Five Days, Sixteen Versions, Nineteen Games: Building With An AI Coding Agent

🔍 Read the full analysis: Five Days, Sixteen Versions, Nineteen Games: Building With An AI Coding Agent on ThorstenMeyerAI.com

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Developer Thorsten Meyer says he built Altersteilzeit, a free web app for people tracking German partial retirement, in five days with Claude Opus 5.5 serving as a coding agent. The finished site combines a working-day countdown, a short narrated film and 19 3D games; Meyer says he directed the work, made product choices and approved releases.

Meyer’s account, published October 13 on ThorstenMeyerAI.com, traces the project from a request on Monday, September 28, for a short animated countdown film to a public launch by Friday, October 2. He says all 24 commits were co-authored by Claude Opus 5.5. The account is a first-person description of the development process, not an independent audit of the code, timeline or agent’s contribution.

Users enter the start date of either their active work phase or passive leave phase and select a model from 1:1 to 5:5. The app calculates the other date and estimates remaining working days, subtracting public holidays for the selected German federal state and leave entered by the user. Its daily view includes milestones, a progress display and a saying of the day; additional features include calendar exports, a printable calendar and a bridge-day finder.

The site also includes a roughly one-minute film, a team countdown, badges and games based on years in the active phase. Meyer describes support for seven languages and five German dialects, while saying the project produced voice clips in 12 languages. He says the app has no accounts, cookies or tracking and calculates dates in the browser. Shared dates are placed after the “#” in a web address, which browsers do not send to a server in ordinary requests.

A Countdown App Becomes a Test Case

The project offers a concrete example of how an AI coding agent may be used across a small web product, from feature development and testing to text, audio and deployment. Meyer reports that the agent produced about 18,000 lines of JavaScript and Python, 15 test scripts, 1,224 voice clips and 17 sound effects. Those figures come from his account and do not by themselves establish how much time or cost the agent saved, or how the work compares with a human-led build.

The app also shows why speed is only part of the story. Its premise depends on choices about tone and audience: Meyer says he chose the sea as an image of freedom, wanted the film to feel cinematic, and decided which voices and dialects to include. In his telling, the agent expanded and implemented a product concept, while the human developer remained responsible for direction and approval. That division is relevant to readers weighing what AI tools can do in software projects—and what still depends on human judgment.

From Partial Retirement to Playable Calendar

Altersteilzeit, or partial retirement, can be arranged in a block model: an employee works as usual during an active phase and is then released from work during a passive phase of equal length. Meyer says the app was prompted by the experience of counting down the remaining working days. The calculation depends on dates, the selected work pattern, local public holidays and any leave entered, rather than simply counting calendar days.

In the games, real time supplies the structure. Meyer says the maze maps months and weeks onto its layout, while Calendar Stack uses two working weeks for a row and treats a public holiday as clearing one. Other games turn working days into emails or months into night shifts. The article describes 19 game modes, though one passage in the source refers to fifteen modes; the exact relationship between modes and games is not explained.

“The interesting part is not how much an AI agent can build, but what was still left for me to do.”

— Thorsten Meyer, developer of Altersteilzeit

The Build Claims Have Limits

Meyer’s article does not provide an external verification of the five-day timeline, commit authorship or production totals. It also does not report independent testing, user numbers, accessibility reviews, operating costs or whether the app has been assessed for date-calculation errors across German states. The privacy description is Meyer’s account of how the site works; no separate technical review is cited.

There are also details left unresolved in the source. It refers to 19 games in its headline and feature list, but later describes the game as playable in fifteen modes. It says the site supports seven languages and five dialects, while citing audio production across 12 languages. The article does not explain whether those counts describe different parts of the product, or how many of the language versions are available throughout the app.

Public Release and Further Verification

Meyer says the app was online by October 2 and describes it as free, but his account does not set out a future development schedule. The next useful evidence for readers would be the live product’s full language and game listings, documentation of its date calculations and privacy practices, and updates from Meyer on fixes or new releases. No independent evaluation or user data is included in the account published October 13.

Key Questions

What is Altersteilzeit?

Altersteilzeit is a German partial-retirement arrangement. In the block model described by Meyer, a person works through an active phase and then has a passive phase away from work.

What does the app calculate?

Users enter the start of one phase and choose a model from 1:1 to 5:5. Meyer says the app calculates the other date and counts working days, accounting for public holidays in a selected German state and leave entered by the user.

What did the AI coding agent do?

Meyer says Claude Opus 5.5 helped with implementation, tests, translations, voice clips, sound effects and deployment. He says he set the direction, made creative and product decisions, and approved releases; the account has not been independently audited.

Is the app independently verified as private?

Meyer says there are no accounts, cookies or tracking and that calculations happen in the browser. The source article provides no independent privacy or security review, so that description remains the developer’s claim.

Source: ThorstenMeyerAI.com