diff --git a/.forgejo/workflows/ci.yaml b/.forgejo/workflows/ci.yaml
index 4a925e84..9046ae7d 100644
--- a/.forgejo/workflows/ci.yaml
+++ b/.forgejo/workflows/ci.yaml
@@ -14,6 +14,12 @@ concurrency:
   group: ${{ forgejo.workflow }}-${{ forgejo.event.pull_request.number || forgejo.ref }}
 
 jobs:
+  check-function:
+    runs-on: native
+    steps:
+      - uses: actions/checkout@v4
+      - run: nix-shell --run 'nix-unit ./deployment/function-test.nix'
+
   check-data-model:
     runs-on: native
     steps:
diff --git a/deployment/function-test.nix b/deployment/function-test.nix
new file mode 100644
index 00000000..73f306ce
--- /dev/null
+++ b/deployment/function-test.nix
@@ -0,0 +1,108 @@
+let
+  inherit (import ../default.nix { }) pkgs;
+  inherit (pkgs) lib;
+  inherit (builtins) tryEval;
+  inherit (lib) mkOption types;
+  inherit (types)
+    int
+    str
+    submodule
+    ;
+  functionType = submodule ./function.nix;
+in
+{
+  _class = "nix-unit";
+
+  test-function-ok = {
+    expr =
+      tryEval
+        (lib.evalModules {
+          modules = [
+            (
+              { config, ... }:
+              {
+                options = {
+                  fn = mkOption {
+                    type = functionType;
+                  };
+                  call = mkOption {
+                    default = config.fn.apply 1;
+                  };
+                };
+                config.fn = {
+                  implementation = x: x;
+                  input-type = int;
+                  output-type = int;
+                };
+              }
+            )
+          ];
+        }).config.call;
+    expected = {
+      success = true;
+      value = 1;
+    };
+  };
+
+  test-bad-input = {
+    expr =
+      tryEval
+        (lib.evalModules {
+          modules = [
+            (
+              { config, ... }:
+              {
+                options = {
+                  fn = mkOption {
+                    type = functionType;
+                  };
+                  call = mkOption {
+                    default = config.fn.apply 1;
+                  };
+                };
+                config.fn = {
+                  implementation = x: x;
+                  input-type = str;
+                  output-type = int;
+                };
+              }
+            )
+          ];
+        }).config.call;
+    expected = {
+      success = false;
+      value = false;
+    };
+  };
+
+  test-bad-output = {
+    expr =
+      tryEval
+        (lib.evalModules {
+          modules = [
+            (
+              { config, ... }:
+              {
+                options = {
+                  fn = mkOption {
+                    type = functionType;
+                  };
+                  call = mkOption {
+                    default = config.fn.apply 1;
+                  };
+                };
+                config.fn = {
+                  implementation = x: x;
+                  input-type = int;
+                  output-type = str;
+                };
+              }
+            )
+          ];
+        }).config.call;
+    expected = {
+      success = false;
+      value = false;
+    };
+  };
+}