What they did
The poster, u/sammyranks, was paying about $100 a year for an online invoicing service and decided to have an agent build a replacement instead. They describe themselves as having close to no coding knowledge. They ran Qwen 3.8 27B (a 4-bit GGUF with a 120K context) in LM Studio on an RTX 5090 and pointed the OpenCode 2 harness at it with default settings. Their part was writing detailed feature requests, usually with screenshots of what they wanted; OpenCode's agents wrote the code, fixed bugs and ran web searches. The agent also built import tools that pulled in more than five years of the poster's old invoices and records. A late feature was done with Qwen 3.6 35B A3B as a test.
How it went
The poster says the finished desktop app covers the paid service's features: invoices, quotes that convert into invoices, templates, discounts, taxes and shipping, partial payments, PDF output, emailing, recurring invoices and overdue reminders, plus a dark mode. They put the total at roughly 50 hours, most of it spent writing prompts and testing, and estimate about $15 in electricity. Context stayed between 57K and 90K thanks to automatic compaction. The reported usage was 294 million input tokens and 1.26 million output tokens.
Worth knowing
Commenters doubted the input token count, and others suggested cached tokens were counted on every turn. Several pointed out that the code has had no security review and that free open-source invoicing tools already exist; the poster said they plan a review and declined to share the code because it holds business data. The setup needs a GPU with about 31GB of free VRAM.