diff --git a/pkg/spec/src/pick.ts b/pkg/spec/src/pick.ts
new file mode 100644
index 0000000..6af6bc2
--- /dev/null
+++ b/pkg/spec/src/pick.ts
@@ -0,0 +1,185 @@
+// The shared deterministic action picker for W2 approach B.
+//
+// walk() traverses a GeneratorNode tree, and nextAction() wraps it with the
+// 16-attempt retry that matches worker.go NextAction. Both engines (the goja
+// verifier and the V8 web runtime) run THIS code, drawing through the shared
+// Pcg, so a given seed yields an identical action stream on every platform.
+//
+// PARITY CONTRACT - draw order.
+// Every random decision goes through the Pcg in a FIXED, pinned order. Changing
+// this order shifts the stream for a seed and breaks cross-engine
+// reproducibility, so treat it as load-bearing:
+//
+//   weighted node:   ONE float64() draw, then an ASCENDING cumulative scan over
+//                    max(0, weight). (matches worker.go pickWeighted.)
+//   actions node:    if the generated list has >1 entry, ONE intN(len) draw;
+//                    a 0- or 1-element list draws nothing. (pickFromResult.)
+//   builtin node, per verb, in this exact sequence:
+//     taps/doubleTaps/longPresses: intN(candidateCount)            [1 draw]
+//     typing:                      intN(candidateCount), intN(corpusLength)
+//     swipes:                      intN(candidateCount),
+//                                  200 + intN(401) magnitude, intN(4) direction
+//     scrolls:                     intN(candidateCount), intN(4) direction
+//     pressKeys:                   intN(keyCount)
+//     waitOnce:                    no draw
+//
+// Builtin targets are resolved to a {x, y} Point by the host BEFORE the picker
+// sees them, so no element handle crosses into this module.
+
+import type { Pcg } from "./pcg.ts";
+import type {
+  ActionDescriptor,
+  BuiltinVerb,
+  GeneratorNode,
+  Host,
+} from "./action-tree.ts";
+import type { Direction, Point } from "./types.ts";
+import { INPUT_CORPUS, NATIVE_PRESS_KEYS, WEB_PRESS_KEYS } from "./corpus.ts";
+import { supports, warnUnsupportedOnce } from "./verbs.ts";
+
+// SWIPE_MIN_MAGNITUDE / SWIPE_MAGNITUDE_SPAN reproduce worker.go's
+// `200 + rng.IntN(401)` swipe distance in pixels (200..600 inclusive).
+const SWIPE_MIN_MAGNITUDE = 200;
+const SWIPE_MAGNITUDE_SPAN = 401;
+const SWIPE_DURATION_MILLIS = 250;
+
+const DIRECTIONS: readonly Direction[] = ["up", "down", "left", "right"];
+
+const MAX_RETRIES = 16;
+
+// nextAction resolves an action for the current step, retrying walk() up to 16
+// times when it yields null (matches worker.go NextAction). Returns null when
+// every attempt comes up empty.
+export function nextAction(
+  root: GeneratorNode,
+  rng: Pcg,
+  host: Host,
+): ActionDescriptor | null {
+  for (let attempt = 0; attempt < MAX_RETRIES; attempt++) {
+    const action = walk(root, rng, host);
+    if (action !== null) return action;
+  }
+  return null;
+}
+
+// walk resolves a single node to an ActionDescriptor or null.
+export function walk(
+  node: GeneratorNode,
+  rng: Pcg,
+  host: Host,
+): ActionDescriptor | null {
+  switch (node.kind) {
+    case "weighted":
+      return walkWeighted(node.branches, rng, host);
+    case "actions":
+      return walkActions(node.generate(), rng);
+    case "builtin":
+      return walkBuiltin(node.verb, rng, host);
+  }
+}
+
+function walkWeighted(
+  branches: ReadonlyArray<readonly [number, GeneratorNode]>,
+  rng: Pcg,
+  host: Host,
+): ActionDescriptor | null {
+  let total = 0;
+  for (const [weight] of branches) total += Math.max(0, weight);
+  if (total <= 0) return null;
+  const draw = rng.float64() * total;
+  let cumulative = 0;
+  for (const [weight, child] of branches) {
+    cumulative += Math.max(0, weight);
+    if (draw < cumulative) return walk(child, rng, host);
+  }
+  // Floating-point slack: draw can equal total. Fall to the last branch,
+  // matching worker.go's trailing `return generators[length-1]`.
+  const last = branches[branches.length - 1];
+  return last ? walk(last[1], rng, host) : null;
+}
+
+function walkActions(
+  generated: ActionDescriptor[],
+  rng: Pcg,
+): ActionDescriptor | null {
+  if (generated.length === 0) return null;
+  if (generated.length === 1) return generated[0] ?? null;
+  return generated[rng.intN(generated.length)] ?? null;
+}
+
+function walkBuiltin(
+  verb: BuiltinVerb,
+  rng: Pcg,
+  host: Host,
+): ActionDescriptor | null {
+  if (!supports(verb, host.platform())) {
+    warnUnsupportedOnce(host, verb);
+    return null;
+  }
+  if (verb === "waitOnce") {
+    return { kind: "Wait", durationMillis: 500 };
+  }
+  if (verb === "pressKeys") {
+    return walkPressKey(rng, host);
+  }
+
+  const candidates = host.queryCandidates(verb);
+  if (candidates.length === 0) return null;
+  const picked = candidates[rng.intN(candidates.length)];
+  if (!picked) return null;
+  const point: Point = { x: picked.x, y: picked.y };
+
+  switch (verb) {
+    case "taps":
+      return { kind: "Tap", on: point };
+    case "doubleTaps":
+      return { kind: "DoubleTap", on: point };
+    case "longPresses":
+      return { kind: "LongPress", on: point };
+    case "typing": {
+      const text = INPUT_CORPUS[rng.intN(INPUT_CORPUS.length)] ?? "";
+      return { kind: "InputText", into: point, text };
+    }
+    case "swipes":
+      return buildSwipe(point, rng);
+    case "scrolls": {
+      const direction = DIRECTIONS[rng.intN(DIRECTIONS.length)] ?? "down";
+      return { kind: "Scroll", direction, in: point };
+    }
+  }
+}
+
+function walkPressKey(rng: Pcg, host: Host): ActionDescriptor | null {
+  const keys = host.platform() === "web" ? WEB_PRESS_KEYS : NATIVE_PRESS_KEYS;
+  if (keys.length === 0) return null;
+  const key = keys[rng.intN(keys.length)] ?? keys[0];
+  if (key === undefined) return null;
+  return { kind: "PressKey", key };
+}
+
+function buildSwipe(from: Point, rng: Pcg): ActionDescriptor {
+  const magnitude = SWIPE_MIN_MAGNITUDE + rng.intN(SWIPE_MAGNITUDE_SPAN);
+  let toX = from.x;
+  let toY = from.y;
+  switch (rng.intN(4)) {
+    case 0:
+      toY = from.y - magnitude;
+      break;
+    case 1:
+      toY = from.y + magnitude;
+      break;
+    case 2:
+      toX = from.x - magnitude;
+      break;
+    case 3:
+      toX = from.x + magnitude;
+      break;
+  }
+  return {
+    kind: "Swipe",
+    from,
+    to: { x: Math.max(0, toX), y: Math.max(0, toY) },
+    durationMillis: SWIPE_DURATION_MILLIS,
+  };
+}
+