not-a-robot

A Python library for building the detector side of a "prove you're not a robot" check: it extracts behavioral-telemetry features (mouse-movement dynamics, keystroke timing, overall pacing) from an interaction session and trains an ML classifier that scores how human-like the session looks.

This is meant to run on infrastructure you control, as one signal alongside your own verification flow — not to defeat verification on someone else's site. See Scope below.

Install

pip install -e ".[dev]"

Quickstart

from not_a_robot import BotDetector, InteractionSession, MouseEvent, KeyEvent

# Sessions you've captured and labeled from your own application.
# label=True for a known-human session, label=False for a known-bot session.
sessions = [
    InteractionSession(
        mouse_events=[MouseEvent(x=10, y=12, t=0), MouseEvent(x=14, y=20, t=35), ...],
        key_events=[KeyEvent(t_down=500, t_up=560), ...],
        page_load_t=0.0,
        submit_t=4200.0,
        label=True,
    ),
    # ... more labeled sessions ...
]

detector = BotDetector()
detector.fit(sessions)
detector.save("bot_detector.joblib")

# Later, score a new session:
detector = BotDetector.load("bot_detector.joblib")
p_human = detector.score(new_session)          # float in [0, 1]
is_human = detector.predict(new_session)        # bool at the default 0.5 threshold

Run the end-to-end example (uses synthetic data, see below) from the repo root:

python -m examples.quickstart

What it extracts

• Mouse dynamics (not_a_robot.features.mouse): path length vs. straight-line distance ("path efficiency"), velocity/acceleration/jerk statistics, turning-angle statistics, direction reversals, pause count.
• Timing / keystroke dynamics (not_a_robot.features.timing): dwell time (key down -> up), flight time (key up -> next key down), time to first interaction, time to submit.
• Scroll behavior (not_a_robot.features.scroll): total distance, direction reversals, interval and delta statistics.
• Click/tap behavior (not_a_robot.features.clicks): click count, interval statistics, position variance (scripted clicks tend to land on the exact same pixel repeatedly).
• Tab-focus and paste behavior (not_a_robot.features.engagement): blur/refocus count, paste count and total pasted characters.
• Enrichment ratios (not_a_robot.features.enrichment): coefficients of variation and per-second rates derived from the feature groups above (e.g. mouse_velocity_cv, key_rate_per_sec, scroll_rate_per_sec, typed_vs_pasted_ratio), which normalize for session length/typing speed and tend to separate scripted, uniform behavior from naturally variable human behavior better than any single raw statistic.

Every field on InteractionSession (mouse_events, key_events, scroll_events, click_events, focus_events, paste_events) is optional and defaults to empty — you don't have to capture all of them to use the library, but the more of them you wire up client-side, the more signal the detector has to work with.

All features are combined into one fixed-order vector (not_a_robot.session.FEATURE_NAMES) that feeds a scikit-learn classifier (RandomForestClassifier by default — pass your own via BotDetector(model=...)).

Training pipeline and success-rate validation

There are three evaluation paths, answering three different questions.

run_training_pipeline() fits the detector you'd actually deploy: one stratified train/test split, fit on train, evaluated once on test. Useful for producing a model + a quick report, but its metrics are a single point estimate — on a dataset in the hundreds of sessions, one 75/25 split can look meaningfully better or worse than another from sampling luck alone, before the model is even a variable.

evaluate_cv() runs repeated stratified k-fold CV (n_splits x n_repeats independent folds, default 5x10=50) on one sample of sessions, and reports mean +/- std per metric, recall pooled by InteractionSession.group with a Wilson 95% confidence interval (not a mean/std of per-fold rates — a rare group can have 0-2 members in a given fold, where std is close to meaningless; pooling raw hit/total counts across all folds is the number that's actually defensible), and the cost-optimal decision threshold for a stated false-accept-vs-reject cost ratio, with per-group recall at that threshold instead of just the classifier's default 0.5 cut.

summarize_across_seeds() (CLI: --seeds 0,1,2,3) is the one to actually quote. Repeated CV within one seed only captures fold-partition variance — every fold in that run shares the same 400 sessions. Running evaluate_cv at several seeds and pooling exposes the variance that matters: how much the numbers move when the sample itself changes.

from not_a_robot import run_training_pipeline, evaluate_cv, summarize_across_seeds

detector, report = run_training_pipeline(sessions, data_source="prod-2026-09")
detector.save("bot_detector.joblib")

cv_report = evaluate_cv(sessions, data_source="prod-2026-09")
print(cv_report.summary())

From the command line, against a real captured session log:

python -m not_a_robot.train --data sessions.jsonl --model-out bot_detector.joblib --report-out report.json
python -m not_a_robot.train --data sessions.jsonl --seeds 0,1,2,3   # the defensible report

--synthetic runs the same pipeline against the bundled demo dataset (see below) so you can see a real, computed report before you have real traffic:

python -m not_a_robot.train --synthetic --n-per-class 200 --seeds 0,1,2,3

That produced (1,600 sessions total: 400/seed x 4 seeds, 5-fold x 10-repeat CV per seed, full feature set, c_fa=10 : c_fr=1 for the cost-optimal threshold, BotDetector's calibrated default model — see below):

  seed    accuracy   human pass   bot catch     FAR     FRR
  0         92.5%        92.9%       92.1%    7.9%    7.1%
  1         96.0%        98.2%       93.7%    6.3%    1.8%
  2         94.7%        95.8%       93.6%    6.4%    4.2%
  3         94.4%        96.5%       92.3%    7.7%    3.5%

Bot catch rate range across seeds: 92.1% - 93.7%  <- the honest operating characteristic

Combined per-group recall (pooled across all seeds, Wilson 95% CI):
  group          weight       n   recall [95% CI]
  human          50.0%    8000   95.9% [95.4%-96.3%]
  naive          21.9%    3500   100.0% [99.9%-100.0%]
  evasive        17.2%    2760   100.0% [99.9%-100.0%]
  headless        5.5%     880   100.0% [99.6%-100.0%]
  sophisticated   5.4%     860   34.3% [31.2%-37.5%]

Read it as: bot catch rate is stable at 92-94% across resamples, not a single point estimate. naive/evasive/headless are caught at ~100% with a tight interval (n in the thousands, pooled). sophisticated is caught at 34.3% [31.2-37.5%] pooled — but per-seed it ranges 10.7% to 54.3%, a ~40-point spread the pooled interval doesn't show on its own. That per-seed spread, not the pooled point estimate, is the honest finding about this group: the only signal separating it from humans is the scroll/click/engagement channels, and it's weak enough that which seed the model happens to train on visibly changes how much of it gets caught. Do not treat any single seed's sophisticated recall as an estimate of real-world performance against mimicry bots — not the 54.3% from seed 2, and not the pooled 34.3% either, without also carrying that per-seed range.

Calibration, and what it did and didn't fix. BotDetector's default model wraps its RandomForestClassifier in CalibratedClassifierCV (isotonic) — a raw random forest's predict_proba is a vote fraction, not a real probability, and a reliability check on the raw model showed the predicted-vs-observed relationship breaking down badly in a sparse mid-range (a handful of test sessions per 0.1-wide probability bin, not tracking the observed human fraction there) while a real, if partial, overlap between sophisticated bots and humans sits in exactly that region. Calibrating did meaningfully improve default-threshold sophisticated recall (23.7% pooled before calibration -> 34.3% after) and nudged overall bot catch rate up a couple points. It did not, however, change the cost-curve behavior at c_fa=10:c_fr=1: the cost-optimal threshold is still 0.85-0.89 across seeds, with FAR pushed to ~0% at the cost of a 13-16% false reject rate on real humans, both before and after calibration. That similarity is itself informative: it means that behavior was never primarily a calibration artifact — it's what a 10:1 cost ratio actually does when sophisticated bots and a minority of real humans (the ones who also don't scroll, blur, or paste in a given session) genuinely overlap in score. Whether trading a ~1-in-7 real-user rejection rate for catching most sophisticated bots is worth it depends entirely on your own false-accept-vs-reject cost, which is why cost_fa/cost_fr are parameters, not constants — the 10:1 default here is illustrative, not a recommendation; pass --cost-fa/--cost-fr with your actual deployment's asymmetry (a login form and a comment form do not have the same one), and don't ship the cost-optimal threshold without deciding you actually want that trade.

--drop-keys ablation (excludes keystroke-timing features, simulating a mouse-only capture surface): removing them barely moved anything — bot catch rate range 91.6-94.2% (vs. 92.1-93.7% with keys), combined sophisticated recall 34.2% [31.1-37.4%] (vs. 34.3% with keys), statistically indistinguishable. This holds both before and after calibration, and contradicts what the single-split top-feature-importance list suggested earlier (keystroke features ranked highest) — that ranking reflected naive/evasive separability, not what actually separates sophisticated. The reason is in the generator: sophisticated reuses the human archetype's keystroke timing and mouse trajectory exactly, so neither channel ever carried separating signal against it — only the scroll/click/engagement features it doesn't fake do. Keystroke timing helps separate naive/evasive (which fake it badly), but mouse geometry alone already separates those too, so dropping keys is redundant there, not costly. The lesson isn't "keystroke timing matters most" — it's "the channels a specific bot doesn't bother faking are what catch it," a property of the bot, not of any one feature group. Run this against your own real data before assuming it transfers; a real mouse-only capture surface (e.g. a slider puzzle with no text field) will likely have worse naive/evasive separability than this synthetic set, since here they still fail on mouse geometry too.

The synthetic generator (examples/synthetic_data.py) draws bots from four weighted archetypes: naive (straight-line path, uniform keystrokes, fixed click coordinate, 45%), evasive (jittered but still tighter than human, scripted scroll, 35%), headless (near-instant submit, little/no activity, 10%), and sophisticated (10%) — which reuses the human archetype's mouse and keyboard distributions exactly, so those two channels carry zero separable signal against it by construction (see description.txt on GAN-generated mouse trajectories and keystroke mimicry for why an attacker would specifically invest there). The non-zero recall it shows comes entirely from the scroll/click/engagement channels it does not mimic, plus (at the cost-optimal threshold) trading human pass rate for sophisticated-bot recall. That is the pipeline correctly recovering the partial signal the generator leaves available — not a demonstration of general robustness against every kind of mimicry.

The report format and numbers above are real, computed output from this repo. The input data is not: it's synthetic, generated locally, with no interaction with any real website. Run python -m not_a_robot.train --data <your sessions.jsonl> --seeds 0,1,2,3 on real, labeled traffic from your own site to get numbers you can actually trust for a production decision.

Auto-retrain per project

AutoRetrainStore automates when a project's detector gets retrained, not what counts as ground truth. Each project gets its own store rooted at its own directory -- no data or model is shared across projects, and there's no code path that trains on anything but a session you've explicitly labeled:

from not_a_robot import AutoRetrainStore

store = AutoRetrainStore("path/to/project/.not_a_robot", min_new_sessions=50)

# From your live scoring path (cheap -- just a file append):
store.record_session(session)  # raises if session.label is None
p_human = store.score(new_session)

# From a separate periodic job (cron, a scheduled task) -- NOT the
# request path: fitting + multi-seed CV takes tens of seconds, not ms.
record = store.maybe_retrain()  # None if under min_new_sessions since last retrain

Or as a scheduled command:

python -m not_a_robot.autoretrain --root path/to/project/.not_a_robot --min-new-sessions 50

Real output from a run (30 sessions recorded, below the 50 threshold, then 20 more crossing it):

pending after 30 sessions: 30
maybe_retrain() result: None
pending after 50 sessions: 50
{
  "timestamp": "2026-09-16T20:40:15.396101+00:00",
  "n_sessions": 50,
  "n_new_sessions": 50,
  "seeds": [0, 1, 2],
  "accuracy_range": [0.942, 0.946],
  "human_pass_rate_range": [0.964, 0.972],
  "bot_catch_rate_range": [0.92, 0.92]
}
model file exists: True

Each retrain fits on every session recorded so far, runs the same multi-seed evaluate_cv used above (so the record's ranges are the defensible cross-seed numbers, not a single split), backs up the model it replaces (model.joblib.<timestamp>.bak, never deleted automatically -- rollback is a file copy), and appends the summary to state.json. Not built here, deliberately: any mechanism that would label sessions from the detector's own predictions or from unverified live traffic. That's the difference between "automates when you retrain" (this) and "trains itself on whatever it sees" (a real risk of training-data poisoning, and out of scope for this library — see Scope).

Capturing real training data

The library only defines the schema and the feature math; you own the client-side capture. On the page you're protecting, record mousemove coordinates + timestamps and keydown/keyup timestamps into MouseEvent/KeyEvent objects, tag each finished session with a label (from a secondary signal you trust — e.g. a CAPTCHA outcome, an email verification, or manual review), and either pass the collected InteractionSession objects straight to run_training_pipeline(), or persist them with not_a_robot.io.save_sessions_jsonl() (one JSON object per line) so python -m not_a_robot.train --data sessions.jsonl can pick them up later.

examples/synthetic_data.py generates crude synthetic sessions (one human archetype and four weighted bot archetypes, see above) purely so the rest of the pipeline has example data to run against before you have real, labeled traffic. It is not a model of real bot or human behavior — replace it with your own data before relying on this for anything.

Scope

This library builds a defensive behavioral classifier for a system you run and control. It intentionally does not include: CAPTCHA-solving (OCR, image-grid classifiers), browser automation for clicking through third-party challenges, integrations with CAPTCHA-solving services, or synthetic mouse-trajectory generation meant to fool someone else's bot detection. Those are a different (and, outside authorized testing of your own systems, frequently abusive) category of tool.

Development

pip install -e ".[dev]"
pytest