoryEntry) {
+	if m.Store == nil || len(entries) == 0 {
+		return
+	}
+	var saved []proto.ChatMessage
+	for _, e := range entries {
+		msg := &proto.ChatMessage{
+			Mid:   e.Mid,
+			MsgID: e.Mid, // mid is already content-addressed for gossipped messages
+			From:  proto.PeerID(e.From),
+			Room:  room,
+			Text:  e.Text,
+			Ts:    e.Ts,
+		}
+		if err := m.Store.SaveMessage(msg); err != nil {
+			continue
+		}
+		saved = append(saved, *msg)
+	}
+	if len(saved) == 0 {
+		return
+	}
+	m.emit(proto.IpcMessage{
+		Type:     proto.EvtHistoryLoaded,
+		Room:     room,
+		Messages: saved,
+	})
+	log.Printf("mesh: history_chunk room=%s: %d/%d new messages", room, len(saved), len(entries))
+}
+
 // Emit sends an event to all IPC subscribers (exported for ipc/nat packages).
 func (m *Mesh) Emit(msg proto.IpcMessage) {
 	m.emit(msg)

internal/mesh/peer.go

@@ -197,6 +197,8 @@ func handleDCMessage(data []byte, from proto.PeerID, localID *crypto.Identity, m
 		})
 		// Tell the new peer about everyone we can currently see.
 		go m.sendGossipTo(from)
+		// Request message history from this peer (EXT-007).
+		go m.RequestHistoryFrom(from)
 		return
 	}
 
@@ -212,11 +214,12 @@ func dispatchPeerMessage(msg proto.PeerMessage, from proto.PeerID, m *Mesh) {
 	switch msg.Type {
 	case proto.MsgChat:
 		chat := &proto.ChatMessage{
-			Mid:  midOrRandom(msg.Mid),
+			Mid:   midOrRandom(msg.Mid),
-			From: from,
+			MsgID: proto.ComputeMsgID(from, msg.Room, msg.Ts, msg.Text),
-			Room: msg.Room,
+			From:  from,
-			Text: msg.Text,
+			Room:  msg.Room,
-			Ts:   msg.Ts,
+			Text:  msg.Text,
+			Ts:    msg.Ts,
 		}
 		m.SaveMessage(chat)
 		m.Emit(proto.IpcMessage{Type: proto.EvtMessageReceived, Message: chat})
@@ -298,6 +301,12 @@ func dispatchPeerMessage(msg proto.PeerMessage, from proto.PeerID, m *Mesh) {
 			}
 		}
 		log.Printf("mesh: gossip from %s: %d hints, %d new", from.Short(), len(msg.Gossip.Peers), newPeers)
+	case proto.MsgHistoryRequest:
+		go m.HandleHistoryRequest(from, msg.Room, msg.Since, msg.Limit)
+
+	case proto.MsgHistoryChunk:
+		go m.HandleHistoryChunk(msg.Room, msg.History)
+
 	case proto.MsgPing:
 		log.Printf("mesh: ping from %s", from.Short())
 	case proto.MsgPong:

internal/proto/proto.go

@@ -3,7 +3,11 @@
 // Binary data (keys, signatures) is hex-encoded; signaling boxes are base64.
 package proto
 
-import "time"
+import (
+	"crypto/sha256"
+	"fmt"
+	"time"
+)
 
 // ── Identity ──────────────────────────────────────────────────────────────────
 
@@ -43,8 +47,10 @@ const (
 	MsgFileAccept   MsgType = "file-accept"
 	MsgFileCancel   MsgType = "file-cancel"
 	MsgFileDone     MsgType = "file-done"
-	MsgPing         MsgType = "ping"
+	MsgPing            MsgType = "ping"
-	MsgPong         MsgType = "pong"
+	MsgPong            MsgType = "pong"
+	MsgHistoryRequest  MsgType = "history_request"
+	MsgHistoryChunk    MsgType = "history_chunk"
 )
 
 // PmMessage is a private message sent directly over a single peer link (§8 "pm").
@@ -83,17 +89,43 @@ type PeerMessage struct {
 	Reason string `json:"reason,omitempty"` // file-cancel
 
 	Seq *uint64 `json:"seq,omitempty"` // ping/pong
+
+	// history_request fields
+	Since int64 `json:"since,omitempty"` // Unix ms; 0 = no lower bound
+	Limit int   `json:"limit,omitempty"`
+
+	// history_chunk fields
+	History     []HistoryEntry `json:"history,omitempty"`
+	HistoryDone bool           `json:"history_done,omitempty"`
+}
+
+// HistoryEntry is one message in a history_chunk response.
+type HistoryEntry struct {
+	Mid       string `json:"mid"`
+	From      string `json:"from"`       // peer ID hex
+	FromAlias string `json:"from_alias"` // advisory
+	Text      string `json:"text"`
+	Ts        int64  `json:"ts"` // Unix ms
 }
 
 // ChatMessage is a group chat message (wire type "chat", §8).
 // Also used internally for persisting PMs after they are received.
 type ChatMessage struct {
-	Mid  string `json:"mid,omitempty"` // optional dedup id (required when relay hops > 0)
+	Mid   string  `json:"mid,omitempty"`   // optional dedup id (required when relay hops > 0)
-	From PeerID `json:"from,omitempty"` // set by receiver from DC context; not on wire for pm
+	MsgID string  `json:"msg_id,omitempty"` // EXT-007: content-addressed gossip ID
-	To   *PeerID `json:"to,omitempty"`  // internal only — not transmitted; set for DMs
+	From  PeerID  `json:"from,omitempty"`  // set by receiver from DC context; not on wire for pm
-	Room string `json:"room"`
+	To    *PeerID `json:"to,omitempty"`    // internal only — not transmitted; set for DMs
-	Text string `json:"text"`
+	Room  string  `json:"room"`
-	Ts   int64  `json:"ts"` // Unix milliseconds
+	Text  string  `json:"text"`
+	Ts    int64   `json:"ts"` // Unix milliseconds
+}
+
+// ComputeMsgID returns the EXT-007 content-addressed ID for a message.
+// sha256(fromID \x00 room \x00 ts_decimal \x00 text)
+func ComputeMsgID(fromID PeerID, room string, ts int64, text string) string {
+	h := sha256.New()
+	fmt.Fprintf(h, "%s\x00%s\x00%d\x00%s", string(fromID), room, ts, text)
+	return fmt.Sprintf("sha256:%x", h.Sum(nil))
 }
 
 // PeerGossip shares known peer addresses.
@@ -259,6 +291,7 @@ const (
 	EvtIdentityImported IpcMsgType = "identity_imported"
 	EvtSharesList       IpcMsgType = "shares_list"
 	EvtRoomCreated      IpcMsgType = "room_created"    // field: room (name)
+	EvtHistoryLoaded    IpcMsgType = "history_loaded"  // fields: room, messages
 )
 
 // NetworkInfo summarises one joined network for state_snapshot and network_joined events.
@@ -312,7 +345,8 @@ type IpcMessage struct {
 	Networks       []NetworkInfo `json:"networks,omitempty"`
 	ErrorMessage   string       `json:"error_message,omitempty"`
 	InviteGenerated string      `json:"invite,omitempty"`
-	Files          []FileEntry  `json:"files,omitempty"`
+	Files          []FileEntry    `json:"files,omitempty"`
+	Messages       []ChatMessage  `json:"messages,omitempty"` // history_loaded
 	Shares         []ShareEntry `json:"shares,omitempty"`
 	ShareNetworks  []string     `json:"network_ids,omitempty"` // for add_share command: scope to specific network IDs, or ["*"] for global
 	// export_identity / import_identity

internal/store/store.go

@@ -5,6 +5,7 @@ package store
 import (
 	"database/sql"
 	"fmt"
+	"strings"
 	"time"
 
 	_ "modernc.org/sqlite"
@@ -35,6 +36,14 @@ CREATE TABLE IF NOT EXISTS rooms (
 );
 `
 
+// migrations run after the base schema. ALTER TABLE ADD COLUMN fails with
+// "duplicate column name" on subsequent opens — we swallow that error.
+var migrations = []string{
+	// EXT-007: canonical message ID for history dedup (NULL for pre-feature messages).
+	`ALTER TABLE messages ADD COLUMN msg_id TEXT`,
+	`CREATE UNIQUE INDEX IF NOT EXISTS idx_messages_msg_id ON messages (msg_id) WHERE msg_id IS NOT NULL`,
+}
+
 // Store is a local SQLite-backed message and peer store.
 type Store struct {
 	db *sql.DB
@@ -51,6 +60,12 @@ func Open(path string) (*Store, error) {
 		db.Close()
 		return nil, fmt.Errorf("migrate db: %w", err)
 	}
+	for _, m := range migrations {
+		if _, err := db.Exec(m); err != nil && !strings.Contains(err.Error(), "duplicate column") {
+			db.Close()
+			return nil, fmt.Errorf("migration %q: %w", m, err)
+		}
+	}
 	return &Store{db: db}, nil
 }
 
@@ -64,9 +79,9 @@ func (s *Store) Close() error {
 func (s *Store) SaveMessage(msg *proto.ChatMessage) error {
 	sentAt := time.UnixMilli(msg.Ts).UTC()
 	_, err := s.db.Exec(
-		`INSERT OR IGNORE INTO messages (mid, room, from_peer, body, sent_at)
+		`INSERT OR IGNORE INTO messages (mid, msg_id, room, from_peer, body, sent_at)
-		 VALUES (?, ?, ?, ?, ?)`,
+		 VALUES (?, ?, ?, ?, ?, ?)`,
-		msg.Mid, msg.Room, string(msg.From), msg.Text, sentAt,
+		msg.Mid, nullableString(msg.MsgID), msg.Room, string(msg.From), msg.Text, sentAt,
 	)
 	return err
 }
@@ -91,14 +106,31 @@ func (s *Store) PeerAlias(peerID proto.PeerID) string {
 
 // RecentMessages returns up to limit messages for a room, oldest first.
 func (s *Store) RecentMessages(room string, limit int) ([]proto.ChatMessage, error) {
-	rows, err := s.db.Query(
+	return s.queryMessages(
-		`SELECT mid, from_peer, body, sent_at
+		`SELECT mid, from_peer, body, sent_at FROM messages
-		 FROM messages
 		 WHERE room = ?
-		 ORDER BY sent_at DESC
+		 ORDER BY sent_at DESC LIMIT ?`,
-		 LIMIT ?`,
 		room, limit,
 	)
+}
+
+// RecentMessagesSince returns up to limit messages for a room with ts > sinceMs, oldest first.
+// Only messages that have a msg_id (i.e. gossip-safe) are returned.
+func (s *Store) RecentMessagesSince(room string, sinceMs int64, limit int) ([]proto.ChatMessage, error) {
+	if limit <= 0 || limit > 500 {
+		limit = 500
+	}
+	since := time.UnixMilli(sinceMs).UTC()
+	return s.queryMessages(
+		`SELECT mid, from_peer, body, sent_at FROM messages
+		 WHERE room = ? AND sent_at > ? AND msg_id IS NOT NULL
+		 ORDER BY sent_at DESC LIMIT ?`,
+		room, since, limit,
+	)
+}
+
+func (s *Store) queryMessages(q string, args ...any) ([]proto.ChatMessage, error) {
+	rows, err := s.db.Query(q, args...)
 	if err != nil {
 		return nil, err
 	}
@@ -113,7 +145,6 @@ func (s *Store) RecentMessages(room string, limit int) ([]proto.ChatMessage, err
 			return nil, err
 		}
 		m.From = proto.PeerID(from)
-		m.Room = room
 		m.Ts = sentAt.UnixMilli()
 		msgs = append(msgs, m)
 	}
@@ -168,3 +199,10 @@ func (s *Store) KnownPeers() (map[proto.PeerID]string, error) {
 	}
 	return out, rows.Err()
 }
+
+func nullableString(s string) any {
+	if s == "" {
+		return nil
+	}
+	return s
+}