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diff --git a/packages/meshbay-node/src/meshbay_node/transport/wire.py b/packages/meshbay-node/src/meshbay_node/transport/wire.py
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+"""
+Wire shapes shared by every node transport.
+
+`file_chunk` lives in `meshbay_common.protocol` — it is pure crypto and shape, so a
+client can use the same encoder. This module is for the messages that also need the
+node's own view of its disk, which `meshbay-common` cannot see.
+
+Why it exists at all: `index_sync` was built twice, and the two copies did not agree.
+WebRTC sent `{group_id, version, entries, dirs, roots}` — the shape the shipping
+client reads — while QUIC sent `{index_b64}`, a signed, compressed, GEK-encrypted
+envelope produced by `GroupIndex.serialize()`. Same message type, two encodings, one
+consumer each and nothing asserting they matched. Same failure mode as the two
+`file_chunk` encoders, and the same fix: one builder, used by both.
+
+`GroupIndex.serialize()`/`deserialize()` are unchanged and still tested — they remain
+a correct signed index envelope — but they no longer describe any MNP message. Read
+them as an at-rest/interchange format, not as a wire contract.
+"""
+
+from __future__ import annotations
+
+from meshbay_common import MNP_VERSION
+from meshbay_common.protocol import MNP, index_entry_wire
+
+from meshbay_node.roots import RootSet
+
+# A group with a deep tree can hold more directories than anyone will navigate in one
+# sitting, and the whole list rides on one message.
+MAX_DIRS = 2000
+
+
+def list_dirs(roots: RootSet | None) -> list[str]:
+    """
+    Every directory in the group, as members address them, sorted.
+
+    Each root appears as a directory in its own right, so a root holding no files yet
+    is still somewhere a member can navigate to and upload into. An unavailable root
+    is listed too — its content is frozen, not gone, and hiding it would look exactly
+    like deletion.
+    """
+    if not roots:
+        return []
+    out: list[str] = []
+    for root in roots:
+        out.append(root.name)
+        if not root.available:
+            continue
+        try:
+            for path in sorted(root.path.rglob("*")):
+                if path.is_dir() and not path.name.startswith("."):
+                    rel = path.relative_to(root.path)
+                    if not any(part.startswith(".") for part in rel.parts):
+                        out.append(f"{root.name}/{rel.as_posix()}")
+        except OSError:
+            continue
+    return sorted(out)[:MAX_DIRS]
+
+
+def index_sync_message(index, roots: RootSet | None) -> dict:
+    """
+    The full `index_sync` message for one group.
+
+    `dirs` and `roots` are here because directories are not index entries: without
+    them a folder someone just created, or one they emptied, does not exist as far as
+    a client is concerned, and a member cannot tell "the drive is unplugged" from "it
+    is all still there".
+    """
+    return {
+        "type": MNP.INDEX_SYNC,
+        "v": MNP_VERSION,
+        "group_id": index.group_id,
+        "version": index.version,
+        "entries": [index_entry_wire(e) for e in index.entries],
+        "dirs": list_dirs(roots),
+        "roots": roots.describe() if roots else [],
+    }