12万分の1 — 野球人生シミュレーション
Live one boy's baseball life from birth to death, year by year, with odds drawn from public statistics; its beginner developer says Claude Code designed and built most of it.
Official site 1in120k.com, verified Nov 3, 2026. Linked as the game's address in the developer's own Qiita post.
Where to play
Every platform this game is on. Links are checked against official sources; we never host or mirror games.
- Play free — official siteBrowserReleasedFree
Runs in a desktop or mobile browser. No download.
Browser (no download) · Mobile · Official link verified Nov 3, 2026.
About 12万分の1 — 野球人生シミュレーション
A free browser life simulation about one baseball life. You are born a boy and play out his whole life year by year: high school, then university, company-team or independent-league baseball, possibly the pros, and life after retirement, with branches such as a gap year to retake university entrance exams, the draft, free agency, trades and posting to an overseas league, plus many lives that leave baseball behind. Each year an event is drawn, you choose a response yourself or let the game choose, and the result feeds into your abilities, money and family. Odds are based on public statistics, with official data preferred; real school and team names appear only where the developer has a source and a check date, and the players are fictional. There are about 1,000 events, a potential mode where you pick a talent level from one to ten stars, an overseas-born mode and a two-generation parent-and-child mode. Lives can be saved and resumed in the browser, shared to X, and replayed exactly from the same random seed. The site is in Japanese, English, Korean and Traditional Chinese. The developer says the idea was borrowed from 1 in 8 Billion (80億分の1), as that game's terms allow for ideas, and that the screen layout and flow were built from scratch. The developer, a programming beginner, says the code was not written by hand: Claude Code designed and built most of the systems and reported back, and the developer approved or stopped each step.
How it was built
VerifiedThe developer, a programming beginner, says the code was not written by hand (“I can't, so I didn't”): most of the systems were thought up, built and reported by Claude Code, and the developer read the reports and said yes or wait. The developer also made almost no technical choices; the plan used an existing Vite + React + TypeScript project, and Cloudflare Pages was picked from a list of candidate hosts that included GitHub Pages and Netlify. Claude Code set up an event format that compiles new events into the game, statistical tests that check stage-by-stage rates (such as staying in high-school baseball for three years) against targets over 400,000 simulated lives, and a “fingerprint” that replays 100,000 fixed-seed lives after code changes to show whose life changed (later run only when relevant). Within an agreed scope it worked without stopping for small checks and reported at the end of each stage; deletions, overwrites, production releases, changes to approved specifications and fingerprint baseline updates needed the developer's approval first, and skipped checks had to be reported as unverified. The developer notes that it sometimes went beyond its scope, for example a fix planned for 5 files that spread to 13, and was then asked for a diff and a list of out-of-scope changes. When usage limits were hit, a new session picked up the work from the plan document. After a traffic surge stopped the play counter, Claude Code traced the cause to a query that grew with traffic and replaced it with 10-second summary tables.
A beginner directing Claude Code by approval. The game is a static Vite + React + TypeScript site on Cloudflare Pages that saves lives in the browser's localStorage; only the counter of lives started uses a server (Cloudflare Workers with a D1 database). Each year draws an event, takes a choice and applies its effects; random numbers are seeded so a life can be replayed exactly. Balance is guarded by statistical tests and a 100,000-life fingerprint; the developer describes finding that adding one line of text used an extra random number and quietly changed later lives, so new text and events now draw from their own random streams. When the test suite passed 1,000 tests and began timing out, the number of parallel workers was cut from 20 to 6. After release, player feedback reported a 68-year-old throwing 160 km/h: draft checks for independent-league and company-team players had no age limit, and the displayed pitch speed never fell after age 23; both were fixed. Versions went from 1.1.0 on 22 Oct to 1.6.0 on 24 Oct 2026. The developer says the work that stayed human was deciding what may and may not change.
- Engine / framework
- Not disclosed (Vite + React web app)
- Languages
- TypeScript
- Team size
- 1 (solo)Developer ReportedQiita (ao_zzz)
- Development time
- Not disclosed
- Development cost
- Not disclosed
- AI-written code
- Not disclosed
“仕組みの多くは Claude Code が考えて、作って、報告してくれたもので、私はそれを読んで「はい」か「待って」を言っていました。”Verification notes: AI use from the developer's Qiita post (published 27 Oct 2026, 17:21 UTC, which is 28 Oct in Japan; updated 28 Oct), which links 1in120k.com as the game. The post says the code was “almost” not written by hand, so no AI code share is recorded. The first public release date is not stated; the post gives version 1.1.0 on 22 Oct and 1.6.0 on 24 Oct 2026, so no release date is recorded. Languages from the site's hreflang alternates (ja, en, ko, zh-TW) and the post's “4 languages”. The post says the idea comes from 1 in 8 Billion under that game's terms (ideas free; copying its program, text or art not allowed) and that the layout and flow were rebuilt; no code, text or art from it is reported to be used, so no IP notice is recorded. The developer reports that the lives-started counter jumped from about 30 registrations the day before to about 30,000 in 24 hours, stopping when it hit the D1 free tier's 5 million rows-read limit (about 7.19 million rows read); this counts lives started, not players, and is not recorded as a metric. The developer's note.com companion post was not opened. No source mentions generative AI running in the game.
Development history
Version 1.1.0.
DeveloperQiita (ao_zzz)Version 1.6.0 adds 16 jobs and about 240 events, two days and five versions after 1.1.0.
DeveloperQiita (ao_zzz)The developer publishes a Qiita post on making the game with Claude Code and fixing it from player feedback.
DeveloperQiita (ao_zzz)
Game performance
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Browser (official site)
Free in the browser with no account; progress is saved only in the browser.
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Sales & revenue (all platforms)
Browser game — there are no downloads to count.
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Sources
| Source | Type | Published | Accessed |
|---|---|---|---|
| Claude Codeに作ってもらい、ユーザーに直してもらったゲーム Qiita (ao_zzz) · Japanese | Devlog | Oct 28, 2026 | Nov 3, 2026 |
| 12万分の1 — 野球人生シミュレーション 1in120k.com · Japanese | Official site | Not confirmed | Nov 3, 2026 |
| Official game page (language check) 1in120k.com (ao_zzz) | Official site | Not confirmed | Nov 3, 2026 |
