diff --git a/adapters/native/backend.go b/adapters/native/backend.go
index 6194ca6..c26d69e 100644
--- a/adapters/native/backend.go
+++ b/adapters/native/backend.go
@@ -45,12 +45,24 @@ type Options struct {
 	MCP *mcp.Manager
 	// SkillsRoots 可选：全局技能根目录；执行时自动并入工作区 .agent/skills/。
 	SkillsRoots []string
+	// Compact 可选：转录超过阈值时把头部历史压成摘要（长会话上下文治理）。
+	Compact *CompactConfig
 	// Log 可选。nil 时丢弃日志。
 	Log *logsys.Logger
 	// ExecutionID 写入日志，不参与执行控制。
 	ExecutionID string
 }
 
+// CompactConfig 声明长会话压缩策略（优化性质：压缩失败不阻断执行，但留日志）。
+type CompactConfig struct {
+	// ThresholdChars 触发阈值（按字节数近似），默认 300_000。
+	ThresholdChars int
+	// KeepLastN 压缩后保留的最近消息条数，默认 8。
+	KeepLastN int
+	// Summarizer 可选注入摘要函数；缺省用后端 stream 做一次无工具调用。
+	Summarizer func(ctx context.Context, messages []llm.Message) (string, error)
+}
+
 // Backend 以自研内核实现 exec.Backend。
 type Backend struct {
 	opts Options
@@ -168,6 +180,13 @@ func (b *Backend) Execute(ctx context.Context, req exec.Request) (*exec.Result,
 		client := &llm.Client{BaseURL: b.opts.APIBase, APIKey: b.opts.APIKey}
 		stream = client.Stream
 	}
+	_ = store.SaveMeta(map[string]string{
+		"model":   model,
+		"scope":   req.SessionScope,
+		"exec_id": req.ExecutionID,
+		"updated": time.Now().UTC().Format(time.RFC3339),
+	})
+	b.maybeCompact(ctx, store, stream, model)
 
 	timeout := req.Timeout
 	if timeout <= 0 {
@@ -301,6 +320,74 @@ func (b *Backend) run(ctx context.Context, model string, budget *runtime.Budget,
 	return runRes, err
 }
 
+// maybeCompact 在转录超过阈值时压缩头部历史。压缩是优化性质：
+// 失败不阻断执行（按原文继续），但必须留日志，不静默。
+func (b *Backend) maybeCompact(ctx context.Context, store *session.Store, stream llm.StreamFunc, model string) {
+	cfg := b.opts.Compact
+	if cfg == nil {
+		return
+	}
+	threshold := int64(cfg.ThresholdChars)
+	if threshold <= 0 {
+		threshold = 300_000
+	}
+	keep := cfg.KeepLastN
+	if keep <= 0 {
+		keep = 8
+	}
+	if store.Stats().Bytes < threshold {
+		return
+	}
+	summarize := cfg.Summarizer
+	if summarize == nil {
+		summarize = defaultSummarizer(stream, model)
+	}
+	done, err := store.Compact(keep, func(messages []llm.Message) (string, error) {
+		return summarize(ctx, messages)
+	})
+	if err != nil {
+		b.logger().Warn("session compaction failed; continuing with full transcript", "error", err.Error())
+		return
+	}
+	if done {
+		b.logger().Info("session compacted", "keep_last", keep)
+	}
+}
+
+// defaultSummarizer 用后端 stream 做一次无工具调用，把历史压成摘要。
+func defaultSummarizer(stream llm.StreamFunc, model string) func(ctx context.Context, messages []llm.Message) (string, error) {
+	return func(ctx context.Context, messages []llm.Message) (string, error) {
+		var b strings.Builder
+		for _, m := range messages {
+			b.WriteString(string(m.Role))
+			b.WriteString(": ")
+			b.WriteString(m.Content)
+			b.WriteString("\n")
+			if b.Len() > 120_000 {
+				break
+			}
+		}
+		prompt := "把以下智能体对话历史压缩成摘要：保留全部关键事实、已做决定、文件变更与未决问题；中文；不超过 800 字；只输出摘要正文。\n\n" + b.String()
+		var out strings.Builder
+		for ev := range stream(ctx, model, []llm.Message{{Role: llm.RoleUser, Content: prompt}}, nil, llm.StreamOptions{}) {
+			switch ev.Kind {
+			case llm.KindTextDelta:
+				out.WriteString(ev.Text)
+			case llm.KindError:
+				return "", ev.Err
+			}
+		}
+		if strings.TrimSpace(out.String()) == "" {
+			return "", errString("summarizer produced empty output")
+		}
+		return out.String(), nil
+	}
+}
+
+type errString string
+
+func (e errString) Error() string { return string(e) }
+
 // resumeUnsafeMarkers 续跑不安全的关键词（会话已被污染：上下文溢出/
 // 迭代上限/非法请求——重试必须换新会话而非续跑）。
 var resumeUnsafeMarkers = []string{
diff --git a/adapters/native/compact_test.go b/adapters/native/compact_test.go
new file mode 100644
index 0000000..9b3683d
--- /dev/null
+++ b/adapters/native/compact_test.go
@@ -0,0 +1,69 @@
+package native
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"agent-native/exec"
+	"agent-native/runtime/llm"
+)
+
+func TestBackendCompactBeforeRun(t *testing.T) {
+	dir := t.TempDir()
+	// 预置一份超阈值的长转录（系统消息 + 12 条历史）。
+	transcript := filepath.Join(dir, ".agentx", "transcript.jsonl")
+	if err := os.MkdirAll(filepath.Dir(transcript), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	var b strings.Builder
+	b.WriteString(`{"role":"system","content":"sys"}` + "\n")
+	for i := 0; i < 12; i++ {
+		b.WriteString(`{"role":"user","content":"` + strings.Repeat("x", 400) + `"}` + "\n")
+	}
+	if err := os.WriteFile(transcript, []byte(b.String()), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	var sawSummary bool
+	// 注入式捕获：验证 Run 收到的消息里有摘要标记。
+	bk2 := New(Options{
+		Model:  "fake",
+		Stream: captureFirstStream(&sawSummary),
+		Compact: &CompactConfig{ThresholdChars: 1000, KeepLastN: 3, Summarizer: func(ctx context.Context, messages []llm.Message) (string, error) {
+			return "预置历史摘要", nil
+		}},
+	})
+	res, err := bk2.Execute(context.Background(), exec.Request{
+		WorkDir: dir, Prompt: "继续", Targets: []string{"output.md"},
+		ToolProfile: exec.ToolProfileReasoning,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if res.Success && !sawSummary {
+		t.Fatal("compaction should have run before the loop")
+	}
+	data, _ := os.ReadFile(transcript)
+	if !strings.Contains(string(data), "[conversation summary]") {
+		t.Fatalf("transcript should contain the summary marker, got:\n%.200s", data)
+	}
+}
+
+func captureFirstStream(saw *bool) llm.StreamFunc {
+	return func(ctx context.Context, model string, messages []llm.Message, decls []llm.ToolDecl, opts llm.StreamOptions) <-chan llm.Event {
+		for _, m := range messages {
+			if strings.Contains(m.Content, "[conversation summary]") {
+				*saw = true
+			}
+		}
+		ch := make(chan llm.Event, 4)
+		go func() {
+			defer close(ch)
+			ch <- llm.Event{Kind: llm.KindMessageEnd, Message: llm.Message{Role: llm.RoleAssistant, Content: "压缩后续写完成", StopReason: llm.StopEnd}}
+		}()
+		return ch
+	}
+}
diff --git a/runtime/session/compact.go b/runtime/session/compact.go
new file mode 100644
index 0000000..1c675d1
--- /dev/null
+++ b/runtime/session/compact.go
@@ -0,0 +1,138 @@
+package session
+
+import (
+	"bufio"
+	"encoding/json"
+	"os"
+
+	"agent-native/runtime/llm"
+)
+
+// Stats 是转录文件的轻量元数据（扫描即得，无副作用）。
+type Stats struct {
+	Lines int
+	Bytes int64
+}
+
+// Stats 统计当前转录行数与字节数（空行不计）。
+func (s *Store) Stats() Stats {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	st := Stats{}
+	f, err := os.Open(s.path)
+	if err != nil {
+		return st
+	}
+	defer f.Close()
+	sc := bufio.NewScanner(f)
+	sc.Buffer(make([]byte, 0, 64*1024), 8*1024*1024)
+	for sc.Scan() {
+		if len(sc.Bytes()) == 0 {
+			continue
+		}
+		st.Lines++
+	}
+	if info, err := f.Stat(); err == nil {
+		st.Bytes = info.Size()
+	}
+	return st
+}
+
+// SaveMeta 把会话元数据写进 <path>.meta.json（创建时间/模型等由调用方定义）。
+func (s *Store) SaveMeta(meta map[string]string) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	raw, err := json.MarshalIndent(meta, "", "  ")
+	if err != nil {
+		return err
+	}
+	tmp := s.path + ".meta.json.tmp"
+	if err := os.WriteFile(tmp, raw, 0o644); err != nil {
+		return err
+	}
+	return os.Rename(tmp, s.path+".meta.json")
+}
+
+// ReadMeta 读取会话元数据；不存在时返回 nil（新会话）。
+func (s *Store) ReadMeta() map[string]string {
+	raw, err := os.ReadFile(s.path + ".meta.json")
+	if err != nil {
+		return nil
+	}
+	var meta map[string]string
+	if err := json.Unmarshal(raw, &meta); err != nil {
+		return nil
+	}
+	return meta
+}
+
+// Compact 把转录改写为「系统消息 + 摘要消息 + 最近 keepLastN 条」。
+// 摘要由 summarize 生成（由适配层注入，通常是一次无工具模型调用）；
+// 改写原子落盘（临时文件 + rename）。
+// 返回是否发生了压缩（消息数不足/已是紧凑形态时返回 false 且不动文件）。
+func (s *Store) Compact(keepLastN int, summarize func(messages []llm.Message) (string, error)) (bool, error) {
+	messages, err := s.Load()
+	if err != nil {
+		return false, err
+	}
+	if len(messages) <= keepLastN+1 { // 系统消息 + 尾部，无可压缩头部
+		return false, nil
+	}
+	var system *llm.Message
+	rest := messages
+	if len(rest) > 0 && rest[0].Role == llm.RoleSystem {
+		copyOf := rest[0]
+		system = &copyOf
+		rest = rest[1:]
+	}
+	tailStart := len(rest) - keepLastN
+	if tailStart <= 0 {
+		return false, nil
+	}
+	head := rest[:tailStart]
+	tail := rest[tailStart:]
+	summary, err := summarize(head)
+	if err != nil {
+		return false, err
+	}
+	summaryMsg := llm.Message{
+		Role:    llm.RoleUser,
+		Content: "[conversation summary]\n" + summary + "\n[recent messages follow]",
+	}
+	var rebuilt []llm.Message
+	if system != nil {
+		rebuilt = append(rebuilt, *system)
+	}
+	rebuilt = append(rebuilt, summaryMsg)
+	rebuilt = append(rebuilt, tail...)
+
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	tmp := s.path + ".compact.tmp"
+	f, err := os.OpenFile(tmp, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0o644)
+	if err != nil {
+		return false, err
+	}
+	w := bufio.NewWriter(f)
+	for _, m := range rebuilt {
+		raw, err := json.Marshal(m)
+		if err != nil {
+			_ = f.Close()
+			_ = os.Remove(tmp)
+			return false, err
+		}
+		w.Write(raw)
+		w.WriteByte('\n')
+	}
+	if err := w.Flush(); err != nil {
+		_ = f.Close()
+		_ = os.Remove(tmp)
+		return false, err
+	}
+	_ = f.Close()
+	if err := os.Rename(tmp, s.path); err != nil {
+		_ = os.Remove(tmp)
+		return false, err
+	}
+	return true, nil
+}
diff --git a/runtime/session/compact_test.go b/runtime/session/compact_test.go
new file mode 100644
index 0000000..b7c742f
--- /dev/null
+++ b/runtime/session/compact_test.go
@@ -0,0 +1,159 @@
+package session
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"agent-native/runtime/llm"
+)
+
+func appendMsgs(t *testing.T, s *Store, msgs ...llm.Message) {
+	t.Helper()
+	for _, m := range msgs {
+		if err := s.Append(m); err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+func TestLoadSkipsCrashResidueTail(t *testing.T) {
+	dir := t.TempDir()
+	s, err := Open(filepath.Join(dir, "t.jsonl"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	appendMsgs(t, s,
+		llm.Message{Role: llm.RoleSystem, Content: "sys"},
+		llm.Message{Role: llm.RoleUser, Content: "hi"},
+	)
+	// 模拟崩溃半行：无换行结尾的残缺 JSON。
+	f, _ := os.OpenFile(s.Path(), os.O_APPEND|os.O_WRONLY, 0o644)
+	_, _ = f.WriteString(`{"role":"assistant","content":"trunca`)
+	_ = f.Close()
+
+	msgs, err := s.Load()
+	if err != nil {
+		t.Fatalf("crash residue must not poison the session: %v", err)
+	}
+	if len(msgs) != 2 || msgs[0].Role != llm.RoleSystem || msgs[1].Content != "hi" {
+		t.Fatalf("want sys+hi, got %+v", msgs)
+	}
+}
+
+func TestLoadStillFailsOnMidFileCorruption(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "t.jsonl")
+	if err := os.WriteFile(path, []byte("{\"bad json}\n{\"role\":\"user\",\"content\":\"ok\"}\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	s := &Store{path: path}
+	if _, err := s.Load(); err == nil {
+		t.Fatal("mid-file corruption must still error")
+	}
+}
+
+func TestStatsAndMeta(t *testing.T) {
+	dir := t.TempDir()
+	s, _ := Open(filepath.Join(dir, "t.jsonl"))
+	appendMsgs(t, s,
+		llm.Message{Role: llm.RoleSystem, Content: "sys"},
+		llm.Message{Role: llm.RoleUser, Content: "q"},
+	)
+	st := s.Stats()
+	if st.Lines != 2 || st.Bytes <= 0 {
+		t.Fatalf("stats wrong: %+v", st)
+	}
+	if err := s.SaveMeta(map[string]string{"model": "m1"}); err != nil {
+		t.Fatal(err)
+	}
+	meta := s.ReadMeta()
+	if meta == nil || meta["model"] != "m1" {
+		t.Fatalf("meta roundtrip failed: %v", meta)
+	}
+	if other := (&Store{path: filepath.Join(dir, "missing.jsonl")}).ReadMeta(); other != nil {
+		t.Fatalf("missing meta must return nil, got %v", other)
+	}
+}
+
+func TestCompactRewritesTranscript(t *testing.T) {
+	dir := t.TempDir()
+	s, _ := Open(filepath.Join(dir, "t.jsonl"))
+	appendMsgs(t, s, llm.Message{Role: llm.RoleSystem, Content: "sys"})
+	for i := 0; i < 10; i++ {
+		appendMsgs(t, s,
+			llm.Message{Role: llm.RoleUser, Content: "early" + string(rune('a'+i))},
+			llm.Message{Role: llm.RoleAssistant, Content: "reply"},
+		)
+	}
+	var seenHead []llm.Message
+	done, err := s.Compact(3, func(messages []llm.Message) (string, error) {
+		seenHead = messages
+		return "这是摘要", nil
+	})
+	if err != nil || !done {
+		t.Fatalf("compact failed: %v done=%v", err, done)
+	}
+	if len(seenHead) != 17 { // 20 - system - 3 tail
+		t.Fatalf("summarizer should receive 17 head messages, got %d", len(seenHead))
+	}
+	msgs, err := s.Load()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(msgs) != 5 { // system + summary + 3 tail
+		t.Fatalf("want 5 messages after compact, got %d", len(msgs))
+	}
+	if msgs[0].Role != llm.RoleSystem {
+		t.Fatal("system message must be preserved")
+	}
+	if !strings.Contains(msgs[1].Content, "[conversation summary]") || !strings.Contains(msgs[1].Content, "这是摘要") {
+		t.Fatalf("summary message wrong: %+v", msgs[1])
+	}
+	if msgs[2].Content != "reply" || msgs[3].Content != "earlyj" || msgs[4].Content != "reply" {
+		t.Fatalf("tail order wrong: %+v", msgs[2:])
+	}
+	// 无 tmp 残留。
+	if _, err := os.Stat(s.Path() + ".compact.tmp"); !os.IsNotExist(err) {
+		t.Fatal("compact tmp file must not remain")
+	}
+}
+
+func TestCompactNoopWhenShort(t *testing.T) {
+	dir := t.TempDir()
+	s, _ := Open(filepath.Join(dir, "t.jsonl"))
+	appendMsgs(t, s,
+		llm.Message{Role: llm.RoleSystem, Content: "sys"},
+		llm.Message{Role: llm.RoleUser, Content: "q"},
+	)
+	done, err := s.Compact(8, func([]llm.Message) (string, error) { return "x", nil })
+	if err != nil || done {
+		t.Fatalf("short transcript must be a no-op: %v %v", done, err)
+	}
+	msgs, _ := s.Load()
+	if len(msgs) != 2 {
+		t.Fatal("no-op compact must not touch the file")
+	}
+}
+
+func TestCompactSummarizerErrorAborts(t *testing.T) {
+	dir := t.TempDir()
+	s, _ := Open(filepath.Join(dir, "t.jsonl"))
+	appendMsgs(t, s, llm.Message{Role: llm.RoleSystem, Content: "sys"})
+	for i := 0; i < 6; i++ {
+		appendMsgs(t, s, llm.Message{Role: llm.RoleUser, Content: "m"})
+	}
+	done, err := s.Compact(2, func([]llm.Message) (string, error) { return "", errStringT("boom") })
+	if err == nil || done {
+		t.Fatal("summarizer error must abort compaction")
+	}
+	msgs, _ := s.Load()
+	if len(msgs) != 7 {
+		t.Fatal("failed compaction must leave the transcript untouched")
+	}
+}
+
+type errStringT string
+
+func (e errStringT) Error() string { return string(e) }
diff --git a/runtime/session/jsonl.go b/runtime/session/jsonl.go
index 2c29fa0..3d84b2c 100644
--- a/runtime/session/jsonl.go
+++ b/runtime/session/jsonl.go
@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"os"
 	"path/filepath"
+	"strings"
 	"sync"
 
 	"agent-native/runtime/llm"
@@ -47,16 +48,28 @@ func (s *Store) Append(msg llm.Message) error {
 	return err
 }
 
+// Load 全量读取转录。崩溃残留容错：Append 恒以换行收尾，因此无换行结尾的
+// 尾部若不是完整 JSON 即为崩溃半行，丢弃而不是让整个会话不可用；
+// 行中间的坏行仍报错（那是文件损坏，不是崩溃残留）。
 func (s *Store) Load() ([]llm.Message, error) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
-	f, err := os.Open(s.path)
+	raw, err := os.ReadFile(s.path)
 	if err != nil {
 		return nil, err
 	}
-	defer f.Close()
+	text := string(raw)
+	if text != "" && !strings.HasSuffix(text, "\n") {
+		if idx := strings.LastIndexByte(text, '\n'); idx >= 0 {
+			if !json.Valid([]byte(text[idx+1:])) {
+				text = text[:idx+1]
+			}
+		} else if !json.Valid([]byte(text)) {
+			text = ""
+		}
+	}
 	var out []llm.Message
-	sc := bufio.NewScanner(f)
+	sc := bufio.NewScanner(strings.NewReader(text))
 	sc.Buffer(make([]byte, 0, 64*1024), 8*1024*1024)
 	for sc.Scan() {
 		line := sc.Bytes()