diff --git a/backend/app/api/ai_suggestions.py b/backend/app/api/ai_suggestions.py
index a3c81efe..273696f8 100644
--- a/backend/app/api/ai_suggestions.py
+++ b/backend/app/api/ai_suggestions.py
@@ -118,7 +118,10 @@ def _apply_kpi_target(db: Session, sug: AISuggestion, params: dict, current_user
     kpi_id = params.get("kpi_id") or sug.target_id
     if not kpi_id:
         raise HTTPException(400, "缺少 kpi_id")
-    kpi = db.query(KPIDefinition).filter(KPIDefinition.id == kpi_id).first()
+    kpi = db.query(KPIDefinition).filter(
+        KPIDefinition.id == kpi_id,
+        KPIDefinition.entity_id == sug.entity_id,   # 2026-09-18 派生单: 目标KPI须归属建议账套
+    ).first()
     if not kpi:
         raise HTTPException(404, f"KPI {kpi_id} 不存在")
     before = kpi.target_value
@@ -157,7 +160,10 @@ def _apply_budget_adjust(db: Session, sug: AISuggestion, params: dict, current_u
     kpi_id = params.get("kpi_id") or sug.target_id
     if not kpi_id:
         raise HTTPException(400, "缺少 kpi_id")
-    kpi = db.query(KPIDefinition).filter(KPIDefinition.id == kpi_id).first()
+    kpi = db.query(KPIDefinition).filter(
+        KPIDefinition.id == kpi_id,
+        KPIDefinition.entity_id == sug.entity_id,   # 2026-09-18 派生单: 目标KPI须归属建议账套
+    ).first()
     if not kpi:
         raise HTTPException(404, f"KPI {kpi_id} 不存在")
 
@@ -397,10 +403,14 @@ def apply_suggestion(
 def dismiss_suggestion(
     suggestion_id: int,
     db: Session = Depends(get_db),
+    entity_id: int = Depends(get_entity_id),
     current_user=Depends(require_auth),
 ):
     """忽略建议"""
-    sug = db.query(AISuggestion).filter(AISuggestion.id == suggestion_id).first()
+    sug = db.query(AISuggestion).filter(
+        AISuggestion.id == suggestion_id,
+        AISuggestion.entity_id == entity_id,   # 2026-09-18 派生单: 建议须归属会话账套
+    ).first()
     if not sug:
         raise HTTPException(404, "建议不存在")
     sug.status = "dismissed"
diff --git a/backend/tests/test_dismiss_suggestion_entity_scope_20260918.py b/backend/tests/test_dismiss_suggestion_entity_scope_20260918.py
new file mode 100644
index 00000000..2268c4c6
--- /dev/null
+++ b/backend/tests/test_dismiss_suggestion_entity_scope_20260918.py
@@ -0,0 +1,275 @@
+"""CMA 跨实体隔离 — AI 建议「忽略」与「应用目标 KPI」账套收敛（2026-09-18 派生单）
+
+任务：fullstack-20260918-2030-dismiss-suggestion-entity-scope
+父单：fullstack-20260918-2025-read-side-entity-scope-gaps
+      （其 §五 范围外发现 第 1/2 条：写侧同族漏网点）
+
+缺陷（同文件 ai_suggestions.py 的写侧漏网点）：
+  1. dismiss_suggestion      — POST /{suggestion_id}/dismiss 按 id 直取、无账套条件，
+                               且会改状态（status="dismissed"）
+     ⇒ 本账套会话带对端账套建议 id 可跨账套忽略对端建议（数据被改写）
+  2. _apply_kpi_target       — 目标 KPIDefinition 只按 id 查，不校验归属会话账套
+  3. _apply_budget_adjust    — 同上（BudgetPlan 侧已按 sug.entity_id，KPI 侧未校验）
+     ⇒ 建议自身归属校验通过后，其 suggestion_data 指向对端账套 KPI 时，
+       应用动作会改到对端账套的 target_value / 预算行（纵深防御缺口）
+
+对端记录口径 = 404「建议不存在」/「KPI {id} 不存在」（对齐同文件
+apply_suggestion 与 1905b/2015 的写法）。
+
+本用例（改前失败条目见 _reports/evidence-fullstack-20260918-2030-red.txt）：
+- T1 忽略建议：对端账套 → 404 且建议状态保持 unapplied（双向）；本账套 → 200
+     且状态 dismissed（双向正对照）
+- T2 忽略幂等：同一建议重复忽略仍 200、状态仍 dismissed（既有语义不得改动）
+- T3 应用 kpi_target：目标 KPI 非本账套 → 404，对端 KPI target_value 逐值未变，
+     建议状态保持 unapplied；本账套应用正常（正对照，目标值已写入）
+- T4 应用 budget_adjust：目标 KPI 非本账套 → 404，不落任何预算行（对端/本账套都不落）
+- T5 鉴权底线：dismiss 无 token → 401（改前改后都应通过）
+"""
+import pytest
+from fastapi.testclient import TestClient
+from sqlalchemy.orm import Session
+
+from app.models import AISuggestion, BudgetPlan, Entity, KPIDefinition
+from tests.conftest import TEST_USER_PASSWORD, auth_header, create_test_user
+
+SUGG = "/api/cma/ai/suggestions"
+
+# kpi_code 全局唯一 → 每账套独立 code
+E1_KPI = "ZZ_DSE1_KPI"
+E2_KPI = "ZZ_DSE2_KPI"
+E1_SUG_TITLE = "ZZ_DS e1 私有建议"
+E2_SUG_TITLE = "ZZ_DS e2 私有建议"
+E1_CROSS_TITLE = "ZZ_DS e2 指向对端 KPI 的建议"
+
+
+# ──────────────────────────────────────────────
+# 夹具（沿用同族风格：双向登录 + 同账套正对照）
+# ──────────────────────────────────────────────
+
+def _ensure_entity(db: Session, eid: int, name: str) -> Entity:
+    ent = db.query(Entity).filter(Entity.id == eid).first()
+    if not ent:
+        ent = Entity(id=eid, name=name, short_name=name, status="active")
+        db.add(ent)
+        db.commit()
+        db.refresh(ent)
+    return ent
+
+
+def _mk_kpi(db: Session, code: str, eid: int, target: float = 1000.0) -> KPIDefinition:
+    kpi = KPIDefinition(
+        entity_id=eid,
+        kpi_code=code,
+        kpi_name=f"{code}-名称",
+        dimension="finance",
+        category="revenue_growth",
+        formula="-",
+        data_source_type="excel",
+        data_source="Excel导入",
+        data_owner="财务部",
+        frequency="monthly",
+        unit="元",
+        target_value=target,
+        kpi_level="operational",
+        status="active",
+    )
+    db.add(kpi)
+    db.commit()
+    db.refresh(kpi)
+    return kpi
+
+
+def _seed_two_entities(db: Session):
+    _ensure_entity(db, 1, "酣客")
+    _ensure_entity(db, 2, "博海")
+    create_test_user(db)
+
+
+def _login(client: TestClient, entity_id: int) -> str:
+    resp = client.post("/api/cma/auth/login", json={
+        "username": "testadmin",
+        "password": TEST_USER_PASSWORD,
+        "entity_id": entity_id,
+    })
+    assert resp.status_code == 200, f"登录 entity{entity_id} 失败: {resp.text[:300]}"
+    return resp.json()["token"]
+
+
+def _mk_suggestion(client: TestClient, token: str, title: str,
+                   target_id: int, kpi_id: int, target_value: float = 2000.0) -> int:
+    body = {
+        "suggestion_type": "kpi_target",
+        "target_type": "kpi",
+        "target_id": target_id,
+        "title": title,
+        "content": "目标偏低，建议上调",
+        "suggestion_data": {"kpi_id": kpi_id, "target_value": target_value},
+    }
+    r = client.post(SUGG, json=body, headers=auth_header(token))
+    assert r.status_code == 200, f"创建建议失败: {r.status_code} {r.text[:300]}"
+    return r.json()["data"]["id"]
+
+
+def _sug_status(db: Session, sid: int) -> str:
+    db.expire_all()
+    row = db.query(AISuggestion).filter(AISuggestion.id == sid).first()
+    assert row is not None, f"建议 #{sid} 不存在"
+    return row.status
+
+
+def _kpi_target(db: Session, kid: int) -> float:
+    db.expire_all()
+    row = db.query(KPIDefinition).filter(KPIDefinition.id == kid).first()
+    assert row is not None, f"KPI #{kid} 不存在"
+    return row.target_value
+
+
+# ──────────────────────────────────────────────
+# T1. 忽略建议按会话账套限定（双向 + 正对照）
+# ──────────────────────────────────────────────
+
+def test_dismiss_suggestion_scoped_to_session_entity(client: TestClient, db: Session):
+    """对端账套建议 → 404 且状态保持 unapplied；本账套 → 200 且 dismissed（双向）"""
+    _seed_two_entities(db)
+    k1 = _mk_kpi(db, E1_KPI, 1)
+    k2 = _mk_kpi(db, E2_KPI, 2)
+    tok1 = _login(client, 1)
+    tok2 = _login(client, 2)
+    s1 = _mk_suggestion(client, tok1, E1_SUG_TITLE, k1.id, k1.id)
+    s2 = _mk_suggestion(client, tok2, E2_SUG_TITLE, k2.id, k2.id)
+
+    # 方向 A：e2 会话忽略 e1 建议 → 404 且 e1 建议状态未改
+    r_a = client.post(f"{SUGG}/{s1}/dismiss", headers=auth_header(tok2))
+    assert r_a.status_code == 404, (
+        f"e2 会话忽略 e1 建议应 404，实际 {r_a.status_code} {r_a.text[:200]}"
+        " —— dismiss_suggestion 按 id 直取、未按会话账套限定"
+    )
+    assert _sug_status(db, s1) == "unapplied", (
+        "跨账套忽略竟改动了对端账套建议状态（应为 unapplied）"
+    )
+
+    # 方向 B：e1 会话忽略 e2 建议 → 404 且 e2 建议状态未改
+    r_b = client.post(f"{SUGG}/{s2}/dismiss", headers=auth_header(tok1))
+    assert r_b.status_code == 404, (
+        f"e1 会话忽略 e2 建议应 404，实际 {r_b.status_code} {r_b.text[:200]}"
+    )
+    assert _sug_status(db, s2) == "unapplied", (
+        "跨账套忽略竟改动了对端账套建议状态（应为 unapplied）"
+    )
+
+    # 正对照：本账套忽略自己的建议 → 200 且状态 dismissed
+    r_ok1 = client.post(f"{SUGG}/{s1}/dismiss", headers=auth_header(tok1))
+    assert r_ok1.status_code == 200, f"e1 忽略自己的建议应 200，实际 {r_ok1.status_code}"
+    assert _sug_status(db, s1) == "dismissed", "本账套忽略未落状态 dismissed"
+
+    r_ok2 = client.post(f"{SUGG}/{s2}/dismiss", headers=auth_header(tok2))
+    assert r_ok2.status_code == 200, f"e2 忽略自己的建议应 200，实际 {r_ok2.status_code}"
+    assert _sug_status(db, s2) == "dismissed", "本账套忽略未落状态 dismissed"
+
+
+# ──────────────────────────────────────────────
+# T2. 忽略幂等：既有语义不得改动
+# ──────────────────────────────────────────────
+
+def test_dismiss_suggestion_repeat_keeps_existing_semantics(client: TestClient, db: Session):
+    """已 dismissed 的建议重复忽略仍 200、状态仍 dismissed（加账套条件不得改动既有语义）"""
+    _seed_two_entities(db)
+    k1 = _mk_kpi(db, E1_KPI, 1)
+    tok1 = _login(client, 1)
+    s1 = _mk_suggestion(client, tok1, E1_SUG_TITLE, k1.id, k1.id)
+
+    assert client.post(f"{SUGG}/{s1}/dismiss",
+                       headers=auth_header(tok1)).status_code == 200
+    r2 = client.post(f"{SUGG}/{s1}/dismiss", headers=auth_header(tok1))
+    assert r2.status_code == 200, (
+        f"重复忽略应维持既有 200 语义，实际 {r2.status_code} {r2.text[:200]}"
+    )
+    assert _sug_status(db, s1) == "dismissed", "重复忽略后状态异常"
+
+
+# ──────────────────────────────────────────────
+# T3. 应用 kpi_target：目标 KPI 归属校验
+# ──────────────────────────────────────────────
+
+def test_apply_kpi_target_foreign_kpi_404(client: TestClient, db: Session):
+    """建议自身属本账套、但其目标 KPI 属对端 → 404；对端 KPI target_value 逐值未变"""
+    _seed_two_entities(db)
+    k1 = _mk_kpi(db, E1_KPI, 1, target=1000.0)
+    _mk_kpi(db, E2_KPI, 2, target=3000.0)
+    tok1 = _login(client, 1)
+    tok2 = _login(client, 2)
+
+    # e2 会话创建建议（归属 e2），但 suggestion_data/target_id 指向 e1 的 KPI
+    # （模拟手填数据 / 迁移遗留 / Bot 通道写入的不自洽数据）
+    s_cross = _mk_suggestion(client, tok2, E1_CROSS_TITLE, k1.id, k1.id, target_value=88888.0)
+
+    before_k1 = _kpi_target(db, k1.id)
+    assert before_k1 == 1000.0, f"前置：e1 KPI 目标值异常 {before_k1}"
+
+    r = client.post(f"{SUGG}/{s_cross}/apply", headers=auth_header(tok2),
+                    json={"action": "kpi_target", "target_value": 99999.0})
+    assert r.status_code == 404, (
+        f"目标 KPI 属对端账套应 404，实际 {r.status_code} {r.text[:200]}"
+        " —— _apply_kpi_target 只按 id 查 KPIDefinition，未校验归属会话账套"
+    )
+    assert _kpi_target(db, k1.id) == before_k1, (
+        f"对端账套 KPI target_value 被改写：{before_k1} → {_kpi_target(db, k1.id)}"
+    )
+    assert _sug_status(db, s_cross) == "unapplied", (
+        "越权应用竟改动了建议状态（应保持 unapplied）"
+    )
+
+    # 正对照：本账套建议 + 本账套 KPI → 200 且目标值已写入
+    k2 = db.query(KPIDefinition).filter(KPIDefinition.kpi_code == E2_KPI).first()
+    s_ok = _mk_suggestion(client, tok2, E2_SUG_TITLE, k2.id, k2.id, target_value=4500.0)
+    r_ok = client.post(f"{SUGG}/{s_ok}/apply", headers=auth_header(tok2),
+                       json={"action": "kpi_target", "target_value": 4500.0})
+    assert r_ok.status_code == 200, f"本账套应用应 200，实际 {r_ok.status_code} {r_ok.text[:200]}"
+    assert _kpi_target(db, k2.id) == 4500.0, "本账套应用未写入目标值"
+
+
+# ──────────────────────────────────────────────
+# T4. 应用 budget_adjust：目标 KPI 归属校验
+# ──────────────────────────────────────────────
+
+def test_apply_budget_adjust_foreign_kpi_404(client: TestClient, db: Session):
+    """budget_adjust 目标 KPI 属对端 → 404 且不落任何预算行（含对端账套）"""
+    _seed_two_entities(db)
+    k1 = _mk_kpi(db, E1_KPI, 1, target=1000.0)
+    _mk_kpi(db, E2_KPI, 2, target=3000.0)
+    tok2 = _login(client, 2)
+    s_cross = _mk_suggestion(client, tok2, E1_CROSS_TITLE, k1.id, k1.id, target_value=7777.0)
+
+    db.expire_all()
+    plans_before = db.query(BudgetPlan).filter(BudgetPlan.kpi_id == k1.id).count()
+
+    r = client.post(f"{SUGG}/{s_cross}/apply", headers=auth_header(tok2),
+                    json={"action": "budget_adjust", "period": "2026-09", "budget_value": 7777.0})
+    assert r.status_code == 404, (
+        f"budget_adjust 目标 KPI 属对端账套应 404，实际 {r.status_code} {r.text[:200]}"
+        " —— _apply_budget_adjust 的 KPI 查询未校验归属会话账套"
+    )
+    db.expire_all()
+    plans_after = db.query(BudgetPlan).filter(BudgetPlan.kpi_id == k1.id).count()
+    assert plans_after == plans_before, (
+        f"越权 budget_adjust 落了预算行：{plans_before} → {plans_after}"
+    )
+    assert _sug_status(db, s_cross) == "unapplied", "越权应用竟改动了建议状态"
+
+
+# ──────────────────────────────────────────────
+# T5. 鉴权底线（dismiss / apply 无 token → 401）
+# ──────────────────────────────────────────────
+
+def test_dismiss_and_apply_without_token_401(client: TestClient, db: Session):
+    """无 token 一律 401（改前改后都应通过）"""
+    _seed_two_entities(db)
+    k1 = _mk_kpi(db, E1_KPI, 1)
+    tok1 = _login(client, 1)
+    s1 = _mk_suggestion(client, tok1, E1_SUG_TITLE, k1.id, k1.id)
+
+    r1 = client.post(f"{SUGG}/{s1}/dismiss")
+    assert r1.status_code == 401, f"dismiss 无 token 应 401，实际 {r1.status_code}"
+
+    r2 = client.post(f"{SUGG}/{s1}/apply", json={"action": "kpi_target", "target_value": 1})
+    assert r2.status_code == 401, f"apply 无 token 应 401，实际 {r2.status_code}"