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Build track: a lookup tool on public data

⏱ 40 min5 steps

A lookup tool answers one question from public records: who owns this company, what's the water quality here, who owns this property. The data is often free but scattered and hard to search. My example is UnmaskLLC.

It's rung 1 on the ladder from What to build if it's free and collects nothing about visitors. Public doesn't always mean free to republish.

Nobody finds quick reading. Claude goes through them with you in step 3.

terms of use The rules a website or data source sets for how you can use what it publishes. All words

Before this, you'll want:

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Steps

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  1. Pick one question people ask, and where the answer lives

    One question, one , one area to start.

    1. Write down the one question your tool answers.
    2. Find the official place the data comes from, and write down its .
    source Where a fact comes from, like the web page, the person or the paper that says it. All wordsURL The address of a page on the internet, like gethomebydark.com. You type it in the bar at the top of your browser. All words
    That's not what I see
    The data I want is spread over lots of websites.
    That's often the opportunity. Start with one source, for one area, and get that right first.

  2. Start Claude Code in your my-idea folder

    1. Open (Cmd+SpaceWindows key, type Terminal, press Enter).

    2. Go to your my-idea . Type this and press Enter:

      cd ~/Backyard/my-idea

      cd ~\Backyard\my-idea

    3. Check the says my-idea.

    4. Start . Type this and press Enter:

      claude
    Terminal An app on your Mac where you type instructions instead of clicking on things. Claude Code works inside it. All wordsfolder A place on your computer that holds files and other folders, like a drawer in a filing cabinet. All wordsprompt line The line in Terminal that ends with a % sign. The word right before the % is the folder you're in. All wordsClaude Code Claude, working inside Terminal. It can make files and run things on your Mac, with a safety checker watching what it does. All words

    You should see:

    Claude Code's name and version at the top, a line starting with ❯ where you type, and ⏵⏵ on in the bar at the bottom.

    auto mode Claude works without asking, and a separate safety checker blocks anything risky. All words
    That's not what I see
    It says "cd: no such file or directoryCannot find path … because it does not exist".
    The my-idea folder isn't there. It comes from Check the idea first and The three files, so do those first, then come back.
    It says "zsh: command not found: claudeThe term 'claude' is not recognized".
    This Terminal window doesn't know about Claude Code. Close the window with Cmd+WCtrl+Shift+W, open a new one and try again. If it still says that, go back to Claude Code and .install Put a new program on your computer so you can use it. All wordssign in Tell Claude Code which Claude account is yours, so it knows you've paid for it. All words

  3. Add the lookup-tool part to your PRD

    This has Claude interview you about the tool and its data source, one question at a time, and add what you say to with a "Done means" list.

    1. Copy the prompt below.
    I'm building a free lookup tool on public data. Add a section to PRD.md called "Track: lookup tool" and interview me, one question at a time, to fill it in: the one question it answers, who asks it and how they answer it today; the official data source, its license or terms of use, and how often it changes; whether it includes people's names, addresses or phone numbers; and how it earns, if it does. Then help me read the source's terms of use and tell me in plain words what they allow. Then add this "Done means" list: one question answered, with the source shown next to every answer; data loaded from the official source; "no result" said honestly instead of guessed; and pages that don't exist show a real "not found". Show me the section before you save it. Don't build anything yet.
    1. Paste it into Claude Code with Cmd+VCtrl+V and press Enter.
    2. Answer each question, then read the section and tell Claude what to fix.
    prompt What you type to Claude. A good one says what you want, how to work and what never to do. All wordsPRD.md One page on the problem, who it's for, and what "done" looks like. PRD stands for product requirements document. All words

    You should see:

    Claude asks you one question, about the question your tool answers, and waits for your answer. Later it reads the source's terms of use and tells you what they allow, then shows you the section before it saves it.

    That's not what I see
    Claude says there's no PRD.md.
    Do The three files first, then come back to this step.
    The terms of use say I can't copy or republish the data.
    Then don't. Look for an official download or with friendlier terms, or pick a different question. Public doesn't always mean free to republish.API A way for one program to ask another for data directly, without going through web pages. All words

  4. Build the first version, one step at a time

    This is the "plan the build" prompt from Starter prompts. Claude shows a numbered plan, waits for your OK, then builds one step at a time. Show the source next to every answer, so people can check it themselves. What good looks like goes at the end.

    Build with made-up test data. Real customers' or people's details go in only once the tool lives in the right accounts.

    1. Copy the prompt below.
    Goal: build the smallest version from PRD.md for the people it names, to the quality bar I give at the end. How to work: 1. Read PRD.md and CLAUDE.md first. 2. Plan before you build. Write the plan as a numbered list of small steps, each one something I can check. Show it to me and wait for my OK. 3. For each step, say how we'll know it works, like "I open the page and see X". 4. Build one step at a time. After each step, check it yourself, tell me what you checked and what you didn't, then stop for me to look. Rules: Never say something works unless you checked it. Never buy, subscribe or sign up for anything. Never delete files without asking. Ask me when you're unsure, one question at a time. Start by reading the files and showing me the plan. The quality bar, what good looks like:
    1. Paste it into Claude Code with Cmd+VCtrl+V.
    2. Type a space, then what good looks like, like every answer shows its source, and a search with no match says so plainly.
    3. Press Enter.
    4. Read the plan. Ask about any step you don't follow, then type OK.
    5. After each step, look at what Claude made before you tell it to go on.

    When the first page is built, go back to Working isn't the same as good and do its steps from step 4 on.

    You should see:

    Claude reads PRD.md and , then shows you a numbered plan of small steps and waits for your OK.

    CLAUDE.md A note to Claude about how to work in this project. Claude Code reads it at the start of every session in that folder. All words
    That's not what I see
    Claude wants to scrape a website.
    Ask it to find an official download or API first, and to read the site's terms before collecting anything.
    Claude says it needs to install something first.
    In auto mode it won't install programs for you. Ask it: Can we build the first version with nothing to install? If the answer is no, ask it to name the program and where it comes from, and install it yourself.
    PRD.md has more than one Track section.
    Tell Claude: Build only the "Track: lookup tool" section for now.

  5. Test the answers you'd least like to get wrong

    1. Copy the message below, paste it into Claude Code and press Enter.
    Open the lookup tool in my browser.
    1. Search for things that don't exist, things with odd spellings, and things you already know the answer to.
    2. Tell Claude each answer that's wrong, in plain words.

    The mistakes I see most: breaking the source's terms, republishing personal details carelessly, and the source changing or disappearing without anyone noticing.

    To put it online for anyone, follow steps 6 to 9 of the comparison-site track. Steps 10 and 11 add visitor stats. They're optional, and they move the tool up a rung, because it then collects something about visitors and needs a privacy page.

    Later: a scheduled job that refreshes the data, and a daily check that tells you when the source stops updating.

    You should see:

    Your opens on the lookup tool. If Claude has to start something first to show it, it says so, and may give you a link to open instead.

    browser The app you use to visit websites, like Safari or Chrome. All words
    That's not what I see
    A search with no result shows something anyway.
    That's the most common bug in tools like this. Tell Claude: When there's no match, say so plainly. Never show a close guess as if it were the answer.
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Your challenge

Find the official source for your data and read its terms of use, all of them, before you write any code. Write down what they allow and what they don't.

Last checked 2026-10-20

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