I build software small businesses actually run on — and dig into the layer underneath.

A working business on a page: it buys used cooking oil from households, warungs and restaurants across Jakarta, Tangerang, Bekasi and Bogor, then passes it on to licensed facilities that turn it into biodiesel, bio-avtur, soap and candles.

The commercial terms are the first thing you see rather than something you have to ask for — Rp6.000-8.000 per litre, 15 litres minimum, free pickup, weighed in front of you, paid on the spot. An offer form and a WhatsApp thread are the only two ways in, because those are the two the sellers already use.

A free assistant for small F&B businesses. It works out cost of goods properly instead of stopping at the price of ingredients, plans the day’s production from past sales patterns with AI suggestions on top, and tracks what is left over so it does not quietly end up as landfill.

The framing is deliberate: the pitch leads with waste, and treats the better margin as the side effect rather than the headline.

Agents rack up API spend quietly — the runaway cost problem. Every provider has its own dashboard, its own billing period and its own unit of account, so the one number that matters, what this agent cost me today, exists nowhere.

The tool logs every call’s token usage, computes the dollar cost from an editable pricing table and renders a dashboard over the lot. It speaks to any OpenAI-compatible endpoint, with rates for roughly twenty providers — OpenRouter, Anthropic, Nous Portal, Groq, DeepInfra, Together, Ollama and more — all editable in one JSON file rather than compiled in.

Two constraints shaped it. No dependencies at all, so installing a cost tracker never becomes its own supply-chain problem. And keys are read from environment variables or a git-ignored config, never committed.

Contraception in Indonesia is treated as the wife’s problem, and most of what men hear about vasectomy is wrong. This works through the myths one at a time — a quiz, articles, an AI you can ask directly, and a way to find a doctor when you are ready.

The privacy stance is the product decision, not a footnote: no account, no personal data, no saved history. Someone reading this page is doing it quietly, and the site is built so it stays that way.

Menu, cart, spice levels, delivery areas and branch locations for an ayam geprek business, with ordering handed off to WhatsApp and Grab because that is where the orders already come from.

The detail that matters on a food page: opening hours are live rather than printed. The header states whether it is open right now and when it closes, so nobody places an order into a closed kitchen.

Story, menu, locations and contact for a specialty coffee and dessert shop, with ordering through WhatsApp, GrabFood or ShopeeFood.

A deliberately different register from the other storefronts: quiet type, a lot of air, illustration instead of photography. A cafe and a fried chicken counter should not look like the same template, and here they do not.

Agent skills are user-contributed and largely unvetted — you install one from a hub and it runs with your shell and your keys. This scans a skill or plugin folder before that happens.

It looks for destructive commands, downloads piped straight into a shell, privilege escalation, obfuscation like base64-decoded eval, outbound POSTs that could be exfiltration, and prompt-injection text hidden in the skill’s own markdown. That last one is the interesting case: the attack is not in the code at all, it is in the prose the agent is about to read.

An agreement between you and yourself, witnessed by a chain. Name a habit, stake MON, set a term in days. Each day you check in onchain earns that day’s share back; each day you miss is sent to the dead address, beyond anyone’s reach including the author’s.

Settlement is pro rata rather than all-or-nothing: ten days with two missed returns eighty percent and burns twenty. The contract has no owner and no backdoor, which is the only reason the promise means anything.

A Merkle tree is usually explained with a diagram you cannot touch. You can read that changing one leaf changes the root and still not feel why that matters, because the chain reaction is the whole idea and a still picture cannot show a reaction.

Edit a block and every hash above it recomputes with real SHA-256. Click a leaf and it marks the sibling hashes that make up its Merkle proof, then replays them step by step to show they reconstruct the same root. Odd levels duplicate the last node the way Bitcoin does, and the padded node is drawn dashed rather than hidden.

The demo further down this page is the same idea rebuilt for this site, with a synchronous hash so the whole tree can be recomputed on every keystroke.

A management sim in the browser: name the airline, give it a slogan, an emblem and two brand colours, choose a home hub from real airport codes, set a difficulty, then run real-time flights, map cargo and fleet operations from there.

An economy is a system, and the fastest way to understand one is to build a small one and watch where it breaks.

Up to five business profiles, each with its own logo and invoice counter. Line items total themselves, tax and discount are editable, and the preview sits beside the form on desktop and behind tabs on mobile. Status runs Draft, Sent, Paid — and flips to Overdue on its own once the due date passes.

Everything lives in the browser’s localStorage, which is the whole design: no account to make, no server to trust with someone’s client list, and the A4 PDF is rendered on the same machine that typed it. Finished invoices can be shared as WhatsApp-ready text or exported to CSV for a spreadsheet.

An agent workspace quietly fills up with things that must never be pushed: API keys in debug output, email addresses in transcripts, home directory paths in memory files. One careless git add . and it is public permanently.

This masks them first — provider keys for OpenAI, Anthropic, OpenRouter, GitHub, Google, Stripe, AWS, Slack and Telegram, plus JWTs, SSH private keys and bare KEY=secret assignments, alongside emails, IP addresses, card numbers and /home/... paths. Extra patterns can be added on the command line, and a --fail exit code turns it into a CI gate rather than a thing you have to remember to run.

A curated list with a strict definition: free means a recurring tier that resets, not a one-off coupon that evaporates in thirty days. Promotional credits are labelled as promotions rather than folded into the same column, and every row carries the date it was last verified.

The table in the README is generated from a single JSON file by a Python script, and a scheduled GitHub Action re-checks the links and rebuilds it. A list like this is worthless the moment it goes stale, so the maintenance is automated rather than promised.

Working in a single tree means a constant churn of stashing and checking out whenever a feature and a PR review want your attention at the same time. Git worktrees fix that by giving each branch its own directory, but the built-in commands are wordy enough that most people never adopt them.

gwt shortens the whole loop: add a branch and its worktree in one command, list everything with dirty and locked status, jump in by name, see the status of every worktree at once, open one in your editor, and clean up the worktrees whose branches are already merged. Shell hooks make the jump an actual cd rather than a subshell.