rendercheck

rendercheck

The worst bugs in generated media don't throw.

"the audio often cuts off the final sentence […] though the API returns

success without error signals"

— a developer on the OpenAI forum,

April 2026, describing production output

If you generate speech or video with a model — TTS, voice agents, podcasts, avatars, AI video — your tests catch the exception that never happens. They do not catch the narration that reads at 300 words per minute, the voice track sitting 18 dB below the footage it's cut against, the clip that rendered at 42% length and got cached as a success, the captions that describe the audio three seconds before it happens, or the file whose audio track is missing entirely.

The 2026 state of the art for catching these is *a person listening to the output*. That works, and it costs more than everything else in your pipeline combined.

rendercheck makes them throw.

from rendercheck import assert_pace, assert_loudness, looks_ok

assert_pace("episode-12.mp3", "episode-12.vtt") assert_loudness("episode-12.mp3") looks_ok("slide-14.png", ["the title fits on one line"])

Plain assert functions. No framework, no runner, no service. They raise AssertionError, so they already work in pytest, in CI, or in a five-line script. Eighteen of the nineteen checks have no dependencies and make no network calls — if you have ffmpeg, you're ready.

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Quickstart

You need ffmpeg on your PATH (brew install ffmpeg, apt-get install ffmpeg, or winget install ffmpeg). Then:

pip install rendercheck

rendercheck demo

Or drop a file into the [playground](https://rogermsc.github.io/rendercheck/playground/) — same checks, running on ffmpeg compiled to WebAssembly, nothing uploaded.

demo synthesises ten defective files and runs the real checks against them, so you can see it fire without owning a broken render. Verbatim, first two of eight:

Narration too fast

A voice picked to match a presenter's face read English at machine-gun speed. Valid audio, correct timing, perfectly in sync.

$ rendercheck check machine-gun.wav --script narration.vtt

FAIL pace narration pace 300 WPM exceeds 245 (300 words in 60.0s) -- this reads as machine-gun delivery and listeners cannot follow it: machine-gun.wav PASS loudness -16.1 LUFS PASS dead air 0.0 s silence PASS truncation 8.9 dB of fall-off at the end PASS clipping 0 samples at 0 dBFS

Levels that don't match Synthesised narration landed 18 dB under the footage it was cut against. Nobody noticed until viewers rode the volume knob.

$ rendercheck check too-quiet.wav

SKIP pace no --script given FAIL loudness -34.2 LUFS is 18.2 dB quieter than the -16 target -- it will sound inaudible next to correctly-levelled audio cut alongside it: too-quiet.wav PASS dead air 0.0 s silence PASS truncation 8.8 dB of fall-off at the end PASS clipping 0 samples at 0 dBFS

…and one of the two added in 0.3.0:

Captions against the wrong clock

A concatenation added three seconds of pre-roll after the captions were written. Both files are perfectly valid on their own.

$ rendercheck check late-captions.wav

SKIP pace no --script given PASS loudness -16.0 LUFS PASS dead air 0.0 s silence PASS truncation 74.3 dB of fall-off at the end PASS clipping 0 samples at 0 dBFS FAIL captions late-captions.vtt runs 3.0s late against late-captions.wav, past the 0.75s limit -- every line arrives at the wrong moment, and both files are individually valid so nothing else catches it

Then point it at your own output:

rendercheck check episode-12.mp3 --script episode-12.vtt --preset podcast

Exit code is 1 if anything failed — or if nothing could be measured, because a run that looked at nothing is not a clean one. A path you typo'd exits 2. --json gives you the same report for pipelines in any language, and --strict rejects partial runs too.

Where is this file going?

"How loud should this be?" has no single answer — it depends entirely on where the file ends up, and every platform publishes a different number. --preset turns that table into something a build can enforce:

$ rendercheck presets

preset target tol peak source youtube -14L 1.0dB -1.0TP YouTube normalises playback to -14 LUFS spotify -14L 1.0dB -1.0TP Spotify, including podcasts, at -14 LUFS tiktok -14L 1.5dB -1.0TP TikTok and Instagram, measured rather than published podcast -16L 1.0dB -1.0TP AES71 / Apple Podcasts: -16 LUFS stereo, -19 mono apple -16L 1.0dB -1.0TP Apple Music Sound Check, -16 LUFS web -16L 2.0dB -- spoken-word web video -- rendercheck's own defaults ebu -23L 1.0dB -1.0TP EBU R128, European broadcast atsc -24L 2.0dB -2.0TP ATSC A/85, North American broadcast netflix -27L 2.0dB -2.0TP Netflix delivery, dialog-gated

None of those numbers are ours. The contribution is that --preset ebu is a decision a reviewer can read, where --target-lufs -23 is a magic number the next person will not dare touch. A preset that states a ceiling also switches on the true-peak check, which catches a master measuring clean locally and distorting after upload. web exists only to *name* the built-in defaults, so it states none and behaves exactly like passing no preset at all.

Project-wide settings go in rendercheck.toml (or [tool.rendercheck] in pyproject.toml) so a CI step is not eight flags on one line:

preset = "podcast"

max_silence = 5.0

Flags you type still beat the file, and the file beats the built-in defaults.

In pytest they're just asserts — no plugin, no fixtures:

@pytest.mark.parametrize("episode", EPISODES)

def test_episode_is_shippable(episode): assert_pace(episode.audio, episode.vtt) assert_loudness(episode.audio) assert_no_dead_air(episode.audio)

"Isn't this forty lines of pyloudnorm?"

For one of the nineteen checks, roughly yes. None of these measurements are novel, and it would be dishonest to imply otherwise:

Most of those hand a number to a researcher. The two that are already gates gate a different thing: MediaConch checks that a file conforms to a container policy, which is a preservation question, not a perceptual one — a file can pass every MediaConch rule and still be narrated at 300 WPM. ffsubsync will happily realign captions that were never wrong, because it has no opinion about whether they needed it.

What is actually missing, and what this is:

• A threshold that came from a defect, not from a paper. 245 WPM because a real voice narrated at 280 and shipped. −16 LUFS because narration landed at −34 against footage at −13.
• A message that says what a person would notice. "−34.0 LUFS" is a reading. "18.0 dB quieter than the −16 target — it will sound inaudible next to correctly-levelled audio cut alongside it" is a bug report.
• Fail-open on infrastructure, fail-closed on a defect, so it can sit in CI without becoming the thing that breaks the build for its own reasons.
• Exit codes and one command over a directory, rather than a notebook.

Against the LLM-eval tools the difference is structural rather than a matter of coverage. promptfoo, DeepEval and RAGAS are excellent and none of them can do this: their test case is a string. There is no assertion to add, because there is nowhere to put the file. Use them for the script; use this for what the script turned into.

And if you already run broadcast QC — Interra BATON, Telestream Vidchecker, QCTools — you have had most of this for twenty years. It just isn't in your git hooks.

In your pipeline

GitHub Actions — installs ffmpeg and fails the build on a defect:

- uses: rogermsc/rendercheck@v0

with: files: out/ preset: podcast strict: "true"

Node, Remotion, anything that renders in a build step:

npx rendercheck check out/

Docker, if you would rather not have a Python toolchain at all — ffmpeg is already in the image:

docker run --rm -v "$PWD:/work" ghcr.io/rogermsc/rendercheck check /work/out.mp4

Coding agents, via MCP. An agent that just wrote a render pipeline and ran it has no way to tell whether the file that came back is any good; the other media MCP servers cut and transcode, which hands it *more* media rather than an answer:

claude mcp add rendercheck -- rendercheck mcp

It is listed in the MCP registry as io.github.rogermsc/rendercheck, so clients that read the registry can install it without being told where it lives. No key is needed — every check except looks ok is deterministic.

check_media returns one verdict per check with the measured value, so the model can act on "−34 LUFS, 18 dB under target" rather than on a file it cannot hear.

promptfoo — its assertions are all string-shaped, so an eval can confirm the narration script and tell you nothing about the audio. examples/promptfoo/ closes that half in thirty lines:

assert:

• type: python value: file://rendercheck_assert.py:get_assert

Anything else — --json on stdout, one object per file, plus exit codes.

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Six checks, six incidents

Each default is a threshold set by a defect that actually shipped, not a number chosen for symmetry. All output below is verbatim. Every threshold is an argument — see the reference for tuning.

Narration too fast. A voice picked to match a presenter's *face* narrated English at ~280 WPM. The audio was valid, correctly timed, perfectly in sync. It just sounded like a machine gun.

assert_pace("episode-12.mp3", "episode-12.vtt", max_wpm=245)

narration pace 300 WPM exceeds 245 (300 words in 60.0s) -- this reads as

machine-gun delivery and listeners cannot follow it: episode-12.mp3

Levels that don't match. Synthesised narration landed at −34 LUFS and was concatenated with footage at