diff --git a/nix/host-modules/hive-forge/default.nix b/nix/host-modules/hive-forge/default.nix
index 7a59a3ce..9cadbf85 100644
--- a/nix/host-modules/hive-forge/default.nix
+++ b/nix/host-modules/hive-forge/default.nix
@@ -23,6 +23,19 @@ let
   # that make the CA reachable are shared with hive-ci via the
   # `hive-ca-trust` helper; only the Go SSL_CERT_FILE concat below is
   # hive-forge-specific.
+  # Forgejo's name for the login source. A constant, not an option: it
+  # is the key this module's own idempotency check looks up, so making
+  # it configurable would buy nothing and add a way for the lookup and
+  # the row to disagree.
+  ssoSourceName = "authelia";
+
+  # `url` is the half of the authelia module that exists on EVERY hive —
+  # null when no SSO provider is configured anywhere, which the
+  # assertion below turns into an eval failure rather than a discovery
+  # request to `null/.well-known/...`.
+  autheliaUrl = config.services.hyperhive.swarm.authelia.url;
+  autheliaDiscoveryUrl = "${toString autheliaUrl}/.well-known/openid-configuration";
+
   caTrust = import ../lib/hive-ca-trust.nix { inherit lib tlsCfg gatewayCfg; };
   useSelfSigned = caTrust.useSelfSigned;
   caContainerPath = caTrust.caContainerPath;
@@ -314,10 +327,86 @@ in
         external DNS, but that's off the CI path).
       '';
     };
+
+    sso = {
+      enable = lib.mkOption {
+        type = lib.types.bool;
+        default = false;
+        example = true;
+        description = ''
+          Register the swarm's authelia as an OpenID Connect login
+          source on this forge.
+
+          **Additive, never exclusive.** Forgejo keeps its local
+          password database and gains an extra "sign in with" button;
+          this does not disable local login. Deliberate: an identity
+          provider that can take the forge offline when it hiccups is a
+          worse forge than one with two ways in.
+        '';
+      };
+
+      clientId = lib.mkOption {
+        type = lib.types.str;
+        default = "forgejo";
+        description = ''
+          OAuth2 client id this forge identifies itself with. Must match
+          the `id` of the corresponding entry in
+          `services.hyperhive.swarm.authelia.oidc.clients`.
+        '';
+      };
+
+      clientSecretFile = lib.mkOption {
+        type = lib.types.nullOr lib.types.str;
+        default = null;
+        example = "/var/lib/hyperhive/forge-oidc-secret";
+        description = ''
+          Path **inside the forge container** holding the client
+          secret's plaintext.
+
+          A path, never a value: an OIDC client secret has two holders
+          in two containers (authelia keeps a hash, this forge needs the
+          plaintext), and a literal written here would be rendered into
+          the world-readable nix store.
+
+          Required when `enable` is set — deliberately no fallback. A
+          forge that boots with SSO half-configured presents as a login
+          button that always fails, which is harder to diagnose than an
+          eval error.
+        '';
+      };
+    };
   };
 
   config = lib.mkIf config.services.hyperhive.enable {
     assertions = [
+      {
+        # Fail at EVAL, not at boot. The alternative failure is a login
+        # button that always 401s, three layers from the missing file.
+        assertion = !cfg.sso.enable || cfg.sso.clientSecretFile != null;
+        message = ''
+          services.hyperhive.swarm.forge.sso.enable requires
+          sso.clientSecretFile — the path (inside the forge container)
+          holding the OIDC client secret's plaintext.
+
+          On a hive that also runs the swarm's authelia this is wired up
+          for you. Set it explicitly when authelia lives on another
+          host: see docs/swarm/ for which secret goes where.
+        '';
+      }
+      {
+        # Without a provider URL there is nothing to discover against,
+        # and the rendered unit would ask `null/.well-known/…`.
+        assertion = !cfg.sso.enable || autheliaUrl != null;
+        message = ''
+          services.hyperhive.swarm.forge.sso.enable requires
+          services.hyperhive.swarm.authelia.url — the base URL of the
+          swarm's SSO provider.
+
+          It defaults to this host's own instance only when this host
+          runs authelia. A hive that federates with a swarm sets it
+          explicitly to wherever that provider lives.
+        '';
+      }
       {
         assertion = cfg.rootUrl == null || lib.hasSuffix "/" cfg.rootUrl;
         message = ''
@@ -648,6 +737,80 @@ in
               fi
             '';
           };
+
+          # Register authelia as an OIDC login source.
+          #
+          # ⚠️ Ordered AFTER forgejo, unlike forgejo-gpg-init above, and
+          # the difference is not stylistic: a login source is a row in
+          # forgejo's database, and on a fresh hive that database does
+          # not exist until forgejo has started and run its migrations.
+          # Running first would either fail or let the CLI initialise a
+          # schema behind the server's back.
+          #
+          # Idempotency is by QUERY, not by a stamp file — same reason
+          # spelled out for the GPG key above: a stamp survives a state
+          # wipe that took the thing it claims exists, and then suppresses
+          # the repair.
+          systemd.services.forgejo-sso-source = lib.mkIf cfg.sso.enable {
+            description = "register authelia as Forgejo's OIDC login source";
+            after = [ "forgejo.service" ];
+            requires = [ "forgejo.service" ];
+            wantedBy = [ "multi-user.target" ];
+            serviceConfig = {
+              Type = "oneshot";
+              RemainAfterExit = true;
+              User = "forgejo";
+              Group = "forgejo";
+              SyslogIdentifier = "forgejo-sso-source";
+            };
+            # `FORGEJO_CUSTOM` (not `GITEA_CUSTOM` — forgejo renamed it)
+            # is how the CLI finds the app.ini upstream's module wrote.
+            environment.FORGEJO_CUSTOM = "/var/lib/forgejo/custom";
+            path = [
+              cfg.package
+              pkgs.coreutils
+              pkgs.gnugrep
+            ];
+            script = ''
+              set -euo pipefail
+
+              secret=$(cat ${lib.escapeShellArg cfg.sso.clientSecretFile})
+              if [ -z "$secret" ]; then
+                echo "empty OIDC client secret at ${cfg.sso.clientSecretFile}" >&2
+                exit 1
+              fi
+
+              # `--secret` is the only input forgejo offers: there is no
+              # --secret-file and no env var, though its sibling
+              # `forgejo-cli actions register` has both. So the value is
+              # on this argv for the length of one exec, inside this
+              # container, where forgejo already stores it at rest in the
+              # login-source row — root and forgejo are the only
+              # principals here, and root can read the resting copy
+              # anyway. Accepted deliberately (see docs/swarm/), not
+              # overlooked; upstream gap filed.
+              args="--provider openidConnect \
+                --key ${lib.escapeShellArg cfg.sso.clientId} \
+                --auto-discover-url ${lib.escapeShellArg autheliaDiscoveryUrl} \
+                --scopes ${lib.escapeShellArg "openid profile email groups"}"
+
+              # shellcheck disable=SC2086
+              if forgejo admin auth list | grep -q "[[:space:]]${ssoSourceName}[[:space:]]"; then
+                id=$(forgejo admin auth list \
+                  | grep "[[:space:]]${ssoSourceName}[[:space:]]" \
+                  | cut -f1)
+                forgejo admin auth update-oauth --id "$id" \
+                  --name ${lib.escapeShellArg ssoSourceName} \
+                  --secret "$secret" $args
+                echo "updated OIDC login source ${ssoSourceName} (id $id)"
+              else
+                forgejo admin auth add-oauth \
+                  --name ${lib.escapeShellArg ssoSourceName} \
+                  --secret "$secret" $args
+                echo "added OIDC login source ${ssoSourceName}"
+              fi
+            '';
+          };
         };
     };