Bitcoin Learning Sandbox - learn UTXOs, fees, mempool & blocks in your browser

[Cinexa]

Hi everyone,

I built a free Bitcoin Learning Sandbox for people who want to understand how Bitcoin actually works, without running a node, without testnet coins, and without risking real funds.

Link: https://cinexacode.fun/bitcoin-sandbox

Everything runs in the browser. Keys, UTXOs, mempool, and blocks are simulated for education. No mainnet connection.

What you can do

• Wallet Generator - generate a mock keypair, see private key (hex), WIF, public key, Legacy (1...) and SegWit (bc1q...) addresses. Starter UTXOs are auto-funded so you can practice immediately.
  
• UTXO model - balance = sum of unspent outputs. Each UTXO shows txid, vout, address, and amount in sats.
  
• Transaction Builder - pick inputs, set receiver + amount, adjust fee rate (sat/vB). Preview raw JSON with vin/vout including change. Dust rule enforced (<546 sats rejected).
  
• Mempool - broadcast a tx and watch queue position. Higher fee rate = higher priority. Fee slider (1-200 sat/vB) updates confirmation estimates live.
  
• Block Explorer - mine the next block, confirm pending txs, increment block height, and follow events in the explorer feed.
  
• Security Lessons - simulate address reuse warnings and private key exposure (exposed keys get swept on the next block).
  
• Documentation tab - architecture overview and flow reference built into the UI.
  
• Guided tutorial - first-visit walkthrough (12 steps) through each tab and button. Mobile-friendly bottom sheet on phones.
  

Typical learning flow

Code:

Generate Wallet → Build TX → Broadcast → Mempool Queue → Mine Block → Confirmed

Fee formula used in the simulator:

Code:

fee = fee_rate × vsize
change = inputs - amount - fee

Rough mempool behavior:

• ≥ 80 sat/vB — likely next block
• ≥ 15 sat/vB — ~2 blocks
• ≥ 1 sat/vB — ~4 blocks
• Very low — sits at the back of the queue

Who is this for?

• Newcomers who hear "wallet" and "UTXO" but have never seen them in action
• Anyone preparing for deeper reading (Mastering Bitcoin, etc.) who wants hands-on practice first

Who is this NOT for?

• Production wallet testing - keys are mock/educational only
• Real signing or broadcasting - no network, no real coins
• Advanced script/path-level accuracy - this is a teaching simulator, not bitcoind

Important: Never use sandbox keys with real Bitcoin. Treat all keys as disposable learning material.

Feedback welcome

This is early and meant to be useful, not perfect. Just for fun. I'd especially appreciate thoughts on:

• Is the UTXO → fee → mempool → confirm flow clear enough?
• What topics should the Security tab cover next? (RBF, CPFP, coin selection, etc.)
• Any bugs on mobile or desktop?

Try it here: https://cinexacode.fun/bitcoin-sandbox

If you find it helpful, a reply or suggestion for improvement is appreciated.

Thanks,
Cinexa
Anyway, we have 5 new tools on our website.

Fee Estimator & RBF/CPFP Lab
Explore virtual size, feerates, mempool position, replace-by-fee bumps, and child-pays-for-parent packages.
- https://cinexacode.fun/fee-playground

Address & Script Decoder
Paste a Bitcoin address or scriptPubKey hex to see format, witness data, fee/privacy notes, and optional on-chain reuse / connection / AML-style heuristics.
https://cinexacode.fun/address-decoder

BIP39 Checksum Lab
Validate English BIP39 checksums, no key derivation, no storage.
https://cinexacode.fun/bip39-lab

PSBT Inspector
Read-only BIP174 decoder, inspect inputs, outputs, partial signatures, and sighash types. No signing.
- https://cinexacode.fun/psbt-inspector

Flow Explorer
Tracer-style fund flow map with zoom/pan, BitList forum usernames on Bitcoin wallets, and all connected counterparties.
- https://cinexacode.fun/flow-explorer

[hd49728]

I built a free Bitcoin Learning Sandbox for people who want to understand how Bitcoin actually works, without running a node, without testnet coins, and without risking real funds.

Link: https://cinexacode.fun/bitcoin-sandbox

Everything runs in the browser. Keys, UTXOs, mempool, and blocks are simulated for education. No mainnet connection.

I can not access your site with Tor so I give up.
I would like to learn from this site https://learnmeabitcoin.com/ and I would like to recommend it to newbies who want to learn about Bitcoin too.

They can click on the Search box, type keyword of things they want to learn, and enter, the site will give them related pages it has for newbies to learn about that.

For example, with UTXO
https://learnmeabitcoin.com/technical/transaction/utxo/

[Shishir99]

I built a free Bitcoin Learning Sandbox for people who want to understand how Bitcoin actually works, without running a node, without testnet coins, and without risking real funds.

Link: https://cinexacode.fun/bitcoin-sandbox

Everything runs in the browser. Keys, UTXOs, mempool, and blocks are simulated for education. No mainnet connection.

I can not access your site with Tor so I give up.
I would like to learn from this site https://learnmeabitcoin.com/ and I would like to recommend it to newbies who want to learn about Bitcoin too.

They can click on the Search box, type keyword of things they want to learn, and enter, the site will give them related pages it has for newbies to learn about that.

For example, with UTXO
https://learnmeabitcoin.com/technical/transaction/utxo/

I checked the website. While LearnMeaBitcoin is an infographic-style website that teaches Bitcoin, OP's website is a sandbox to simulate transactions with fake Bitcoin. It is actually an interesting concept for newbies to learn. But I am not sure if newbies would like to test such a thing. OP needs to explain this in more detail on each page. I see this page has instructions, but I don't think it's enough for the newbies.

Op, if you are good at making tools, I would recommend making some useful tools for the community. I am not saying the current one is not good. I appreciate your effort, and I am asking you to continue the work by making more tools.

[joniboini]

Have you made similar tool before? I swear I've seen a project like this just a few weeks or months ago. I can't remember the exact name but it's also a sandboxed Bitcoin environment.

As for your website, for me the font makes it hard to read. Either make the font bigger or change the style to something else. Maybe the color scheme also affects this. It would be even better if you can visualize some of the info though, like making one block to another connected, change the receiving address, and so on. So far it feels like a click - read - click - read steps, which can be boring for some people.

[Cinexa]

Thank you.

No, I did not make any tools before. Thank you for the suggestion. I also realized that the font doesn't look good. I have changed the fonts. I am just expecting some people to try it. Otherwise, it will be a complete waste of time and resources.

[dkbit98]

This looks like interesting bitcoin testing tool for newbies, but I can't find any option to generate new BTC addresses.
It would be a good idea to create demo wallet that could be used for this and simulating coin sending and transactions.

[joniboini]

I visited the site again and the fonts definitely looks better for me. Would be nice if there's a zoom option. I'm still wondering about the mempool option though. Is it expected that the tx shown there won't be mined in the mined blocks ever? I think it's been like this since you launched this website. I thought it was a bug at first, since it shows that the generated tx would be confirmed within the next 2 blocks or so.

[Cinexa]

@dkbit98,

Thanks for the feedback.

Generating new address feature has been added.
Click on "Generate New Address" for more receive addresses (auto-funded with demo coins).
You can click "Send to this" on an address, then use TX Builder → Broadcast → Explorer → Mine Next Block to simulate sending/confirming.

@joniboni

Glad the fonts are better. There is now a zoom control in the header (A− / A+). It remembers your preference.

As for the mempool, That was a real bug (sorry). After mining, the confirmed txs were incorrectly restored into the mempool, so they looked stuck forever, even though the fee estimate said "~2 blocks".

How it works:
- Broadcast puts the tx in the mempool (sorted by sat/vB).
- This sandbox does NOT auto-mine. You must click Explorer → "Mine Next Block".
- Each mine confirms the top 6 fee-rate transactions and removes them from the mempool.
- If you're alone (or in the top 6), the next Mine click confirms your tx immediately.
- Fee labels are estimates for when blocks are mined — not a timer that runs by itself.

Flow: Create Demo Wallet → (optional) Generate New Address → TX Builder → Broadcast → Mempool → Mine Next Block → Confirmed

[Cinexa]

I have added two tools to verify PGP-signed messages and Bitcoin-signed messages.

https://cinexacode.fun/bitcoin-verifier

https://cinexacode.fun/pgp-verifier

Please let me know if there are any bugs or improvements needed.

[joniboini]

As for the mempool, That was a real bug (sorry). After mining, the confirmed txs were incorrectly restored into the mempool, so they looked stuck forever, even though the fee estimate said "~2 blocks".

I can confirm that it works now. Created a new wallet and tx, click the mine button and it seems like any confirmed tx disappeared from the mempool now. Glad to hear that it's no longer bugged. I wonder if the starting balance is changeable too? Tried to generate multiple times and the balance always start from 3.15.

As for the verification tools, does it work client-side only or does it send data to your server? I assume people would dislike it if the later is true.

[Cinexa]

I can confirm that it works now. Created a new wallet and tx, click the mine button and it seems like any confirmed tx disappeared from the mempool now. Glad to hear that it's no longer bugged. I wonder if the starting balance is changeable too? Tried to generate multiple times and the balance always start from 3.15.

As for the verification tools, does it work client-side only or does it send data to your server? I assume people would dislike it if the later is true.

Glad the mine/mempool flow is working for you now.

On the starting balance: it used to always seed 3.15 BTC (four practice UTXOs for coin control). That’s now changeable - on the Wallet tab use the Start with dropdown (1 / 3.15 / 5 / 10 BTC) before creating a new demo wallet. Default is still 3.15.

On the verification tools: you’re right to ask. They are not client-side only right now. The web tools, Bitcointalk userscript, and browser extension send the signed message / signature / public key to cinexacode.fun over HTTPS so the server can verify. We do not ask for private keys, and the app doesn’t store verification results. We’ve added an explicit privacy note on the pages so that’s clear. Don’t paste private keys, seeds, or highly sensitive plaintext.

[bitmover]

Interesting website and nice project Cinexa.

I was taking a look at the address decoder, and you mentioned privacy notes.

I was expecting some analysis about address reuse, connections with other address, maybe some  AML score.

It would be an interesting addition.

[Cinexa]

Interesting website and nice project Cinexa.

I was taking a look at the address decoder, and you mentioned privacy notes.

I was expecting some analysis about address reuse, connections with other address, maybe some  AML score.

It would be an interesting addition.

Thank you bitmover.
Good idea. I have added them.
Please check with some real address and let me know if you liked it.

However, I did not post about these new tools yet since I am still working on them.

[bitmover]

Interesting website and nice project Cinexa.

I was taking a look at the address decoder, and you mentioned privacy notes.

I was expecting some analysis about address reuse, connections with other address, maybe some  AML score.

It would be an interesting addition.

Thank you bitmover.
Good idea. I have added them.
Please check with some real address and let me know if you liked it.

However, I did not post about these new tools yet since I am still working on them.

That was fast

I liked the result. I pasted an addrssses which is my "dump " address, which i paste in every contest, review etc anything where i need to publicly add an address

Results were nice.

Address reuse

74 separate deposits hit this address and it has been spent from, which links those payments.

Risk: high · funded UTXOs: 74 · spent UTXOs: 51

Connections

145 other address(es) co-appear in recent transactions — shared inputs/outputs can link activity.

Where does the aml score came from? Your own calculation?

[Cinexa]

Where does the aml score came from? Your own calculation?

Good to know you liked it bitmover.
Yes, it is from our own calculation, not a third-party AML/KYT feed.
There is no chain analysis for that. The tool pulls public mainnet stats from mempool.space and then runs a simple internal heuristic on our server. It checks, transaction count, cumulative inflow, repeated deposits to the same address, how many distinct counterparties show up in the recent txs and whether there’s both inbound and outbound activity.

So for your address, a medium band with reuse/deposit factors is basically the tool saying: this address is used publicly and often, privacy/linkability risk is higher than a one-time fresh receive. That’s intentional, not a claim that the address is dirty.

[joniboini]

Btw, does your server block access from some IPs? Especially VPNs. I noticed that I can't visit your web properly now if I activate my VPN. I'll have to change to a different region (from Japan btw) or disable it entirely to make it loads properly. I wonder if there's an increase on visits so you have to enable some DDOS protection or there's something else going on.

[Cinexa]

Btw, does your server block access from some IPs? Especially VPNs. I noticed that I can't visit your web properly now if I activate my VPN. I'll have to change to a different region (from Japan btw) or disable it entirely to make it loads properly. I wonder if there's an increase on visits so you have to enable some DDOS protection or there's something else going on.

We don’t block VPNs/IPs. The site is on Hostinger + Cloudflare. So some VPN/data center routes may encounter default CDN/WAF behavior or fail to load external CSS/fonts (Tailwind/Google Fonts), which can make pages look broken. Not from a manual DDoS toggle on my side. If you can share VPN provider, exit region, and what you see (timeout vs unstyled vs challenge page), I can dig further. Also, I heard that people from many regions are unable to access sites which are hosted on Hostinger.
Anyway, we have 5 new tools on our website.

Fee Estimator & RBF/CPFP Lab
Explore virtual size, feerates, mempool position, replace-by-fee bumps, and child-pays-for-parent packages.
- https://cinexacode.fun/fee-playground

Address & Script Decoder
Paste a Bitcoin address or scriptPubKey hex to see format, witness data, fee/privacy notes, and optional on-chain reuse / connection / AML-style heuristics.
https://cinexacode.fun/address-decoder

BIP39 Checksum Lab
Validate English BIP39 checksums, no key derivation, no storage.
https://cinexacode.fun/bip39-lab

PSBT Inspector
Read-only BIP174 decoder, inspect inputs, outputs, partial signatures, and sighash types. No signing.
- https://cinexacode.fun/psbt-inspector

Flow Explorer
Tracer-style fund flow map with zoom/pan, BitList forum usernames on Bitcoin wallets, and all connected counterparties.
- https://cinexacode.fun/flow-explorer