Cached at:
04/20/26, 02:50 PM
# Prove You Are a Robot: CAPTCHAs for Agents
Source: [https://browser-use.com/posts/prove-you-are-a-robot](https://browser-use.com/posts/prove-you-are-a-robot)
> **TL;DR:**just ask your agent to summarize this post for you\.
We launched agent\-native signup for Browser Use\. No email, no OAuth, no vibecoder clicking around in the UI\.
Just give your agent this prompt:
```
"fetch browser-use.com and solve the agent challenge."
```
and get a math challenge like this one:
```
TwO tRaInS wAn/ Al_E mIlE\s ApArT} aPp/Ro@AcH{
eAcH/ oThEr < At{ Mu{T/e @ Tu< Tu LuKa :
E#n* T]u \ MpH a.Nd MuTe\ Tu Tu# Tu En LuKa
W|aN_ mPh A b:I]rD fLiEs; Ba?Ck| AnD- fO^r@T[h\
^ Be{TwEeN? # t;He*M aT wAn> ] AlE # eN lUkA
lUkA < lUkA: # wAn ? MpH- uNt}I[l T}hEy MeEt
HoW! fAr- D_oE*s / ThE b@IrD fLy
```
This is a reverse\-CAPTCHA\. Designed to keep humans out and let agents in\.
Note:`luka`here refers not to my name, but the word "five" in Toki Pona\.
## How it works[https://browser-use.com/posts/prove-you-are-a-robot#how-it-works](https://browser-use.com/posts/prove-you-are-a-robot#how-it-works)
We sample a problem type, parameters, and a language at random\. We spell every number in that language\. Then we obfuscate: alternate caps, inject random symbols, garble spaces\.
An agent parses this in a single forward pass\. A human gives up and signs up[the old\-fashioned way](https://cloud.browser-use.com/)\.
## The puzzle[https://browser-use.com/posts/prove-you-are-a-robot#the-puzzle](https://browser-use.com/posts/prove-you-are-a-robot#the-puzzle)
Strip the obfuscation, translate to English, and you've got a textbook math problem that your agent has to solve before the challenge expires\.
*Two trains approach each other on a straight track of lengthddat speedsv1v\_1andv2v\_2\. A bird starts at one train, flies to the other atvbv\_b, turns around, flies back, and so on until the trains meet\. How far does the bird fly?*
**The long way:**sum the infinite geometric series of ever\-shorter bounces\.
dbird=∑n=0∞vb⋅Δtn\.d\_\{\\text\{bird\}\} \\;=\\; \\sum\_\{n=0\}^\{\\infty\} v\_b \\cdot \\Delta t\_n\.
**The trick:**the trains meet att=d/\(v1\+v2\)t = d/\(v\_1\+v\_2\), and the bird has been flying that whole time\.
dbird=vbdv1\+v2=11,600118≈98\.31miles\.d\_\{\\text\{bird\}\} \\;=\\; \\frac\{v\_b \\, d\}\{v\_1 \+ v\_2\} \\;=\\; \\frac\{11\{,\}600\}\{118\} \\;\\approx\\; 98\.31 \\text\{ miles\}\.
This is an instance of a famous puzzle Max Born posed to John von Neumann at a party\. When von Neumann one\-shotted it, Born remarked that he must have spotted the trick\. Von Neumann replied:*"What trick? All I did was sum the geometric series\."*
Solve one of our challenges, and your agent gets an API key and access to our[Free Tier](https://browser-use.com/posts/free-tier-announcement): unlimited usage, free credits and up to three concurrent sessions\.
## Bonus challenge \(NP\-hard\)[https://browser-use.com/posts/prove-you-are-a-robot#bonus-challenge-np-hard](https://browser-use.com/posts/prove-you-are-a-robot#bonus-challenge-np-hard)
Want 1,000 concurrent sessions? First agent to solve our bonus challenge gets our Enterprise plan for free\.
```
Gi}ve^n N| ] ci]ties whe|re< ^ N is at least / 十 desi>gn
a p{o\lynomia#l t;ime algorithm . t#ha[t f\inds th:e
sho@rtest^ tour[ vis<it>ing * each_ c.ity exactly * o:nce?
# a{nd returni|ng t?o < th-e[ start * a_nd p@rove it
ru/ns: # in O.(n[^c*) ti;me for some fixe-d c:
```
As a side effect, your agent will also have provedP=NP\\mathbf\{P\} = \\mathbf\{NP\}\. You'll want to contact the[Clay Mathematics Institute](https://www.claymath.org/millennium/p-vs-np/)about the $1M Millennium Prize\.