diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index cc38804287..c8f7acb03b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -372,6 +372,9 @@ jobs:
           node-version: 22
       - uses: ./.github/actions/prepare-ffmpeg-bin
       - run: bun install --frozen-lockfile
+      # Runtime coverage now imports core modules that consume workspace
+      # subpaths. Build their dist exports before Vitest resolves them.
+      - run: bun run --filter '@hyperframes/{parsers,lint,studio-server}' build
       - run: bun run --filter @hyperframes/core test:hyperframe-runtime-ci
 
   studio-load-smoke:
diff --git a/packages/core/package.json b/packages/core/package.json
index a5bfd89cee..239ba6b209 100644
--- a/packages/core/package.json
+++ b/packages/core/package.json
@@ -402,8 +402,10 @@
     "test": "bun run check:position-edits-render && vitest run",
     "test:watch": "vitest",
     "test:coverage": "vitest run --coverage",
-    "typecheck": "tsc --noEmit",
-    "lint:runtime-preview-guards": "tsx scripts/lint-runtime-preview-guards.ts",
+    "test:runtime-coverage": "vitest run --coverage src/runtime",
+    "typecheck": "tsc --noEmit && bun run typecheck:runtime",
+    "typecheck:runtime": "tsc --noEmit -p tsconfig.runtime.json",
+    "lint:runtime-preview-guards": "bun scripts/lint-runtime-preview-guards.ts",
     "build:hyperframes-runtime": "tsx scripts/build-hyperframes-runtime-artifact.ts",
     "build:position-edits-render": "tsx scripts/build-position-edits-render.ts",
     "check:position-edits-render": "bun run build:position-edits-render && git diff --exit-code -- src/generated/position-edits-render-inline.ts",
@@ -415,9 +417,9 @@
     "test:hyperframe-runtime-duration-guards": "tsx scripts/test-hyperframe-runtime-duration-guards.ts",
     "test:hyperframe-runtime-parity": "tsx scripts/test-hyperframe-runtime-parity.ts",
     "test:hyperframe-runtime-security": "tsx scripts/test-hyperframe-runtime-security.ts",
-    "test:hyperframe-linter": "tsx scripts/test-hyperframe-linter.ts",
-    "test:hyperframe-runtime-ci": "bun run build:hyperframes-runtime && bun run test:hyperframe-runtime-contract && bun run test:hyperframe-runtime-behavior && bun run test:hyperframe-runtime-seek && bun run test:hyperframe-runtime-duration-guards && bun run test:hyperframe-runtime-parity && bun run test:hyperframe-runtime-security",
-    "check:hyperframe-html": "tsx scripts/check-hyperframe-static.ts",
+    "test:hyperframe-linter": "bun scripts/test-hyperframe-linter.ts",
+    "test:hyperframe-runtime-ci": "bun run typecheck:runtime && bun run lint:runtime-preview-guards && bun run build:hyperframes-runtime && bun run test:hyperframe-runtime-contract && bun run test:hyperframe-runtime-behavior && bun run test:hyperframe-runtime-seek && bun run test:hyperframe-runtime-duration-guards && bun run test:hyperframe-runtime-parity && bun run test:hyperframe-runtime-security && bun run test:runtime-coverage && bun run test:hyperframe-linter",
+    "check:hyperframe-html": "bun scripts/check-hyperframe-static.ts",
     "debug:timeline": "tsx scripts/debug-timeline.ts",
     "prepublishOnly": "echo skip"
   },
diff --git a/packages/core/scripts/check-hyperframe-static.ts b/packages/core/scripts/check-hyperframe-static.ts
index e61e1ec754..ecf17b5be5 100644
--- a/packages/core/scripts/check-hyperframe-static.ts
+++ b/packages/core/scripts/check-hyperframe-static.ts
@@ -1,7 +1,6 @@
 import fs from "node:fs";
 import path from "node:path";
-import { lintHyperframeHtml } from "../src/lint/hyperframeLinter";
-import type { HyperframeLintResult } from "../src/lint/types";
+import { lintHyperframeHtml, type HyperframeLintResult } from "@hyperframes/lint";
 
 function formatCounts(result: HyperframeLintResult): string {
   const parts = [`${result.warningCount} warning${result.warningCount === 1 ? "" : "s"}`];
diff --git a/packages/core/scripts/lint-runtime-preview-guards.ts b/packages/core/scripts/lint-runtime-preview-guards.ts
index fdfc481557..bebba4227c 100644
--- a/packages/core/scripts/lint-runtime-preview-guards.ts
+++ b/packages/core/scripts/lint-runtime-preview-guards.ts
@@ -13,6 +13,9 @@ type GuardCheckResult = {
   failed: GuardSpec[];
 };
 
+// Keep only temporary source-shape guards that do not yet have behavioral
+// coverage. Timeline replacement and early-play rebinding are exercised by
+// init.test.ts and timelineRebindPolicy.test.ts instead of regexes.
 const GUARD_SPECS: GuardSpec[] = [
   {
     id: "external_compositions_gate",
@@ -20,12 +23,6 @@ const GUARD_SPECS: GuardSpec[] = [
     filePath: "src/runtime/init.ts",
     pattern: /if\s*\(\s*!externalCompositionsReady\s*\)\s*return\s+false;/,
   },
-  {
-    id: "usable_timeline_gate",
-    description: "Skip rebinding when current timeline is already usable",
-    filePath: "src/runtime/init.ts",
-    pattern: /if\s*\(\s*currentTimeline\s*&&\s*currentTimelineUsable\s*\)\s*return\s+false;/,
-  },
   {
     id: "child_timeline_activation",
     description: "Force root child timelines active before composition binding",
@@ -39,18 +36,6 @@ const GUARD_SPECS: GuardSpec[] = [
     pattern:
       /if\s*\(\s*!isUsableTimelineDuration\(rootDurationSeconds\)\s*&&\s*rootChildCandidates\.length\s*>\s*0\s*\)/,
   },
-  {
-    id: "loop_guard_rebind",
-    description: "Enable loop guard based timeline rebinding",
-    filePath: "src/runtime/init.ts",
-    pattern: /if\s*\(\s*rebindTimelineFromResolution\(resolution,\s*"loop_guard"\)\s*\)/,
-  },
-  {
-    id: "early_play_rebind_hold",
-    description: "Hold rebinding during first playback seconds",
-    filePath: "src/runtime/init.ts",
-    pattern: /shouldHoldRebindDuringEarlyPlay/,
-  },
   {
     id: "external_script_ordering",
     description: "Inject external composition scripts with deterministic ordering",
diff --git a/packages/core/scripts/test-hyperframe-linter.ts b/packages/core/scripts/test-hyperframe-linter.ts
index bdb8a5027c..a6ce3559ba 100644
--- a/packages/core/scripts/test-hyperframe-linter.ts
+++ b/packages/core/scripts/test-hyperframe-linter.ts
@@ -1,21 +1,36 @@
 import assert from "node:assert/strict";
 import { execFileSync } from "node:child_process";
-import fs from "node:fs";
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
 import path from "node:path";
-import { lintHyperframeHtml } from "../src/lint/hyperframeLinter";
+import { fileURLToPath } from "node:url";
+import { lintHyperframeHtml } from "@hyperframes/lint";
 
-const ROOT = path.resolve(path.dirname(new URL(import.meta.url).pathname), "..");
+const ROOT = path.resolve(path.dirname(fileURLToPath(import.meta.url)), "..");
+const VALID_COMPOSITION = `
+<html>
+<body>
+  <div id="root" data-composition-id="comp-1" data-width="1920" data-height="1080" data-start="0">
+    <div id="stage"></div>
+  </div>
+  <script src="https://cdn.gsap.com/gsap.min.js"></script>
+  <script>
+    window.__timelines = window.__timelines || {};
+    const tl = gsap.timeline({ paused: true });
+    tl.to("#stage", { opacity: 1, duration: 1 }, 0);
+    window.__timelines["comp-1"] = tl;
+  </script>
+</body>
+</html>`;
 
-function testCleanFixturePasses() {
-  const fixturePath = path.join(ROOT, "src/tests/chat-project-9/index.html");
-  const html = fs.readFileSync(fixturePath, "utf8");
-  const result = lintHyperframeHtml(html, { filePath: fixturePath });
+async function testCleanFixturePasses() {
+  const result = await lintHyperframeHtml(VALID_COMPOSITION, { filePath: "valid.html" });
 
-  assert.equal(result.ok, true, "chat-project-9 should pass without lint errors");
-  assert.equal(result.errorCount, 0, "chat-project-9 should have zero lint errors");
+  assert.equal(result.ok, true, "valid composition should pass without lint errors");
+  assert.equal(result.errorCount, 0, "valid composition should have zero lint errors");
 }
 
-function testDetectsMissingCompositionHostId() {
+async function testDetectsMissingCompositionHostId() {
   const html = `
     <html>
       <body>
@@ -31,7 +46,7 @@ function testDetectsMissingCompositionHostId() {
     </html>
   `;
 
-  const result = lintHyperframeHtml(html);
+  const result = await lintHyperframeHtml(html);
   const codes = result.findings.map((finding) => finding.code);
 
   assert.equal(result.ok, false, "missing composition ids should fail lint");
@@ -39,7 +54,7 @@ function testDetectsMissingCompositionHostId() {
   assert.ok(codes.includes("host_missing_composition_id"));
 }
 
-function testDetectsOverlappingGsapTweens() {
+async function testDetectsOverlappingGsapTweens() {
   const html = `
     <html>
       <body>
@@ -57,7 +72,7 @@ function testDetectsOverlappingGsapTweens() {
     </html>
   `;
 
-  const result = lintHyperframeHtml(html);
+  const result = await lintHyperframeHtml(html);
   const overlapFinding = result.findings.find(
     (finding) => finding.code === "overlapping_gsap_tweens",
   );
@@ -67,29 +82,30 @@ function testDetectsOverlappingGsapTweens() {
 }
 
 function testCliJsonOutput() {
-  const fixturePath = path.join(ROOT, "src/tests/chat-project-9/index.html");
-  const tsxBin = path.join(ROOT, "node_modules/.bin/tsx");
-  const stdout = execFileSync(
-    tsxBin,
-    ["scripts/check-hyperframe-static.ts", "--json", fixturePath],
-    {
+  const tempDir = mkdtempSync(path.join(tmpdir(), "hf-core-lint-script-"));
+  try {
+    const fixturePath = path.join(tempDir, "index.html");
+    writeFileSync(fixturePath, VALID_COMPOSITION, "utf8");
+    const stdout = execFileSync("bun", ["run", "check:hyperframe-html", "--json", fixturePath], {
       cwd: ROOT,
       encoding: "utf8",
-    },
-  );
-  const payload = JSON.parse(stdout);
+    });
+    const payload = JSON.parse(stdout);
 
-  assert.equal(payload.ok, true);
-  assert.equal(typeof payload.errorCount, "number");
-  assert.ok(Array.isArray(payload.findings));
+    assert.equal(payload.ok, true);
+    assert.equal(typeof payload.errorCount, "number");
+    assert.ok(Array.isArray(payload.findings));
+  } finally {
+    rmSync(tempDir, { recursive: true, force: true });
+  }
 }
 
-function main() {
-  testCleanFixturePasses();
-  testDetectsMissingCompositionHostId();
-  testDetectsOverlappingGsapTweens();
+async function main() {
+  await testCleanFixturePasses();
+  await testDetectsMissingCompositionHostId();
+  await testDetectsOverlappingGsapTweens();
   testCliJsonOutput();
   console.log("hyperframe linter tests passed");
 }
 
-main();
+await main();
diff --git a/packages/core/src/runtime/adapters/css.ts b/packages/core/src/runtime/adapters/css.ts
index b884e928a4..51b560abfa 100644
--- a/packages/core/src/runtime/adapters/css.ts
+++ b/packages/core/src/runtime/adapters/css.ts
@@ -37,7 +37,7 @@ export function createCssAdapter(params?: {
     animation: Animation,
     startSeconds: number,
   ): { endSeconds?: number; unbounded?: true } => {
-    let timing: { endTime?: number | string } | null = null;
+    let timing: ComputedEffectTiming | null = null;
     try {
       timing = animation.effect?.getComputedTiming?.() ?? null;
     } catch (err) {
diff --git a/packages/core/src/runtime/adapters/waapi.ts b/packages/core/src/runtime/adapters/waapi.ts
index f17af71540..0ed5b1fa97 100644
--- a/packages/core/src/runtime/adapters/waapi.ts
+++ b/packages/core/src/runtime/adapters/waapi.ts
@@ -100,7 +100,7 @@ export function createWaapiAdapter(): RuntimeDeterministicAdapter {
         value: original,
         configurable: true,
       });
-      const wrappedAnimate = function (...args: Parameters<Element["animate"]>) {
+      const wrappedAnimate = function (this: Element, ...args: Parameters<Element["animate"]>) {
         const animation = original.apply(this, args);
         trackAnimation(animation, lastSeekTimeMs);
         return animation;
@@ -126,7 +126,7 @@ export function createWaapiAdapter(): RuntimeDeterministicAdapter {
   const inferAnimationEndSeconds = (
     animation: Animation,
   ): { endSeconds?: number; unbounded?: true } => {
-    let timing: { endTime?: number | string } | null = null;
+    let timing: ComputedEffectTiming | null = null;
     try {
       timing = animation.effect?.getComputedTiming?.() ?? null;
     } catch (err) {
diff --git a/packages/core/src/runtime/applyVariableBindings.ts b/packages/core/src/runtime/applyVariableBindings.ts
index 79a37211a7..7b704f9338 100644
--- a/packages/core/src/runtime/applyVariableBindings.ts
+++ b/packages/core/src/runtime/applyVariableBindings.ts
@@ -60,7 +60,8 @@ function isSafeMediaUrl(url: string): boolean {
   const normalized = url.replace(/[\u0000-\u0020]/g, "");
   const scheme = /^([a-z][a-z0-9+.-]*):/i.exec(normalized);
   if (!scheme) return true;
-  const proto = scheme[1].toLowerCase();
+  const proto = scheme[1]?.toLowerCase();
+  if (!proto) return false;
   if (proto === "https" || proto === "http" || proto === "blob") return true;
   if (proto === "data") return /^data:image\//i.test(normalized);
   return false;
diff --git a/packages/core/src/runtime/captionOverrides.ts b/packages/core/src/runtime/captionOverrides.ts
index bb115fec4d..d90c049227 100644
--- a/packages/core/src/runtime/captionOverrides.ts
+++ b/packages/core/src/runtime/captionOverrides.ts
@@ -135,7 +135,7 @@ export function applyCaptionOverrides(): void {
 
           // Use the first tween's color as the dim baseline — if no tweens,
           // fall back to computed style.
-          const dimBaseline = colorTweens.length > 0 ? String(colorTweens[0].vars.color) : "";
+          const dimBaseline = colorTweens[0] ? String(colorTweens[0].vars.color) : "";
 
           for (const tw of colorTweens) {
             const tweenColor = String(tw.vars.color);
diff --git a/packages/core/src/runtime/colorGrading.ts b/packages/core/src/runtime/colorGrading.ts
index 8ec35806b6..4fd00faf0b 100644
--- a/packages/core/src/runtime/colorGrading.ts
+++ b/packages/core/src/runtime/colorGrading.ts
@@ -542,7 +542,7 @@ function createProgram(
   return program;
 }
 
-function createTexture(gl: WebGLRenderingContext, filter = gl.LINEAR): WebGLTexture | null {
+function createTexture(gl: WebGLRenderingContext, filter: number = gl.LINEAR): WebGLTexture | null {
   const texture = gl.createTexture();
   if (!texture) return null;
   gl.bindTexture(gl.TEXTURE_2D, texture);
diff --git a/packages/core/src/runtime/compositionLoader.ts b/packages/core/src/runtime/compositionLoader.ts
index 3b6264dd52..c86236c236 100644
--- a/packages/core/src/runtime/compositionLoader.ts
+++ b/packages/core/src/runtime/compositionLoader.ts
@@ -366,14 +366,15 @@ async function mountCompositionContent(params: {
     details: Record<string, string | number | boolean | null | string[]>;
   }) => void;
 }): Promise<void> {
-  let innerRoot: Element | null = null;
+  let innerRoot: HTMLElement | null = null;
   if (params.authoredCompositionId) {
     const candidateRoots = Array.from(
-      params.sourceNode.querySelectorAll<Element>("[data-composition-id]"),
+      params.sourceNode.querySelectorAll<HTMLElement>("[data-composition-id]"),
     );
     innerRoot =
       candidateRoots.find(
         (candidate) =>
+          candidate instanceof HTMLElement &&
           candidate.getAttribute("data-composition-id") === params.authoredCompositionId,
       ) ?? null;
   }
diff --git a/packages/core/src/runtime/init.test.ts b/packages/core/src/runtime/init.test.ts
index 3f66788c1f..378a6a9bb4 100644
--- a/packages/core/src/runtime/init.test.ts
+++ b/packages/core/src/runtime/init.test.ts
@@ -12,11 +12,13 @@ function createMockTimeline(duration: number): RuntimeTimelineLike {
     pause: () => {
       state.paused = true;
     },
-    seek: (time: number) => {
-      state.time = time;
+    seek: (time?: number) => {
+      if (time !== undefined) state.time = time;
+      return state.time;
     },
-    totalTime: (time: number) => {
-      state.time = time;
+    totalTime: (time?: number) => {
+      if (time !== undefined) state.time = time;
+      return state.time;
     },
     time: () => state.time,
     duration: () => state.duration,
@@ -1756,6 +1758,26 @@ describe("initSandboxRuntimeModular", () => {
     expect(seekTimes.length).toBeGreaterThan(beforeResume);
   });
 
+  it("keeps a usable bound timeline when the registry entry is replaced", () => {
+    const raf = createManualRaf();
+    vi.spyOn(performance, "now").mockImplementation(() => raf.now());
+    window.requestAnimationFrame = raf.requestAnimationFrame as typeof window.requestAnimationFrame;
+    window.cancelAnimationFrame = raf.cancelAnimationFrame as typeof window.cancelAnimationFrame;
+
+    document.body.innerHTML = `
+      <div data-composition-id="root" data-start="0" data-duration="5" data-width="1920" data-height="1080"></div>
+    `;
+    const originalTimeline = createMockTimeline(5);
+    window.__timelines = { root: originalTimeline };
+    initSandboxRuntimeModular();
+
+    const replacementTimeline = createMockTimeline(8);
+    window.__timelines.root = replacementTimeline;
+    for (let frame = 0; frame < 60; frame += 1) raf.step(16);
+
+    expect(window.__player?.getDuration()).toBe(5);
+  });
+
   // applyClipLayout force-absolutizes authored root-level timed clips so they
   // stack as overlays. But in Studio/preview the runtime also stamps `data-start`
   // onto ID'd / GSAP-targeted *flow* children (a <header>/<footer> in a column)
diff --git a/packages/core/src/runtime/init.ts b/packages/core/src/runtime/init.ts
index 38f1dfc58a..acaf614e9e 100644
--- a/packages/core/src/runtime/init.ts
+++ b/packages/core/src/runtime/init.ts
@@ -44,6 +44,7 @@ import type {
 } from "./types";
 import type { PlayerAPI } from "../core.types";
 import { swallow } from "./diagnostics";
+import { shouldAttemptPeriodicTimelineBind } from "./timelineRebindPolicy";
 
 const AUTHORED_DURATION_ATTR = "data-hf-authored-duration";
 const AUTHORED_END_ATTR = "data-hf-authored-end";
@@ -186,7 +187,7 @@ export function initSandboxRuntimeModular(): void {
     } else {
       for (let i = 0; i < arr.length; i++) normalized[`tl-${i}`] = arr[i];
     }
-    (window as Record<string, unknown>).__timelines = normalized;
+    (window as unknown as Record<string, unknown>).__timelines = normalized;
   }
 
   // Agents sometimes omit data-start on the root composition element. The
@@ -295,7 +296,6 @@ export function initSandboxRuntimeModular(): void {
 
   const MIN_VALID_TIMELINE_DURATION_SECONDS = 1 / 60;
   const TIMELINE_FLOOR_COVERAGE_RATIO = 0.75;
-  const PLAY_REBIND_HOLD_SECONDS = 2;
   const METADATA_REBIND_MIN_DURATION_GAIN_SECONDS = 0.05;
   const METADATA_REBIND_DEBOUNCE_MS = 100;
   const MAX_DIAGNOSTIC_MESSAGE_LENGTH = 240;
@@ -776,7 +776,9 @@ export function initSandboxRuntimeModular(): void {
           !!entry[1] && typeof entry[1].play === "function" && typeof entry[1].pause === "function",
       );
       if (usable.length !== 1) return { timeline: null };
-      const [soleId, soleTimeline] = usable[0];
+      const sole = usable[0];
+      if (!sole) return { timeline: null };
+      const [soleId, soleTimeline] = sole;
       return {
         timeline: soleTimeline,
         selectedTimelineIds: [soleId],
@@ -1223,7 +1225,7 @@ export function initSandboxRuntimeModular(): void {
       // reapplyPositionEditsAfterSeek to un-bake it. Call the apply hook
       // directly here as well, since the wrapper may not be installed yet
       // during initial rebind (timing race on first load / soft reload).
-      const applyFn = (window as Record<string, unknown>).__hfStudioManualEditsApply;
+      const applyFn = (window as unknown as Record<string, unknown>).__hfStudioManualEditsApply;
       if (typeof applyFn === "function") applyFn();
 
       // SDK moveElement edits (data-hf-edit-base-x/y markers) render as a
@@ -1996,8 +1998,10 @@ export function initSandboxRuntimeModular(): void {
     // handler. Identity is stable as long as the inputs are stable (each
     // adapter is expected to return the same promise on repeat calls while
     // its work is in flight).
+    const firstPromise = promises[0];
+    if (!firstPromise) return true;
     const combined: PromiseLike<unknown> =
-      promises.length === 1 ? promises[0] : Promise.all(promises);
+      promises.length === 1 ? firstPromise : Promise.all(promises);
     if (combined !== trackedAdapterReadyPromise) {
       trackedAdapterReadyPromise = combined;
       trackedAdapterReadySettled = false;
@@ -2512,10 +2516,10 @@ export function initSandboxRuntimeModular(): void {
     }
 
     for (const child of children) {
-      if (!isObjectRecord(child) || !isObjectRecord(child.vars)) continue;
-      const hasCallback = GSAP_CALLBACK_NAMES.some(
-        (name) => typeof child.vars[name] === "function",
-      );
+      if (!isObjectRecord(child)) continue;
+      const vars = child.vars;
+      if (!isObjectRecord(vars)) continue;
+      const hasCallback = GSAP_CALLBACK_NAMES.some((name) => typeof vars[name] === "function");
       if (!hasCallback) continue;
 
       const totalDuration = readGsapDuration(child, "totalDuration");
@@ -2629,24 +2633,25 @@ export function initSandboxRuntimeModular(): void {
       transportTickCount += 1;
 
       // Slower operations: timeline binding (~every 60 frames / ~1s at 60fps)
-      if (transportTickCount % 60 === 0) {
-        const shouldHoldRebind =
-          clock.isPlaying() &&
-          state.capturedTimeline != null &&
-          clock.now() < PLAY_REBIND_HOLD_SECONDS;
-        if (!shouldHoldRebind) {
-          const prevTimeline = state.capturedTimeline;
-          if (bindRootTimelineIfAvailable()) {
-            if (state.capturedTimeline && !player._timeline) {
-              player._timeline = state.capturedTimeline;
-            }
-            if (state.capturedTimeline && state.capturedTimeline !== prevTimeline) {
-              state.capturedTimeline.pause();
-            }
-            const dur = getSafeTimelineDurationSeconds(state.capturedTimeline, 0);
-            if (dur > 0) clock.setDuration(dur);
-            postTimeline();
+      if (
+        shouldAttemptPeriodicTimelineBind({
+          tick: transportTickCount,
+          isPlaying: clock.isPlaying(),
+          hasCapturedTimeline: state.capturedTimeline != null,
+          currentTimeSeconds: clock.now(),
+        })
+      ) {
+        const prevTimeline = state.capturedTimeline;
+        if (bindRootTimelineIfAvailable()) {
+          if (state.capturedTimeline && !player._timeline) {
+            player._timeline = state.capturedTimeline;
           }
+          if (state.capturedTimeline && state.capturedTimeline !== prevTimeline) {
+            state.capturedTimeline.pause();
+          }
+          const dur = getSafeTimelineDurationSeconds(state.capturedTimeline, 0);
+          if (dur > 0) clock.setDuration(dur);
+          postTimeline();
         }
       }
       if (transportTickCount % 20 === 0) {
diff --git a/packages/core/src/runtime/mediaVolumeEnvelope.ts b/packages/core/src/runtime/mediaVolumeEnvelope.ts
index 00e0094d78..94856bb04a 100644
--- a/packages/core/src/runtime/mediaVolumeEnvelope.ts
+++ b/packages/core/src/runtime/mediaVolumeEnvelope.ts
@@ -1,3 +1,5 @@
+import type { RuntimeTimelineLike } from "./types";
+
 /**
  * Shared volume-automation utilities used by both the renderer (offline PCM
  * baking in audioVolumeEnvelope.ts) and the preview runtime (per-tick gain
@@ -125,10 +127,7 @@ export function probeElementVolumeKeyframes(
   return hasAutomation ? keyframes : null;
 }
 
-export interface RuntimeTimelineRef {
-  totalTime?: ((t?: number, suppressEvents?: boolean) => unknown) | undefined;
-  seek?: ((t?: number, suppressEvents?: boolean) => unknown) | undefined;
-}
+export type RuntimeTimelineRef = Partial<Pick<RuntimeTimelineLike, "totalTime" | "seek">>;
 
 /**
  * Probe a media element and, if volume automation is detected, store the
diff --git a/packages/core/src/runtime/picker.ts b/packages/core/src/runtime/picker.ts
index 52921fb82e..5d73102526 100644
--- a/packages/core/src/runtime/picker.ts
+++ b/packages/core/src/runtime/picker.ts
@@ -142,8 +142,7 @@ export function createPickerModule(deps: PickerModuleDeps): PickerModule {
     if (blocksPickerAtPoint(raw[0] ?? null)) return [];
     const dedupe: Record<string, true> = {};
     const candidates: Element[] = [];
-    for (let i = 0; i < raw.length; i += 1) {
-      const node = raw[i];
+    for (const [i, node] of raw.entries()) {
       if (!isPickableElement(node)) continue;
       const key = `${node.tagName}::${(node as HTMLElement).id || ""}::${i}`;
       if (dedupe[key]) continue;
@@ -157,8 +156,7 @@ export function createPickerModule(deps: PickerModuleDeps): PickerModule {
   function extractElementInfo(el: Element): RuntimePickerElementInfo {
     const rect = el.getBoundingClientRect();
     const dataAttributes: Record<string, string> = {};
-    for (let i = 0; i < el.attributes.length; i += 1) {
-      const attr = el.attributes[i];
+    for (const attr of Array.from(el.attributes)) {
       if (attr.name.startsWith("data-")) {
         dataAttributes[attr.name] = attr.value;
       }
diff --git a/packages/core/src/runtime/player.test.ts b/packages/core/src/runtime/player.test.ts
index 78c3dfcf08..f0b5fa7d3f 100644
--- a/packages/core/src/runtime/player.test.ts
+++ b/packages/core/src/runtime/player.test.ts
@@ -11,11 +11,13 @@ function createMockTimeline(opts?: { time?: number; duration?: number }): Runtim
     pause: vi.fn(() => {
       state.paused = true;
     }),
-    seek: vi.fn((t: number) => {
-      state.time = t;
+    seek: vi.fn((t?: number) => {
+      if (t !== undefined) state.time = t;
+      return state.time;
     }),
-    totalTime: vi.fn((t: number) => {
-      state.time = t;
+    totalTime: vi.fn((t?: number) => {
+      if (t !== undefined) state.time = t;
+      return state.time;
     }),
     time: vi.fn(() => state.time),
     duration: vi.fn(() => state.duration),
@@ -64,11 +66,13 @@ function createNestedTimelineHarness() {
       pause: vi.fn(() => {
         state.paused = true;
       }),
-      seek: vi.fn((t: number) => {
-        state.time = t;
+      seek: vi.fn((t?: number) => {
+        if (t !== undefined) state.time = t;
+        return state.time;
       }),
-      totalTime: vi.fn((t: number) => {
-        state.time = t;
+      totalTime: vi.fn((t?: number) => {
+        if (t !== undefined) state.time = t;
+        return state.time;
       }),
       time: vi.fn(() => state.time),
       duration: vi.fn(() => duration),
@@ -95,14 +99,16 @@ function createNestedTimelineHarness() {
     pause: vi.fn(() => {
       masterState.paused = true;
     }),
-    seek: vi.fn((t: number) => {
+    seek: vi.fn((t?: number) => {
+      if (t === undefined) return masterState.time;
       masterState.time = t;
       for (const child of children) {
         if (child.state.paused) continue;
         child.state.time = Math.max(0, Math.min(t - child.start, child.duration));
       }
     }),
-    totalTime: vi.fn((t: number) => {
+    totalTime: vi.fn((t?: number) => {
+      if (t === undefined) return masterState.time;
       masterState.time = t;
       for (const child of children) {
         if (child.state.paused) continue;
diff --git a/packages/core/src/runtime/timeline.ts b/packages/core/src/runtime/timeline.ts
index d7ea927f7c..0cd123fc0c 100644
--- a/packages/core/src/runtime/timeline.ts
+++ b/packages/core/src/runtime/timeline.ts
@@ -385,8 +385,7 @@ export function collectRuntimeTimelinePayload(params: {
     ),
   );
   let maxEnd = 0;
-  for (let i = 0; i < nodes.length; i += 1) {
-    const node = nodes[i];
+  for (const [i, node] of nodes.entries()) {
     if (node === root) continue;
     if (["SCRIPT", "STYLE", "LINK", "META", "TEMPLATE", "NOSCRIPT"].includes(node.tagName))
       continue;
diff --git a/packages/core/src/runtime/timelineRebindPolicy.test.ts b/packages/core/src/runtime/timelineRebindPolicy.test.ts
new file mode 100644
index 0000000000..2b54b9a63c
--- /dev/null
+++ b/packages/core/src/runtime/timelineRebindPolicy.test.ts
@@ -0,0 +1,57 @@
+import { describe, expect, it } from "vitest";
+import {
+  PLAY_REBIND_HOLD_SECONDS,
+  TIMELINE_REBIND_INTERVAL_FRAMES,
+  shouldAttemptPeriodicTimelineBind,
+} from "./timelineRebindPolicy";
+
+describe("shouldAttemptPeriodicTimelineBind", () => {
+  it("only checks for replacements at the periodic boundary", () => {
+    expect(
+      shouldAttemptPeriodicTimelineBind({
+        tick: TIMELINE_REBIND_INTERVAL_FRAMES - 1,
+        isPlaying: false,
+        hasCapturedTimeline: true,
+        currentTimeSeconds: 0,
+      }),
+    ).toBe(false);
+    expect(
+      shouldAttemptPeriodicTimelineBind({
+        tick: TIMELINE_REBIND_INTERVAL_FRAMES,
+        isPlaying: false,
+        hasCapturedTimeline: true,
+        currentTimeSeconds: 0,
+      }),
+    ).toBe(true);
+  });
+
+  it("holds a captured timeline during the first playback seconds", () => {
+    expect(
+      shouldAttemptPeriodicTimelineBind({
+        tick: TIMELINE_REBIND_INTERVAL_FRAMES,
+        isPlaying: true,
+        hasCapturedTimeline: true,
+        currentTimeSeconds: PLAY_REBIND_HOLD_SECONDS - 0.001,
+      }),
+    ).toBe(false);
+    expect(
+      shouldAttemptPeriodicTimelineBind({
+        tick: TIMELINE_REBIND_INTERVAL_FRAMES,
+        isPlaying: true,
+        hasCapturedTimeline: true,
+        currentTimeSeconds: PLAY_REBIND_HOLD_SECONDS,
+      }),
+    ).toBe(true);
+  });
+
+  it("does not hold when no timeline has bound yet", () => {
+    expect(
+      shouldAttemptPeriodicTimelineBind({
+        tick: TIMELINE_REBIND_INTERVAL_FRAMES,
+        isPlaying: true,
+        hasCapturedTimeline: false,
+        currentTimeSeconds: 0,
+      }),
+    ).toBe(true);
+  });
+});
diff --git a/packages/core/src/runtime/timelineRebindPolicy.ts b/packages/core/src/runtime/timelineRebindPolicy.ts
new file mode 100644
index 0000000000..c3380bb224
--- /dev/null
+++ b/packages/core/src/runtime/timelineRebindPolicy.ts
@@ -0,0 +1,22 @@
+export const TIMELINE_REBIND_INTERVAL_FRAMES = 60;
+export const PLAY_REBIND_HOLD_SECONDS = 2;
+
+export function shouldAttemptPeriodicTimelineBind(input: {
+  tick: number;
+  isPlaying: boolean;
+  hasCapturedTimeline: boolean;
+  currentTimeSeconds: number;
+}): boolean {
+  if (
+    !Number.isInteger(input.tick) ||
+    input.tick <= 0 ||
+    input.tick % TIMELINE_REBIND_INTERVAL_FRAMES !== 0
+  ) {
+    return false;
+  }
+  return !(
+    input.isPlaying &&
+    input.hasCapturedTimeline &&
+    input.currentTimeSeconds < PLAY_REBIND_HOLD_SECONDS
+  );
+}
diff --git a/packages/core/src/runtime/types.ts b/packages/core/src/runtime/types.ts
index b2b6648149..4f54a1a2f7 100644
--- a/packages/core/src/runtime/types.ts
+++ b/packages/core/src/runtime/types.ts
@@ -228,17 +228,32 @@ export type RuntimeSeekOptions = {
   suppressEvents?: boolean;
 };
 
+export type RuntimeTimelineChildLike = {
+  targets?: () => unknown[];
+  vars?: unknown;
+  startTime?: () => number;
+  duration?: () => number;
+  parent?: RuntimeTimelineChildLike;
+};
+
 export type RuntimeTimelineLike = {
   play: () => void;
   pause: () => void;
-  seek: (timeSeconds: number, suppressEvents?: boolean) => void;
-  totalTime?: (timeSeconds: number, suppressEvents?: boolean) => void;
+  seek: (timeSeconds?: number, suppressEvents?: boolean) => unknown;
+  totalTime?: (timeSeconds?: number, suppressEvents?: boolean) => unknown;
+  progress?: (value?: number, suppressEvents?: boolean) => unknown;
   time: () => number;
   duration: () => number;
   add: (timeline: RuntimeTimelineLike, startAtSeconds: number) => void;
   paused: (paused?: boolean) => void;
   timeScale?: (rate: number) => void;
   set: (target: RuntimeGsapSetTarget, vars: RuntimeGsapSetVars, atSeconds?: number) => void;
+  getChildren?: (
+    nested?: boolean,
+    tweens?: boolean,
+    timelines?: boolean,
+    ignoreBeforeTime?: number,
+  ) => RuntimeTimelineChildLike[];
 };
 
 export type RuntimeDeterministicAdapter = {
diff --git a/packages/core/tsconfig.runtime.json b/packages/core/tsconfig.runtime.json
new file mode 100644
index 0000000000..2b7e0e8b0c
--- /dev/null
+++ b/packages/core/tsconfig.runtime.json
@@ -0,0 +1,9 @@
+{
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    "noEmit": true
+  },
+  "files": [],
+  "include": ["src/runtime/**/*.ts"],
+  "exclude": ["node_modules", "dist", "**/*.test.ts"]
+}
diff --git a/packages/core/vitest.config.ts b/packages/core/vitest.config.ts
index 58fade59e5..f8e447f6ee 100644
--- a/packages/core/vitest.config.ts
+++ b/packages/core/vitest.config.ts
@@ -12,7 +12,6 @@ export default defineConfig({
         "src/runtime/types.ts",
         "src/runtime/window.d.ts",
         "src/runtime/entry.ts",
-        "src/runtime/init.ts",
         "src/runtime/README.md",
       ],
       thresholds: {
diff --git a/packages/producer/scripts/test-classification.mjs b/packages/producer/scripts/test-classification.mjs
index aa6fc43365..ec07e0330c 100644
--- a/packages/producer/scripts/test-classification.mjs
+++ b/packages/producer/scripts/test-classification.mjs
@@ -9,6 +9,7 @@ export const PRODUCER_ROOT = resolve(dirname(fileURLToPath(import.meta.url)), ".
 // or local sockets. Keep the list explicit so a filename-only rename does not
 // make Git/fallow re-audit thousands of unchanged test lines as new code.
 const INTEGRATION_TEST_FILES = new Set([
+  "src/services/coreRuntimeBrowser.test.ts",
   "src/services/deterministicFonts-systemCapture.test.ts",
   "src/services/distributed/assemble.test.ts",
   "src/services/distributed/chunkBoundary.test.ts",
diff --git a/packages/producer/src/services/coreRuntimeBrowser.test.ts b/packages/producer/src/services/coreRuntimeBrowser.test.ts
new file mode 100644
index 0000000000..9ca5d5053c
--- /dev/null
+++ b/packages/producer/src/services/coreRuntimeBrowser.test.ts
@@ -0,0 +1,93 @@
+import { afterAll, beforeAll, describe, expect, it } from "vitest";
+import { readFileSync } from "node:fs";
+import { resolve } from "node:path";
+import puppeteer, { type Browser, type Page } from "puppeteer";
+
+const RUNTIME_PATH = resolve(import.meta.dirname, "../../../core/dist/hyperframe.runtime.iife.js");
+
+describe("core runtime browser contract", () => {
+  let browser: Browser;
+  let page: Page;
+
+  beforeAll(async () => {
+    browser = await puppeteer.launch({
+      headless: true,
+      args: ["--no-sandbox", "--disable-setuid-sandbox"],
+    });
+    page = await browser.newPage();
+    await page.setContent(`<!doctype html>
+      <style>
+        @keyframes slide { from { transform: translateX(0); } to { transform: translateX(100px); } }
+        #box { animation: slide 2s linear both; }
+      </style>
+      <div data-composition-id="root" data-start="0" data-duration="2" data-width="320" data-height="180">
+        <div id="box"></div>
+      </div>`);
+    await page.addScriptTag({ content: readFileSync(RUNTIME_PATH, "utf8") });
+    await page.waitForFunction(
+      () =>
+        (window as unknown as { __playerReady?: boolean }).__playerReady === true &&
+        (window as unknown as { __renderReady?: boolean }).__renderReady === true,
+    );
+  }, 30_000);
+
+  afterAll(async () => {
+    await browser?.close();
+  });
+
+  it("initializes the public player contract and seeks the CSS adapter", async () => {
+    const result = await page.evaluate(() => {
+      const runtimeWindow = window as unknown as {
+        __player?: {
+          play?: () => void;
+          pause?: () => void;
+          renderSeek?: (timeSeconds: number) => void;
+          getDuration?: () => number;
+          isPlaying?: () => boolean;
+        };
+      };
+      const player = runtimeWindow.__player;
+      player?.renderSeek?.(1);
+      const animation = document.getElementById("box")?.getAnimations()[0];
+      return {
+        hasPlay: typeof player?.play === "function",
+        hasPause: typeof player?.pause === "function",
+        hasRenderSeek: typeof player?.renderSeek === "function",
+        duration: player?.getDuration?.(),
+        animationTime: Number(animation?.currentTime),
+      };
+    });
+
+    expect(result).toEqual({
+      hasPlay: true,
+      hasPause: true,
+      hasRenderSeek: true,
+      duration: 2,
+      animationTime: 1000,
+    });
+  });
+
+  it("removes the control bridge during teardown", async () => {
+    const result = await page.evaluate(async () => {
+      const runtimeWindow = window as unknown as {
+        __hfRuntimeTeardown?: (() => void) | null;
+        __player?: { isPlaying?: () => boolean };
+      };
+      const hadTeardown = typeof runtimeWindow.__hfRuntimeTeardown === "function";
+      runtimeWindow.__hfRuntimeTeardown?.();
+      window.dispatchEvent(
+        new MessageEvent("message", {
+          data: { source: "hf-parent", type: "control", action: "play" },
+        }),
+      );
+      await new Promise((resolveFrame) => requestAnimationFrame(() => resolveFrame(undefined)));
+      return {
+        hadTeardown,
+        teardownCleared: runtimeWindow.__hfRuntimeTeardown === null,
+        isPlaying: runtimeWindow.__player?.isPlaying?.(),
+      };
+    });
+
+    expect(result).toEqual({ hadTeardown: true, teardownCleared: true, isPlaying: false });
+  });
+});