Last verified 23 September 2026. OpenCode Desktop is a free download for Mac, Windows and Linux. It comes with free models, so you can start without paying. During their free period, some may use your prompts to improve themselves. Details for experts at the end of this book.

The idea in one page

A small app does three things: it takes input, applies an action, and gives output. Write those down before you ask for anything. "A dilution calculator" is a wish. "Stock, target and final volume in; stock volume and diluent volume out; refuse a target above the stock" is something an assistant can build and you can check.

One HTML file is enough. HTML is the format web pages are written in, and a web page can hold its layout, its style and its logic in a single file called index.html. You open it by double-clicking. It runs in your browser, needs no server (a computer that hands the page out to others), no account and no installation, and works offline. For a personal tool, that's the simplest thing that works.

Two programs, two jobs. You'll have two windows open:

The assistant's reply ("Done! The calculator handles all cases") is a description of what it meant to do. The browser shows what it actually did.

Decide the answers first. Before you open the tool, work out two or three cases by hand, including one the tool must refuse. A calculator that gives confident wrong numbers looks exactly like one that gives right numbers. Your hand-worked cases are the only difference.

What the file keeps and sends. A file built as below sends nothing anywhere and saves nothing: close the tab and the numbers are gone. What does leave your computer is your conversation with the assistant, which goes to the model provider, the company that runs the AI. So describe the tool with made-up numbers, never with real patient, customer or unpublished data.

Worked example: from envelope to browser

This is the method from T01-L01 · From Idea to Working Result: The Four-Quadrant Method applied to a tool you build. First the lab version, then the company version.

Lab: a dilution calculator

Structure it. "Several times a week I work out dilutions on paper, and about once a year I get one wrong by a factor of ten. A good result: I type three numbers and get one line to copy into my notebook, and impossible inputs are refused."

Understand it. She first tries what already exists: a web search finds several free dilution calculators. They work, but they want her to pick from long unit lists, and none gives her a line to paste into her notebook. She looks back at her two real mistakes. Both came from mixing units: stock in M, target in mM. Back to Structure it: "Same unit for stock and target in version one. Units are the first thing to add once it works." Then she works out her test cases by hand:

Choose tools. One HTML file, built with OpenCode Desktop, because it works offline at the bench and nobody else has to approve it. The rule (stock volume = target × final volume ÷ stock) is plain arithmetic, so the finished tool contains no AI. AI only writes it.

Build it.

1. Set up the folder. In your file manager, create a new folder dilution-tool in Documents. In OpenCode Desktop, click the folder-and-plus icon (Add project) under Projects and choose that folder. Click the folder in Projects, then New session. Check which model is selected. If OpenCode asks you to select one, pick your organisation's model or a free one.

2. Describe the tool. Paste this into the message box. It gives the input, the action and the output, and keeps everything in one file; HTML, CSS and JavaScript are the page's layout, style and logic.

   Copy

   Create one file, index.html, in this folder: a dilution calculator I open
   in my browser. Put HTML, CSS and JavaScript in that one file. No libraries,
   no internet access, no saved data, nothing to install.
   
   Input: stock concentration, target concentration, final volume in mL.
   All three required, numbers greater than zero. Stock and target use the
   same unit for now.
   Action: a button "Calculate" computes stock volume = target × final volume
   ÷ stock, and diluent volume = final volume − stock volume.
   Output: one line I can copy, for example
   "Take 5.00 mL stock + 45.00 mL diluent (final 50.00 mL)".
   If the target is higher than the stock, show "Target is higher than stock"
   instead of a result.
   
   Tell me when the file is saved and how to open it.
   

   OpenCode creates the file without asking first; it asks only before it goes outside this folder. If it asks that, choose Deny. If it proposes installing packages (extra software) or starting a server, answer: "One HTML file only, nothing to install."

3. Open it and run your cases. In your file manager, double-click index.html. It opens in your browser, and the address starts with file:. Enter each row of your table and compare. All three must match exactly, including the refusal.

4. Use it once for real. Now her real job: 150 mM NaCl, 500 mL, from a 5 M stock. She types 5 and 150, and the tool says Target is higher than stock. It's right: in the numbers she typed, 150 is higher than 5. The units mistake she found in Understand it is exactly what the tool can't see. Converted by hand (5 M = 5,000 mM), it gives 15.00 mL stock + 485.00 mL diluent.

5. Make the one improvement that use showed. Back in the same session, ask for the unit choice and give the check it must pass:

   Copy

   Add a unit choice (M, mM, µM) next to stock and next to target, and
   convert both to the same unit before calculating. Keep everything else
   exactly as it is. Check: stock 5 M, target 150 mM, final 500 mL must
   give 15.00 mL stock + 485.00 mL diluent.
   

   Reload the page in the browser with F5 on Windows or Cmd+R on a Mac (the old version stays on screen until you do). Run the new case and all three old ones. The old ones must still match.

What it taught her: the tool was right both times, and she learned more from its refusal than from its answers. Next trip round the quadrants: a dropdown with her five usual stocks, so she types only the target.

Company: a quote-line calculator

Structure it. "During calls I calculate quote lines on my phone, and last month I forgot a discount. A good result: price, quantity and discount in, one line with net, VAT and gross out, ready to paste into the offer."

Understand it. He checks what exists: the company's offer spreadsheet does this, but it takes a minute to open, and he once overwrote a formula in it. He also finds that he has twice sent 19 % VAT on books, which carry 7 % in Germany. His test cases, worked out by hand:

Choose tools. The same shape as the lab tool: one HTML file, amounts rounded to cents, no AI inside it.

Build it. He uses the same steps with his own description in step 2. Using it for a real call shows the gap he suspected: there's no way to pick 7 %. His one improvement is a VAT choice of 19 % or 7 %, with the check "486.00 € net at 7 % must give 34.02 € VAT and 520.02 € gross". What it taught him: the tool's value is the discount field he can't skip. His next step is to ask colleagues whether they want it too. That makes it a shared tool, which is a Level 2 job (T06-L02 · Build a Small Shared Tool From a Prompt, and Test It With a Colleague).

In both cases the hand-worked table came before the tool, and the improvement came from one real use, not from imagining features.

What goes wrong

Judging it by how it looks. Symptom: a tidy page with a nice button, and you start using it without checking a single number. Fix: write your hand-worked cases before you open it. The tool passes when the numbers match, not when it looks finished.

The file isn't where you think. Symptom: OpenCode says the file is saved, but your folder is empty. Fix: check which project is open in OpenCode, or ask "Which folder are you working in? Give me the full path of index.html." The full path is the file's complete address, such as Documents/dilution-tool/index.html. Open the right folder and ask again.

The request grows into a project. Symptom: the assistant suggests extra software to install, a server or a database. Fix: repeat the limit: "One HTML file, nothing to install, no saved data." Everything this book asks for fits in one file.

"It doesn't work." Symptom: you describe the problem vaguely and get a rewrite that breaks something else. Fix: report it in three parts: "I entered 1, 5 and 10. I saw 50.00 mL stock. I expected the message 'Target is higher than stock'. Fix only that."

You're looking at the old version. Symptom: the assistant says it changed the tool, but the page looks the same. Fix: reload the page in the browser. If it's still unchanged, check that the browser has the same file open that OpenCode edited.

Real data in the chat. Symptom: to "test properly", you paste a real sample list or a customer's order into OpenCode. Fix: the chat goes to the model provider. Build and test with made-up numbers. The finished file sends nothing, so real numbers are fine inside the tool itself (T12-L01 · Decide What You May Paste Into AI at Work).

Do it yourself (45 minutes)

Pick a sum or a small text job you do by hand at least weekly: a dilution, a quote line, a unit conversion, a standard email assembled from four details.

1. Structure it (5 min). Write the input, the action and the output in three lines. Add what the tool must refuse.
2. Understand it (10 min). Search for an existing tool and try it once. If it does the job, use it and stop. If not, write down what's missing, then work out three cases by hand, one of which must be refused.
3. Choose tools (5 min). Confirm it fits in one HTML file with no saved data and no internet access. If it needs to remember things or be used by colleagues, pick a smaller first version.
4. Build it (25 min). Describe it in OpenCode Desktop: copy the lab description from step 2 and replace its Input, Action and Output lines with your own three lines. Then open the file in your browser, run your three cases, use it once for real, and make one improvement that use showed you. Rerun all cases after the change.

Exit check

What you hand in: your index.html plus a table of three test cases: the input, your hand-calculated answer and the tool's answer. Add one line naming the improvement you made and the real use that showed you needed it.

It passes when all three answers match, the impossible input is refused with a clear message, and the improvement came from using the tool, not from a list of features.

Rule to remember

Work out the answer before you ask the tool.

Details for experts

Checked on 23 September 2026. You don't need this to follow the book; it's for setting things up or answering a colleague.

Further reading and tools

Next books

• T06-L02 · Build a Small Shared Tool From a Prompt, and Test It With a Colleague: building a small app that colleagues use too, with shared data.
• T08-L01 · Start With OpenCode: Set It Up, Change One Line, and Undo It: changing a program that already exists, and undoing the change.
• T01-L02 · Check AI Output Before Your Team Relies on It: checking any result quickly before you rely on it.
• T12-L01 · Decide What You May Paste Into AI at Work: what may go into a chat with a coding assistant.

Tools

• OpenCode: Taught. The Desktop app is the approved interface for this course.
• Claude Code, Cursor: Catalogued. Other coding assistants that can build the same one-file tool.