diff --git a/.forgejo/workflows/ci.yaml b/.forgejo/workflows/ci.yaml
index 18925ab8..203eca03 100644
--- a/.forgejo/workflows/ci.yaml
+++ b/.forgejo/workflows/ci.yaml
@@ -15,6 +15,12 @@ jobs:
       - uses: actions/checkout@v4
       - run: nix-build -A tests
 
+  check-data-model:
+    runs-on: native
+    steps:
+      - uses: actions/checkout@v4
+      - run: nix-shell --run 'nix-unit ./deployment/data-model-test.nix'
+
   check-peertube:
     runs-on: native
     steps:
diff --git a/README.md b/README.md
index 3b19751c..d0a94395 100644
--- a/README.md
+++ b/README.md
@@ -154,6 +154,3 @@ details as to what they are for. As an overview:
 
 - [`services/`](./services) contains our effort to make Fediverse applications
   work seemlessly together in our specific setting.
-
-- [`website/`](./website) contains the framework and the content of [the
-  Fediversity website](https://fediversity.eu/)
diff --git a/default.nix b/default.nix
index 4c71ec49..7c536a03 100644
--- a/default.nix
+++ b/default.nix
@@ -41,6 +41,23 @@ in
   shell = pkgs.mkShellNoCC {
     inherit (pre-commit-check) shellHook;
     buildInputs = pre-commit-check.enabledPackages;
+    packages =
+      let
+        test-loop = pkgs.writeShellApplication {
+          name = "test-loop";
+          runtimeInputs = [
+            pkgs.watchexec
+            pkgs.nix-unit
+          ];
+          text = ''
+            watchexec -w ${builtins.toString ./.} -- nix-unit ${builtins.toString ./deployment/data-model-test.nix} "$@"
+          '';
+        };
+      in
+      [
+        pkgs.nix-unit
+        test-loop
+      ];
   };
 
   tests = {
diff --git a/deployment/README.md b/deployment/README.md
index f3e24276..784273dd 100644
--- a/deployment/README.md
+++ b/deployment/README.md
@@ -3,6 +3,13 @@
 This directory contains work to generate a full Fediversity deployment from a minimal configuration.
 This is different from [`../services/`](../services) that focuses on one machine, providing a polished and unified interface to different Fediverse services.
 
+## Data model
+
+The core piece of the project is the [Fediversity data model](./data-model.nix), which describes all entities and their interactions.
+
+What can be done with it is exemplified in the [evaluation tests](./data-model-test.nix).
+Run `test-loop` in the development environment when hacking on the data model or adding tests.
+
 ## Checks
 
 There are three levels of deployment checks: `basic`, `cli`, `panel`.
diff --git a/deployment/data-model-test.nix b/deployment/data-model-test.nix
new file mode 100644
index 00000000..17c85f77
--- /dev/null
+++ b/deployment/data-model-test.nix
@@ -0,0 +1,45 @@
+let
+  inherit (import ../default.nix { }) pkgs;
+  inherit (pkgs) lib;
+  eval =
+    module:
+    (lib.evalModules {
+      modules = [
+        module
+        ./data-model.nix
+      ];
+    }).config;
+in
+{
+  test-eval = {
+    expr =
+      let
+        example = eval {
+          runtime-environments.bar.nixos = {
+            module =
+              { ... }:
+              {
+                system.stateVersion = "25.05";
+              };
+          };
+          applications.foo = {
+            module =
+              { pkgs, ... }:
+              {
+                environment.systemPackages = [
+                  pkgs.hello
+                ];
+              };
+          };
+        };
+      in
+      {
+        has-runtime = lib.isAttrs example.runtime-environments.bar.nixos.module;
+        has-application = lib.isAttrs example.applications.foo.module;
+      };
+    expected = {
+      has-runtime = true;
+      has-application = true;
+    };
+  };
+}
diff --git a/deployment/data-model.nix b/deployment/data-model.nix
new file mode 100644
index 00000000..af867d55
--- /dev/null
+++ b/deployment/data-model.nix
@@ -0,0 +1,43 @@
+{
+  lib,
+  ...
+}:
+let
+  inherit (lib) types mkOption;
+in
+with types;
+{
+  options = {
+    runtime-environments = mkOption {
+      description = "Collection of runtime environments into which applications can be deployed";
+      type = attrsOf (attrTag {
+        nixos = mkOption {
+          description = "A single NixOS machine";
+          type = submodule {
+            options = {
+              module = mkOption {
+                description = "The NixOS module describing the base configuration for that machine";
+                type = deferredModule;
+              };
+            };
+          };
+        };
+      });
+    };
+    applications = mkOption {
+      description = "Collection of Fediversity applications";
+      type = attrsOf (submoduleWith {
+        modules = [
+          {
+            options = {
+              module = mkOption {
+                description = "The NixOS module for that application, for configuring that application";
+                type = deferredModule;
+              };
+            };
+          }
+        ];
+      });
+    };
+  };
+}