diff --git a/pkg/spec/test/runtime-entry.test.ts b/pkg/spec/test/runtime-entry.test.ts
new file mode 100644
index 0000000..1b0a7a9
--- /dev/null
+++ b/pkg/spec/test/runtime-entry.test.ts
@@ -0,0 +1,104 @@
+import assert from "node:assert/strict";
+import { test } from "node:test";
+
+import { serializeAction } from "../src/runtime-entry.ts";
+import type { ActionDescriptor } from "../src/action-tree.ts";
+
+// decodeWire mirrors the Go decodeAction reading the unified flat contract: a
+// field rename (fromX vs from_x) would make these assertions fail here, the
+// same way it would silently no-op web actions in production.
+const POINT = { x: 12, y: 34 };
+
+test("Tap serializes to the flat point contract", () => {
+  assert.deepEqual(serializeAction({ kind: "Tap", on: POINT }), {
+    kind: "Tap",
+    x: 12,
+    y: 34,
+  });
+});
+
+test("Tap carries a resolved selector when present", () => {
+  const target = { x: 1, y: 2, selector: "id:save" };
+  assert.deepEqual(serializeAction({ kind: "Tap", on: target }), {
+    kind: "Tap",
+    x: 1,
+    y: 2,
+    selector: "id:save",
+  });
+});
+
+test("DoubleTap and LongPress share the point contract", () => {
+  assert.deepEqual(serializeAction({ kind: "DoubleTap", on: POINT }), {
+    kind: "DoubleTap",
+    x: 12,
+    y: 34,
+  });
+  assert.deepEqual(serializeAction({ kind: "LongPress", on: POINT }), {
+    kind: "LongPress",
+    x: 12,
+    y: 34,
+  });
+});
+
+test("InputText carries x, y and text", () => {
+  assert.deepEqual(serializeAction({ kind: "InputText", into: POINT, text: "hi" }), {
+    kind: "InputText",
+    x: 12,
+    y: 34,
+    text: "hi",
+  });
+});
+
+test("Swipe emits camelCase fromX/fromY/toX/toY/durationMillis", () => {
+  const action: ActionDescriptor = {
+    kind: "Swipe",
+    from: { x: 5, y: 6 },
+    to: { x: 7, y: 8 },
+    durationMillis: 300,
+  };
+  assert.deepEqual(serializeAction(action), {
+    kind: "Swipe",
+    fromX: 5,
+    fromY: 6,
+    toX: 7,
+    toY: 8,
+    durationMillis: 300,
+  });
+});
+
+test("Swipe defaults durationMillis to 250", () => {
+  const action = serializeAction({
+    kind: "Swipe",
+    from: { x: 0, y: 0 },
+    to: { x: 0, y: 1 },
+  }) as { durationMillis: number };
+  assert.equal(action.durationMillis, 250);
+});
+
+test("Scroll emits direction plus from/to point and duration", () => {
+  assert.deepEqual(serializeAction({ kind: "Scroll", direction: "down", in: POINT }), {
+    kind: "Scroll",
+    direction: "down",
+    fromX: 12,
+    fromY: 34,
+    toX: 12,
+    toY: 34,
+    durationMillis: 250,
+  });
+});
+
+test("PressKey and Wait pass through their fields", () => {
+  assert.deepEqual(serializeAction({ kind: "PressKey", key: "enter" }), {
+    kind: "PressKey",
+    key: "enter",
+  });
+  assert.deepEqual(serializeAction({ kind: "Wait", durationMillis: 500 }), {
+    kind: "Wait",
+    durationMillis: 500,
+  });
+});
+
+test("an unresolved (string) target drops the action", () => {
+  assert.equal(serializeAction({ kind: "Tap", on: "id:never-resolved" }), null);
+  assert.equal(serializeAction(null), null);
+});