diff --git a/.forgejo/workflows/ci.yml b/.forgejo/workflows/ci.yml
new file mode 100644
index 00000000..9ea39dec
--- /dev/null
+++ b/.forgejo/workflows/ci.yml
@@ -0,0 +1,17 @@
+name: CI
+
+on:
+  pull_request:
+    branches: ["**"]
+
+jobs:
+  check:
+    name: nix flake check
+    runs-on: [hive-ci]
+    steps:
+      - uses: actions/checkout@v3
+      - name: check
+        # Runs all flake checks: formatting (treefmt+rustfmt), cargo test,
+        # cargo clippy, and module evaluation. No --no-build: the checks
+        # derivations are the canonical source of truth.
+        run: nix flake check
diff --git a/flake.nix b/flake.nix
index 840653fd..0169702c 100644
--- a/flake.nix
+++ b/flake.nix
@@ -246,6 +246,7 @@
           agentBaseToplevel = self.packages.x86_64-linux.agent-base-toplevel;
           managerToplevel = self.packages.x86_64-linux.manager-toplevel;
         };
+        hive-ci = ./nix/modules/hive-ci.nix;
         hive-forge = ./nix/modules/hive-forge.nix;
         # Convenience alias: one import covers the full hyperhive host
         # stack (hive-c0re + hive-forge, since hive-c0re already pulls
diff --git a/nix/modules/hive-c0re.nix b/nix/modules/hive-c0re.nix
index 8de18a11..2565e27e 100644
--- a/nix/modules/hive-c0re.nix
+++ b/nix/modules/hive-c0re.nix
@@ -22,6 +22,7 @@ in
   # opt-in (off by default) and asserts that `services.hyperhive.domain`
   # is set before it can be enabled.
   imports = [
+    ./hive-ci.nix
     ./hive-forge.nix
     ./hive-gateway.nix
     ./hive-matrix.nix
@@ -99,34 +100,40 @@ in
   # `identity.rs::peers()` + the dashboard's `peer_hives` state field
   # (feeds the P33RS dashboard tab).
   options.services.hyperhive.peers = lib.mkOption {
-    type = lib.types.attrsOf (lib.types.submodule {
-      options = {
-        domain = lib.mkOption {
-          type = lib.types.str;
-          example = "lab.example.com";
-          description = ''
-            DNS domain of the peer hive. Used to construct the peer's
-            dashboard URL (`http://''${domain}/`) and for Matrix
-            federation auto-discovery (`matrix.''${domain}`).
-            Must be reachable from this host.
-          '';
+    type = lib.types.attrsOf (
+      lib.types.submodule {
+        options = {
+          domain = lib.mkOption {
+            type = lib.types.str;
+            example = "lab.example.com";
+            description = ''
+              DNS domain of the peer hive. Used to construct the peer's
+              dashboard URL (`http://''${domain}/`) and for Matrix
+              federation auto-discovery (`matrix.''${domain}`).
+              Must be reachable from this host.
+            '';
+          };
+          tlsCertFile = lib.mkOption {
+            type = lib.types.nullOr lib.types.path;
+            default = null;
+            description = ''
+              Optional path to a PEM cert/bundle to trust for this peer's
+              TLS. Null = system CA bundle (for Let's Encrypt peers). Set
+              to the peer's self-signed cert for `selfSignedTls = true`
+              peers. Forward-compat slot; not yet used in v0.
+            '';
+          };
         };
-        tlsCertFile = lib.mkOption {
-          type = lib.types.nullOr lib.types.path;
-          default = null;
-          description = ''
-            Optional path to a PEM cert/bundle to trust for this peer's
-            TLS. Null = system CA bundle (for Let's Encrypt peers). Set
-            to the peer's self-signed cert for `selfSignedTls = true`
-            peers. Forward-compat slot; not yet used in v0.
-          '';
-        };
-      };
-    });
+      }
+    );
     default = { };
     example = {
-      lab = { domain = "lab.example.com"; };
-      edge = { domain = "edge.corp"; };
+      lab = {
+        domain = "lab.example.com";
+      };
+      edge = {
+        domain = "edge.corp";
+      };
     };
     description = ''
       Peer hives in the same swarm. The attrset key is a short label
@@ -345,24 +352,29 @@ in
         # links; when false it falls back to direct `<host>:<port>` TCP.
         HIVE_GATEWAY_ENABLED = "1";
       }
-      // lib.optionalAttrs (config.services.hyperhive.forge.enable && config.services.hyperhive.forge.behindGateway) {
-        # Public URL of the forge vhost served by hive-gateway. The
-        # dashboard uses this to build browser-facing forge links
-        # instead of hardcoding `<hostname>:3000`, which breaks when
-        # the operator accesses the dashboard through the gateway
-        # (forge sub-domain has no port; direct port URL would be
-        # wrong). Absent when `behindGateway = false` — dashboard
-        # falls back to `<hostname>:3000`.
-        HIVE_FORGE_PUBLIC_URL = "https://${config.services.hyperhive.forge.domain}";
-      }
+      //
+        lib.optionalAttrs
+          (config.services.hyperhive.forge.enable && config.services.hyperhive.forge.behindGateway)
+          {
+            # Public URL of the forge vhost served by hive-gateway. The
+            # dashboard uses this to build browser-facing forge links
+            # instead of hardcoding `<hostname>:3000`, which breaks when
+            # the operator accesses the dashboard through the gateway
+            # (forge sub-domain has no port; direct port URL would be
+            # wrong). Absent when `behindGateway = false` — dashboard
+            # falls back to `<hostname>:3000`.
+            HIVE_FORGE_PUBLIC_URL = "https://${config.services.hyperhive.forge.domain}";
+          }
       // lib.optionalAttrs (config.services.hyperhive.peers != { }) {
         # Peer hives serialised as a JSON array of {label, domain} objects.
         # Consumed by hive-ag3nt::identity::peers() + the dashboard's
         # peer_hives StateSnapshot field (P33RS tab). tlsCertFile is
         # nix-side-only (host nginx/trust config); rust never needs the path.
         HYPERHIVE_PEERS = builtins.toJSON (
-          lib.mapAttrsToList (label: p: { inherit label; inherit (p) domain; })
-            config.services.hyperhive.peers
+          lib.mapAttrsToList (label: p: {
+            inherit label;
+            inherit (p) domain;
+          }) config.services.hyperhive.peers
         );
       };
       serviceConfig = {
diff --git a/nix/modules/hive-ci.nix b/nix/modules/hive-ci.nix
new file mode 100644
index 00000000..4eeabf04
--- /dev/null
+++ b/nix/modules/hive-ci.nix
@@ -0,0 +1,213 @@
+{
+  pkgs,
+  lib,
+  config,
+  ...
+}:
+let
+  cfg = config.services.hyperhive.ci;
+  forgeCfg = config.services.hyperhive.forge;
+
+  # hive-c0re writes its own admin token here on first forge startup.
+  # The token has read:admin + write:admin scopes — sufficient to call
+  # the runner registration-token API endpoint.
+  coreTokenPath = "/var/lib/hyperhive/forge-core-token";
+
+  # Script run before the gitea-runner-hive service starts.
+  # On first boot (no .runner credentials yet) it fetches a fresh
+  # runner registration token from the forge admin API and writes it to
+  # /run/hive-ci/runner-token so gitea-actions-runner can register.
+  # On subsequent boots the .runner credentials file already exists and
+  # the runner ignores the token file entirely, so we write a dummy to
+  # satisfy the file-existence check in the NixOS module.
+  autoRegisterScript = pkgs.writeShellScript "hive-ci-autoregister" ''
+    set -euo pipefail
+    TOKEN_FILE=/run/hive-ci/runner-token
+    STATE_FILE=/var/lib/gitea-runner/hive/.runner
+
+    mkdir -p /run/hive-ci
+    chmod 700 /run/hive-ci
+
+    if [ -f "$STATE_FILE" ]; then
+      # Already registered — dummy token satisfies the module's path check.
+      echo "already-registered" > "$TOKEN_FILE"
+      chmod 600 "$TOKEN_FILE"
+      exit 0
+    fi
+
+    CORE_TOKEN=$(cat /run/hive-ci/core-token)
+    FORGE_URL="http://127.0.0.1:${toString forgeCfg.httpPort}"
+
+    # Retry up to 30s for forge to come up (containers autoStart in parallel).
+    for i in $(seq 1 30); do
+      REG_TOKEN=$(${pkgs.curl}/bin/curl -sf \
+        "$FORGE_URL/api/v1/admin/runners/registration-token" \
+        -H "Authorization: token $CORE_TOKEN" \
+        | ${pkgs.jq}/bin/jq -r .token) && break
+      sleep 1
+    done
+
+    if [ -z "''${REG_TOKEN:-}" ] || [ "$REG_TOKEN" = "null" ]; then
+      echo "hive-ci: failed to fetch runner registration token from forge" >&2
+      exit 1
+    fi
+
+    echo "$REG_TOKEN" > "$TOKEN_FILE"
+    chmod 600 "$TOKEN_FILE"
+  '';
+in
+{
+  # Forgejo Actions runner in a `hive-ci` nixos-container.
+  # Shares host netns (same as hive-forge), so the runner reaches
+  # the forge at `http://127.0.0.1:<httpPort>` without extra plumbing.
+  # Container is non-ephemeral: the runner's registered credentials
+  # survive restarts (gitea-actions-runner writes them to its stateDir
+  # on first registration and reuses them on every subsequent start).
+  #
+  # Auto-registration: on first boot the container fetches a runner
+  # registration token from the forge's admin API using the core token
+  # hive-c0re writes to /var/lib/hyperhive/forge-core-token. No manual
+  # token handling needed — `ci.enable = true` is the full operator
+  # bootstrap. Registration flow: preStart calls the Forgejo admin API,
+  # writes the token to /run/hive-ci/runner-token, runner registers and
+  # persists credentials to stateDir — token file ignored on next boot.
+  #
+  # Nix builds inside the container use the shared /nix/store (standard
+  # nixos-container behaviour) with sandbox-fallback = true, because
+  # nspawn containers can't create the user-namespaces that nix sandboxing
+  # requires. See docs/gotchas.md.
+
+  options.services.hyperhive.ci = {
+    enable = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      example = true;
+      description = ''
+        Run a Forgejo Actions runner in a `hive-ci` nixos-container.
+        Disabled by default; requires `services.hyperhive.forge.enable = true`.
+
+        On first start the container auto-registers against hive-forge using
+        hive-c0re's admin token — no manual token provisioning needed.
+        Runner credentials are persisted in the container's state dir and
+        reused on every subsequent boot.
+      '';
+    };
+
+    name = lib.mkOption {
+      type = lib.types.str;
+      default = "hive-ci";
+      example = "prod-hive";
+      description = ''
+        Runner name as shown in the Forgejo admin panel. Defaults to
+        "hive-ci"; override when multiple hives share a Forgejo instance.
+      '';
+    };
+
+    concurrency = lib.mkOption {
+      type = lib.types.ints.positive;
+      default = 1;
+      example = 4;
+      description = ''
+        Maximum number of workflow jobs the runner executes in parallel.
+        Each job gets its own temporary working directory; multiple parallel
+        jobs share the container's nix store and cargo registry cache.
+        Higher values trade memory + CPU headroom for throughput.
+      '';
+    };
+
+    labels = lib.mkOption {
+      type = lib.types.listOf lib.types.str;
+      default = [ "hive-ci:host" ];
+      example = [
+        "hive-ci:host"
+        "nix:host"
+      ];
+      description = ''
+        Runner labels in `<name>:<scheme>` format. The `host` scheme runs
+        commands directly in the container (no docker/podman). Workflow
+        files target this runner with `runs-on: [hive-ci]`.
+      '';
+    };
+
+    package = lib.mkOption {
+      type = lib.types.package;
+      default = pkgs.gitea-actions-runner;
+      defaultText = lib.literalExpression "pkgs.gitea-actions-runner";
+      description = "gitea-actions-runner package.";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    assertions = [
+      {
+        assertion = forgeCfg.enable;
+        message = ''
+          services.hyperhive.ci.enable = true requires
+          services.hyperhive.forge.enable = true — the runner registers
+          against the hive-forge Forgejo instance.
+        '';
+      }
+    ];
+
+    containers.hive-ci = {
+      autoStart = true;
+      ephemeral = false;
+      # Shared host netns: runner reaches hive-forge at localhost.
+      privateNetwork = false;
+
+      bindMounts = {
+        # Core token — used by the auto-register script on first boot.
+        # Read-only: the container only reads it, never modifies it.
+        "/run/hive-ci/core-token" = {
+          hostPath = coreTokenPath;
+          isReadOnly = true;
+        };
+      };
+
+      config =
+        { pkgs, lib, ... }:
+        {
+          system.stateVersion = "26.05";
+
+          # nspawn containers can't create user-namespaces, so nix
+          # sandboxing always fails. Fall back to unsandboxed builds.
+          # See docs/gotchas.md.
+          nix.settings.sandbox-fallback = lib.mkForce true;
+          nix.settings.experimental-features = [
+            "nix-command"
+            "flakes"
+          ];
+
+          services.gitea-actions-runner.instances.hive = {
+            enable = true;
+            name = cfg.name;
+            url = "http://127.0.0.1:${toString forgeCfg.httpPort}";
+            # Token file is written by the ExecStartPre script below.
+            # On first boot: real registration token fetched from forge API.
+            # On subsequent boots: dummy value (runner uses .runner creds).
+            tokenFile = "/run/hive-ci/runner-token";
+            labels = cfg.labels;
+            settings = {
+              runner.capacity = cfg.concurrency;
+              # Generous timeout for cold-cache nix builds.
+              runner.timeout = "3h";
+            };
+            package = cfg.package;
+          };
+
+          # Prepend auto-register script before the runner service starts.
+          # `+` prefix runs with elevated privileges so it can read the
+          # bind-mounted core-token (owned by root on the host).
+          systemd.services."gitea-runner-hive".serviceConfig.ExecStartPre = lib.mkBefore [
+            "+${autoRegisterScript}"
+          ];
+
+          # git is required by the runner's checkout step.
+          # Everything else (nix, rust tools) is either part of NixOS
+          # by default or pulled in hermetically by nix flake check.
+          # curl/jq in the autoregister preStart use absolute store paths.
+          environment.systemPackages = [ pkgs.git ];
+        };
+    };
+  };
+}