fix(mnp): give a reply an id, so it stops being routed by luck

MNP carried no correlation id. A reply named its own type and nothing
else, so a client with more than one request in flight worked out which
one a message answered from the message itself — and for the replies
that name nothing it could not. `_dispatch` fell through to matching by
arrival order, which is a guess. `_sendAndWait` had the right value all
along: it keys `_pending` by `this._seqId++` and never put it on the
wire.

The guess fails asymmetrically, which is why it hid. The victim is not
the request that was answered wrongly — it is the unrelated one that now
waits out its own 30s timeout for a reply already delivered elsewhere.
Live on 2026-09-06: five `music_meta_req` sat pending for over 100
seconds behind a failing MusicBrainz, and a `device_list_result` was
handed to one of them. The composer is disabled while a send is in
flight, so a chat message whose reply went astray the same way left the
Chat tab looking frozen for thirty seconds, then unfroze on its own.

The `ack` half of this was fixed on 2026-08-30 by matching on request
type. That closed the instance and left the class open: a refusal has no
type to match on either, and `_dispatch_message`'s catch-all answers
every unforeseen failure with `{"type": "error", "detail": "Request
failed"}` — 238 of this module's 240 error sends name nothing at all.

`req_id` now rides on the request and comes back on the reply. On the
node it is published for the whole handler in a ContextVar and stamped
by `_send`: a parameter would have meant threading an argument through
all 240 send sites, and asyncio copies the context into a task, so a
handler that `_spawn`s its real work still answers under the right id.
It is never stamped on a broadcast — those answer nothing, and the
owner check in `_send` is what keeps a chat broadcast or an index push
from reaching another peer looking like a reply.

On the client, `_dispatch` resolves on `req_id` first and the
arrival-order fallback is gone the moment a node proves it stamps
(`_correlates`, armed by the handshake's own reply). The fallback stays
for an MNP 1.0 node, unchanged and no wider: there it is the only thing
there is, and removing it would leave device_list_result, join_result
and the handshake replies reaching nobody.

Two things fall out. `sendChat` refuses an `error` reply like every
other request in the file — it returned it as success, which did not
matter while a refusal reached the wrong caller anyway and would now
show a rejected message as sent. And `_group_ctx` uses `.get`: a reload
pops a removed group while sessions connected to it are open, and every
request they had left raised KeyError into that same catch-all.

Sealed index messages are the one exception to the fast path. They
cannot be handed over until they are opened, which is asynchronous while
`_dispatch` is not — resolving on the id alone gave `fetchIndex` the
envelope and skipped `onIndexSync` entirely. Caught by extending
`index_seal_probe.mjs` to stamp a reply the way a current node does,
after the hub suite passed over it: the probe built its own frames and
had never seen one.

Tests, all failing before and passing after: `test_chat_send.py` drives
the real ChatPanel over the real transport for both shapes of reply with
an older request pending (3 of its 6 are new, and the 3 for `ack` pass
either way, so it discriminates); `test_reply_correlation.py` pins the
node's half — the refusals that name nothing else, the broadcast that
must not be stamped, and a late reply from a spawned task answering
under its own id rather than the most recent request's.

Full suite: 1897 passed, same 11 pre-existing failures as before.

QUIC keeps its own dispatch and is not stamped. It is disabled by
default and no browser request reaches it, but the asymmetry is real.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Dn1xYx9uT69mCB6UDvyKAN

1 files changed, 147 insertions, 0 deletions

diff --git a/packages/meshbay-node/tests/test_reply_correlation.py b/packages/meshbay-node/tests/test_reply_correlation.py
new file mode 100644
index 0000000..7c2138b
--- /dev/null
+++ b/packages/meshbay-node/tests/test_reply_correlation.py
@@ -0,0 +1,147 @@
+"""
+A reply names the request it answers.
+
+MNP carried no correlation id until 2026-09-07. A reply named its own type and
+nothing else, so a client with more than one request outstanding had to work out
+which one a message answered from the message itself — and for the replies that
+name nothing, it could not. This module sends `{"type": "error"}` from 240
+places and two of them say what they are about; `_dispatch_message`'s catch-all
+is one of the 238. Such a refusal reached no caller at all: the browser handed
+it to whichever request happened to be waiting, and the request it belonged to
+sat until its own 30s timeout. Live symptom (2026-09-06): the Chat composer is
+disabled while a send is in flight, so a chat message whose refusal went astray
+froze the tab for thirty seconds.
+
+`req_id` is the client's own pending-map key, put on the wire and stamped back
+onto the reply by `_send`. What matters here, and what the browser cannot check
+for itself:
+
+  * a reply carries it, including the refusals that name nothing else;
+  * a *broadcast* does not — it answers no request, and stamping it would hand
+    another peer's client a reply to a request it never made;
+  * work handed to a background task still answers under the right id, which is
+    why this is a ContextVar and not an attribute on the session.
+"""
+import asyncio
+
+import msgpack
+import pytest
+from meshbay_common.protocol import MNP
+from meshbay_node.transport.webrtc_server import WebRTCPeerSession
+
+pytestmark = pytest.mark.asyncio
+
+
+class _Channel:
+    readyState = "open"
+
+    def __init__(self):
+        self.sent = []
+
+    def send(self, framed):
+        # Skip the 4-byte length prefix _pack writes.
+        self.sent.append(msgpack.unpackb(framed[4:], raw=False))
+
+
+def _session(peer_id="p"):
+    s = WebRTCPeerSession.__new__(WebRTCPeerSession)
+    s._ctx = {}
+    s._peer_id = peer_id
+    s._user_id = None
+    s._group_id = ""
+    s._channel = _Channel()
+    s._tasks = set()
+    return s
+
+
+async def test_a_refusal_that_names_nothing_else_names_the_request():
+    """The reply at the root of the defect: no type of its own to match on."""
+    s = _session()
+    # No handshake yet, so any other message is refused — a bare `error`, the
+    # same shape the catch-all sends and the same shape a browser could not
+    # route.
+    s._handle_message({"type": MNP.CHAT_MESSAGE, "req_id": 41})
+
+    (reply,) = s._channel.sent
+    assert reply["type"] == "error"
+    assert reply["req_id"] == 41, (
+        "a refusal that names neither the request nor a type of its own is a "
+        "reply no caller can claim")
+
+
+async def test_the_catch_all_refusal_names_the_request_too():
+    """Every failure in the dispatch loop funnels into one generic reply."""
+    s = _session()
+    s._user_id = "u"
+
+    def _boom(msg):
+        raise RuntimeError("filesystem path that must not reach the peer")
+    s._do_chat_message = _boom
+
+    s._handle_message({"type": MNP.CHAT_MESSAGE, "req_id": 7})
+
+    (reply,) = s._channel.sent
+    assert reply == {"type": "error", "detail": "Request failed", "req_id": 7}, (
+        "the catch-all is where an unforeseen failure ends up, so it is exactly "
+        "the reply that must still be routable")
+
+
+async def test_a_request_without_an_id_is_answered_without_one():
+    """An older client sends none; nothing may be invented for it."""
+    s = _session()
+    s._handle_message({"type": MNP.CHAT_MESSAGE})
+
+    (reply,) = s._channel.sent
+    assert "req_id" not in reply
+
+
+async def test_a_broadcast_to_another_peer_is_not_stamped():
+    """The reply goes to the asker; the broadcast goes to everyone else.
+
+    They travel out of the same handler, and only the first answers anything.
+    Stamping the second would hand another browser a reply keyed to a pending
+    request of its own that it never sent — the very confusion this fixes.
+    """
+    asker, other = _session("asker"), _session("other")
+    asker._user_id, other._user_id = "a", "b"
+    registry = {"a": asker, "b": other}
+    asker._peer_registry = lambda: registry
+    asker._user_names = lambda: {}
+    asker._audit = lambda *a, **k: None
+    asker._group_ctx = lambda: {}
+
+    asker._handle_message({"type": MNP.CHAT_MESSAGE, "payload": "hi", "req_id": 3})
+
+    (ack,) = asker._channel.sent
+    assert ack["type"] == "ack" and ack["req_id"] == 3
+    (broadcast,) = other._channel.sent
+    assert broadcast["type"] == MNP.CHAT_MESSAGE
+    assert "req_id" not in broadcast, (
+        "a broadcast answers no request and must not look like a reply")
+
+
+async def test_work_handed_to_a_task_still_answers_under_the_right_id():
+    """Most handlers `_spawn` their real work, and the reply leaves long after
+    the dispatch call that started it has returned.
+
+    This is the reason the id lives in a ContextVar: asyncio copies the current
+    context into a task, so the answer keeps the id even though nothing passed
+    it along. An attribute on the session would have been overwritten by the
+    next message to arrive in the meantime.
+    """
+    s = _session()
+    s._user_id = "u"
+
+    async def _late(reply):
+        await asyncio.sleep(0.01)
+        s._send({"type": "roster_read_resp", "detail": reply})
+    s._do_chat_message = lambda msg: s._spawn(_late(msg["payload"]))
+
+    s._handle_message({"type": MNP.CHAT_MESSAGE, "payload": "first", "req_id": 11})
+    # A second request arrives while the first one's task is still asleep.
+    s._handle_message({"type": MNP.CHAT_MESSAGE, "payload": "second", "req_id": 12})
+    await asyncio.gather(*list(s._tasks))
+
+    by_id = {m["req_id"]: m["detail"] for m in s._channel.sent}
+    assert by_id == {11: "first", 12: "second"}, (
+        "a late reply answered under whichever request arrived most recently")