Align context trimming to tool groups and take the N2 WP1 pins

Why: trimOldest dropped single messages without tool-pair awareness —
after leading users were evicted it could orphan tool results or leave
dangling calls (acceptance clause 2's product-side gap).
Evidence: go test -count=1 ./... green; TestTrimOldestKeepsToolPairs
(pairing holds across repeated trims), TestTrimOldestDropsLeadingOrphanToolRun,
TestTrimOldestStopsWhenGroupCrossesCurrentTurn, TestPrefixHashStableForIdenticalInput;
native pins carry the spill and cache-contract work (Agent-Native#25).
Impact: trimming now evicts whole tool groups; the window may be
slightly exceeded when a group would cross the current turn — pairing
is never broken. Report.Truncations still records the count.
Refs: Agent-Native#25, Agent-Squad#29, N2 plan WP1

internal/ctxasm/finish.go

@@ -207,12 +207,12 @@ func trimOldest(msgs []agentllm.Message, window, reserve int, rep *Report) []age
 	}
 	dropped := 0
 	for estimateAll(msgs) > limit {
-		idx := oldestDrop(msgs)
-		if idx < 0 {
+		start, end := oldestDropRange(msgs)
+		if start < 0 {
 			break
 		}
-		msgs = append(msgs[:idx], msgs[idx+1:]...)
-		dropped++
+		dropped += end - start
+		msgs = append(msgs[:start], msgs[end:]...)
 	}
 	if dropped > 0 {
 		rep.Truncations = append(rep.Truncations, fmtDropped(dropped))
@@ -226,9 +226,14 @@ func fmtDropped(n int) string {
 
 func itoa(n int) string { return strconv.Itoa(n) }
 
-func oldestDrop(msgs []agentllm.Message) int {
+// oldestDropRange 返回下一次丢弃的区间 [start, end)（找不到返回 -1,-1）。
+// 丢弃按工具组对齐：带 tool_calls 的 assistant 与其连续 tool 结果同退，
+// 开头的孤立 tool 结果组整组同退——裁切后既不剩无结果的调用，
+// 也不剩无调用的结果（与库层 alignToolTail 同语义、方向相反）。
+// 组越过最后一条（当前轮）时不再丢弃：窗口允许略超，配对不可破坏。
+func oldestDropRange(msgs []agentllm.Message) (int, int) {
 	if len(msgs) < 3 {
-		return -1
+		return -1, -1
 	}
 	// 保留开头的系统前缀和最后一条（当前轮）。
 	start := 0
@@ -236,9 +241,26 @@ func oldestDrop(msgs []agentllm.Message) int {
 		start++
 	}
 	if start >= len(msgs)-1 {
-		return -1
+		return -1, -1
 	}
-	return start
+	end := start + 1
+	m := msgs[start]
+	switch {
+	case m.Role == agentllm.RoleAssistant && len(m.ToolCalls) > 0:
+		// assistant 及其连续 tool 结果整组同退。
+		for end < len(msgs) && msgs[end].Role == agentllm.RoleTool {
+			end++
+		}
+	case m.Role == agentllm.RoleTool:
+		// 开头即孤立 tool 结果（其 assistant 必已被丢或不存在）：整组同退。
+		for end < len(msgs) && msgs[end].Role == agentllm.RoleTool {
+			end++
+		}
+	}
+	if end > len(msgs)-1 {
+		return -1, -1
+	}
+	return start, end
 }
 
 func estimateAll(msgs []agentllm.Message) int {

internal/ctxasm/finish_test.go

@@ -62,3 +62,118 @@ func TestTrimDropsOldestHistory(t *testing.T) {
 		t.Fatal("expected truncation")
 	}
 }
+
+// ---- N2 WP1：裁切工具组对齐（验收② 产品侧机器证据）----
+
+// assertNoOrphanToolPairs 双向断言：每个 tool_call 有结果，每个结果有调用。
+func assertNoOrphanToolPairs(t *testing.T, msgs []agentllm.Message) {
+	t.Helper()
+	calls := map[string]bool{}
+	results := map[string]bool{}
+	for _, m := range msgs {
+		for _, c := range m.ToolCalls {
+			calls[c.ID] = true
+		}
+		if m.Role == agentllm.RoleTool && m.ToolCallID != "" {
+			results[m.ToolCallID] = true
+		}
+	}
+	for id := range calls {
+		if !results[id] {
+			t.Fatalf("孤立 tool_call：%s（无结果）", id)
+		}
+	}
+	for id := range results {
+		if !calls[id] {
+			t.Fatalf("悬空 tool 结果：%s（无调用）", id)
+		}
+	}
+}
+
+func toolMsgs() []agentllm.Message {
+	call := func(id, name string) agentllm.ToolCall {
+		return agentllm.ToolCall{ID: id, Name: name, Arguments: "{}"}
+	}
+	return []agentllm.Message{
+		{Role: agentllm.RoleSystem, Content: "s"},
+		{Role: agentllm.RoleUser, Content: strings.Repeat("旧一", 300)},
+		{Role: agentllm.RoleAssistant, ToolCalls: []agentllm.ToolCall{call("tc1", "read")}},
+		{Role: agentllm.RoleTool, ToolCallID: "tc1", Name: "read", Content: strings.Repeat("结果一", 200)},
+		{Role: agentllm.RoleUser, Content: strings.Repeat("旧二", 300)},
+		{Role: agentllm.RoleAssistant, ToolCalls: []agentllm.ToolCall{call("tc2", "grep"), call("tc3", "read")}},
+		{Role: agentllm.RoleTool, ToolCallID: "tc2", Name: "grep", Content: strings.Repeat("结果二", 200)},
+		{Role: agentllm.RoleTool, ToolCallID: "tc3", Name: "read", Content: strings.Repeat("结果三", 200)},
+		{Role: agentllm.RoleUser, Content: "现在"},
+	}
+}
+
+// 连续多轮裁切后配对仍成立：assistant 与其结果同退，不逐条拆散。
+func TestTrimOldestKeepsToolPairs(t *testing.T) {
+	out, rep := FinishAssembly(toolMsgs(), &Report{}, Tail{Window: 1600})
+	assertNoOrphanToolPairs(t, out)
+	if len(rep.Truncations) == 0 {
+		t.Fatal("expected truncation")
+	}
+	if out[len(out)-1].Content != "现在" {
+		t.Fatal("当前轮必须保留")
+	}
+	// 窗口再压：旧二组也须整组同退，仍不出现孤立配对。
+	out2, _ := FinishAssembly(toolMsgs(), &Report{}, Tail{Window: 900})
+	assertNoOrphanToolPairs(t, out2)
+}
+
+// 开头即孤立 tool 结果组：整组同退（其 assistant 已不存在）。
+func TestTrimOldestDropsLeadingOrphanToolRun(t *testing.T) {
+	msgs := []agentllm.Message{
+		{Role: agentllm.RoleSystem, Content: "s"},
+		{Role: agentllm.RoleTool, ToolCallID: "ghost", Name: "read", Content: strings.Repeat("孤", 500)},
+		{Role: agentllm.RoleUser, Content: strings.Repeat("旧", 500)},
+		{Role: agentllm.RoleUser, Content: "现在"},
+	}
+	out, _ := FinishAssembly(msgs, &Report{}, Tail{Window: 700})
+	assertNoOrphanToolPairs(t, out)
+	for _, m := range out {
+		if m.ToolCallID == "ghost" {
+			t.Fatal("孤立结果组应整组丢弃")
+		}
+	}
+}
+
+// 工具组越过当前轮时不再丢弃：窗口可略超，配对不可破坏。
+func TestTrimOldestStopsWhenGroupCrossesCurrentTurn(t *testing.T) {
+	msgs := []agentllm.Message{
+		{Role: agentllm.RoleSystem, Content: "s"},
+		{Role: agentllm.RoleAssistant, ToolCalls: []agentllm.ToolCall{{ID: "tc9", Name: "read", Arguments: "{}"}}},
+		{Role: agentllm.RoleTool, ToolCallID: "tc9", Name: "read", Content: strings.Repeat("结果", 900)},
+	}
+	start, end := oldestDropRange(msgs)
+	if start != -1 || end != -1 {
+		t.Fatalf("组越过当前轮应返回 -1,-1，得到 %d,%d", start, end)
+	}
+}
+
+// ---- N2 WP1：PrefixHash 稳定契约（同一输入 → 同一前缀哈希）----
+
+func TestPrefixHashStableForIdenticalInput(t *testing.T) {
+	build := func() []agentllm.Message {
+		return []agentllm.Message{
+			{Role: agentllm.RoleSystem, Content: "系统前缀A"},
+			{Role: agentllm.RoleSystem, Content: "系统前缀B"},
+			{Role: agentllm.RoleUser, Content: "历史一"},
+			{Role: agentllm.RoleAssistant, Content: "回复一"},
+			{Role: agentllm.RoleUser, Content: "现在"},
+		}
+	}
+	_, rep1 := FinishAssembly(build(), &Report{}, Tail{Instructions: "个人指令"})
+	_, rep2 := FinishAssembly(build(), &Report{}, Tail{Instructions: "个人指令"})
+	if rep1.PrefixHash == "" || rep1.PrefixHash != rep2.PrefixHash {
+		t.Fatalf("同一输入的前缀哈希必须一致：%q vs %q", rep1.PrefixHash, rep2.PrefixHash)
+	}
+	// 前缀变化必须改变哈希（钉住哈希语义，防退化为常量）。
+	other := build()
+	other[0].Content = "系统前缀X"
+	_, rep3 := FinishAssembly(other, &Report{}, Tail{Instructions: "个人指令"})
+	if rep3.PrefixHash == rep1.PrefixHash {
+		t.Fatal("前缀变化后哈希必须不同")
+	}
+}