feat(history): 会话列表lastMessage预览(B3) — 单查询无N+1

- history/session_preview.go: SessionMeta.LastMessage + 单条SQL(LEFT JOIN取每会话ORDER BY created_at DESC,rowid DESC LIMIT 1,走覆盖索引)
- ListSessions 迁出重写(签名不变), content折叠空白截前60rune
- gateway: historySessionSummary 增 lastMessage{role,content,createdAt}(omitempty), /sessions与/admin/sessions共用
- 新增测试(session_preview_test, history_lastmessage_test)
- Docs/api-client.md v0.22 同步

Docs/api-client.md

@@ -24,7 +24,7 @@
 | 1 | `/api/v1/auth/login` | POST | 否 | ✅ | `{username,password}` → `{data:{token,expiresAt,role}}`（24h，role=owner） |
 | 2 | `/api/v1/health` | GET | 否 | ✅ | 健康检查 `{data:{status:"ok"}}` |
 | 3 | `/api/v1/chat` | POST | JWT | ✅ | `{text,image?,deviceId?,mode?,stream?}` → `{data:{reply}}`；`stream:true` 时经 WS 触发流式。`image` 为 base64 data: URI 或 http(s) URL，入视觉路由（design/06）——**真视觉需配 `LLM_VISION_MODEL`/凭据，无凭据时降级为主模型直接看图或视觉·OCR 工具文本描述**；`deviceId` 为**客户端生成的具体端标识**（design/16 §4.5，如 `web:chrome:a1b2c3`，可选），落库到消息、便于主人知道从哪个端发来；`mode` 为**行为模式**（design/01 §3 / design/16 §5.1，如 `chat`/`code`/`home`/`docs`/…，可选），随该轮次落库——IM 恒 `chat`、缺省回落 `chat`，供前端按模式渲染气泡（仅 `chat` 才解析 `<p>` 拆气泡）。**会话编排（design/15）**：非流式且该会话「正忙」时**不阻塞**，返回 `202` `{data:{queued:true,queueDepth}}`（稍后按上下文续接）；待处理队列满时返回 `409` `{error:{code:"chat_busy"}}` |
-| 4 | `/api/v1/sessions` | GET | JWT | ✅ | `{data:[session...]}`。**会话持久化（design/16）**：注入 HistoryStore 后返回持久化会话元数据 `{data:[{id,type("main"|"sub"),title?,createdAt}]}`（主会话 `id="web"` 恒在首位，次级 `web:<uuid>`）；未注入时退化回渠道会话列表 |
+| 4 | `/api/v1/sessions` | GET | JWT | ✅ | `{data:[session...]}`。**会话持久化（design/16）**：注入 HistoryStore 后返回持久化会话元数据 `{data:[{id,type("main"|"sub"),title?,createdAt,lastMessage?}]}`（主会话 `id="web"` 恒在首位，次级 `web:<uuid>`；`lastMessage` 为**最后一条消息预览** `{content,role(user|assistant),createdAt}`，content 折叠空白截前 60 rune，空会话省略）；未注入时退化回渠道会话列表 |
 | 5 | `/api/v1/sessions` | POST | JWT(owner/admin) | ✅ | **新建次级会话** → `201 {data:{sessionId}}`（`sessionId="web:<uuid>"`）；未注入 HistoryStore 回 `501 not_implemented` |
 | 6 | `/api/v1/sessions/{id}/messages` | GET | JWT(owner/admin) | ✅ | **热分页拉历史**：`?limit=&before=&platform=` 默认 `limit=50`（上限 200）、`before`=上一页最旧 `createdAt`（毫秒），按时间倒序 `{data:[{id,sessionId,role,content,mode?,mood?,image?,thinking?,platform?,deviceId?,senderName?,createdAt}]}`；`platform` 为**可选**来源平台过滤（design/16 §4.4）——Web 端不传（=全量共享主会话），IM 适配器传**本平台**（如 `telegram`）只拉本渠道消息；`deviceId` 为**客户端具体端标识**（design/16 §4.5，如 `web:chrome:a1b2c3` / `weixin-ilink:conf0:wx_uid`）；`senderName` 为**第三方发送者昵称**（design/16 §4.6，群聊/陌生人私聊消息带，主人私聊为空，`omitempty`）；非法 `limit`/`before` 回 `400`；未注入 HistoryStore 回 `501` |
 | 7 | `/api/v1/sessions/{id}` | DELETE | JWT(owner/admin) | ✅ | **删除次级会话及其消息** → `{data:{deleted:<id>}}`；主会话不可删回 `403 forbidden`；未注入 HistoryStore 回 `501` |
@@ -205,6 +205,7 @@
 - 语音：上行 `voice_input`（base64 音频或已转写 text），下行 `voice_output`（合成音频 base64 + text）。
 
 ## 5. 变更记录
+- `v0.22`（**B3 会话列表 lastMessage 预览**）：`GET /api/v1/sessions` 与 `/api/v1/admin/sessions` 每个会话新增可选 **`lastMessage`** `{content,role,createdAt}`（content 折叠空白截前 60 rune + `…`；空会话省略），**单条 SQL**（LEFT JOIN 取每会话 `ORDER BY created_at DESC,rowid DESC LIMIT 1`，无 N+1，走 `idx_chat_msgs_session_created` 覆盖索引）。`unreadCount` 需已读游标设计，后置。
 - `v0.21`（**A4 会话 title**）：`PATCH /api/v1/sessions/{id}`（**新增**，body `{title}` ≤80 rune → `{data:{id,title}}`；空/超长 400、不存在 404、主会话可改名）；会话**首条用户消息到达时自动命名**（`history.append` 前 `ensureTitle`：折叠空白取前 20 rune，纯空白回落 `主会话`/`次级会话`），仅首次命名、不覆盖。`HistoryStore` 新增 `RenameSession`；隔离会话（`sess-xxx`）不在 `chat_sessions` 有行、不自动命名（其本不在 `/sessions` 列表）。**边界**：前端 `createSubSession(title?)` 的 POST body 含 `title` 但后端暂忽略（会被首条消息自动命名覆盖），未处理。
 - `v0.20`（**A3 消息 mode 落库**）：`POST /api/v1/chat` 与 `ws.chat` 上行新增**可选 `mode`**（行为模式，design/01 §3 / design/16 §5.1）。后端把每条持久化消息的 `Mode` **真实写入**（IM 路径恒 `chat`；Web 路径经 ctx 透传本轮 `mode`，缺省回落 `chat`；恢复/续接路径沿用发起轮次的 `mode`），消息 DTO、`message_persisted`、`channel_persisted` 的 `mode` 不再恒空，供前端按模式渲染气泡（**仅 `chat` 解析 `<p>` 拆气泡**，其它模式原样整条）。
 - `v0.19`（**百炼声音复刻 create_voice + TTS instruction 情绪驱动，主 TTS 用昔涟复刻音色**）：`POST /api/v1/audio/voice-clone`（**新增**，owner-only + 审计，body `{url:"<公网参考音频URL>", prefix?}` → `{data:{voiceId}}`）——走后端 `voice.DashscopeCloner`（百炼 `voice-enrollment` create_voice，`POST {base}/services/audio/tts/customization`，`input{action:create_voice, target_model:VOICE_TTS_MODEL, prefix, url}`，**只收公网 URL 不收 base64**，返回 `output.voice_id`；**URL 不打日志不写审计**）。`POST /api/v1/audio/speech` 新增**可选 `instruction`**（per-request 情绪/语气指令，如 `"语气开朗俏皮，像女朋友撒娇"`），经 ctx 透传给 TTS adapter；**只对复刻/设计音色生效**（`cosyvoice-v3-flash` 复刻音色可任意指令；系统音色 `longxiaochun_v3` 需固定格式，服务端 `canInstruct` 判定后**永不发送**）。config 新增 `VOICE_TTS_INSTRUCTION`（默认 `""`）、`VOICE_CLONE_PREFIX`（默认 `cyrene`，≤10 字母/数字）；`voice.Chain`/`voice.Service` 新增 `CloneVoice`，`VoiceCloner` 接口（cloud rung 提供；未配 cloud 回 `voice_unavailable`）。**⚠️ 真建昔涟音色仍待主人提供公网参考音频 URL（当前无）**，且 CosyVoice 声音复刻仅华北2北京地域（确认 key 地域）；TTS instruction 用现有复刻音色或先仅单测请求体，避免改系统音色行为。

internal/runtime/history/session_preview.go

@@ -0,0 +1,103 @@
+package history
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"strings"
+)
+
+// This file owns the last-message preview carried in the session list
+// (GET /api/v1/sessions): a display-only summary of each session's newest message so a
+// client can render a conversation row without one request per session. The preview is
+// truncated for display and never feeds identity or permission decisions — identity
+// comes from channel actor metadata / JWT (project red line).
+
+// lastMessagePreviewRunes bounds the preview content so a long message cannot bloat the
+// session-list response. A longer message is whitespace-collapsed and truncated to this
+// many runes with an ellipsis.
+const lastMessagePreviewRunes = 60
+
+// MessageSummary is a display-only preview of a session's most recent message.
+type MessageSummary struct {
+	Role      string // "user" | "assistant"
+	Content   string // whitespace-collapsed, truncated preview
+	CreatedAt int64  // Unix milliseconds
+}
+
+// listSessionsQuery reads every session plus its newest message in one statement. The
+// LEFT JOIN's correlated subquery resolves each session's newest row via the
+// (session_id, created_at DESC) index (rowid DESC breaks a same-millisecond tie by
+// insertion order), so a session with no messages still appears with NULL preview
+// columns. Ordering keeps the main session first, then subs oldest-first.
+const listSessionsQuery = `SELECT s.id, s.type, s.title, s.created_at,
+		m.role, m.content, m.created_at
+	FROM chat_sessions s
+	LEFT JOIN chat_messages m ON m.rowid = (
+		SELECT rowid FROM chat_messages
+		WHERE session_id = s.id
+		ORDER BY created_at DESC, rowid DESC
+		LIMIT 1
+	)
+	ORDER BY (s.type = ?) DESC, s.created_at ASC`
+
+// ListSessions returns the main session first, then every sub session, oldest first,
+// each carrying a LastMessage preview of its newest message (nil when the session has
+// none). Sessions and previews are read in a single query, so the query count does not
+// grow with the number of sessions. The returned slice is non-nil.
+func (s *Store) ListSessions(ctx context.Context) ([]SessionMeta, error) {
+	rows, err := s.db.QueryContext(ctx, listSessionsQuery, SessionMain)
+	if err != nil {
+		return nil, fmt.Errorf("history: list sessions: %w", err)
+	}
+	defer func() { _ = rows.Close() }()
+
+	sessions := make([]SessionMeta, 0)
+	for rows.Next() {
+		meta, err := scanSessionMeta(rows)
+		if err != nil {
+			return nil, fmt.Errorf("history: scan session: %w", err)
+		}
+		sessions = append(sessions, meta)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, fmt.Errorf("history: iterate sessions: %w", err)
+	}
+	return sessions, nil
+}
+
+// scanSessionMeta reads one joined chat_sessions + last-message row. The three message
+// columns are NULL for a session that has never received a message, so they scan into
+// nullable holders and only produce a LastMessage when a row is present.
+func scanSessionMeta(sc rowScanner) (SessionMeta, error) {
+	var (
+		meta    SessionMeta
+		typeStr string
+		role    sql.NullString
+		content sql.NullString
+		lastAt  sql.NullInt64
+	)
+	if err := sc.Scan(&meta.ID, &typeStr, &meta.Title, &meta.CreatedAt, &role, &content, &lastAt); err != nil {
+		return SessionMeta{}, err
+	}
+	meta.Type = typeStr
+	if role.Valid && lastAt.Valid {
+		meta.LastMessage = &MessageSummary{
+			Role:      role.String,
+			Content:   previewContent(content.String),
+			CreatedAt: lastAt.Int64,
+		}
+	}
+	return meta, nil
+}
+
+// previewContent collapses whitespace runs (newlines included) to single spaces and
+// truncates to lastMessagePreviewRunes runes with an ellipsis. It is display-only.
+func previewContent(content string) string {
+	joined := strings.Join(strings.Fields(content), " ")
+	runes := []rune(joined)
+	if len(runes) <= lastMessagePreviewRunes {
+		return joined
+	}
+	return string(runes[:lastMessagePreviewRunes]) + "…"
+}

internal/runtime/history/session_preview_test.go

@@ -0,0 +1,134 @@
+package history
+
+import (
+	"context"
+	"strings"
+	"testing"
+)
+
+// sessionOf returns the SessionMeta of one session from ListSessions, failing the test
+// when the session is absent.
+func sessionOf(t *testing.T, st *Store, id string) SessionMeta {
+	t.Helper()
+	sessions, err := st.ListSessions(context.Background())
+	if err != nil {
+		t.Fatalf("list sessions: %v", err)
+	}
+	for _, m := range sessions {
+		if m.ID == id {
+			return m
+		}
+	}
+	t.Fatalf("session %q not found in %+v", id, sessions)
+	return SessionMeta{}
+}
+
+func TestListSessionsLastMessagePreview(t *testing.T) {
+	t.Parallel()
+	st := newStore(t)
+	ctx := context.Background()
+
+	// The empty main session has no preview.
+	if got := sessionOf(t, st, MainSessionID); got.LastMessage != nil {
+		t.Fatalf("empty main LastMessage = %+v, want nil", got.LastMessage)
+	}
+
+	sub, err := st.NewSubSession(ctx, "owner")
+	if err != nil {
+		t.Fatalf("new sub: %v", err)
+	}
+	if err := st.Append(ctx, Message{SessionID: MainSessionID, Role: RoleUser, Content: "first", CreatedAt: 1000}); err != nil {
+		t.Fatalf("append main user: %v", err)
+	}
+	if err := st.Append(ctx, Message{SessionID: MainSessionID, Role: RoleAssistant, Content: "latest reply", CreatedAt: 2000}); err != nil {
+		t.Fatalf("append main assistant: %v", err)
+	}
+	if err := st.Append(ctx, Message{SessionID: sub, Role: RoleUser, Content: "sub question", CreatedAt: 1500}); err != nil {
+		t.Fatalf("append sub: %v", err)
+	}
+
+	// Main stays first and carries its newest message, not the first one.
+	sessions, err := st.ListSessions(ctx)
+	if err != nil {
+		t.Fatalf("list sessions: %v", err)
+	}
+	if len(sessions) != 2 || sessions[0].ID != MainSessionID {
+		t.Fatalf("sessions = %+v, want main first", sessions)
+	}
+	main := sessions[0].LastMessage
+	if main == nil || main.Role != RoleAssistant || main.Content != "latest reply" || main.CreatedAt != 2000 {
+		t.Fatalf("main LastMessage = %+v, want assistant/latest reply/2000", main)
+	}
+	subPreview := sessions[1].LastMessage
+	if subPreview == nil || subPreview.Role != RoleUser || subPreview.Content != "sub question" || subPreview.CreatedAt != 1500 {
+		t.Fatalf("sub LastMessage = %+v, want user/sub question/1500", subPreview)
+	}
+
+	// A sub session created after the main one but never written has no preview.
+	empty, err := st.NewSubSession(ctx, "owner")
+	if err != nil {
+		t.Fatalf("new empty sub: %v", err)
+	}
+	if got := sessionOf(t, st, empty); got.LastMessage != nil {
+		t.Fatalf("empty sub LastMessage = %+v, want nil", got.LastMessage)
+	}
+}
+
+func TestListSessionsLastMessageTruncatesAndCollapses(t *testing.T) {
+	t.Parallel()
+	st := newStore(t)
+	ctx := context.Background()
+
+	long := strings.Repeat("字", lastMessagePreviewRunes+10) + "\n\n尾部"
+	if err := st.Append(ctx, Message{SessionID: MainSessionID, Role: RoleUser, Content: long, CreatedAt: 1000}); err != nil {
+		t.Fatalf("append long: %v", err)
+	}
+	want := strings.Repeat("字", lastMessagePreviewRunes) + "…"
+	if got := sessionOf(t, st, MainSessionID).LastMessage; got == nil || got.Content != want {
+		t.Fatalf("truncated preview = %+v, want %q", got, want)
+	}
+
+	// Whitespace runs (including newlines) collapse to single spaces, and a short
+	// message is returned whole.
+	sub, err := st.NewSubSession(ctx, "owner")
+	if err != nil {
+		t.Fatalf("new sub: %v", err)
+	}
+	if err := st.Append(ctx, Message{SessionID: sub, Role: RoleUser, Content: "  你好\n\t世界  ", CreatedAt: 1000}); err != nil {
+		t.Fatalf("append spaced: %v", err)
+	}
+	if got := sessionOf(t, st, sub).LastMessage; got == nil || got.Content != "你好 世界" {
+		t.Fatalf("collapsed preview = %+v, want %q", got, "你好 世界")
+	}
+}
+
+func TestListSessionsLastMessageTieBreaksByInsertion(t *testing.T) {
+	t.Parallel()
+	st := newStore(t)
+	ctx := context.Background()
+
+	// Two messages sharing a millisecond timestamp: the later insert is the newest.
+	for _, content := range []string{"older", "newer"} {
+		if err := st.Append(ctx, Message{SessionID: MainSessionID, Role: RoleAssistant, Content: content, CreatedAt: 5000}); err != nil {
+			t.Fatalf("append %s: %v", content, err)
+		}
+	}
+	if got := sessionOf(t, st, MainSessionID).LastMessage; got == nil || got.Content != "newer" {
+		t.Fatalf("tie-break preview = %+v, want newer", got)
+	}
+}
+
+func TestPreviewContentBounds(t *testing.T) {
+	t.Parallel()
+	if got := previewContent(""); got != "" {
+		t.Fatalf("empty preview = %q, want empty", got)
+	}
+	exact := strings.Repeat("a", lastMessagePreviewRunes)
+	if got := previewContent(exact); got != exact {
+		t.Fatalf("exact-length preview changed: %q", got)
+	}
+	over := strings.Repeat("a", lastMessagePreviewRunes+1)
+	if got := previewContent(over); got != exact+"…" {
+		t.Fatalf("over-length preview = %q, want ellipsis", got)
+	}
+}

internal/runtime/history/store.go

@@ -109,6 +109,11 @@ type SessionMeta struct {
 	Type      string // "main" | "sub"
 	Title     string
 	CreatedAt int64
+	// LastMessage is a display-only preview of the session's newest message, filled
+	// by ListSessions in the same query. It is nil for a session with no messages.
+	// A client uses it to render the conversation row without a per-session request;
+	// identity and permissions never derive from it.
+	LastMessage *MessageSummary
 }
 
 // HistoryStore abstracts the chat-history store so callers (the gateway) and tests can
@@ -481,36 +486,6 @@ func (s *Store) NewSubSession(ctx context.Context, ownerID string) (string, erro
 	return sessionID, nil
 }
 
-// ListSessions returns the main session first, then every sub session, oldest first. The
-// returned slice is non-nil.
-func (s *Store) ListSessions(ctx context.Context) ([]SessionMeta, error) {
-	rows, err := s.db.QueryContext(ctx, `SELECT id, type, title, created_at FROM chat_sessions
-		ORDER BY (type = ?) DESC, created_at ASC`, SessionMain)
-	if err != nil {
-		return nil, fmt.Errorf("history: list sessions: %w", err)
-	}
-	defer func() { _ = rows.Close() }()
-
-	sessions := make([]SessionMeta, 0)
-	for rows.Next() {
-		var (
-			meta      SessionMeta
-			typeStr   string
-			createdAt int64
-		)
-		if err := rows.Scan(&meta.ID, &typeStr, &meta.Title, &createdAt); err != nil {
-			return nil, fmt.Errorf("history: scan session: %w", err)
-		}
-		meta.Type = typeStr
-		meta.CreatedAt = createdAt
-		sessions = append(sessions, meta)
-	}
-	if err := rows.Err(); err != nil {
-		return nil, fmt.Errorf("history: iterate sessions: %w", err)
-	}
-	return sessions, nil
-}
-
 // ensureMainSession inserts the fixed main session row when it is absent, so ListSessions
 // always reports the main session even before any message is written.
 func (s *Store) ensureMainSession(ctx context.Context) error {

internal/services/gateway/history.go

@@ -20,6 +20,18 @@ type historySessionSummary struct {
 	Type      string `json:"type"` // "main" | "sub"
 	Title     string `json:"title,omitempty"`
 	CreatedAt int64  `json:"createdAt"`
+	// LastMessage is a display-only preview of the session's newest message, so the
+	// client can render a conversation row (e.g. the Android session list) without a
+	// per-session request. It is omitted for a session with no messages. Content is
+	// already truncated by the store.
+	LastMessage *historyLastMessage `json:"lastMessage,omitempty"`
+}
+
+// historyLastMessage is the wire shape of the last-message preview in a session summary.
+type historyLastMessage struct {
+	Role      string `json:"role"` // "user" | "assistant"
+	Content   string `json:"content"`
+	CreatedAt int64  `json:"createdAt"`
 }
 
 // historyMessage is the wire shape of one persisted chat message in the
@@ -60,15 +72,25 @@ func (s *Server) handleHistorySessions(w http.ResponseWriter, r *http.Request) {
 	out := make([]historySessionSummary, 0, len(meta))
 	for _, m := range meta {
 		out = append(out, historySessionSummary{
-			ID:        m.ID,
-			Type:      m.Type,
-			Title:     m.Title,
-			CreatedAt: m.CreatedAt,
+			ID:          m.ID,
+			Type:        m.Type,
+			Title:       m.Title,
+			CreatedAt:   m.CreatedAt,
+			LastMessage: toHistoryLastMessage(m.LastMessage),
 		})
 	}
 	writeJSON(w, http.StatusOK, out)
 }
 
+// toHistoryLastMessage maps a stored last-message preview to its wire shape, returning
+// nil for a session that has no messages (so the field is omitted from the JSON).
+func toHistoryLastMessage(m *history.MessageSummary) *historyLastMessage {
+	if m == nil {
+		return nil
+	}
+	return &historyLastMessage{Role: m.Role, Content: m.Content, CreatedAt: m.CreatedAt}
+}
+
 // handleCreateSubSession creates a new sub session and returns its sessionId. It is a
 // user-side write, so it is owner/admin-gated like /api/v1/chat.
 func (s *Server) handleCreateSubSession(w http.ResponseWriter, r *http.Request) {

internal/services/gateway/history_lastmessage_test.go

@@ -0,0 +1,61 @@
+package gateway
+
+import (
+	"context"
+	"net/http"
+	"testing"
+
+	"git.yeij.top/AskaEth/Cyrene_v2/internal/auth"
+	"git.yeij.top/AskaEth/Cyrene_v2/internal/runtime/history"
+)
+
+// TestHistorySessionsLastMessage asserts the GET /api/v1/sessions wire contract for the
+// last-message preview (B3): every session carries lastMessage {role,content,createdAt}
+// with the newest message's whitespace-collapsed content, and a message-less session
+// omits the field.
+func TestHistorySessionsLastMessage(t *testing.T) {
+	t.Parallel()
+	_, ts, jwt, st := newHistoryEnv(t)
+	ctx := context.Background()
+	if err := st.Append(ctx, history.Message{SessionID: history.MainSessionID, Role: history.RoleUser, Content: "你好", CreatedAt: 1000}); err != nil {
+		t.Fatalf("seed user: %v", err)
+	}
+	if err := st.Append(ctx, history.Message{SessionID: history.MainSessionID, Role: history.RoleAssistant, Content: "  在的\n请说  ", CreatedAt: 2000}); err != nil {
+		t.Fatalf("seed assistant: %v", err)
+	}
+	sub, err := st.NewSubSession(ctx, "owner")
+	if err != nil {
+		t.Fatalf("new sub: %v", err)
+	}
+	tok := signAny(t, jwt, "acct", auth.RoleOwner)
+
+	resp, body := doReq(t, http.MethodGet, ts.URL+"/api/v1/sessions", nil, tok)
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf("status = %d, want 200 (body=%s)", resp.StatusCode, body)
+	}
+	// Decode into a JSON-tagged shape so the wire contract (key names) is asserted,
+	// not just the in-package struct.
+	var sessions []struct {
+		ID          string `json:"id"`
+		LastMessage *struct {
+			Role      string `json:"role"`
+			Content   string `json:"content"`
+			CreatedAt int64  `json:"createdAt"`
+		} `json:"lastMessage"`
+	}
+	decodeDataSlice(t, body, &sessions)
+	if len(sessions) != 2 || sessions[0].ID != history.MainSessionID {
+		t.Fatalf("sessions = %+v, want main first", sessions)
+	}
+	main := sessions[0].LastMessage
+	if main == nil || main.Role != history.RoleAssistant || main.Content != "在的 请说" || main.CreatedAt != 2000 {
+		t.Fatalf("main lastMessage = %+v, want assistant/在的 请说/2000", main)
+	}
+	// The empty sub session omits lastMessage entirely.
+	if sessions[1].ID != sub {
+		t.Fatalf("sessions[1].id = %q, want %q", sessions[1].ID, sub)
+	}
+	if sessions[1].LastMessage != nil {
+		t.Fatalf("empty sub lastMessage = %+v, want omitted", sessions[1].LastMessage)
+	}
+}