ai-lens

Evals for AI apps that make text, images, video or audio. Add one decorator, say in plain English what you care about, and ai-lens tells you whether every change made your outputs better or worse, why, and what it costs.

$ lens inspect

are my videos getting better?

Latest change
  WORSE  a1f3c2d → a1f3c2d +edit 9e04b1

    The faster model breaks the motion: the cat's legs melt into the board mid-jump, and
    the fur smears whenever it moves. The old version won 9 of 12 side-by-side comparisons.

    Scores
    ↓ c3  Is the motion smooth, with no warping?  0.81 → 0.48  -0.33 ±0.08
           "Frames 2 and 3 show the cat's legs melting into the board."
    ~ c4  Does the cat stay in frame?             0.70 → 0.77  +0.07 ±0.12  could be noise
      rubric: Sharp subject -0.40

    side by side  new better 1 · old better 9 · tie 2
      "The old clip keeps the fur sharp; the new one smears it in motion."

    changed  uncommitted edits to prompts/motion.py, model gen-v3 → gen-v3-fast
    cost/run $0.0310 → $0.0170
    12 matched inputs

Versions
  version               runs  pass  c3     c4    cost/run  tokens/run  time
  a1f3c2d               12    100%  0.81   0.70  $0.0310   3.2k        41.0s
  a1f3c2d +edit 9e04b1  12    100%  0.48↓  0.77  $0.0170   3.2k        28.5s

Why ai-lens

• One decorator. @lens.trace records inputs, prompt, input assets, output, latency, errors, git commit, tokens and cost for every run and step.
• Any output. Text, JSON, images, video (judged from sampled frames) and audio.
• Plain-English goals. lens track "are my videos getting better?" writes the eval plan for you.
• Your best results become the bar. Point ai-lens at examples you love. It learns a rubric from them and grades every new output against them.
• Verdicts you can trust. Versions are compared on the same inputs, with 95% confidence intervals. Old and new outputs are compared side by side, in both orders, so position bias can't decide. When there's too little data, it says so.
• It names the cause. Each regression is tied to the commit, model, parameter, prompt or asset that changed.
• It finds token waste. The most expensive calls, repeated identical calls, and cost per run over time.
• It suggests fixes. lens suggest reads your report, diff and code, and proposes changes with file and line numbers.
• Your model, your keys. ai-lens ships no model and never sees an API key. All judging runs through a function you provide.

Install

pip install ailens-evals

You need Python 3.10+. For image, video and audio outputs, also install ffmpeg (brew install ffmpeg or apt install ffmpeg). The package itself has no dependencies.

Set up with a coding agent

Paste this into Claude Code, Cursor, Copilot or Codex, inside your project:

Set up ai-lens in this project by following https://raw.githubusercontent.com/techwarq/ai-lens/main/docs/setup-with-ai.md

The agent installs the package, traces your AI feature, wires in your own model, and checks that it works. Or follow the steps below yourself.

Quickstart

1. Give ai-lens your model

ai-lens never calls a model on its own. Write one function in your project that takes a list of parts and returns the model's reply. Parts are text strings and JPEG image bytes, in order. Use a vision model if you generate images or video.

# lens_model.py
import base64
import anthropic

client = anthropic.Anthropic()

def judge(parts):
    content = [
        {"type": "image", "source": {"type": "base64", "media_type": "image/jpeg", "data": base64.b64encode(part).decode()}}
        if isinstance(part, bytes) else {"type": "text", "text": part}
        for part in parts
    ]
    return client.messages.create(model="claude-opus-5-5", max_tokens=16000, messages=[{"role": "user", "content": content}])

# OpenAI
from openai import OpenAI
client = OpenAI()

def judge(parts):
    content = [
        {"type": "image_url", "image_url": {"url": "data:image/jpeg;base64," + base64.b64encode(part).decode()}}
        if isinstance(part, bytes) else {"type": "text", "text": part}
        for part in parts
    ]
    return client.chat.completions.create(model="gpt-4o", messages=[{"role": "user", "content": content}])

# Anything else: return the reply as a string
def judge(parts):
    return my_model.generate(parts)

Return a plain string, or return the Claude, OpenAI or Gemini response object as is. For Claude responses, and other models you've priced with lens.configure(prices=...), the cost is counted as eval spend.

2. Trace your app

import ai_lens as lens

lens.configure(model="lens_model:judge")

@lens.trace
def make_video(prompt: str, start_frame: str, seed: int = 42) -> str:
    script = write_script(prompt)
    lens.log(params={"seed": seed, "steps": 50}, model="gen-v3")
    return render(script, start_frame)   # e.g. "out/video.mp4"

@lens.step
def write_script(prompt):
    return client.messages.create(...)   # tokens and cost picked up automatically

@lens.step(type="tool")
def search_stock_footage(query): ...

Run your app as usual. Each call adds one line to .ai-lens/runs.jsonl.

3. Say what you want to know

lens track "are my videos getting better or worse with my changes?" --type video

Tracking: are my videos getting better or worse with my changes?
Output type: video
  c1  success
  c2  video_integrity
  c3  judge: Does the motion stay smooth with no warping between frames?
  c4  judge: Does the video follow the prompt and the start frame?

The plan is saved in .ai-lens/plan.json. Edit it if you want different checks.

4. Change your code, run it again, and inspect

lens inspect

Only new runs are scored, so each run is paid for once. Then you get the versions table, what changed and why, rubric scores, side-by-side results, token hotspots and failures.

5. Get fixes

lens suggest

1. Revert gen-v3-fast for the motion pass  [quality]
   app/render.py:42
   Problem:  smoothness fell from 0.82 to 0.49 when the model changed
   Change:   model="gen-v3" in render_motion(); keep gen-v3-fast for the thumbnail pass only
   Expected: smoothness back to ~0.8, cost +$0.004/run

Tracing

Guarantees:

• Tracing never changes what your function returns or raises.
• If saving fails, you get a warning and your app keeps running.
• Async functions and threads are supported.
• Media outputs and input assets are copied and fingerprinted (sha256), so overwriting out/video.mp4 never corrupts history.

Graders

Model graders see the prompt, input assets and the output. They see images directly and video as 4 frames in order. Audio is shown as a waveform and a spectrogram picture, plus measured length, loudness and silence.

To tune a G-Eval check, edit its steps in .ai-lens/plan.json.

Grade against your best results

lens.configure(references="evals/best.jsonl")

{"output": "best/cat_surfing.mp4", "inputs": {"prompt": "a cat surfing"}, "notes": "smooth motion, sharp fur"}
{"output": "best/city.png", "notes": "this is the colour grade we want"}
{"output": "Captions are under 12 words and name the subject."}

• output is a file or a text description of what good looks like.
• inputs (optional) limits a reference to runs made from the same inputs.
• notes (optional) say what makes it good.
• Paths are relative to the .jsonl file. You can also pass a list of these dicts.

lens track studies up to 4 references with your model and writes 3–6 specific criteria. Every new output is compared with its closest references, and the report tracks each criterion per version. If your references change, lens inspect tells you to run lens track again.

How verdicts are made

1. Versions. Runs are grouped by git commit, model and params, plus a fingerprint of your uncommitted edits. Each set of edits you try, like a prompt change, is its own version, labelled e.g. a1f3c2d +edit 9e04b1. New untracked files, like your app's outputs, don't count as edits.
2. Same inputs first. If two versions share at least 3 inputs, they're compared only on those inputs. This way a version isn't judged worse just because it got harder prompts. Otherwise both versions are compared as a whole, and the report says "different inputs".
3. Each check on its own. Every scored check gets its own mean difference with a 95% bootstrap interval. A check moved only when the whole interval is on one side of zero. Pass/fail checks (success, valid JSON, file integrity) are gates: they are kept out of quality scores, and any drop in their pass rate is flagged.
4. The headline.
   • WORSE if a pass/fail check started failing
   • otherwise, a lopsided side-by-side result (at least 80% one way) decides
   • otherwise, MIXED if some checks went up and others down, BETTER or WORSE if they all moved one way
   • NO CLEAR CHANGE when nothing moved beyond noise, with how many more matched inputs you'd need to see a 0.10 change
   • NOT ENOUGH DATA below 3 runs
5. Side by side. For up to 5 matched inputs per change, your model sees both outputs and picks the better one, twice, with the order swapped. Disagreements count as ties, which cancels position bias.
6. In plain words. For a BETTER, WORSE or MIXED result, your model writes 2–3 sentences on what the outputs now do differently, from what the judge saw, shown above the scores. It's written once per result and cached, so it adds about one cheap call per change. Without a model, or with --no-eval, you get a plain summary built from the scores.
7. Locked judge. The plan records which model function judged it. If you switch models, lens inspect warns that new scores aren't comparable with old ones.

Commands

ailens works as an alias for lens.

Configuration

What's stored

.ai-lens/
  runs.jsonl        one line per traced run
  plan.json         goal, checks, rubric, judge
  evals.jsonl       one score per run and check
  pairwise.jsonl    side-by-side results
  config.json       where your model function lives (never a key)
  references.json   your normalised reference set
  artifacts/        outputs, by run id
  assets/           input files, by content hash
  references/       reference files, by content hash

Everything stays on your machine, in plain JSON you can read, diff or commit.

Limitations

• Video is judged from 4 sampled frames, so judgements about audio and timing within a clip are limited.
• Your model can't hear audio. It sees the audio's shape: silence, clipping, noise, rhythm and pitch. It can't tell what words are spoken.
• G-Eval in the paper weights scores by token probabilities. ai-lens only gets text back from your model, so it uses the whole-number score.
• Steps inside threads you start yourself aren't attached to the run. Async code works.
• Judge scores are only as good as your model. Use your best vision model for media, and compare versions on the same inputs for a fair verdict.

Coming from @techwarq/ailens on npm?

That was the earlier TypeScript version and it's no longer developed. ai-lens is now a Python package with multimodal evals, statistics and reference grading. Install it with pip install ailens-evals.

Development

git clone https://github.com/techwarq/ai-lens && cd ai-lens
python -m venv .venv && .venv/bin/pip install -e ".[dev]"
.venv/bin/pytest -q
.venv/bin/basedpyright --pythonpath .venv/bin/python ai_lens tests examples

To release, bump version in pyproject.toml, add a CHANGELOG.md entry, and push a tag like v0.2.1. GitHub Actions builds the package and publishes it to PyPI through trusted publishing, so no token is stored anywhere.

License

MIT