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diff --git a/packages/meshbay-common/src/meshbay_common/protocol.py b/packages/meshbay-common/src/meshbay_common/protocol.py
new file mode 100644
index 0000000..c3f2b1a
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+++ b/packages/meshbay-common/src/meshbay_common/protocol.py
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+"""
+MeshBay protocol constants and message type definitions.
+
+MNP (Mesh Node Protocol) — v0.1
+MHP (Mesh Bay Hub Protocol) — v0.1
+
+All wire messages are length-prefixed msgpack (4-byte big-endian length header).
+Every message carries a "v" field for protocol version.
+"""
+
+from dataclasses import dataclass, field
+from typing import Any
+
+MNP_VERSION = "0.1"
+MHP_VERSION = "0.1"
+
+
+# ── MNP message types ─────────────────────────────────────────────────────────
+
+class MNP:
+    HANDSHAKE       = "handshake"
+    HANDSHAKE_ACK   = "handshake_ack"
+    INDEX_SYNC      = "index_sync"       # full Mesh Group Index
+    INDEX_DELTA     = "index_delta"      # incremental update
+    FILE_REQUEST    = "file_req"         # request chunk(s)
+    FILE_CHUNK      = "file_chunk"       # encrypted chunk response
+    STREAM_SEGMENT  = "stream_seg"       # HLS/DASH segment
+    CHAT_MESSAGE    = "chat_msg"         # Double Ratchet message
+    CHAT_ATTACHMENT = "chat_attach"      # attachment metadata
+    EPHEMERAL_STREAM = "ephemeral_stream"  # reserved — mobile live push
+
+
+# ── Index entry ───────────────────────────────────────────────────────────────
+
+@dataclass
+class IndexEntry:
+    id:        str    # blake3 hash of file (hex)
+    name:      str    # filename
+    path:      str    # path relative to shared directory
+    size:      int    # bytes
+    type:      str    # video | audio | image | document | archive | other
+    added_at:  int    # unix timestamp
+    duration:  int | None = None   # seconds, for media
+    thumb_hash: str | None = None  # blake3 of thumbnail
+
+
+@dataclass
+class IndexDelta:
+    base_version: int
+    version:      int
+    additions:    list[IndexEntry] = field(default_factory=list)
+    deletions:    list[str] = field(default_factory=list)  # list of ids
+
+
+# ── Chunk request/response ────────────────────────────────────────────────────
+
+@dataclass
+class ChunkRequest:
+    file_id:     str   # blake3 hash of file (hex)
+    chunk_index: int
+
+@dataclass
+class ChunkResponse:
+    chunk_index:    int
+    plaintext_size: int
+    nonce_b64:      str
+    ct_b64:         str
+    ct_hash_b64:    str
+    pt_hash_b64:    str
+    sig_b64:        str
+    pk_node_b64:    str
+    file_hash_b64:  str