{
  pkgs,
  lib,
  config,
  ...
}:
let
  cfg = config.services.hyperhive.matrix;
  networkCfg = config.services.hyperhive.network;
  hyperhiveDomain = config.services.hyperhive.domain;
  effectiveServerName = if cfg.serverName != null then cfg.serverName else hyperhiveDomain;

  # fluffychat-web build fixes: nixpkgs's `flutter341.buildFlutterApplication`
  # skips the dart web-worker compile + the emscripten native_imaging
  # build. Two derivations below cover both. Full rationale (why
  # passthru.pubspecLock.dependencySources, why `dontConfigure`, why
  # `make -C js`, why build-CWD-relative dart path): docs/matrix.md::
  # fluffychat-web build fixes.

  fluffychat-web-imaging = pkgs.stdenv.mkDerivation {
    pname = "fluffychat-web-imaging";
    version = pkgs.fluffychat-web.passthru.pubspecLock.dependencyVersions.native_imaging;
    src = pkgs.fluffychat-web.passthru.pubspecLock.dependencySources.native_imaging;

    nativeBuildInputs = with pkgs; [
      emscripten
      cmake
      gnumake
      jq
    ];

    # cmake runs inside js/Makefile via `emcmake cmake`; the default
    # configurePhase would invoke cmake at the package root (no
    # CMakeLists) and fail.
    dontConfigure = true;

    buildPhase = ''
      runHook preBuild
      # emscripten on-demand sysroot build needs writable HOME + cache.
      export HOME=$TMPDIR
      export EM_CACHE=$TMPDIR/.emscriptencache
      mkdir -p $EM_CACHE
      # `make -C js` keeps pwd at source root for the installPhase.
      make -C js Imaging.js Imaging.wasm
      runHook postBuild
    '';

    installPhase = ''
      runHook preInstall
      mkdir -p $out
      install -m 644 js/Imaging.js  $out/Imaging.js
      install -m 644 js/Imaging.wasm $out/Imaging.wasm
      runHook postInstall
    '';

    meta = with pkgs.lib; {
      description = "Imaging.js + Imaging.wasm built from the native_imaging dart package for fluffychat-web";
      homepage = "https://pub.dev/packages/native_imaging";
      license = licenses.agpl3Plus;
    };
  };

  fluffychat-web-fixed = pkgs.fluffychat-web.overrideAttrs (old: {
    # dart from the flutter341 closure (already pulled, no incremental
    # cost) to compile the web-worker entry point.
    nativeBuildInputs = (old.nativeBuildInputs or [ ]) ++ [ pkgs.flutter341.dart ];

    postInstall = (old.postInstall or "") + ''
      # `web/...` is BUILD-CWD-relative (not `$src/...`) so dart's
      # package_config walk-up hits buildFlutterApplication's
      # pub-get output `.dart_tool/`.
      ${pkgs.flutter341.dart}/bin/dart compile js \
        -o $out/native_executor.js \
        web/native_executor.dart

      install -m 644 ${fluffychat-web-imaging}/Imaging.js  $out/Imaging.js
      install -m 644 ${fluffychat-web-imaging}/Imaging.wasm $out/Imaging.wasm
    '';
  });
in
{
  # Private matrix-tuwunel homeserver wrapped in a nixos-container,
  # optional fluffychat-web client at matrix.<hive>/. Container shape,
  # serverName vs gatewayHost split, provisioning flow (registration
  # token + LoadCredential), assertion rationale, initial rollout
  # settings: docs/matrix.md. Vhost map + discovery flow + tuning
  # knobs: docs/gateway.md.

  options.services.hyperhive.matrix = {
    enable = lib.mkOption {
      type = lib.types.bool;
      default = false;
      description = ''
        Run hive-matrix — a private matrix-tuwunel homeserver (in a
        nixos-container) for hyperhive agents. Off by default while
        the integration phases in; flip to `true` once the operator
        has set `services.hyperhive.domain` and is ready to onboard agents.
      '';
    };

    package = lib.mkOption {
      type = lib.types.package;
      default = pkgs.matrix-tuwunel;
      defaultText = lib.literalExpression "pkgs.matrix-tuwunel";
      description = ''
        matrix-tuwunel package to run inside the container. Defaults
        to nixpkgs's `pkgs.matrix-tuwunel`. Override to pin a
        specific upstream if you need an unreleased feature.
      '';
    };

    serverName = lib.mkOption {
      type = lib.types.nullOr lib.types.str;
      default = null;
      example = "chat.example.org";
      description = ''
        Matrix `server_name` — the host part of every user ID
        (`@argus:<server_name>`) and room ID minted on this
        homeserver. CRITICAL: must be stable from day one because
        it's embedded irrevocably in the identifiers. Defaults to
        `services.hyperhive.domain` (the bare hive domain). Combined
        with the `.well-known/matrix/{client,server}` routes the
        hive-gateway serves at that domain, clients auto-discover the
        actual matrix endpoint without needing a subdomain. Override
        here only if you need a different server_name shape (e.g.
        `matrix.<domain>` if you want the subdomain split, or
        `chat.example.org` for a bespoke hostname).

        **Breaking change**: this used to default to
        `matrix.''${services.hyperhive.domain}`. matrix IDs embed
        the server_name irrevocably, so existing homeservers must
        set `services.hyperhive.matrix.serverName = "matrix.''${services.hyperhive.domain}";`
        explicitly to preserve their existing user / room IDs
        before rebuilding.
      '';
    };

    httpPort = lib.mkOption {
      type = lib.types.port;
      default = 8008;
      description = ''
        TCP port tuwunel serves the matrix client-server API on.
        Default 8008 is the matrix-spec well-known port. Sits
        outside hyperhive's claimed ranges (dashboard 7000, every
        agent in 8100..8999 via FNV-1a hash). Federation listens on
        `federationPort` separately.
      '';
    };

    gatewayHost = lib.mkOption {
      type = lib.types.nullOr lib.types.str;
      default = "matrix.${hyperhiveDomain}";
      defaultText = lib.literalExpression ''"matrix.''${services.hyperhive.domain}"'';
      example = "matrix.example.com";
      description = ''
        Public hostname for the matrix homeserver behind the gateway.
        Defaults to `matrix.''${services.hyperhive.domain}` (sub-domain
        shape — see `docs/gateway.md`). Set to `null` to skip the
        gateway vhost (tuwunel stays direct on `httpPort`). See
        `docs/gateway.md` for the vhost map + matrix discovery flow,
        and the federation port-8448 caveat at the bottom of that doc.

        Note: `gatewayHost` is the API listener hostname (where nginx
        proxies `/_matrix/*`); `serverName` is the matrix-identifier
        domain embedded irrevocably in user/room IDs (default = bare
        hive-domain). The two are distinct.
      '';
    };

    openFirewall = lib.mkOption {
      type = lib.types.bool;
      default = false;
      example = true;
      description = ''
        Open `httpPort` in the host firewall. Off by default
        (secure-by-default): the host reaches the homeserver on
        loopback, and agent containers reach it at `matrix.<domain>`
        via the gateway — so the firewall open only matters for
        access from outside the host. Flip to `true` when announcing
        the homeserver to other hives or when an external matrix
        client needs to reach the client-server API directly.

        **Breaking change**: this used to default to `true`. If you
        relied on the old default for external reach, add
        `services.hyperhive.matrix.openFirewall = true;` to your host
        config before rebuilding.

        Note: federation (the matrix-spec well-known port 8448) is
        intentionally not opened here. tuwunel serves the federation
        API on the same `httpPort` as the client-server API by
        default; reaching it on 8448 requires either binding tuwunel
        to that port explicitly OR a reverse-proxy + `.well-known/
        matrix/server` delegation, neither of which lives in this
        module. Add that proxy config alongside whatever serves your
        dashboard or forge on 443.
      '';
    };

    trustedServers = lib.mkOption {
      type = lib.types.listOf lib.types.str;
      default = [ ];
      example = [ "matrix.org" ];
      description = ''
        List of trusted matrix servers (homeservers whose signing
        keys this server will fetch identity-server-style). Empty
        by default — federation is enabled at the protocol level
        but no peer is trusted until listed here, so the homeserver
        is effectively closed until the operator declares hive
        peers explicitly.
      '';
    };

    maxRequestSize = lib.mkOption {
      type = lib.types.ints.positive;
      default = 20000000;
      description = ''
        Maximum size in bytes of a single matrix client request body.
        Default 20 MB matches the matrix-spec recommendation for
        media uploads + the upstream tuwunel default.
      '';
    };

    registrationTokenFile = lib.mkOption {
      type = lib.types.path;
      default = "/var/lib/hyperhive/matrix-register-token";
      description = ''
        Host path to a file containing the matrix registration token
        tuwunel reads to authorise new-account creation. The token is
        generated automatically by `hive-c0re` on first boot (32-byte
        random hex, mode 0600) and is bind-mounted read-only into the
        tuwunel container at the same path. Agents never see this
        token — hive-c0re uses it to provision per-agent accounts
        and the agent only receives the resulting `access_token`.
        Override only when integrating with externally-managed
        registration tokens.
      '';
    };

    allowEncryption = lib.mkOption {
      type = lib.types.bool;
      default = false;
      description = ''
        Server-side switch for matrix end-to-end encryption — sets
        tuwunel's `allow_encryption`. Off by default: on the hive-internal
        homeserver the operator already controls the transport, so server
        E2EE adds key-management overhead (cross-signing, device
        verification, undecryptable-message recovery) without a clear
        threat-model win for the common single-hive case. Turn on when
        agents join encrypted rooms on external / federated homeservers,
        or when the operator wants message contents opaque to the
        homeserver admin. Independent of the agent matrix client, which
        always supports decryption so it can read encrypted rooms it is
        invited to regardless of this flag; this option only governs
        whether THIS homeserver permits room encryption.
      '';
    };

    gui = {
      enable = lib.mkOption {
        type = lib.types.bool;
        default = cfg.enable;
        defaultText = lib.literalExpression "config.services.hyperhive.matrix.enable";
        description = ''
          Serve a matrix web client at `matrix.''${services.hyperhive.domain}/`.
          Requires `matrix.gatewayHost != null` (default `matrix.<hive>`
          when hive-domain set); the gateway itself always runs. When
          off, the dashboard's `M4TR1X →` tab is hidden. See
          `docs/gateway.md` for the discovery flow that lets clients
          auto-find the sub-domain.
        '';
      };

      package = lib.mkOption {
        type = lib.types.package;
        default = fluffychat-web-fixed;
        defaultText = lib.literalMD ''
          `pkgs.fluffychat-web` with a `postInstall` patch that adds
          the three files `flutter341.buildFlutterApplication` skips.
        '';
        description = ''
          Static web client dist served at `matrix.<hive>/`. Override
          to swap fluffychat for hydrogen-web, cinny, element-web, or
          an out-of-tree dist — any replacement is mounted at the
          sub-domain root with the upstream-default `<base href "/">`,
          no sub-path gymnastics needed.
        '';
      };
    };
  };

  config = lib.mkIf cfg.enable {
    # `serverName` is irrevocably embedded in user/room IDs; it derives
    # from `services.hyperhive.domain` (required, asserted in
    # hive-network.nix) when not set explicitly, so no separate
    # domain/serverName assertion is needed here. gatewayHost may not be
    # "" (same footgun as forge.domain — nginx rejects an empty
    # server_name). docs/matrix.md::Assertion rationale.
    assertions = [
      {
        assertion = cfg.gatewayHost == null || cfg.gatewayHost != "";
        message = ''
          services.hyperhive.matrix.gatewayHost = "" is rejected. The
          rendered URLs would be invalid (nginx wildcard catch-all for
          an empty server_name, /etc/hosts rejects empty entries).
          Use `null` to disable the gateway vhost entirely (tuwunel
          stays direct on httpPort), or set a non-empty hostname like
          "matrix.example.com" or "homeserver.internal".
        '';
      }
    ];

    # Activation-time token generation — without this the bind-mount
    # would hand tuwunel an empty file on first boot and break every
    # registration until restart. Idempotent;
    # docs/matrix.md::Provisioning flow.
    system.activationScripts.hive-matrix-register-token = lib.stringAfter [ "var" ] ''
      tokenFile=${lib.escapeShellArg (toString cfg.registrationTokenFile)}
      if [ ! -s "$tokenFile" ]; then
        mkdir -p "$(dirname "$tokenFile")"
        head -c 32 /dev/urandom | od -An -tx1 | tr -d ' \n' > "$tokenFile"
        echo >> "$tokenFile"
        echo "hive-matrix: generated registration token at $tokenFile"
      fi
      # Re-apply 0600 (normalises any pre-LoadCredential carry-over).
      chmod 0600 "$tokenFile"
    '';

    containers.hive-matrix = {
      autoStart = true;
      ephemeral = false;
      # Shared host netns — agents reach tuwunel at localhost:<port>.
      privateNetwork = false;
      # Read-only bind of the host-managed registration token; tuwunel
      # reads it via systemd LoadCredential below (not directly).
      bindMounts.${cfg.registrationTokenFile} = {
        hostPath = cfg.registrationTokenFile;
        isReadOnly = true;
      };
      config =
        { ... }:
        {
          system.stateVersion = "26.05";

          # Peer-hive root CAs (`swarm.peers.<domain>.caCert`) added to THIS
          # container's trust bundle so tuwunel validates *federation* TLS
          # from a self-signed peer hive (it checks the peer's federation
          # cert against its trust bundle). Peer CAs are trusted everywhere
          # the hive's own internal CA is — agents get them via the
          # meta-flake renderer (`HIVE_PEER_CA_PATHS` → each agent's
          # `security.pki.certificateFiles`); this block is the matrix
          # container's copy, since the host `security.pki` store doesn't
          # cross the container boundary. They are never installed in the
          # HOST trust store. Null entries (CA-bundle / fingerprint-pinned
          # peers) drop out.
          security.pki.certificateFiles = lib.filter (c: c != null) (
            lib.mapAttrsToList (_domain: p: p.caCert) config.services.hyperhive.swarm.peers
          );

          # tuwunel hard-fails to boot if `/etc/resolv.conf` has no
          # `nameserver` line (`Failed to configure DNS resolver ... no
          # nameservers found in config` → exit 1). This declarative
          # nixos-container comes up with an EMPTY resolv.conf even with
          # `networking.nameservers` set: the nixos-container default
          # `useHostResolvConf = true` puts in-container resolvconf in
          # host-tracking mode (ignores `networking.nameservers`, and never
          # gets the host file across the shared-netns boundary), so it
          # regenerates an empty file and tuwunel dies at boot.
          #
          # The earlier fix turned host-tracking off and trusted resolvconf
          # to honour `networking.nameservers` — but that's a RUNTIME
          # resolvconf behaviour, not verifiable at eval time, and it STILL
          # came up empty in practice. So take resolvconf out of the
          # loop entirely and write a STATIC `/etc/resolv.conf` from
          # `bridgeIp` that nothing regenerates. Eval-proven: the generated
          # `environment.etc."resolv.conf".text` is `nameserver <bridgeIp>`.
          # This container always shares the host netns
          # (`privateNetwork = false`), so it reaches `bridgeIp` regardless
          # of agent-container isolation. Network module off → inherit the host's
          # resolv.conf. See `docs/network.md`.
          networking = lib.mkMerge [
            (lib.mkIf networkCfg.enable {
              # resolvconf is taken out of the loop entirely; the static
              # `environment.etc."resolv.conf"` below is the sole source of
              # the resolver file (no `nameservers` — nothing would read it).
              useHostResolvConf = lib.mkForce false;
              resolvconf.enable = lib.mkForce false;
            })
            (lib.mkIf (!networkCfg.enable) {
              useHostResolvConf = true;
            })
          ];

          # resolvconf is disabled above, so write the static resolver file
          # explicitly — NixOS won't synthesise one from `nameservers` once
          # resolvconf is off, and this is the file tuwunel parses at boot.
          environment.etc = lib.mkIf networkCfg.enable {
            "resolv.conf".text = ''
              nameserver ${networkCfg.bridgeIp}
              options edns0
            '';
          };

          services.matrix-tuwunel = {
            enable = true;
            package = cfg.package;
            settings.global = {
              server_name = effectiveServerName;
              # `address` + `port` are upstream `listOf` — wrap singles.
              address = [ "0.0.0.0" ];
              port = [ cfg.httpPort ];
              max_request_size = cfg.maxRequestSize;
              # Federation enabled at the protocol level; empty
              # trustedServers keeps it effectively closed.
              allow_federation = true;
              trusted_servers = cfg.trustedServers;
              # Token-gated registration. The absent
              # `yes_i_am_very_very_sure_…_open_registration_…` flag
              # keeps the server closed to anyone without the token.
              allow_registration = true;
              # LoadCredential below copies the host file into a
              # 0400 dynamic-user-owned path; tuwunel reads from there.
              registration_token_file = "/run/credentials/tuwunel.service/registration_token";
              # Server-side E2EE is opt-in (default off); the agent matrix
              # client always supports decryption regardless.
              allow_encryption = cfg.allowEncryption;
              # Tuwunel's default suffix is " 💕" — suppress it so agent
              # display names are clean (just the agent name, no emoji).
              new_user_displayname_suffix = "";
            };
          };
          # Keeps DynamicUser=true + PrivateUsers=true intact — no
          # host-side chown :tuwunel / GID-pin gymnastics needed.
          # See `man systemd.exec` → LoadCredential.
          systemd.services.tuwunel.serviceConfig.LoadCredential = [
            "registration_token:${toString cfg.registrationTokenFile}"
          ];
          environment.systemPackages = [ cfg.package ];
        };
    };

    networking.firewall = lib.mkIf cfg.openFirewall {
      allowedTCPPorts = [
        cfg.httpPort
      ];
    };

    # When the hive network module is on, the matrix container's resolver
    # is the dnsmasq that runs in the gateway container (bound at
    # `bridgeIp`). Order the matrix container start after the gateway
    # container so the resolver is up before tuwunel's first federation
    # lookups. tuwunel boots fine without this — it configures the resolver
    # from `/etc/resolv.conf` at startup and only queries on-demand (the
    # boot failure this module fixes was an *empty* resolv.conf, a parse
    # error, not a connectivity one) — so this is robustness, not a boot
    # requirement. Soft `after` ordering (not `requires`) keeps the matrix
    # container's lifecycle decoupled from the gateway's. The gateway
    # always runs alongside hyperhive, so the gateway container unit always
    # exists here. (Declarative `containers.<n>` → `container@<n>.service` — the
    # nspawn template NixOS generates, confirmed from the live
    # `container@hive-matrix.service` host unit.)
    systemd.services."container@hive-matrix".after = lib.mkIf networkCfg.enable [
      "container@hive-gateway.service"
    ];
  };
}