diff --git a/agent/src/main.rs b/agent/src/main.rs
index 0352963..c500559 100644
--- a/agent/src/main.rs
+++ b/agent/src/main.rs
@@ -51,14 +51,20 @@ fn arg(flag: &str, default: &str) -> String {
 
 fn main() {
     let server = arg("--server", "https://tether.pecord.io");
-    let room = arg("--room", "default");
     let source = arg("--source", "agent");
     let from = arg("--from", &format!("agent-{source}"));
     // Friendly name for other devices' nearby lists.
     let host = std::env::var("HOSTNAME").unwrap_or_else(|_| "tether-agent".into());
     let name = arg("--name", &host);
-    // Shared identity: same-account devices pair on the LAN regardless of room.
+    // Shared identity: same-account devices pair on the LAN regardless of room,
+    // and share an account-derived room everywhere (so cross-network works too).
     let account = arg("--account", "pecord@gmail.com");
+    let default_room = if account.is_empty() {
+        "default".to_string()
+    } else {
+        tethercore::room_for_account(account.clone())
+    };
+    let room = arg("--room", &default_room);
 
     eprintln!("tether-agent → {server}  room={room}  source={source}");
 
diff --git a/android/app/src/main/java/io/pecord/tether/TetherStore.kt b/android/app/src/main/java/io/pecord/tether/TetherStore.kt
index 9a97f68..94605b6 100644
--- a/android/app/src/main/java/io/pecord/tether/TetherStore.kt
+++ b/android/app/src/main/java/io/pecord/tether/TetherStore.kt
@@ -19,7 +19,11 @@ data class FeedItem(val text: String, val source: String, val ts: Long, val id:
 class TetherStore(private val context: Context) {
     val serverURL = mutableStateOf(prefs().getString("serverURL", "https://tether.pecord.io")!!)
     val account = mutableStateOf(prefs().getString("account", "pecord@gmail.com")!!)
-    val room = mutableStateOf(RoomIdentity.derive(context))
+    // Room follows the account → all our devices share one room everywhere.
+    val room = mutableStateOf(
+        if (account.value.isEmpty()) RoomIdentity.derive(context)
+        else uniffi.tethercore.roomForAccount(account.value)
+    )
     val feed = mutableStateListOf<FeedItem>()
     val connected = mutableStateOf(false)
 
diff --git a/core/Cargo.lock b/core/Cargo.lock
index dbd788d..015c5e8 100644
--- a/core/Cargo.lock
+++ b/core/Cargo.lock
@@ -8,7 +8,7 @@ version = "0.5.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d122413f284cf2d62fb1b7db97e02edb8cda96d769b16e443a4f6195e35662b0"
 dependencies = [
- "crypto-common",
+ "crypto-common 0.1.7",
  "generic-array",
 ]
 
@@ -20,7 +20,7 @@ checksum = "b169f7a6d4742236a0a00c541b845991d0ac43e546831af1249753ab4c3aa3a0"
 dependencies = [
  "cfg-if",
  "cipher",
- "cpufeatures",
+ "cpufeatures 0.2.17",
 ]
 
 [[package]]
@@ -271,6 +271,15 @@ dependencies = [
  "generic-array",
 ]
 
+[[package]]
+name = "block-buffer"
+version = "0.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d2f6c7dbe95a6ed67ad9f18e57daf93a2f034c524b99fd2b76d18fdfeb6660aa"
+dependencies = [
+ "hybrid-array",
+]
+
 [[package]]
 name = "block-padding"
 version = "0.3.3"
@@ -404,7 +413,7 @@ checksum = "c3613f74bd2eac03dad61bd53dbe620703d4371614fe0bc3b9f04dd36fe4e818"
 dependencies = [
  "cfg-if",
  "cipher",
- "cpufeatures",
+ "cpufeatures 0.2.17",
 ]
 
 [[package]]
@@ -426,7 +435,7 @@ version = "0.4.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "773f3b9af64447d2ce9850330c473515014aa235e6a783b02db81ff39e4a3dad"
 dependencies = [
- "crypto-common",
+ "crypto-common 0.1.7",
  "inout",
  "zeroize",
 ]
@@ -483,6 +492,12 @@ version = "0.9.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c2459377285ad874054d797f3ccebf984978aa39129f6eafde5cdc8315b612f8"
 
+[[package]]
+name = "const-oid"
+version = "0.10.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a6ef517f0926dd24a1582492c791b6a4818a4d94e789a334894aa15b0d12f55c"
+
 [[package]]
 name = "cpufeatures"
 version = "0.2.17"
@@ -492,6 +507,15 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "cpufeatures"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8b2a41393f66f16b0823bb79094d54ac5fbd34ab292ddafb9a0456ac9f87d201"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "crc"
 version = "3.4.0"
@@ -530,6 +554,15 @@ dependencies = [
  "typenum",
 ]
 
+[[package]]
+name = "crypto-common"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ce6e4c961d6cd6c9a86db418387425e8bdeaf05b3c8bc1411e6dca4c252f1453"
+dependencies = [
+ "hybrid-array",
+]
+
 [[package]]
 name = "ctr"
 version = "0.9.2"
@@ -546,7 +579,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "97fb8b7c4503de7d6ae7b42ab72a5a59857b4c937ec27a3d4539dba95b5ab2be"
 dependencies = [
  "cfg-if",
- "cpufeatures",
+ "cpufeatures 0.2.17",
  "curve25519-dalek-derive",
  "fiat-crypto",
  "rustc_version",
@@ -577,7 +610,7 @@ version = "0.7.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e7c1832837b905bbfb5101e07cc24c8deddf52f93225eee6ead5f4d63d53ddcb"
 dependencies = [
- "const-oid",
+ "const-oid 0.9.6",
  "pem-rfc7468",
  "zeroize",
 ]
@@ -608,12 +641,23 @@ version = "0.10.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9ed9a281f7bc9b7576e61468ba615a66a5c8cfdff42420a70aa82701a3b1e292"
 dependencies = [
- "block-buffer",
- "const-oid",
- "crypto-common",
+ "block-buffer 0.10.4",
+ "const-oid 0.9.6",
+ "crypto-common 0.1.7",
  "subtle",
 ]
 
+[[package]]
+name = "digest"
+version = "0.11.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f1dd6dbb5841937940781866fa1281a1ff7bd3bf827091440879f9994983d5c2"
+dependencies = [
+ "block-buffer 0.12.1",
+ "const-oid 0.10.2",
+ "crypto-common 0.2.2",
+]
+
 [[package]]
 name = "displaydoc"
 version = "0.2.6"
@@ -653,7 +697,7 @@ dependencies = [
  "rustls",
  "sec1",
  "sha1",
- "sha2",
+ "sha2 0.10.9",
  "thiserror 1.0.69",
  "tokio",
  "webrtc-util",
@@ -668,7 +712,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ee27f32b5c5292967d2d4a9d7f1e0b0aed2c15daded5a60300e4abb9d8020bca"
 dependencies = [
  "der",
- "digest",
+ "digest 0.10.7",
  "elliptic-curve",
  "rfc6979",
  "signature",
@@ -683,7 +727,7 @@ checksum = "b5e6043086bf7973472e0c7dff2142ea0b680d30e18d9cc40f267efbf222bd47"
 dependencies = [
  "base16ct",
  "crypto-bigint",
- "digest",
+ "digest 0.10.7",
  "ff",
  "generic-array",
  "group",
@@ -977,7 +1021,7 @@ version = "0.12.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6c49c37c09c17a53d937dfbb742eb3a961d65a994e6bcdcf37e7399d0cc8ab5e"
 dependencies = [
- "digest",
+ "digest 0.10.7",
 ]
 
 [[package]]
@@ -1019,6 +1063,15 @@ version = "1.10.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6dbf3de79e51f3d586ab4cb9d5c3e2c14aa28ed23d180cf89b4df0454a69cc87"
 
+[[package]]
+name = "hybrid-array"
+version = "0.4.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9155a582abd142abc056962c29e3ce5ff2ad5469f4246b537ed42c5deba857da"
+dependencies = [
+ "typenum",
+]
+
 [[package]]
 name = "hyper"
 version = "1.10.1"
@@ -1307,7 +1360,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d89e7ee0cfbedfc4da3340218492196241d89eefb6dab27de5df917a6d2e78cf"
 dependencies = [
  "cfg-if",
- "digest",
+ "digest 0.10.7",
 ]
 
 [[package]]
@@ -1487,7 +1540,7 @@ dependencies = [
  "ecdsa",
  "elliptic-curve",
  "primeorder",
- "sha2",
+ "sha2 0.10.9",
 ]
 
 [[package]]
@@ -1499,7 +1552,7 @@ dependencies = [
  "ecdsa",
  "elliptic-curve",
  "primeorder",
- "sha2",
+ "sha2 0.10.9",
 ]
 
 [[package]]
@@ -1590,7 +1643,7 @@ version = "0.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8159bd90725d2df49889a078b54f4f79e87f1f8a8444194cdca81d38f5393abf"
 dependencies = [
- "cpufeatures",
+ "cpufeatures 0.2.17",
  "opaque-debug",
  "universal-hash",
 ]
@@ -1602,7 +1655,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9d1fe60d06143b2430aa532c94cfe9e29783047f06c0d7fd359a9a51b729fa25"
 dependencies = [
  "cfg-if",
- "cpufeatures",
+ "cpufeatures 0.2.17",
  "opaque-debug",
  "universal-hash",
 ]
@@ -2175,8 +2228,8 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e3bf829a2d51ab4a5ddf1352d8470c140cadc8301b2ae1789db023f01cedd6ba"
 dependencies = [
  "cfg-if",
- "cpufeatures",
- "digest",
+ "cpufeatures 0.2.17",
+ "digest 0.10.7",
 ]
 
 [[package]]
@@ -2186,8 +2239,19 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a7507d819769d01a365ab707794a4084392c824f54a7a6a7862f8c3d0892b283"
 dependencies = [
  "cfg-if",
- "cpufeatures",
- "digest",
+ "cpufeatures 0.2.17",
+ "digest 0.10.7",
+]
+
+[[package]]
+name = "sha2"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "446ba717509524cb3f22f17ecc096f10f4822d76ab5c0b9822c5f9c284e825f4"
+dependencies = [
+ "cfg-if",
+ "cpufeatures 0.3.0",
+ "digest 0.11.3",
 ]
 
 [[package]]
@@ -2212,7 +2276,7 @@ version = "2.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "77549399552de45a898a580c1b41d445bf730df867cc44e6c0233bbc4b8329de"
 dependencies = [
- "digest",
+ "digest 0.10.7",
  "rand_core 0.6.4",
 ]
 
@@ -2397,6 +2461,7 @@ dependencies = [
  "reqwest",
  "serde",
  "serde_json",
+ "sha2 0.11.0",
  "tokio",
  "uniffi",
  "webrtc",
@@ -2799,7 +2864,7 @@ version = "0.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fc1de2c688dc15305988b563c3854064043356019f97a4b46276fe734c4f07ea"
 dependencies = [
- "crypto-common",
+ "crypto-common 0.1.7",
  "subtle",
 ]
 
@@ -3004,7 +3069,7 @@ dependencies = [
  "sdp",
  "serde",
  "serde_json",
- "sha2",
+ "sha2 0.10.9",
  "smol_str",
  "stun",
  "thiserror 1.0.69",
diff --git a/core/Cargo.toml b/core/Cargo.toml
index 51f818c..8352276 100644
--- a/core/Cargo.toml
+++ b/core/Cargo.toml
@@ -24,3 +24,4 @@ tokio = { version = "1", features = ["rt-multi-thread", "time", "sync", "macros"
 futures-util = "0.3"
 webrtc = "0.17.1"
 mdns-sd = "0.20.0"
+sha2 = "0.11.0"
diff --git a/core/src/lib.rs b/core/src/lib.rs
index 8151bde..2df7fd1 100644
--- a/core/src/lib.rs
+++ b/core/src/lib.rs
@@ -74,6 +74,20 @@ pub struct Peer {
 /// engine's `nearby()` (reader). Instant is last-seen for staleness pruning.
 type Discovered = Arc<Mutex<HashMap<String, (Peer, Instant)>>>;
 
+/// Canonical room id for a shared identity: sha256(account)[..4] as 8 hex chars.
+/// Defined once here and exported so every client (Swift/Kotlin/agent) derives
+/// the SAME room — that's what puts all of one account's devices together across
+/// LAN, relay, and RTC. Empty account → empty (caller falls back to a device id).
+#[uniffi::export]
+pub fn room_for_account(account: String) -> String {
+    use sha2::{Digest, Sha256};
+    if account.is_empty() {
+        return String::new();
+    }
+    let digest = Sha256::digest(account.as_bytes());
+    digest[..4].iter().map(|b| format!("{b:02x}")).collect()
+}
+
 #[derive(Clone)]
 struct Config {
     server: String,
diff --git a/ios/MacTetherApp/Sources/MacTetherStore.swift b/ios/MacTetherApp/Sources/MacTetherStore.swift
index b337042..6af1e6c 100644
--- a/ios/MacTetherApp/Sources/MacTetherStore.swift
+++ b/ios/MacTetherApp/Sources/MacTetherStore.swift
@@ -19,7 +19,11 @@ final class MacTetherStore {
     var serverURL: String = UserDefaults.standard.string(forKey: "serverURL") ?? "https://tether.pecord.io"
     // Prefilled shared identity — for now it's just us, so all devices pair.
     var account: String = UserDefaults.standard.string(forKey: "account") ?? "pecord@gmail.com"
-    var room: String = RoomIdentity.derive()
+    // Room follows the account → all our devices share one room everywhere.
+    var room: String = {
+        let a = UserDefaults.standard.string(forKey: "account") ?? "pecord@gmail.com"
+        return a.isEmpty ? RoomIdentity.derive() : roomForAccount(account: a)
+    }()
     var feed: [FeedItem] = []
     var nearby: [Peer] = []   // tether devices on the LAN, for the picker
     var connected = false
diff --git a/ios/TetherApp/Sources/TetherStore.swift b/ios/TetherApp/Sources/TetherStore.swift
index 57874df..e4be746 100644
--- a/ios/TetherApp/Sources/TetherStore.swift
+++ b/ios/TetherApp/Sources/TetherStore.swift
@@ -23,7 +23,12 @@ final class TetherStore {
     // Prefilled shared identity — for now it's just us, so all devices pair.
     // Becomes the Sign-in-with-Apple `sub` once the dev program clears.
     var account: String = UserDefaults.standard.string(forKey: "account") ?? "pecord@gmail.com"
-    var room: String = RoomIdentity.derive()
+    // Room follows the account, so all our devices share one room everywhere
+    // (LAN, relay, RTC). Falls back to a device id only when signed out.
+    var room: String = {
+        let a = UserDefaults.standard.string(forKey: "account") ?? "pecord@gmail.com"
+        return a.isEmpty ? RoomIdentity.derive() : roomForAccount(account: a)
+    }()
     var feed: [FeedItem] = []
     var connected = false
     var lastError: String?