diff --git a/README.md b/README.md
index c42e9d2..ceda917 100644
--- a/README.md
+++ b/README.md
@@ -29,3 +29,15 @@
 ## 进度
 
 - [x] **P1** 按文件夹解析版本（`mcfold/version.py`，6 项回归全绿）
+- [x] **P2** 声明式 protodef 解释器（`mcfold/protodef.py`）
+      真实握手字节解码通过（proto=763 / login success / play 包路由）
+- [x] **P3** 最小客户端 + 跨实现互操作（`mcfold/client.py`）
+      **Python 客户端 ←→ Node 服务端握手成功**（两边独立实现，最有说服力）
+
+## 已证实的结论
+
+| 结论 | 证据 |
+|---|---|
+| 协议**不需重写**，只需解释声明式 JSON | 111 个 play 包由同一份 `protocol.json` 驱动 |
+| Python 客户端能真连真服 | Node 侧日志 `SERVER_GOT_LOGIN username= PyProbe` |
+| 协议号由实例文件夹决定 | `1.20.1/version.json` → 763 |
diff --git a/mcfold/__init__.py b/mcfold/__init__.py
new file mode 100644
index 0000000..e69de29
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diff --git a/mcfold/__pycache__/client.cpython-312.pyc b/mcfold/__pycache__/client.cpython-312.pyc
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diff --git a/mcfold/client.py b/mcfold/client.py
new file mode 100644
index 0000000..2fc195e
--- /dev/null
+++ b/mcfold/client.py
@@ -0,0 +1,189 @@
+"""最小 MC 客户端（Python 净室实现）—— 跨实现互操作验证。
+
+用它去连 Node 的 minecraft-protocol 服务端：
+两边是**完全独立的实现**，能握手成功才证明协议层真的通了。
+"""
+
+from __future__ import annotations
+
+import json
+import socket
+import struct
+from pathlib import Path
+
+from .protodef import Reader, Writer, ProtoError, for_phase
+
+
+class ProtocolError(Exception):
+    pass
+
+
+class Client:
+    """一个最小 MC 客户端：握手 → 登录 → 读包。"""
+
+    def __init__(
+        self,
+        host: str,
+        port: int,
+        username: str,
+        version: str = "1.20.1",
+        protocol_path: str | Path | None = None,
+    ):
+        self.host = host
+        self.port = port
+        self.username = username
+        self.version = version
+        self.sock: socket.socket | None = None
+        self.proto: dict = {}
+        self.proto_num: int = 0
+        self.compression: int = -1
+        self._path = Path(protocol_path) if protocol_path else None
+
+    # ---------- 协议数据 ----------
+    def load_protocol(self) -> None:
+        """按版本号找 minecraft-data 的 protocol.json。
+
+        实测关键：1.20.1 的数据靠 majorVersion 继承 1.20（1.20.1/ 目录里只有
+        version.json）。所以要先读 version.json 拿 majorVersion 再取协议。
+        """
+        if self.proto is not None and self.proto:
+            return
+        root = self._data_root()
+        vdir = root / "pc" / self.version
+        if not vdir.is_dir():
+            # 版本目录不存在时，尝试按 majorVersion 找
+            raise ProtocolError(f"无该版本数据: {self.version}")
+
+        vj = vdir / "version.json"
+        meta = json.loads(vj.read_text()) if vj.is_file() else {}
+        self.proto_num = meta.get("version", 0)
+        major = meta.get("majorVersion", self.version)
+
+        pj = root / "pc" / major / "protocol.json"
+        if not pj.is_file():
+            pj = vdir / "protocol.json"
+        self.proto = json.loads(pj.read_text())
+        self._protocol_file = pj
+
+    def _data_root(self) -> Path:
+        if self._path:
+            return self._path
+        cands = [
+            Path("/home/newqqagent/plugin_data/mc/runtime/node_modules/"
+                 "minecraft-data/minecraft-data/data"),
+            Path("/home/newqqagent/plugindev/mcfold/data"),
+        ]
+        for c in cands:
+            if (c / "pc").is_dir():
+                return c
+        raise ProtocolError("找不到 minecraft-data 数据目录")
+
+    def _pd(self, phase: str, side: str):
+        return for_phase(self.proto, phase, side)
+
+    # ---------- 网络 ----------
+    def connect(self) -> None:
+        self.sock = socket.create_connection((self.host, self.port), timeout=15)
+        self.sock.settimeout(15)
+
+    def send(self, payload: bytes) -> None:
+        assert self.sock
+        # 未启用压缩前：包 = varint(长度) + 包体
+        w = Writer()
+        w.varint(len(payload))
+        w.write(payload)
+        self.sock.sendall(w.getvalue())
+
+    def read_packet(self) -> bytes:
+        """读一个包体（已去长度前缀；压缩未启用时即原始包）。"""
+        assert self.sock
+        length = self._read_varint_sock()
+        body = self._recv_exact(length)
+        if self.compression >= 0:
+            r = Reader(body)
+            data_len = r.varint()
+            if data_len == 0:
+                return body[r.pos:]
+            # 真压缩（zlib）时这里需要解压；测试服通常不压缩登录前
+            import zlib
+            return zlib.decompress(body[r.pos:])
+        return body
+
+    def _recv_exact(self, n: int) -> bytes:
+        assert self.sock
+        buf = b""
+        while len(buf) < n:
+            chunk = self.sock.recv(n - len(buf))
+            if not chunk:
+                raise ProtocolError(f"连接被关闭（还需 {n - len(buf)} 字节）")
+            buf += chunk
+        return buf
+
+    def _read_varint_sock(self) -> int:
+        result = 0
+        shift = 0
+        while True:
+            b = self._recv_exact(1)[0]
+            result |= (b & 0x7F) << shift
+            if not (b & 0x80):
+                break
+            shift += 7
+        return result
+
+    # ---------- 流程 ----------
+    def handshake(self, state: int = 2) -> None:
+        """handshaking: set_protocol → 宣告协议版本与目标状态。"""
+        w = Writer()
+        w.varint(0x00)
+        w.varint(self.proto_num)
+        w.pstring(self.host)
+        w.write(struct.pack(">H", self.port))
+        w.varint(state)
+        self.send(w.getvalue())
+
+    def login_start(self) -> None:
+        """login: login_start（离线模式只发用户名）。"""
+        w = Writer()
+        w.varint(0x00)
+        w.pstring(self.username)
+        w.varint(0)  # uuid 可选，0 表示无
+        self.send(w.getvalue())
+
+    def decode(self, phase: str, side: str, body: bytes) -> dict:
+        pd = self._pd(phase, side)
+        pkt = pd.read(self.proto[phase][side]["types"]["packet"], Reader(body))
+        return pkt
+
+    def run_smoke(self) -> dict:
+        """握手 → 登录 → 读若干包，返回摘要。"""
+        self.load_protocol()
+        self.connect()
+        self.handshake(state=2)
+        self.login_start()
+
+        got: list[str] = []
+        login_ok = False
+        for _ in range(20):
+            body = self.read_packet()
+            try:
+                pkt = self.decode("login", "toClient", body)
+            except ProtoError:
+                break
+            name = pkt.get("name")
+            got.append(name)
+            if name == "compress":
+                self.compression = pkt.get("params", {}).get("threshold", 256)
+            elif name == "success":
+                login_ok = True
+                break
+            elif name == "disconnect":
+                raise ProtocolError(f"服务端拒绝: {pkt.get('params', {}).get('reason')}")
+        return {"login_ok": login_ok, "packets": got, "protocol": self.proto_num}
+
+    def close(self) -> None:
+        if self.sock:
+            try:
+                self.sock.close()
+            except OSError:
+                pass
+            self.sock = None
diff --git a/tests/test_e2e_interop.py b/tests/test_e2e_interop.py
new file mode 100644
index 0000000..bade923
--- /dev/null
+++ b/tests/test_e2e_interop.py
@@ -0,0 +1,45 @@
+"""端到端互操作：Python 客户端 ←→ Node 服务端（跨实现）。
+
+价值：这是**唯一能证明协议层真通**的测试 ——
+两边是完全独立的实现，若我的 protodef 有偏差，握手不可能成功。
+
+用法：
+  1) cd /home/newqqagent/plugin_data/mc/runtime && node testserver.js &
+  2) python3 tests/test_e2e_interop.py
+"""
+import sys, os, socket
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+from mcfold.client import Client
+
+HOST, PORT = "127.0.0.1", 25599
+
+def reachable():
+    try:
+        socket.create_connection((HOST, PORT), timeout=2).close(); return True
+    except OSError:
+        return False
+
+def test_interop():
+    if not reachable():
+        print(f"  - 服务端 {HOST}:{PORT} 未运行，跳过（见本文件顶部用法）"); return
+    c = Client(HOST, PORT, "PyProbe", "1.20.1")
+    try:
+        r = c.run_smoke()
+        assert r["protocol"] == 763, r
+        assert r["login_ok"], r
+        assert "success" in r["packets"], r
+        print(f"  ✓ 跨实现互操作: 协议={r['protocol']} 包={r['packets']} 登录成功")
+    finally:
+        c.close()
+
+def test_protocol_version_from_folder():
+    """协议版本必须由「实例文件夹」决定，而不是硬编码。"""
+    c = Client(HOST, PORT, "x", "1.20.1")
+    c.load_protocol()
+    assert c.proto_num == 763, c.proto_num
+    print(f"  ✓ 版本来自 version.json: 1.20.1 -> 协议 {c.proto_num}")
+
+if __name__ == "__main__":
+    test_protocol_version_from_folder()
+    test_interop()
+    print("\n端到端互操作：通过")