@@ -151,7 +151,8 @@ func openChatV3At(launch atLaunch) error {
 		Label:    pair.ThisMachineLabel(),
 		// EVERY LINE OF THE PAIRING GOES TO STDERR, so that `--once` piped into
 		// a file is the reply and nothing else.
-		Say: func(line string) { fmt.Fprintln(os.Stderr, line) },
+		Say:  func(line string) { fmt.Fprintln(os.Stderr, line) },
+		Sync: syncRelayURL(),
 	}
 	if stdinIsTerminal(os.Stdin) {
 		reach.AskCode = askPairingCode
@@ -261,7 +262,7 @@ func askPairingCode(name string) (string, error) {
 func runServe(args []string) error {
 	flags := commandFlags("serve")
 	workspace := flags.String("workspace", "", "directory a connection works in when it does not name one; empty is the directory this command was run in")
-	relayAddress := flags.String("via", "", "the sync address to be reachable through; empty reads "+pair.RelayEnv)
+	relayAddress := flags.String("via", "", "the sync address to be reachable through; empty reads "+pair.RelayEnv+", then the address this machine syncs through")
 	renamedFlag(flags, "relay", "via")
 	if err := parseCommandFlags(flags, args); err != nil {
 		return err
@@ -272,11 +273,11 @@ func runServe(args []string) error {
 
 	service := strings.TrimSpace(*relayAddress)
 	if service == "" {
-		service = pair.Relay()
+		service = pair.Relay(syncRelayURL())
 	}
 	if service == "" {
-		// THE HONEST STATE, AND THE ONE MOST PEOPLE WILL MEET. The relay is a
-		// service and this build does not assume one exists.
+		// Only a machine whose person turned sync off and named no relay
+		// gets here; every other machine has sync's address.
 		return errors.New("no sync address is set up on this machine, so there is nowhere to be reachable from — set " + pair.RelayEnv + " to your sync address, or let people in over ssh with `codeaf chat --host` from their side")
 	}
 
 
@@ -3,6 +3,10 @@ package main
 import (
 	"strings"
 	"testing"
+
+	"github.com/Agent-Field/codeaf/internal/home"
+	"github.com/Agent-Field/codeaf/internal/pair"
+	"github.com/Agent-Field/codeaf/internal/syncsetup"
 )
 
 // The target is read the way the ssh door reads its own, because it is the same
@@ -58,12 +62,13 @@ func TestTheFlagsThatBuildASessionCannotTravelOverAt(t *testing.T) {
 }
 
 // WITH NO RELAY SET UP, BOTH DOORS SAY SO IN ONE SENTENCE AND NAME THE ROAD
-// THAT DOES WORK. The relay is a service, this build does not assume one is
-// running, and a person who meets this must not be left guessing whether their
-// network is broken.
+// THAT DOES WORK. Sync falls back to the hosted relay, so the only machine
+// with no relay is one whose person turned sync off, and they must not be
+// left guessing whether their network is broken.
 func TestWithNoRelayBothDoorsSayWhatIsWrongAndWhatToDoInstead(t *testing.T) {
 	t.Setenv("CODEAF_HOME", t.TempDir())
 	t.Setenv("CODEAF_RELAY", "")
+	t.Setenv(syncsetup.URLVar, "off")
 
 	err := openChatV3At(atLaunch{target: "otter-lamp-42"})
 	if err == nil {
@@ -91,6 +96,39 @@ func TestWithNoRelayBothDoorsSayWhatIsWrongAndWhatToDoInstead(t *testing.T) {
 	}
 }
 
+// SERVE, --at, PAIRING AND SYNC AGREE ABOUT THE RELAY. A machine whose chats
+// sync is reachable through that same relay with nothing else set up, whether
+// sync's address came from the variable, a saved pairing or the hosted default.
+func TestServeAtPairingAndSyncResolveOneRelay(t *testing.T) {
+	cases := []struct{ name, variable, saved, want string }{
+		{"the hosted default", "", "", syncsetup.HostedRelayURL},
+		{"the relay a pairing saved", "", "https://saved.example/fabric", "https://saved.example/fabric"},
+		{"the variable", "https://variable.example", "https://saved.example/fabric", "https://variable.example"},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			t.Setenv("CODEAF_HOME", t.TempDir())
+			t.Setenv(pair.RelayEnv, "")
+			t.Setenv(syncsetup.URLVar, c.variable)
+			if c.saved != "" {
+				if err := syncsetup.SaveRelayURL(home.Dir(), c.saved); err != nil {
+					t.Fatal(err)
+				}
+			}
+			mailbox, err := pairMailbox("")
+			if err != nil {
+				t.Fatal(err)
+			}
+			sync := syncsetup.Resolve(home.Dir()).URL
+			serve := pair.Relay(syncRelayURL())
+			at := pair.RelayFor(pair.Known{}, false, syncRelayURL())
+			if sync != c.want || mailbox.URL != c.want || serve != c.want || at != c.want {
+				t.Fatalf("sync %q, pairing %q, serve %q, --at %q; want all %q", sync, mailbox.URL, serve, at, c.want)
+			}
+		})
+	}
+}
+
 // `codeaf devices` on a machine nothing has paired with is one sentence and an
 // invitation, not a heading over an empty table.
 func TestDevicesOnAFreshMachineSaysNothingIsPaired(t *testing.T) {
 
@@ -24,6 +24,16 @@ import (
 // wait and this client never cuts a healthy one short.
 const pairPollTimeout = pairbox.MaxWait + 10*time.Second
 
+// syncRelayURL is the relay this computer syncs through, empty when sync is
+// off. `codeaf serve` and `--at` fall back to it, so a machine whose chats sync
+// is reachable through the same relay with nothing else set up.
+func syncRelayURL() string {
+	if relay := syncsetup.Resolve(home.Dir()); relay.On() {
+		return relay.URL
+	}
+	return ""
+}
+
 // pairMailbox is the relay a pairing runs through: the --relay flag, else the
 // relay this computer syncs through. A relay named by flag is the one the other
 // device is told to pass, so it is `Shown`; the default needs no telling.
 
@@ -0,0 +1,15 @@
+---
+kind: fixed
+title: codeaf serve and chat --at work through the relay your chats already sync through
+pr: 2073
+surface: [remote, docs]
+invalidates:
+  - "codeaf serve and chat --at needed CODEAF_RELAY or a saved relay file; with neither they said no sync address was set up. They now fall back to the relay sync resolves (CODEAF_SYNC_URL, the relay a pairing saved, then the hosted relay)."
+  - "The relay client dropped the path of a relay address, so https://host/fabric was called at https://host/v1/engine. It now calls https://host/fabric/v1/engine and /fabric/v1/dial/<name>, as every other client does."
+  - "The machine carrier was only the raw aforge-relay/1 HTTP upgrade. It now rides a WebSocket offering that protocol and falls back to the raw upgrade when a relay answers 426; codeaf relay accepts both."
+  - "The hosted relay had no machine doors and answered /v1/engine with 404. It now serves /v1/engine and /v1/dial/<name> through one Durable Object per machine name, which needs the v4-machines migration and the MACHINE binding in a deployment's config."
+  - "codeaf serve said lost sync on every retry and quit when the relay still held its own name from a dropped connection. It now says it once per outage and waits the stale name out."
+---
+
+A relay that is down, or one that cannot connect machines, is now one sentence naming the relay and what
+to try, never an upstream body such as `error code: 522`.
@@ -375,7 +375,7 @@ clear refusal. Run it anyway; the refusals are the deliverable.
 
 ```sh
 codeaf devices                     # who may reach this machine, and its name
-codeaf serve                       # be reachable (needs CODEAF_RELAY set)
+codeaf serve                       # be reachable (through the relay sync uses, or CODEAF_RELAY)
 codeaf chat --at otter-lamp-42     # reach a machine by its name
 ```
 
 
@@ -34,12 +34,10 @@ laptop$ codeaf chat --at otter-lamp-42
 router setting. That machine dials *out* to a relay and stays connected, which is why
 firewalls and NAT stop mattering.
 
-**This needs a relay service, and there is not one running yet.** Until one is switched
-on, `--at` prints:
-
-```
-no service address is set up on this machine, so --at has nowhere to look for otter-lamp-42 — set CODEAF_RELAY to a service address, or reach that machine with --host over ssh
-```
+**There is nothing to set up.** Both machines go through the relay their chats already
+sync through: the hosted relay, unless sync was pointed somewhere else with
+`CODEAF_SYNC_URL` or by pairing. `CODEAF_RELAY`, or `--via` on `codeaf serve`, picks a
+different relay for this door alone, such as one you run yourself.
 
 `--host` works today and is not going anywhere. If you can already reach the machine that
 way, use that; the two doors are compared at the bottom of this page.
@@ -65,19 +63,39 @@ digits, the same every time, on any relay. You cannot pick a nicer one, and nobo
 can register under yours while your machine is connected.
 
 `--workspace` is the directory a connection works in when it does not name one; empty
-means the directory you ran the command in. `--via` beats the `CODEAF_RELAY`
-environment variable.
+means the directory you ran the command in. The relay is, in order: `--via`, the
+`CODEAF_RELAY` environment variable, then the relay this machine syncs through.
 
 Running it twice on one machine is refused, because there is only one of it:
 
 ```
-this machine is already connected to the service as otter-lamp-42 — there is only one of it, so close the other `codeaf serve`
+this machine is already connected to sync as otter-lamp-42 — there is only one of it, so close the other `codeaf serve`
+```
+
+## codeaf serve when the network drops or the relay says no
+
+If the network drops while it runs, it says once:
+
+```
+lost sync — trying again
 ```
 
-With no relay set up at all it says:
+and keeps trying, waiting a little longer each time up to a minute, until it prints its
+name and code again. It does not need restarting, and devices reach it as soon as it has.
+
+A relay that answers but cannot connect machines, such as an older sync service, stops
+it with:
 
 ```
-no service address is set up on this machine, so there is nowhere to be reachable from — set CODEAF_RELAY to your service address, or let people in over ssh with `codeaf chat --host` from their side
+the service at https://service.example.com syncs chats but does not connect machines — run your own with `codeaf relay` and set CODEAF_RELAY to it, or reach that machine with --host over ssh
+```
+
+A relay that cannot be reached when it starts stops it with the sentence in *When --at
+will not open*. With sync turned off (`CODEAF_SYNC_URL=off`) and no `CODEAF_RELAY`, there
+is no relay at all:
+
+```
+no sync address is set up on this machine, so there is nowhere to be reachable from — set CODEAF_RELAY to your sync address, or let people in over ssh with `codeaf chat --host` from their side
 ```
 
 Ctrl+c ends it, and the machine gives its name back on the way out:
@@ -351,9 +369,9 @@ that still has it in the list, which is what revoking removes.
 
 ## When --at will not open
 
-There are four different reasons, and each says which one it is, in one sentence.
+There are five different reasons, and each says which one it is, in one sentence.
 
-**No relay is set up on this machine:**
+**No relay is set up on this machine** (sync is off and `CODEAF_RELAY` is empty):
 
 ```
 no service address is set up on this machine, so --at has nowhere to look for otter-lamp-42 — set CODEAF_RELAY to a service address, or reach that machine with --host over ssh
@@ -365,6 +383,12 @@ no service address is set up on this machine, so --at has nowhere to look for ot
 the service at https://service.example.com cannot be reached from here — check this machine's network, or reach that machine with --host over ssh
 ```
 
+**The relay answers but cannot connect machines:**
+
+```
+the service at https://service.example.com syncs chats but does not connect machines — run your own with `codeaf relay` and set CODEAF_RELAY to it, or reach that machine with --host over ssh
+```
+
 **The relay is fine and that machine is not connected to it:**
 
 ```
@@ -464,15 +488,18 @@ sitting at. They differ only in how the two halves reach each other.
 
 | | `--host` | `--at` |
 | --- | --- | --- |
-| Needs | `ssh <machine>` already works | a relay service, and one pairing |
+| Needs | `ssh <machine>` already works | one pairing; the relay is the one sync uses |
 | Set up on the far machine | nothing | `codeaf serve` running |
 | Works through NAT and firewalls | only if ssh does | yes — that machine dials out |
 | Anything in the middle | nothing of ours | a relay that cannot read the conversation |
-| Available today | yes | not until a relay is running |
+| Available today | yes | yes |
 
 **Prefer `--host` when ssh works.** It has nothing between the two machines, nothing to
 set up, and no service to depend on. `--at` is for the machine ssh cannot reach.
 
+Both machines must use the same relay. If one syncs through a relay of your own and the
+other through the hosted one, set `CODEAF_RELAY` to the same address on both.
+
 Both open the same surface with the same limits — what does and does not work over a
 connection is a property of the connection, not of which of these two doors opened it.
 
 
@@ -72,6 +72,9 @@ type Reach struct {
 	AskCode func(name string) (string, error)
 	// Say is where the pairing's own lines go. Nil says nothing.
 	Say func(string)
+	// Sync is the relay this device syncs through, used when nothing more
+	// particular names one; see [Relay].
+	Sync string
 	// Now is the clock, swapped by tests.
 	Now func() time.Time
 }
@@ -102,7 +105,7 @@ func (r Reach) Open(ctx context.Context) (*Tunnel, error) {
 	if err != nil {
 		return nil, err
 	}
-	service := RelayFor(known, found)
+	service := RelayFor(known, found, r.Sync)
 	if strings.TrimSpace(service) == "" {
 		return nil, NoRelay(r.Name)
 	}
@@ -204,6 +207,8 @@ func (r Reach) dial(ctx context.Context, service string) (io.ReadWriteCloser, er
 		return nil, NotConnected(r.Name)
 	case errors.Is(err, relay.ErrUnreachable):
 		return nil, Unreachable(service)
+	case errors.Is(err, relay.ErrNoDoor):
+		return nil, NoDoor(service)
 	case errors.Is(err, relay.ErrTooMany):
 		return nil, Busy()
 	default: