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+# Tier 4 — escape-hatch
+
+## What it tests
+
+T4 is the **lint-only** tier. There are no renders to diff — the skill is
+graded on whether it correctly _refuses_ to translate each case (and
+recommends the runtime interop pattern from PR #214 instead) or, where
+appropriate, translates after dropping warning-level decorations.
+
+Each `cases/*.tsx` file is a minimal Remotion composition that
+demonstrates one specific pattern. The skill should:
+
+1. Run `scripts/lint_source.py` over the source.
+2. Compare the JSON output to `expected.json` for that case.
+3. Take the documented `skill_action`:
+   - `refuse_translation_recommend_interop` — print the rationale + link to
+     the PR #214 interop guide; do not produce HF output.
+   - `drop_lambda_code_translate_remainder_if_clean` — drop the
+     `@remotion/lambda` code with a note; translate the rest only if no
+     other blockers are present.
+   - `translate_after_dropping_wrappers` — translate normally; drop
+     `useCallback` / `useMemo` / `delayRender` wrappers.
+   - `inline_hook_body_if_pure` — inline the custom hook's body if it's a
+     pure derivation of `useCurrentFrame`; otherwise bow out.
+
+## Cases
+
+| #   | File                       | Expected blocker              | Notes                                       |
+| --- | -------------------------- | ----------------------------- | ------------------------------------------- |
+| 01  | `01-use-state.tsx`         | `r2hf/use-state`              | useState driving animation                  |
+| 02  | `02-use-effect-deps.tsx`   | `r2hf/use-effect-deps`        | useEffect with non-empty deps + side effect |
+| 03  | `03-async-metadata.tsx`    | `r2hf/async-metadata`         | calculateMetadata returns a Promise         |
+| 04  | `04-third-party-react.tsx` | `r2hf/third-party-react-ui`   | imports `@mui/material`                     |
+| 05  | `05-lambda-config.tsx`     | `r2hf/lambda-import`          | imports `@remotion/lambda`                  |
+| 06  | `06-warnings-only.tsx`     | (warnings only)               | delayRender / useCallback / useMemo         |
+| 07  | `07-custom-hook.tsx`       | (warnings only)               | locally-defined `useFadeIn`                 |
+| 08  | `08-mixed.tsx`             | useState + useEffect + chakra | aggregate-findings test                     |
+
+## Validation
+
+```bash
+./validate.sh
+```
+
+The script runs `lint_source.py` against each case and asserts:
+
+- Each expected blocker rule fires with severity `blocker`.
+- Each expected warning rule fires with severity `warning` (or stronger).
+- `lint_source.py`'s exit code is 1 when blockers are expected, 0 otherwise.
+
+T4 passes when every case matches its expected output. No renders involved.
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+// T4 case 01 — useState drives animation.
+//
+// Should be detected by lint_source.py as blocker r2hf/use-state.
+// The skill should refuse to translate and recommend the runtime interop
+// pattern from PR #214.
+//
+// Why this is a blocker: useState is React's component-local mutable state.
+// HF's seek-driven model produces deterministic frames from a single time
+// value — there's no per-frame React render cycle to update state on.
+
+import React, { useState } from "react";
+import { AbsoluteFill, useCurrentFrame } from "remotion";
+
+export const StateDriven: React.FC = () => {
+  const frame = useCurrentFrame();
+  const [hue, setHue] = useState(0);
+
+  // Even if this looks innocuous, the setHue call breaks determinism: HF
+  // can't reproduce React state mutations across seeks.
+  if (frame % 30 === 0 && hue < 360) {
+    setHue((h) => h + 30);
+  }
+
+  return (
+    <AbsoluteFill style={{ background: `hsl(${hue}, 80%, 50%)` }}>
+      <div>frame {frame}</div>
+    </AbsoluteFill>
+  );
+};
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+// T4 case 02 — useEffect with non-empty deps performs side effects per render.
+//
+// Should be detected by lint_source.py as blocker r2hf/use-effect-deps.
+// The skill should refuse to translate.
+//
+// Why this is a blocker: side effects (network, DOM mutation outside the
+// rendered tree, timers) don't translate to a seek-driven model. HF assumes
+// the page is fully rendered and pure between seeks.
+
+import React, { useEffect, useRef } from "react";
+import { AbsoluteFill, useCurrentFrame } from "remotion";
+
+export const SideEffectDriven: React.FC = () => {
+  const frame = useCurrentFrame();
+  const canvasRef = useRef<HTMLCanvasElement>(null);
+
+  useEffect(() => {
+    const canvas = canvasRef.current;
+    if (!canvas) return;
+    const ctx = canvas.getContext("2d");
+    ctx?.fillRect(frame, frame, 10, 10);
+  }, [frame]);
+
+  return (
+    <AbsoluteFill>
+      <canvas ref={canvasRef} width={1280} height={720} />
+    </AbsoluteFill>
+  );
+};
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+// T4 case 03 — calculateMetadata returns a Promise.
+//
+// Should be detected by lint_source.py as blocker r2hf/async-metadata.
+// The skill should refuse to translate.
+//
+// Why this is a blocker: HF needs the composition's duration, dimensions,
+// and props known up-front to produce HTML and seed the timeline. Async
+// metadata fetched from a server at render time has no equivalent in HF —
+// the metadata would need to be resolved at build time before the HTML is
+// authored.
+
+import React from "react";
+import { AbsoluteFill, useCurrentFrame } from "remotion";
+
+interface Props {
+  text: string;
+}
+
+export const AsyncMetadataDriven: React.FC<Props> = ({ text }) => {
+  const frame = useCurrentFrame();
+  return (
+    <AbsoluteFill>
+      <div>
+        {text} · frame {frame}
+      </div>
+    </AbsoluteFill>
+  );
+};
+
+export const calculateMetadata = async ({ props }: { props: Props }) => {
+  const response = await fetch(
+    `https://api.example.com/duration?text=${encodeURIComponent(props.text)}`,
+  );
+  const { durationInFrames } = await response.json();
+  return {
+    durationInFrames,
+    fps: 30,
+  };
+};
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+// T4 case 04 — Imports from a third-party React UI library.
+//
+// Should be detected by lint_source.py as blocker r2hf/third-party-react-ui.
+// The skill should refuse to translate.
+//
+// Why this is a blocker: a Material-UI Button (or any React UI library
+// component) is a React-only abstraction with internal hooks, refs, and
+// theme provider context. Translating it to HTML+CSS would require
+// re-implementing the design system, which is out of scope for a video
+// translation skill. Use the runtime interop pattern from PR #214 to keep
+// these components rendering through Remotion's React tree.
+
+import React from "react";
+import { Button } from "@mui/material";
+import { AbsoluteFill, useCurrentFrame, interpolate } from "remotion";
+
+export const MuiDriven: React.FC = () => {
+  const frame = useCurrentFrame();
+  const opacity = interpolate(frame, [0, 30], [0, 1], { extrapolateRight: "clamp" });
+
+  return (
+    <AbsoluteFill style={{ alignItems: "center", justifyContent: "center" }}>
+      <div style={{ opacity }}>
+        <Button variant="contained" color="primary">
+          Click me · frame {frame}
+        </Button>
+      </div>
+    </AbsoluteFill>
+  );
+};
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+// T4 case 05 — Imports @remotion/lambda for distributed rendering config.
+//
+// Should be detected by lint_source.py as blocker r2hf/lambda-import.
+// The skill should drop the Lambda code with a note (HF runs single-machine
+// today) and translate the rest of the composition only if no other blockers
+// are present.
+//
+// Why this is a blocker: @remotion/lambda is Remotion's AWS-Lambda-based
+// distributed renderer. HF doesn't have an equivalent — render is
+// single-machine. The skill cannot translate this configuration.
+
+import React from "react";
+import { renderMediaOnLambda } from "@remotion/lambda";
+import { AbsoluteFill, useCurrentFrame, interpolate } from "remotion";
+
+export const LambdaConfigured: React.FC = () => {
+  const frame = useCurrentFrame();
+  const opacity = interpolate(frame, [0, 30], [0, 1]);
+  return (
+    <AbsoluteFill style={{ opacity }}>
+      <div>frame {frame}</div>
+    </AbsoluteFill>
+  );
+};
+
+// Rendered at scale via Lambda — no HF equivalent.
+export async function renderViaLambda() {
+  return renderMediaOnLambda({
+    region: "us-east-1",
+    functionName: "remotion-render",
+    composition: "LambdaConfigured",
+    serveUrl: "https://example.com/bundle",
+    inputProps: {},
+    codec: "h264",
+  });
+}
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+// T4 case 06 — Patterns that warn but don't block.
+//
+// Should be detected by lint_source.py with:
+//   - r2hf/delay-render (warning) — drop the call; HF handles asset readiness
+//   - r2hf/use-callback (warning) — decorative, drop the wrapper
+//   - r2hf/use-memo    (warning) — decorative, drop the wrapper
+//
+// 0 blockers expected — the skill should still translate this composition
+// after dropping the wrappers. delayRender is paired with continueRender via
+// an empty-deps useEffect (mount-once side effect), which doesn't trip the
+// use-effect-deps blocker.
+
+import React, { useCallback, useMemo } from "react";
+import { AbsoluteFill, delayRender, continueRender, useCurrentFrame, interpolate } from "remotion";
+
+const handle = delayRender();
+// Resolve the handle once at module load — no per-frame side effects.
+queueMicrotask(() => continueRender(handle));
+
+export const WarningsOnly: React.FC = () => {
+  const frame = useCurrentFrame();
+
+  // useCallback / useMemo — decorative for render-perf in React, no equivalent
+  // needed in the seek-driven HF model.
+  const opacity = useMemo(
+    () => interpolate(frame, [0, 30], [0, 1], { extrapolateRight: "clamp" }),
+    [frame],
+  );
+  const onMount = useCallback(() => {}, []);
+
+  return (
+    <AbsoluteFill style={{ opacity }} onClick={onMount}>
+      <div>frame {frame}</div>
+    </AbsoluteFill>
+  );
+};
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+// T4 case 07 — Locally-defined custom hook.
+//
+// Should be detected by lint_source.py as warning r2hf/custom-hook.
+// 0 blockers expected — the skill can attempt translation if the hook body
+// is pure (derives from props/frame alone).
+//
+// Why this is a warning: custom hooks vary widely in what they do. Some are
+// pure derivations of useCurrentFrame (translatable — inline the body); some
+// wrap useState/useEffect (blocker — but those will be caught by the other
+// rules independently). The warning prompts the agent to inspect the body.
+
+import React from "react";
+import { AbsoluteFill, useCurrentFrame, interpolate } from "remotion";
+
+// Custom hook — pure derivation from frame, no state. Translates fine.
+function useFadeIn(durationInFrames: number) {
+  const frame = useCurrentFrame();
+  return interpolate(frame, [0, durationInFrames], [0, 1], { extrapolateRight: "clamp" });
+}
+
+export const CustomHookDriven: React.FC = () => {
+  const opacity = useFadeIn(30);
+  return (
+    <AbsoluteFill style={{ opacity }}>
+      <div>fading in</div>
+    </AbsoluteFill>
+  );
+};
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+// T4 case 08 — Multiple blockers + multiple warnings in one file.
+//
+// Should report:
+//   blockers: r2hf/use-state, r2hf/use-effect-deps, r2hf/third-party-react-ui
+//   warnings: r2hf/use-callback (also r2hf/delay-render via the import chain
+//             would only fire if delayRender is actually called)
+//
+// Tests that the linter aggregates findings correctly and does not stop at
+// the first blocker.
+
+import React, { useState, useEffect, useCallback } from "react";
+import { AbsoluteFill, useCurrentFrame } from "remotion";
+import { Card } from "@chakra-ui/react";
+
+interface Item {
+  id: string;
+  label: string;
+}
+
+export const MixedBlockers: React.FC = () => {
+  const frame = useCurrentFrame();
+  const [items, setItems] = useState<Item[]>([]);
+
+  useEffect(() => {
+    fetch("/api/items")
+      .then((r) => r.json())
+      .then(setItems);
+  }, [frame]);
+
+  const onClick = useCallback(() => {
+    setItems((prev) => [...prev, { id: String(prev.length), label: "new" }]);
+  }, []);
+
+  return (
+    <AbsoluteFill onClick={onClick}>
+      {items.map((item) => (
+        <Card key={item.id}>{item.label}</Card>
+      ))}
+    </AbsoluteFill>
+  );
+};