A challenge is issued
The small loader asks for a signed, short-lived challenge.
A verification layer that adapts to the traffic in front of it
Most visitors never see OTCaptcha. Those who look automated face rising cost, and your team does not have to step in.
What happens on each attempt
The device does the work
Background threads solve it without blocking the page.
Signals are checked
The answer and signals are assessed, then a pass is issued.
Your server validates
Your backend checks the pass once, then continues.
Three checks, chosen by risk
Invisible check
Normal visitors. The browser solves a small proof-of-work in the background while signals are scored. There is nothing to click.
Press and hold
Suspicious traffic. A short hold on top of harder background work. It works with touch, mouse and keyboard.
Slide puzzle
Riskiest traffic. Slide the piece into place. Anyone who cannot drag, or misses three times, moves to a press and hold.
The invisible check is a small proof-of-work: the visitor's device solves it in background threads in a fraction of a second, measured on a laptop, while a bot farm pays for it on every attempt. During an attack the work doubles, then doubles again, and the hold is shown at a lower risk score. Try all three in your browser.
What the risk engine looks at
No single signal decides. Evidence adds up, and legitimate unusual setups are not blocked for one odd detail.
Two scores are kept for every attempt: one for the network it came from and one for the browser it claims to be. Each names the checks behind it, so a decision can always be explained.
New checks run in measuring mode first. They are recorded on real traffic and only start to count once they are known not to catch real visitors.
Automation traces
Markers left by browser automation tools.
Device consistency
Whether the claimed device matches how it behaves.
Network origin
Hosting networks and rapidly repeating sources.
Page behaviour
Whether time on the page and input look human.
Repeats within the day
The same network, device or near-empty browser profile coming back again and again.
How the puzzle was solved
A drag that is too even, too fast or too straight reads as a machine, not a hand.
Designed for the bad days
Automatic attack mode
Volume or suspicious spikes raise work to 2x, then 4x. Each level eases off after ten quiet minutes.
Designed to fail open
If our service is unreachable, your sign-ups continue. Blocking our domain alone does not skip the check.
Replay protection
Each pass works once and expires after five minutes.
Your brand on the box
When a visitor does see a check, it should look like part of your page. Four settings per site, applied the moment the box opens, with no change to your code.
Accent colour
Your brand colour on the fill, the handle and the tick. Colours that would be hard to read are refused.
Corner radius
From square to fully rounded, so the box matches your buttons.
Light, dark or automatic
Follow the visitor’s system setting, or fix one mode to match your page.
Trust line
A small "Protected by OTCaptcha" line under the box. Hide it per site if you prefer.
Accessibility and languages
A check that some people cannot pass is a check that costs you customers. The hold and the puzzle work with a keyboard and a screen reader, and anyone who cannot drag gets the hold instead. Read the full accessibility statement.
Keyboard
The hold works with the space bar or Enter. The puzzle piece moves with the arrow keys. Tab stays inside the box while it is open, Escape closes it, and focus returns to where it was.
Screen readers
The box is announced as a dialog. Controls carry labels, and status messages such as "Verified" are read out.
Motion and touch
Animations and vibration are skipped when the device asks for reduced motion. On phones the box rises from the bottom of the screen, within easy reach of a thumb.
Six languages
The box speaks the visitor's language, taken from the page: English, Spanish, Portuguese, Arabic, Hindi and Chinese. Arabic is laid out right to left.
Performance and resilience
The loader is about 12 KB gzipped, and the work happens off the main thread, so your page stays responsive. On the server side the service is built to lose a machine, or a whole location, without your form noticing.
Two regions in the EU
The service runs in Germany and Finland, behind two load balancers. If one location fails, traffic moves to the other by itself.
Replicated state
One-time passes live in a replicated store with automatic failover across three locations.
Measured capacity
About 800 full sign-ups a second per server, measured on one 4-core server at 76% CPU. Capacity is added by adding servers.
Error reporting without visitor data
Faults on our servers are reported to an error tracker. Those reports carry no IP addresses, tokens, headers or page content.
Solve times measured on an Apple M3 Max across five desktop browsers ranged from about 30 to 130 ms with background workers. No phone has been measured yet; weak phones are the reason the work is raised in steps rather than all at once.
Data and privacy
The service needs to see enough to tell a bot from a person, and no more. The security page lists what is processed in full.
What a record holds
Time, site, which check ran, the outcome, the scores and the signals that contributed. Network and device identifiers are stored as salted hashes that change every day.
What it never holds
Form contents, passwords, e-mail addresses, cookies or anything typed on your page.
How long
Detailed records are kept for 90 days, then deleted. Daily totals without identifiers are kept so that long-term trends survive.
Every rule is versioned
Each record names the version of the scoring that produced it, so a change in the rules can never be mistaken for a change in your traffic.
Reports you can act on
Protection you can read about. Nothing in the reports identifies a visitor.
Weekly report
Traffic, outcomes, attacks, new suspicious networks and solve times, once a week.
Attack timelines
Every automatic escalation is kept as one event: when it started, what raised it, which networks and countries were involved, and when it ended.
Challenge funnel
How many visitors who saw a hold or a puzzle finished it, gave up, or switched to the other check. Visitors who leave never reach your server, so this is the only place they show up.
Sites that never verify
A site that shows the check but never confirms passes on its server is unprotected. We name it, so an integration gap is noticed before an attacker does.