diff --git a/pkg/spec-api/test/api.test.ts b/pkg/spec-api/test/api.test.ts
new file mode 100644
index 0000000..310e6c7
--- /dev/null
+++ b/pkg/spec-api/test/api.test.ts
@@ -0,0 +1,113 @@
+import assert from "node:assert/strict";
+import { test } from "node:test";
+
+import { Tap, InputText, actions, always, extract, swipes, taps, weighted } from "../src/index.ts";
+import type {
+  AccessibilityElement,
+  Action,
+  ActionGenerator,
+  Extracted,
+  Formula,
+  State,
+  UatuRuntime,
+  WeightedEntry,
+} from "../src/types.ts";
+
+function installFakeRuntime(): UatuRuntime & {
+  extracts: Array<(state: State) => unknown>;
+  alwaysPredicates: Array<() => boolean>;
+  actionGenerators: Array<() => Action[]>;
+  weightedCalls: WeightedEntry[][];
+} {
+  const calls = {
+    extracts: [] as Array<(state: State) => unknown>,
+    alwaysPredicates: [] as Array<() => boolean>,
+    actionGenerators: [] as Array<() => Action[]>,
+    weightedCalls: [] as WeightedEntry[][],
+  };
+  const runtime: UatuRuntime = {
+    extract: <T>(getter: (state: State) => T): Extracted<T> => {
+      calls.extracts.push(getter as (state: State) => unknown);
+      return { current: undefined as unknown as T, previous: undefined };
+    },
+    always: (predicate: () => boolean): Formula => {
+      calls.alwaysPredicates.push(predicate);
+      return { __uatuFormula: true };
+    },
+    actions: (generator: () => Action[]): ActionGenerator => {
+      calls.actionGenerators.push(generator);
+      return { __uatuActionGenerator: true, generate: generator };
+    },
+    weighted: (...entries: WeightedEntry[]): ActionGenerator => {
+      calls.weightedCalls.push(entries);
+      return { __uatuActionGenerator: true, generate: () => [] };
+    },
+    tap: ({ on }) => ({ kind: "Tap", on }),
+    inputText: ({ into, text }) => ({ kind: "InputText", into, text }),
+    taps: { __uatuActionGenerator: true, generate: () => [] },
+    swipes: { __uatuActionGenerator: true, generate: () => [] },
+  };
+  globalThis.__uatu__ = runtime;
+  return Object.assign(runtime, calls);
+}
+
+test("extract forwards the getter to the runtime", () => {
+  const runtime = installFakeRuntime();
+  const getter = (state: State) => state.snapshots["balance"];
+  extract<unknown>(getter);
+  assert.equal(runtime.extracts.length, 1);
+  assert.equal(runtime.extracts[0], getter);
+});
+
+test("always wraps a predicate into a formula via the runtime", () => {
+  const runtime = installFakeRuntime();
+  const predicate = () => true;
+  const formula = always(predicate);
+  assert.equal(runtime.alwaysPredicates[0], predicate);
+  assert.equal(formula.__uatuFormula, true);
+});
+
+test("Tap returns a TapAction with the supplied selector", () => {
+  installFakeRuntime();
+  const action = Tap({ on: "id:login_continue" });
+  assert.deepEqual(action, { kind: "Tap", on: "id:login_continue" });
+});
+
+test("Tap accepts an AccessibilityElement", () => {
+  installFakeRuntime();
+  const element: AccessibilityElement = { id: "login_continue" };
+  const action = Tap({ on: element });
+  assert.equal(action.kind, "Tap");
+  assert.equal(action.on, element);
+});
+
+test("InputText returns an InputTextAction", () => {
+  installFakeRuntime();
+  const action = InputText({ into: "id:phone", text: "+1234567890" });
+  assert.deepEqual(action, { kind: "InputText", into: "id:phone", text: "+1234567890" });
+});
+
+test("actions wraps a generator into the runtime's ActionGenerator", () => {
+  const runtime = installFakeRuntime();
+  const generator = () => [Tap({ on: "id:x" })];
+  const wrapped = actions(generator);
+  assert.equal(runtime.actionGenerators[0], generator);
+  assert.equal(wrapped.__uatuActionGenerator, true);
+});
+
+test("weighted forwards weighted entries to the runtime", () => {
+  const runtime = installFakeRuntime();
+  const entries: WeightedEntry[] = [
+    [80, taps],
+    [20, swipes],
+  ];
+  weighted(...entries);
+  assert.deepEqual(runtime.weightedCalls[0], entries);
+});
+
+test("taps and swipes proxy through to the runtime defaults", () => {
+  installFakeRuntime();
+  assert.equal(taps.__uatuActionGenerator, true);
+  assert.equal(swipes.__uatuActionGenerator, true);
+  assert.equal(typeof taps.generate, "function");
+});