Klaviyo's "Dark Factory" writes its own specs and builds through the weekend, and all 2,300 employees had to hit "L3" agent fluency by June.

Co-CEO Andrew Bialecki described a system that takes a prompt, writes the spec, decomposes the problem into subsystems, writes the contractual API interfaces between them, then dispatches subagents against each piece, interrupting only when requirements are ambiguous. Every employee, including PMs, designers, sales and marketing, was required to reach L3, meaning constantly running multiple agent sessions, by end of June. All of them committing code. Its marketing agent Composer, first prototyped over a single weekend by other agents, hit 95,000+ users in month one with about 25% weekly return and credit consumption growing 30% week over week. SaaStr The contractual-interfaces step is the part I'd copy. Writing the API contracts between subsystems before dispatching subagents is what makes parallel agent work merge cleanly.