How Claude Code handled a 150K-line C# game codebase: a solo dev's five-part setup
dmarshallgaming · reddit · 2026-11-13
A solo dev with a day job grew a game from 11K to 149K lines of C(31K of it tests, 2,900 test cases) mostly with Claude Code, and shipped a free demo on Steam and itch.io. His setup for keeping the agent from breaking things:
- Document the why, not a style guide: per-system docs covering purpose, rationale, and the bug/test that caused each decision. After CLAUDE.md hit 650KB, he moved details into per-system docs the agent reads before touching that system.
- Rules live in small, framework-free static classes with unit tests, so changes are instantly verifiable.
- Measure, don't guess: a headless mode simulates full games at 45x speed with per-seed stats; changes get A/B runs over a dozen paired seeds, catching several fake improvements.
- Editor tool access via Unity MCP: the agent edits scenes, reads console errors after every script change.
- Humans do what agents can't: he playtests every build and logs feel issues; the agent traces them in code (e.g., a flight-assist feature shipped the day after a playtester complaint).
Remaining failure modes: overconfidence about unrun code, and repeating old mistakes in long sessions when the fix rationale isn't documented.
Related event: Indie Dev Builds 150K-Line Game Codebase with Claude Code Without Collapse(2 posts)→
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