diff --git a/pkg/spec/src/actions.ts b/pkg/spec/src/actions.ts
index 7ced33e..2b85eea 100644
--- a/pkg/spec/src/actions.ts
+++ b/pkg/spec/src/actions.ts
@@ -1,7 +1,13 @@
+// Author-facing action factories. Each returns plain GeneratorNode /
+// ActionDescriptor DATA (see action-tree.ts); the shared picker (pick.ts) walks
+// the tree and the runtime entry (runtime-entry.ts) serializes the result. No
+// factory forwards to globalThis.__sanderling__ anymore: the same data tree
+// drives both the goja verifier and the V8 web runtime.
+
+import type { Pcg } from "./pcg.ts";
 import type {
   AccessibilityElement,
   Action,
-  ActionGenerator,
   Direction,
   DoubleTapAction,
   InputTextAction,
@@ -16,16 +22,17 @@ import type {
   WaitAction,
   WeightedEntry,
 } from "./types.ts";
+import type { ActionDescriptor, GeneratorNode } from "./action-tree.ts";
 
-export function actions(generator: () => Action[]): ActionGenerator {
-  return globalThis.__sanderling__.actions(generator);
+export function actions(generator: () => Action[]): GeneratorNode {
+  return { kind: "actions", generate: generator as () => ActionDescriptor[] };
 }
 
 export function whenRoute(
   routeExtractor: { readonly current: string | null },
   routes: string | readonly string[],
   body: () => Action[],
-): ActionGenerator {
+): GeneratorNode {
   const allowed = typeof routes === "string" ? [routes] : routes;
   return actions(() => {
     const current = routeExtractor.current;
@@ -34,38 +41,54 @@ export function whenRoute(
   });
 }
 
-export function weighted(...entries: WeightedEntry[]): ActionGenerator {
-  return globalThis.__sanderling__.weighted(...entries);
+export function weighted(...entries: WeightedEntry[]): GeneratorNode {
+  return { kind: "weighted", branches: entries };
+}
+
+// samplerRng is the picker's Pcg while it evaluates an `actions` node's
+// generator (set by pick.ts walkActions). `from(...).generate()` draws from it
+// so sampling shares the single deterministic stream. It is null outside a
+// walk; eager spec-time generate() calls then fall back to the first item.
+let samplerRng: Pcg | null = null;
+
+export function setSamplerRng(rng: Pcg | null): void {
+  samplerRng = rng;
 }
 
 export function from<T>(items: readonly T[]): Sampler<T> {
-  return globalThis.__sanderling__.from(items);
+  return {
+    generate(): T {
+      if (items.length <= 1) return items[0] as T;
+      const index = samplerRng ? samplerRng.intN(items.length) : 0;
+      return items[index] as T;
+    },
+  };
 }
 
 export function Tap(parameters: { on: string | AccessibilityElement }): TapAction {
-  return globalThis.__sanderling__.tap(parameters);
+  return { kind: "Tap", on: parameters.on };
 }
 
 export function DoubleTap(parameters: { on: string | AccessibilityElement }): DoubleTapAction {
-  return globalThis.__sanderling__.doubleTap(parameters);
+  return { kind: "DoubleTap", on: parameters.on };
 }
 
 export function LongPress(parameters: { on: string | AccessibilityElement }): LongPressAction {
-  return globalThis.__sanderling__.longPress(parameters);
+  return { kind: "LongPress", on: parameters.on };
 }
 
 export function Scroll(parameters: {
   direction: Direction;
   in?: string | AccessibilityElement;
 }): ScrollAction {
-  return globalThis.__sanderling__.scroll(parameters);
+  return { kind: "Scroll", direction: parameters.direction, in: parameters.in };
 }
 
 export function InputText(parameters: {
   into: string | AccessibilityElement;
   text: string;
 }): InputTextAction {
-  return globalThis.__sanderling__.inputText(parameters);
+  return { kind: "InputText", into: parameters.into, text: parameters.text };
 }
 
 export function Swipe(parameters: {
@@ -73,44 +96,27 @@ export function Swipe(parameters: {
   to: Point | AccessibilityElement;
   durationMillis?: number;
 }): SwipeAction {
-  return globalThis.__sanderling__.swipe(parameters);
+  return {
+    kind: "Swipe",
+    from: parameters.from,
+    to: parameters.to,
+    durationMillis: parameters.durationMillis,
+  };
 }
 
 export function PressKey(parameters: { key: Key }): PressKeyAction {
-  return globalThis.__sanderling__.pressKey(parameters);
+  return { kind: "PressKey", key: parameters.key };
 }
 
 export function Wait(parameters: { durationMillis: number }): WaitAction {
-  return globalThis.__sanderling__.wait(parameters);
+  return { kind: "Wait", durationMillis: parameters.durationMillis };
 }
 
-function builtinGenerator(
-  name:
-    | "taps"
-    | "doubleTaps"
-    | "longPresses"
-    | "scrolls"
-    | "typing"
-    | "swipes"
-    | "waitOnce"
-    | "pressKeys",
-): ActionGenerator {
-  return new Proxy({} as ActionGenerator, {
-    get(_target, property) {
-      const runtime = globalThis.__sanderling__[name] as unknown as Record<
-        string | symbol,
-        unknown
-      >;
-      return runtime[property];
-    },
-  });
-}
-
-export const taps: ActionGenerator = builtinGenerator("taps");
-export const doubleTaps: ActionGenerator = builtinGenerator("doubleTaps");
-export const longPresses: ActionGenerator = builtinGenerator("longPresses");
-export const scrolls: ActionGenerator = builtinGenerator("scrolls");
-export const typing: ActionGenerator = builtinGenerator("typing");
-export const swipes: ActionGenerator = builtinGenerator("swipes");
-export const waitOnce: ActionGenerator = builtinGenerator("waitOnce");
-export const pressKey: ActionGenerator = builtinGenerator("pressKeys");
+export const taps: GeneratorNode = { kind: "builtin", verb: "taps" };
+export const doubleTaps: GeneratorNode = { kind: "builtin", verb: "doubleTaps" };
+export const longPresses: GeneratorNode = { kind: "builtin", verb: "longPresses" };
+export const scrolls: GeneratorNode = { kind: "builtin", verb: "scrolls" };
+export const typing: GeneratorNode = { kind: "builtin", verb: "typing" };
+export const swipes: GeneratorNode = { kind: "builtin", verb: "swipes" };
+export const waitOnce: GeneratorNode = { kind: "builtin", verb: "waitOnce" };
+export const pressKey: GeneratorNode = { kind: "builtin", verb: "pressKeys" };
diff --git a/pkg/spec/src/types.ts b/pkg/spec/src/types.ts
index 384be42..c556e0e 100644
--- a/pkg/spec/src/types.ts
+++ b/pkg/spec/src/types.ts
@@ -175,10 +175,10 @@ export type Key =
   | "left"
   | "right";
 
-export interface ActionGenerator {
-  readonly __sanderlingActionGenerator: true;
-  generate(): Action[];
-}
+// ActionGenerator is a node in the action-generator tree (see action-tree.ts).
+// Author specs treat it as an opaque handle they compose with weighted(); the
+// shared picker walks the underlying GeneratorNode.
+export type ActionGenerator = GeneratorNode;
 
 export interface Formula {
   readonly __sanderlingFormula: true;
@@ -196,45 +196,21 @@ export interface Sampler<T> {
   generate(): T;
 }
 
+// SanderlingRuntime is the host-bound surface that stays on
+// globalThis.__sanderling__: extract plus the LTL formula constructors. Action
+// factories no longer live here; they return plain data trees (see actions.ts).
 export interface SanderlingRuntime {
   extract: <T>(getter: (state: State) => T, name?: string) => Extracted<T>;
   always: (predicateOrFormula: (() => boolean) | Formula) => Formula;
   now: (predicate: () => boolean) => Formula;
   next: (predicate: () => boolean) => Formula;
   eventually: (predicate: () => boolean) => EventuallyFormula;
-  actions: (generator: () => Action[]) => ActionGenerator;
-  weighted: (...entries: WeightedEntry[]) => ActionGenerator;
-  from: <T>(items: readonly T[]) => Sampler<T>;
-  tap: (parameters: { on: string | AccessibilityElement }) => TapAction;
-  doubleTap: (parameters: { on: string | AccessibilityElement }) => DoubleTapAction;
-  longPress: (parameters: { on: string | AccessibilityElement }) => LongPressAction;
-  scroll: (parameters: {
-    direction: Direction;
-    in?: string | AccessibilityElement;
-  }) => ScrollAction;
-  inputText: (parameters: {
-    into: string | AccessibilityElement;
-    text: string;
-  }) => InputTextAction;
-  swipe: (parameters: {
-    from: Point | AccessibilityElement;
-    to: Point | AccessibilityElement;
-    durationMillis?: number;
-  }) => SwipeAction;
-  pressKey: (parameters: { key: Key }) => PressKeyAction;
-  wait: (parameters: { durationMillis: number }) => WaitAction;
-  taps: ActionGenerator;
-  doubleTaps: ActionGenerator;
-  longPresses: ActionGenerator;
-  scrolls: ActionGenerator;
-  typing: ActionGenerator;
-  swipes: ActionGenerator;
-  waitOnce: ActionGenerator;
-  pressKeys: ActionGenerator;
 }
 
 export type WeightedEntry = readonly [number, ActionGenerator];
 
+import type { GeneratorNode } from "./action-tree.ts";
+
 declare global {
   // eslint-disable-next-line no-var
   var __sanderling__: SanderlingRuntime;