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, 61 insertions, 1 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
index 8e357c9..aaf3f81 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
@@ -24,6 +24,7 @@ Signaling flow (handled externally by the hub):
 
 import asyncio
 import base64
+import contextvars
 import hashlib
 import hmac
 import logging
@@ -258,6 +259,32 @@ def _pack(obj: dict) -> bytes:
 _WEBRTC_TRACE = os.environ.get("MESHBAY_WEBRTC_TRACE") == "1"
 _WEBRTC_TRACE_INTERVAL_S = 30.0
 
+# The request this session is currently answering, as (session, req_id).
+#
+# MNP has never carried a correlation id: a reply named its own type and
+# nothing else, so a client with more than one request outstanding had to guess
+# which one a message answered — by arrival order, for every reply the client
+# could not key off a field of its own. The guess is wrong whenever two replies
+# reorder, and catastrophically wrong for the replies that name *nothing*: this
+# module sends `{"type": "error"}` from 240 places and two of them name what
+# they are about. A refusal therefore reached no caller at all, and the request
+# it belonged to waited out the client's 30s timeout while some unrelated
+# request was resolved with the refusal instead. Live symptom, found 2026-09-06:
+# the Chat composer is disabled while a send is in flight, so a chat message
+# whose reply went astray froze the tab for 30 seconds.
+#
+# `req_id` closes it: whatever the caller put on the request is stamped on the
+# reply. A ContextVar rather than a parameter because the alternative is
+# threading an argument through all 240 send sites — and asyncio copies the
+# current context into a task, so a handler that `_spawn`s its real work still
+# answers under the id of the request that started it.
+#
+# The session is held alongside the id because a handler may send to *other*
+# sessions as well as its own (a chat broadcast, an index push): those are not
+# replies to anything and must not be stamped. _send checks the owner.
+_REPLY_TO: contextvars.ContextVar[tuple] = contextvars.ContextVar(
+    "meshbay_reply_to", default=(None, None))
+
 
 class _DataChannelBuffer:
     """
@@ -393,6 +420,22 @@ class WebRTCPeerSession:
             )
 
     def _handle_message(self, msg: dict) -> None:
+        """Answer one MNP message, under the correlation id it carries.
+
+        The id is published for the whole handler — see _REPLY_TO — so that
+        every reply _send puts on the wire, including the ones a spawned task
+        sends much later and the generic refusal below, names the request it
+        answers. Resetting on the way out only clears it for *this* call: a
+        task spawned in between captured its own copy of the context when it
+        was created and keeps answering under the right id.
+        """
+        token = _REPLY_TO.set((self, msg.get("req_id")))
+        try:
+            self._dispatch_message(msg)
+        finally:
+            _REPLY_TO.reset(token)
+
+    def _dispatch_message(self, msg: dict) -> None:
         mtype = msg.get("type")
         log.debug("WebRTC recv: %s", mtype)
         try:
@@ -2941,7 +2984,14 @@ class WebRTCPeerSession:
 
     def _group_ctx(self) -> dict:
         if "groups" in self._ctx and self._group_id:
-            return self._ctx["groups"][self._group_id]
+            # `.get`, not a bare subscript. A config reload removes a group
+            # from this map (daemon.py's reload does `groups_ctx.pop`) while
+            # sessions connected to it are still open, and the next request
+            # any of them made raised KeyError into _dispatch_message's
+            # catch-all. An absent group now reads the way an unconfigured
+            # one already does — the handlers all test for what they need —
+            # instead of failing every request the session has left.
+            return self._ctx["groups"].get(self._group_id) or {}
         return self._ctx
 
     def _indexing_status(self) -> dict:
@@ -4918,6 +4968,16 @@ class WebRTCPeerSession:
         self._audit("stream_video", entry.name)
 
     def _send(self, obj: dict) -> None:
+        # Stamp the reply with the id of the request being answered, so the
+        # caller never has to guess. Only for this session's own replies: a
+        # handler that also pushes to other peers (a chat broadcast, an index
+        # delta) reaches them through *their* _send, where the owner no longer
+        # matches and nothing is stamped — those messages answer no request.
+        # An explicit req_id already on the object wins, and an unsolicited
+        # push (no request in scope) carries none, exactly as before.
+        owner, req_id = _REPLY_TO.get()
+        if req_id is not None and owner is self and "req_id" not in obj:
+            obj = {**obj, "req_id": req_id}
         if self._channel and self._channel.readyState == "open":
             self._channel.send(_pack(obj))
         else: