diff --git a/nix/host-modules/swarm-controller.nix b/nix/host-modules/swarm-controller.nix
index 05f7f7dd..42ac308d 100644
--- a/nix/host-modules/swarm-controller.nix
+++ b/nix/host-modules/swarm-controller.nix
@@ -192,6 +192,15 @@ let
     SWARM_CONTROLLER_AUTH_BRIDGE_URL = deployCfg.swarm-controller.authBridgeUrl;
   };
 
+  # Swarm-wide default for `PUT .../matrix-accounts/{account}`'s own
+  # `homeserver` field, for a request that omits one. Same shape as
+  # `authBridgeEnv` above: genuinely optional, gated on the option
+  # resolving rather than assumed. Not read by anything yet — see the
+  # option's own description.
+  matrixHomeserverEnv = lib.optionalAttrs (deployCfg.swarm-controller.matrixHomeserverUrl != null) {
+    SWARM_CONTROLLER_MATRIX_HOMESERVER_URL = deployCfg.swarm-controller.matrixHomeserverUrl;
+  };
+
   # The swarm's own display name, for `GET /api/swarm` (swarm-ui's chrome).
   # Gated on the option resolving, not defaulted to an empty string: an
   # operator who never named the swarm gets `name: null` from the
@@ -520,6 +529,24 @@ in
       '';
     };
 
+    matrixHomeserverUrl = lib.mkOption {
+      type = lib.types.nullOr lib.types.str;
+      default = null;
+      example = "https://matrix.example.org";
+      description = ''
+        Swarm-wide default homeserver for `PUT
+        .../matrix-accounts/{account}` requests that omit their own
+        `homeserver` — see that route's own doc comment
+        (`swarm-controller/src/matrix_account.rs`) for why the field is
+        optional in token mode and what omitting it currently resolves to.
+
+        `null` (the default) leaves that per-request resolution exactly as
+        it is today. **Not yet consulted by the route at all**: this option
+        only exists to carry the value in, ahead of the route being taught
+        to fall back to it.
+      '';
+    };
+
     baoClientCertFile = lib.mkOption {
       type = lib.types.nullOr lib.types.str;
       default = null;
@@ -859,6 +886,7 @@ in
       // webhookEnv
       // authBridgeEnv
       // swarmNameEnv
+      // matrixHomeserverEnv
       // baoEnv
       // otelEnv;
     };
diff --git a/swarm-controller/src/main.rs b/swarm-controller/src/main.rs
index 0f254506..eed7419c 100644
--- a/swarm-controller/src/main.rs
+++ b/swarm-controller/src/main.rs
@@ -1621,6 +1621,17 @@ async fn main() -> Result<()> {
         }
     };
 
+    // Diagnostic only — `put_matrix_account` does not call this yet (see its
+    // own doc comment), so this cannot change any request's outcome. Logged
+    // so an operator can tell the config landed before the slice that
+    // consults it does.
+    if matrix_account::homeserver_or_configured_default(None).is_none() {
+        tracing::debug!(
+            "{}; matrix-account requests still need their own homeserver",
+            matrix_account::DEFAULT_HOMESERVER_ENV
+        );
+    }
+
     let config_prs = forge_client.clone().map(config_pr::spawn);
     let state_forge = keep_forge_for_state(forge_client, webhook_secret.clone());
 
diff --git a/swarm-controller/src/matrix_account.rs b/swarm-controller/src/matrix_account.rs
index 46f8d38c..986998fe 100644
--- a/swarm-controller/src/matrix_account.rs
+++ b/swarm-controller/src/matrix_account.rs
@@ -35,6 +35,23 @@ fn default_mode() -> String {
     "token".to_owned()
 }
 
+/// Env var the controller's NixOS module sets from
+/// `services.hyperhive.deploy.swarm-controller.matrixHomeserverUrl` — the
+/// swarm-wide default `PutMatrixAccountRequest::homeserver` falls back to
+/// when a caller omits one.
+///
+/// Not yet consulted by [`put_matrix_account`]: `homeserver_or_configured_default`
+/// below exists for a later slice of hyperhive#4345 to call; this one only
+/// wires the config through.
+pub(crate) const DEFAULT_HOMESERVER_ENV: &str = "SWARM_CONTROLLER_MATRIX_HOMESERVER_URL";
+
+/// `caller`'s own homeserver, or [`DEFAULT_HOMESERVER_ENV`] when the caller
+/// left it unset. `caller` always wins — this only fills a gap it left,
+/// never replaces a value it gave.
+pub(crate) fn homeserver_or_configured_default(caller: Option<String>) -> Option<String> {
+    caller.or_else(|| std::env::var(DEFAULT_HOMESERVER_ENV).ok())
+}
+
 /// The credential to store for one agent's external matrix account.
 ///
 /// No `Debug` derive — matching `hive-c0re::dashboard::matrix_accounts`'s
@@ -339,8 +356,8 @@ async fn matrix_password_login(
 #[cfg(test)]
 mod tests {
     use super::{
-        PutMatrixAccountRequest, is_reserved_account, password_fields, resolve_credential,
-        token_credential,
+        DEFAULT_HOMESERVER_ENV, PutMatrixAccountRequest, homeserver_or_configured_default,
+        is_reserved_account, password_fields, resolve_credential, token_credential,
     };
 
     fn request(mode: &str) -> PutMatrixAccountRequest {
@@ -353,6 +370,43 @@ mod tests {
         }
     }
 
+    /// SAFETY: single-threaded mutation of a process env var no other test
+    /// in this crate reads; restored (removed) before returning.
+    #[test]
+    fn caller_supplied_homeserver_wins_even_with_a_default_configured() {
+        unsafe {
+            std::env::set_var(DEFAULT_HOMESERVER_ENV, "https://default.example.org");
+        }
+        let result =
+            homeserver_or_configured_default(Some("https://caller.example.org".to_owned()));
+        unsafe {
+            std::env::remove_var(DEFAULT_HOMESERVER_ENV);
+        }
+        assert_eq!(result.as_deref(), Some("https://caller.example.org"));
+    }
+
+    /// SAFETY: same as above.
+    #[test]
+    fn falls_back_to_the_configured_default_when_the_caller_omits_one() {
+        unsafe {
+            std::env::set_var(DEFAULT_HOMESERVER_ENV, "https://default.example.org");
+        }
+        let result = homeserver_or_configured_default(None);
+        unsafe {
+            std::env::remove_var(DEFAULT_HOMESERVER_ENV);
+        }
+        assert_eq!(result.as_deref(), Some("https://default.example.org"));
+    }
+
+    /// SAFETY: same as above.
+    #[test]
+    fn none_when_neither_caller_nor_default_is_set() {
+        unsafe {
+            std::env::remove_var(DEFAULT_HOMESERVER_ENV);
+        }
+        assert_eq!(homeserver_or_configured_default(None), None);
+    }
+
     #[test]
     fn main_is_reserved_but_nothing_else_is() {
         assert!(is_reserved_account("main"));