diff --git a/gateway/sense/classifier.py b/gateway/sense/classifier.py
new file mode 100644
index 0000000..0737afa
--- /dev/null
+++ b/gateway/sense/classifier.py
@@ -0,0 +1,72 @@
+"""分级头（T-G4）：LinearHead 纯 Python 推理 + LoraRemote 预留（T-G8）。
+
+D-G5：serving 路径零新依赖——线性头推理是纯 Python 点积 + softmax（768 维
+≈0.1ms）。工件为 JSON（weights 3×dim / bias 3 / dim / version），由
+scripts/train_tier_head.py（numpy 离线训练）产出。
+缺失/损坏 -> ArtifactMissing（D-G4 降级信号）。
+"""
+from __future__ import annotations
+
+import json
+import math
+from pathlib import Path
+from typing import Any, Dict, List, Optional
+
+from gateway.sense.errors import ArtifactMissing
+
+TIERS = ("t1", "t2", "t3")
+
+
+class LinearHead:
+    """线性有序三分类头（softmax；类别序 t1<t2<t3 与升级阶梯一致）。"""
+
+    def __init__(self, weights: List[List[float]], bias: List[float],
+                 version: str = "dev", labels: Optional[List[str]] = None):
+        self.weights = weights
+        self.bias = bias
+        self.version = version
+        self.labels = labels or ["t1", "t2", "t3"]
+        if len(self.weights) != len(self.bias):
+            raise ArtifactMissing("线性头权重与偏置维度不一致")
+
+    @classmethod
+    def load(cls, path: str | Path, version: str = "") -> "LinearHead":
+        p = Path(path)
+        if not p.exists():
+            raise ArtifactMissing(f"线性头工件不存在: {p}")
+        try:
+            data = json.loads(p.read_text(encoding="utf-8"))
+            weights = data["weights"]
+            bias = data["bias"]
+            if not weights or not bias:
+                raise ValueError("空权重")
+            return cls(weights=weights, bias=bias,
+                       version=str(data.get("version") or version or p.parent.name),
+                       labels=data.get("labels"))
+        except ArtifactMissing:
+            raise
+        except Exception as e:  # noqa: BLE001
+            raise ArtifactMissing(f"线性头工件损坏: {type(e).__name__}: {e}") from e
+
+    def predict(self, vec: List[float]) -> Dict[str, float]:
+        """点积 + softmax -> {t1, t2, t3} 概率（和为 1）。"""
+        logits = []
+        for w, b in zip(self.weights, self.bias):
+            n = min(len(w), len(vec))
+            logits.append(sum(wi * vi for wi, vi in zip(w[:n], vec[:n])) + b)
+        m = max(logits)
+        exps = [math.exp(z - m) for z in logits]
+        total = sum(exps)
+        return {label: e / total for label, e in zip(self.labels, exps)}
+
+
+class LoraRemote:
+    """LoRA 远程分类（vLLM /v1/classify）——T-G8 可选实验，本版预留。"""
+
+    def __init__(self, base_url: str, model: str, api_key: Optional[str] = None):
+        self.base_url = base_url.rstrip("/")
+        self.model = model
+        self.api_key = api_key
+
+    def predict(self, vec: List[float]) -> Dict[str, float]:
+        raise NotImplementedError("T-G8 可选实验（LoRA/vLLM 分类服务）")
diff --git a/requirements-ml.txt b/requirements-ml.txt
index 1708756..6f037fe 100644
--- a/requirements-ml.txt
+++ b/requirements-ml.txt
@@ -1,11 +1,15 @@
-# 可选：接入真实开源小模型（RTX 4060 8GB 可跑 0.5B-4B 量化模型）
-# 安装：pip install -r requirements-ml.txt
-torch>=2.2
-transformers>=4.40
-accelerate>=0.30
-peft>=0.11
-sentencepiece
-protobuf
-bitsandbytes
-# 轻量推理引擎（可选）
-# llama-cpp-python

+# 可选：接入真实开源小模型（RTX 4060 8GB 可跑 0.5B-4B 量化模型）
+# 安装：pip install -r requirements-ml.txt
+torch>=2.2
+transformers>=4.40
+accelerate>=0.30
+peft>=0.11
+sentencepiece
+protobuf
+bitsandbytes
+# 轻量推理引擎（可选）
+# llama-cpp-python
+
+# 语义分析器离线训练（T-G4）：仅 scripts/train_tier_head.py 使用；
+# serving 路径零新依赖（D-G5：线性头推理纯 Python 点积）
+numpy>=1.26
diff --git a/scripts/train_tier_head.py b/scripts/train_tier_head.py
new file mode 100644
index 0000000..20e9336
--- /dev/null
+++ b/scripts/train_tier_head.py
@@ -0,0 +1,185 @@
+"""离线训练三分类线性头（T-G4）：softmax 回归（numpy），产出 JSON 工件。
+
+用法：
+    python scripts/train_tier_head.py --db data/sense.sqlite3 --version v1
+    python scripts/train_tier_head.py --synthetic          # 合成数据自测（无需真实标签）
+
+输入：sense.sqlite3 tier_observations 中 true_tier 非空且 embedding（int8 BLOB）
+可解析的行；按时间 70/15/15 切分 train/val/calib。
+输出：data/sense_models/{version}/head.json + metrics.json + 工件登记（active=0，
+人工 /sense/admin/promote 切换）。
+
+依赖：numpy（可选，requirements-ml.txt）；serving 推理不依赖 numpy（D-G5）。
+"""
+from __future__ import annotations
+
+import argparse
+import json
+import sys
+import time
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(ROOT))
+
+if hasattr(sys.stdout, "reconfigure"):
+    sys.stdout.reconfigure(encoding="utf-8")
+    sys.stderr.reconfigure(encoding="utf-8")
+
+try:
+    import numpy as np
+except ImportError:
+    print("缺少 numpy：pip install numpy（或 requirements-ml.txt）；训练必需，serving 不需要。")
+    sys.exit(1)
+
+from gateway.sense.store import SenseStore  # noqa: E402
+
+TIERS = ["t1", "t2", "t3"]
+
+
+def _deblob(blob) -> list[float]:
+    return [b - 256 if b > 127 else b for b in blob]
+
+
+def load_rows(db_path: str):
+    """true_tier 非空且 embedding 可解析的行 -> (X, y, versions)。"""
+    store = SenseStore.init_db(db_path)
+    X, y, vers = [], [], []
+    for row in store.labeled_rows(limit=200000):
+        blob = row["embedding"]
+        if not blob:
+            continue
+        X.append(_deblob(blob))
+        y.append(TIERS.index(row["true_tier"].lower()))
+        vers.append(row.get("policy_version") or "")
+    return np.array(X, dtype=float), np.array(y), vers
+
+
+def split(n: int, seed: int = 42):
+    idx = np.random.default_rng(seed).permutation(n)
+    t = int(n * 0.7)
+    v = int(n * 0.85)
+    return idx[:t], idx[t:v], idx[v:]
+
+
+def softmax(z):
+    z = z - z.max(axis=1, keepdims=True)
+    e = np.exp(z)
+    return e / e.sum(axis=1, keepdims=True)
+
+
+def train(X, y, epochs=300, lr=0.5, seed: int = 42):
+    """L2 正则 softmax 回归（全量梯度下降，类别权重均衡）。"""
+    n, d = X.shape
+    k = 3
+    Y = np.zeros((n, k))
+    for i, c in enumerate(y):
+        Y[i, c] = 1.0
+    cls_weight = n / (k * np.array([(y == c).sum() or 1 for c in range(k)]))
+    W = np.zeros((d, k))
+    b = np.zeros(k)
+    sw = np.array([cls_weight[c] for c in y])
+    sw = sw / sw.sum() * n
+    rng = np.random.default_rng(seed)
+    for _ in range(epochs):
+        probs = softmax(X @ W + b)
+        grad = (probs - Y) * sw[:, None]
+        W -= lr * (X.T @ grad / n + 0.01 * W)
+        b -= lr * (grad.sum(axis=0) / n)
+    return W, b
+
+
+def metrics(X, y, W, b) -> dict:
+    probs = softmax(X @ W + b)
+    pred = probs.argmax(axis=1)
+    macro_f1s = []
+    for c in range(3):
+        tp = float(((pred == c) & (y == c)).sum())
+        fp = float(((pred == c) & (y != c)).sum())
+        fn = float(((pred != c) & (y == c)).sum())
+        precision = tp / (tp + fp) if tp + fp else 0.0
+        recall = tp / (tp + fn) if tp + fn else 0.0
+        f1 = 2 * precision * recall / (precision + recall) if precision + recall else 0.0
+        macro_f1s.append(f1)
+    acc = float((pred == y).mean())
+    # ordinal AUC 简化：P(判>=c 与 真>=c 一致) 均值
+    return {"accuracy": round(acc, 4),
+            "macro_f1": round(sum(macro_f1s) / 3, 4),
+            "per_tier_f1": [round(f, 4) for f in macro_f1s]}
+
+
+def run(db_path: str, version: str, models_dir: str, epochs: int) -> int:
+    store = SenseStore.init_db(db_path)
+    X, y, _ = load_rows(db_path)
+    if len(X) < 30:
+        print(f"[train] 标签样本不足（{len(X)} < 30），跳过训练。"
+              "先以 collect 模式积累标签（夜间推导）。")
+        return 1
+    tr, va, ca = split(len(X))
+    W, b = train(X[tr], y[tr], epochs=epochs)
+    m_val = metrics(X[va], y[va], W, b)
+    probs_cal = (softmax(X[ca] @ W + b) * 1000).round().astype(int) / 1000
+    calib_rows = [{"probs": json.dumps({TIERS[i]: float(probs_cal[j, i])
+                                        for i in range(3)}),
+                   "true_tier": TIERS[y[ca][j]]} for j in range(len(y[ca]))]
+
+    out = Path(models_dir) / version
+    out.mkdir(parents=True, exist_ok=True)
+    (out / "head.json").write_text(json.dumps({
+        "version": version, "dim": int(X.shape[1]), "labels": TIERS,
+        "weights": W.T.tolist(), "bias": b.tolist()}, ensure_ascii=False), encoding="utf-8")
+    metrics_data = {"version": version, "n": int(len(X)), "val": m_val,
+                    "created": time.strftime("%Y-%m-%d %H:%M:%S")}
+    (out / "metrics.json").write_text(json.dumps(metrics_data, ensure_ascii=False,
+                                                 indent=2), encoding="utf-8")
+
+    # calib 阈值（用 T-G3 校准逻辑）
+    from gateway.sense.calibrate import compute_thresholds, save_thresholds
+    th = compute_thresholds(calib_rows, alpha=0.05, min_labels=5)
+    save_thresholds(store, models_dir, version, th, activate=False)
+
+    store.register_artifact(version, "head", str(out / "head.json"),
+                            metrics_data, active=False)
+    print(f"[train] version={version} n={len(X)} val={m_val}")
+    print(f"[train] 阈值={th}")
+    print(f"[train] 工件已登记（active=0），/sense/admin/promote 切换")
+    return 0
+
+
+def run_synthetic(models_dir: str) -> int:
+    """合成数据自测：三类可分高斯簇，验证训练-工件-LinearHead 推理链路。"""
+    rng = np.random.default_rng(7)
+    n, d = 300, 64
+    centers = [rng.normal(0, 1, d), rng.normal(4, 1, d), rng.normal(-4, 1, d)]
+    X = np.vstack([c + rng.normal(0, 0.8, (n, d)) for c in centers])
+    y = np.repeat([0, 1, 2], n)
+    W, b = train(X, y, epochs=200)
+    m = metrics(X, y, W, b)
+    assert m["macro_f1"] > 0.9, f"合成集 macro_F1 过低: {m}"
+    version = "synthetic"
+    out = Path(models_dir) / version
+    out.mkdir(parents=True, exist_ok=True)
+    (out / "head.json").write_text(json.dumps({
+        "version": version, "dim": d, "labels": TIERS,
+        "weights": W.T.tolist(), "bias": b.tolist()}), encoding="utf-8")
+    print(f"[synthetic] macro_f1={m['macro_f1']} accuracy={m['accuracy']}"
+          f" -> {out/'head.json'}")
+    return 0
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser(description="训练三分类线性头（T-G4）")
+    ap.add_argument("--db", default="data/sense.sqlite3")
+    ap.add_argument("--version", default=time.strftime("v%Y%m%d"))
+    ap.add_argument("--models-dir", default="data/sense_models")
+    ap.add_argument("--epochs", type=int, default=300)
+    ap.add_argument("--synthetic", action="store_true",
+                    help="合成数据自测训练链路（不需要真实标签）")
+    args = ap.parse_args()
+    if args.synthetic:
+        return run_synthetic(args.models_dir)
+    return run(args.db, args.version, args.models_dir, args.epochs)
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/tests/test_sense_classifier.py b/tests/test_sense_classifier.py
new file mode 100644
index 0000000..9a19cd0
--- /dev/null
+++ b/tests/test_sense_classifier.py
@@ -0,0 +1,83 @@
+"""线性头测试（T-G4）：纯 Python 推理与黄金向量一致 / 工件缺失 ArtifactMissing。"""
+import json
+import math
+import tempfile
+from pathlib import Path
+
+import pytest
+
+from gateway.sense.classifier import LinearHead
+from gateway.sense.errors import ArtifactMissing
+
+W = [[0.5, -0.5, 0.0], [0.0, 0.5, -0.5], [-0.5, 0.0, 0.5]]
+B = [0.1, 0.0, -0.1]
+
+
+def _head(tmp_path):
+    p = tmp_path / "head.json"
+    p.write_text(json.dumps({"version": "test", "dim": 3, "labels": ["t1", "t2", "t3"],
+                             "weights": W, "bias": B}), encoding="utf-8")
+    return p
+
+
+def test_predict_matches_reference_softmax(tmp_path):
+    """纯 Python 点积 + softmax 与手工参考实现一致（黄金向量）。"""
+    head = LinearHead.load(_head(tmp_path))
+    vec = [1.0, 2.0, 3.0]
+    logits = [sum(wi * vi for wi, vi in zip(w, vec)) + b for w, b in zip(W, B)]
+    m = max(logits)
+    exps = [math.exp(z - m) for z in logits]
+    total = sum(exps)
+    expected = {"t1": exps[0] / total, "t2": exps[1] / total, "t3": exps[2] / total}
+    probs = head.predict(vec)
+    assert sum(probs.values()) == pytest.approx(1.0, abs=1e-9)
+    for k, v in expected.items():
+        assert probs[k] == pytest.approx(v, rel=1e-9)
+
+
+def test_predict_argmax_consistent_with_logit_order(tmp_path):
+    head = LinearHead.load(_head(tmp_path))
+    # [3,1,2] 点积最大维度是 0 -> t1 概率最大
+    probs = head.predict([3.0, 1.0, 2.0])
+    assert max(probs, key=probs.get) == "t1"
+
+
+def test_load_missing_raises_artifact_missing(tmp_path):
+    with pytest.raises(ArtifactMissing):
+        LinearHead.load(tmp_path / "ghost.json")
+
+
+def test_load_corrupted_raises_artifact_missing(tmp_path):
+    p = tmp_path / "bad.json"
+    p.write_text('{"weights": [], "bias": []}', encoding="utf-8")
+    with pytest.raises(ArtifactMissing):
+        LinearHead.load(p)
+
+
+def test_train_synthetic_roundtrip(tmp_path):
+    """T-G4 验收：合成集训练 macro-F1 > 0.9，head.json 可被 LinearHead 推理。"""
+    import importlib.util
+    import numpy as np
+
+    script = Path(__file__).resolve().parent.parent / "scripts" / "train_tier_head.py"
+    spec = importlib.util.spec_from_file_location("train_tier_head", script)
+    mod = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(mod)
+
+    rng = np.random.default_rng(7)
+    n, d = 150, 32
+    centers = [rng.normal(0, 1, d), rng.normal(5, 1, d), rng.normal(-5, 1, d)]
+    X = np.vstack([c + rng.normal(0, 0.7, (n, d)) for c in centers])
+    y = np.repeat([0, 1, 2], n)
+    W, b = mod.train(X, y, epochs=150)
+    m = mod.metrics(X, y, W, b)
+    assert m["macro_f1"] > 0.9, m
+
+    out = tmp_path / "head.json"
+    out.write_text(json.dumps({"version": "synthetic", "dim": d,
+                               "labels": ["t1", "t2", "t3"],
+                               "weights": W.T.tolist(), "bias": b.tolist()}),
+                   encoding="utf-8")
+    head = LinearHead.load(out)
+    probs = head.predict(X[0].tolist())
+    assert sum(probs.values()) == pytest.approx(1.0, abs=1e-9)
diff --git a/任务拆解与执行计划.md b/任务拆解与执行计划.md
index c539ef6..337c2c3 100644
--- a/任务拆解与执行计划.md
+++ b/任务拆解与执行计划.md
@@ -159,7 +159,7 @@ P0 完成后的能力：干净的后端抽象 + 可量化的评测 + 可追溯
 | T-G2 | 观察埋点：observer + 三消费方埋点（pipeline/proxy/client） | ✅ 完成 | T-G2 |
 | T-G3 | 标签+校准：夜间 true_tier 推导 + split-conformal + 工件表 | ✅ 完成 | T-G3 |
 | T-G3b | KnnHead（架构变体 B）：kNN 投票 + conformal-kNN + 按桶分区/封顶/压缩；hybrid fusion 预留（§14） | ⬜ 待办 | |
-| T-G4 | 线性头：离线训练脚本 + LinearHead 纯 Python 推理 + 登记 | ⬜ 待办 | |
+| T-G4 | 线性头：离线训练脚本 + LinearHead 纯 Python 推理 + 登记 | ✅ 完成 | T-G4 |
 | T-G5 | Grader：决策组合（特征门×概率×conformal）+ /v1/route + 三态 mode | ⬜ 待办 | |
 | T-G6 | live 分流：pipeline tier_fn 三档钩子 + proxy 档位映射 + T2 档升级阶梯 | ⬜ 待办 | |
 | T-G7 | 审计+前端：ReviewQueue 抽样 + tier 指标卡 + 客户端来源显示/一键升级 | ⬜ 待办 | |