diff --git a/pkgs/mruby/src/assert.c b/pkgs/mruby/src/assert.c
new file mode 100644
index 0000000..48ce83c
--- /dev/null
+++ b/pkgs/mruby/src/assert.c
@@ -0,0 +1,50 @@
+#include <kernaux.h>
+
+#include <mruby.h>
+#include <mruby/variable.h>
+
+static void assert_cb(const char *file, int line, const char *msg);
+
+static mrb_value rb_KernAux_assert_cb(mrb_state *mrb, mrb_value self);
+static mrb_value rb_KernAux_assert_cb_EQ(mrb_state *mrb, mrb_value self);
+static mrb_value rb_KernAux_assert_do(mrb_state *mrb, mrb_value self);
+
+void init_assert(mrb_state *const mrb)
+{
+    kernaux_assert_cb = assert_cb;
+
+    struct RClass *const rb_KernAux = mrb_define_module(mrb, "KernAux");
+    mrb_define_class_method(mrb, rb_KernAux, "assert_cb",
+                            rb_KernAux_assert_cb, MRB_ARGS_NONE());
+    mrb_define_class_method(mrb, rb_KernAux, "assert_cb=",
+                            rb_KernAux_assert_cb_EQ, MRB_ARGS_REQ(1));
+    mrb_define_class_method(mrb, rb_KernAux, "assert_do",
+                            rb_KernAux_assert_do, MRB_ARGS_REQ(3));
+}
+
+void assert_cb(const char *const file, const int line, const char *const msg)
+{
+}
+
+mrb_value rb_KernAux_assert_cb(mrb_state *const mrb, const mrb_value self_rb)
+{
+    return mrb_iv_get(mrb, self_rb, mrb_intern_lit(mrb, "@assert_cb"));
+}
+
+mrb_value rb_KernAux_assert_cb_EQ(mrb_state *const mrb, const mrb_value self_rb)
+{
+    mrb_value assert_cb_rb = mrb_nil_value();
+    mrb_get_args(mrb, "o", &assert_cb_rb);
+    mrb_iv_set(mrb, self_rb, mrb_intern_lit(mrb, "@assert_cb"), assert_cb_rb);
+    return assert_cb_rb;
+}
+
+mrb_value rb_KernAux_assert_do(mrb_state *const mrb, const mrb_value self_rb)
+{
+    const char *file = NULL;
+    mrb_int line_rb = 0;
+    const char *msg = NULL;
+    mrb_get_args(mrb, "ziz", &file, &line_rb, &msg);
+    kernaux_assert_do(file, 0, msg);
+    return mrb_nil_value();
+}
diff --git a/pkgs/mruby/src/main.c b/pkgs/mruby/src/main.c
index e4521ef..e2f357e 100644
--- a/pkgs/mruby/src/main.c
+++ b/pkgs/mruby/src/main.c
@@ -1,8 +1,10 @@
 #include <mruby.h>
 
-void mrb_mruby_kernaux_gem_final(mrb_state *mrb) {}
+void init_assert(mrb_state *mrb);
 
-void mrb_mruby_kernaux_gem_init(mrb_state *mrb)
+void mrb_mruby_kernaux_gem_final(mrb_state *const mrb) {}
+
+void mrb_mruby_kernaux_gem_init(mrb_state *const mrb)
 {
-    /* struct RClass *const rb_KernAux = */ mrb_define_module(mrb, "KernAux");
+    init_assert(mrb);
 }
diff --git a/pkgs/mruby/test/dummy.rb b/pkgs/mruby/test/dummy.rb
index 6a2b795..3c07417 100644
--- a/pkgs/mruby/test/dummy.rb
+++ b/pkgs/mruby/test/dummy.rb
@@ -1,3 +1,6 @@
-assert 'Main module exists' do
-  assert_equal KernAux, ::KernAux
+assert 'Dummy' do
+  assert_equal nil, KernAux.assert_cb
+  KernAux.assert_cb = 123
+  assert_equal 123, KernAux.assert_cb
+  assert_equal nil, KernAux.assert_do(__FILE__, __LINE__, 'hello')
 end