I was intrigued by Jev and the self-hostable projects appearing around it, such as OpenJev and SemIf. Reading about them introduced me to a neat trick: reading an LLM's token probabilities.
Apparently this is an old trick for some people. See e.g. OpenAI's logprobs cookbook. But it was new to me.
I believe the basic idea is to write a prompt like this:
State: My order arrived broken and I want a refund. Question: Which team should handle this? [A] billing [B] shipping [C] returns Answer with the letter of the best option only.
Then add a few JSON request parameters to a compatible Chat Completions request:
{
"max_completion_tokens": 1,
"logprobs": true,
"top_logprobs": 20
}
The LLM API will return the letter plus the model's log probabilities for alternative tokens.
Repeat for each question. Forcing it to generating only one token avoids a lengthy answer and is super quick, though processing the input still costs time. Though for each of the questions a shared state prefix can be KV-cached if the backend supports it.
The fun part: this works with vision models too. Jev's documented request format currently describes only text/JSON state. I added an attachments field for images for my local experiments.
My example captures webcam frames, sends base64 JPEGs, and prints a table: is a person visible, are we indoors or outdoors, and how bright is the scene? With Gemma 4 12B on my RTX 3090, I get around 1 frames per second, with three questions per frame. I also ran it against OpenAI gpt-6-luna and got around 0.2 FPS. Presumably because I didn't make any effort to avoid the cost of a separate connection through their system per question per frame.
Specialized computer vision models surely are much more efficient, but what I like here is the flexibility: change a condition by describing it in plain text.
Here's the standalone Python example (OpenCV is just used for convenient access to the webcam, not for any actual computer vision):
#!/usr/bin/env -S uv run --script
# /// script
# dependencies = ["opencv-python"]
# ///
"""Preview and score webcam frames with llama.cpp or OpenAI.
uv run webcam.py
uv run webcam.py https://api.openai.com/v1 gpt-6-luna
OpenAI reads OPENAI_API_KEY.
"""
import argparse
import base64
import concurrent.futures
import datetime
import json
import math
import mimetypes
import os
import pathlib
import time
import urllib.parse
import urllib.request
import cv2
# attachments is our custom addition to the Jev request format.
data = json.loads("""
{
"state": "Inspect this webcam frame. Judge only what is visibly present.",
"attachments": [],
"questions": {
"person": {
"type": "noul",
"instructions": "Is a person visible?"
},
"plant": {
"type": "noul",
"instructions": "Is a plant visible?"
},
"setting": {
"type": "choice",
"instructions": "Where is the camera?",
"criteria": {
"indoors": null,
"outdoors": null,
"unclear": null
}
},
"light": {
"type": "score",
"instructions": "How bright is the scene?",
"criteria": [
"dark",
"dim",
"bright"
]
}
}
}
""")
def score(data, url, model):
state = data["state"]
if not isinstance(state, str):
state = json.dumps(state)
# Attachments are our extension to the Jev-style request format:
# image file paths or base64 data URLs. Load them once for all questions.
images = []
for attachment in data.get("attachments", []):
if attachment.startswith("data:image/"):
images.append(attachment)
continue
path = pathlib.Path(attachment).expanduser()
mime_type, _ = mimetypes.guess_type(path)
if mime_type not in {"image/png", "image/jpeg", "image/webp", "image/gif"}:
raise ValueError(f"Unsupported image file: {path}")
encoded = base64.b64encode(path.read_bytes()).decode()
images.append(f"data:{mime_type};base64,{encoded}")
# Send the API key only to OpenAI.
is_openai = urllib.parse.urlsplit(url).hostname == "api.openai.com"
headers = {"Content-Type": "application/json"}
if is_openai:
headers["Authorization"] = "Bearer " + os.environ["OPENAI_API_KEY"]
answers = {}
for name, question in data["questions"].items():
# Represent choices, booleans, and ordinal levels as lettered options.
if question["type"] == "choice":
options = question["criteria"]
elif question["type"] == "noul":
options = {"true": None, "false": None} | question.get("criteria", {})
elif question["type"] == "score":
options = {str(i): description for i, description in enumerate(question["criteria"])}
else:
raise ValueError(f"Unknown question type: {question['type']}")
if not 2 <= len(options) <= 20:
raise ValueError("Provide 2 to 20 criteria per question.")
letters = "ABCDEFGHIJKLMNOPQRST"[:len(options)]
# Ask for a single option letter, so its logprob represents that option.
instructions = question["instructions"]
if not isinstance(instructions, str):
instructions = json.dumps(instructions)
lines = [f"State:\n{state}\n\nQuestion: {instructions}\nOptions:"]
for letter, (key, description) in zip(letters, options.items()):
line = f"[{letter}] {key}"
if description is not None:
line += f": {description}"
lines.append(line)
prompt = "\n".join(lines) + "\n\nAnswer with the letter of the best option only."
# OpenAI needs Responses for enough alternatives; llama.cpp needs Chat for logprobs.
# top_p=1 avoids pruning alternatives.
if is_openai:
endpoint = "/responses"
content = [{"type": "input_text", "text": prompt}]
content.extend({"type": "input_image", "image_url": image} for image in images)
body = {
"model": model,
"input": [{"role": "user", "content": content}],
"reasoning": {"effort": "none"},
"max_output_tokens": 16,
"top_p": 1,
"top_logprobs": 20,
"include": ["message.output_text.logprobs"],
}
else:
endpoint = "/chat/completions"
content = [{"type": "text", "text": prompt}]
content.extend({"type": "image_url", "image_url": {"url": image}} for image in images)
body = {
"model": model,
"messages": [{"role": "user", "content": content}],
"max_completion_tokens": 1,
"temperature": 0,
"reasoning_effort": "none",
"logprobs": True,
"top_logprobs": 1024,
}
# Send the request and read the first output token's alternatives.
request = urllib.request.Request(
url.rstrip("/") + endpoint,
headers=headers,
data=json.dumps(body).encode(),
)
with urllib.request.urlopen(request) as response:
result = json.load(response)
if is_openai:
message = next(item for item in result["output"] if item["type"] == "message")
candidates = message["content"][0]["logprobs"][0]["top_logprobs"]
else:
candidates = result["choices"][0]["logprobs"]["content"][0]["top_logprobs"]
logprobs = {item["token"]: item["logprob"] for item in candidates}
# Normalize the returned option scores; missing options initially get zero.
missing = [letter for letter in letters if letter not in logprobs or logprobs[letter] <= -9999]
if len(missing) == len(letters):
raise ValueError("API did not return usable scores for any option")
peak = max(logprobs[letter] for letter in letters if letter not in missing)
weights = [math.exp(logprobs[letter] - peak) if letter not in missing else 0 for letter in letters]
total = sum(weights)
# An omitted token cannot outrank the last returned alternative.
# Allow zero only when their combined normalized probability is below 1e-6.
if missing:
cutoff = min(value for value in logprobs.values() if value > -9999)
missing_weight = len(missing) * math.exp(cutoff - peak)
if missing_weight / (total + missing_weight) >= 1e-6:
raise ValueError(f"API omitted non-negligible option scores for: {', '.join(missing)}")
probabilities = {key: weight / total for key, weight in zip(options, weights)}
# Return the winning choice, probability of true, or expected ordinal level.
if question["type"] == "choice":
answers[name] = {
"type": "choice",
"choice": max(probabilities, key=probabilities.get),
"probabilities": probabilities,
}
elif question["type"] == "noul":
answers[name] = {"type": "noul", "noul": probabilities["true"]}
else:
answers[name] = {
"type": "score",
"score": sum(int(key) * probability for key, probability in probabilities.items()),
"legend": options,
"probabilities": probabilities,
}
return {"answers": answers}
# Choose the server and model before opening the camera.
parser = argparse.ArgumentParser(description=__doc__)
parser.add_argument("url", nargs="?", default="http://localhost:8060/v1")
parser.add_argument("model", nargs="?", default="gemma-4-12b")
args = parser.parse_args()
# Point OpenCV's bundled Qt at the installed system fonts.
os.environ["QT_QPA_FONTDIR"] = "/usr/share/fonts/truetype/noto"
# Open the default Linux webcam with a small capture buffer.
camera = cv2.VideoCapture(0, cv2.CAP_V4L2)
if not camera.isOpened():
raise RuntimeError("Could not open /dev/video0")
camera.set(cv2.CAP_PROP_BUFFERSIZE, 1)
print(f"Webcam -> {args.model}. Noul: yes %; score: value/max. Ctrl-C or Esc to stop.", flush=True)
print(f"{'time':<8}" + "".join(f"{name:>10}" for name in data["questions"]) + f"{'fps':>10}", flush=True)
# Preview continuously while a background worker scores one frame at a time.
executor = concurrent.futures.ThreadPoolExecutor(max_workers=1)
pending = None
try:
while True:
ok, frame = camera.read()
if not ok:
raise RuntimeError("Could not read a webcam frame")
cv2.imshow("Webcam", frame)
if cv2.waitKey(1) == 27 or cv2.getWindowProperty("Webcam", cv2.WND_PROP_VISIBLE) < 1:
break
# Print a completed result, then submit the latest frame.
if pending is not None:
if not pending.done():
continue
result = pending.result()
columns = []
for name in data["questions"]:
answer = result["answers"][name]
if answer["type"] == "noul":
value = f"{answer['noul']:.1%}"
elif answer["type"] == "choice":
value = answer["choice"]
else:
value = f"{answer['score']:.2f}/{len(data['questions'][name]['criteria']) - 1}"
columns.append(f"{value:>10}")
columns.append(f"{1 / (time.perf_counter() - started):>10.2f}")
print(captured + "".join(columns), flush=True)
# Measure throughput for evaluated frames, including image encoding.
started = time.perf_counter()
captured = datetime.datetime.now().strftime("%H:%M:%S")
ok, jpeg = cv2.imencode(".jpg", frame)
if not ok:
raise RuntimeError("Could not encode the webcam frame")
image = "data:image/jpeg;base64," + base64.b64encode(jpeg.tobytes()).decode()
data["attachments"] = [image]
pending = executor.submit(score, data, args.url, args.model)
except KeyboardInterrupt:
print("\nStopped.")
finally:
camera.release()
cv2.destroyAllWindows()
executor.shutdown()
The script handles the API differences: llama.cpp uses Chat Completions and OpenAI uses Responses to get it to show alternatives.
I ran Gemma 4 12B QAT through llama.cpp. On Linux with NVIDIA drivers, curl, zstd, and uv installed:
# Model (~7 GB) and multimodal projector (~175 MB). mkdir -p ~/models/gemma-4-12b/ cd ~/models/gemma-4-12b/ curl -fL -C - -o gemma-4-12b-it-qat-q4_0.gguf https://huggingface.co/google/gemma-4-12B-it-qat-q4_0-gguf/resolve/main/gemma-4-12b-it-qat-q4_0.gguf curl -fL -C - -o mmproj-gemma-4-12b-it-qat-q4_0.gguf https://huggingface.co/google/gemma-4-12B-it-qat-q4_0-gguf/resolve/main/mmproj-gemma-4-12b-it-qat-q4_0.gguf # Standalone llama.cpp binary for RTX 3090 (CUDA architecture 86). curl -fL -o llama.zst https://huggingface.co/buckets/ggml-org/install.sh/resolve/b11160/x86_64/linux/cuda/86/llama-app.zst mkdir -p ~/bin/ zstd -d llama.zst -o ~/bin/llama chmod +x ~/bin/llama ~/bin/llama serve --models-dir ~/models/ --port 8060
Save the Python example as webcam.py. In another terminal, from that directory:
uv run webcam.py http://localhost:8060/v1 gemma-4-12b # Or use OpenAI, with OPENAI_API_KEY set in your environment. uv run webcam.py https://api.openai.com/v1 gpt-6-luna
