00 · ORIGIN
Built by a founder and a fleet of agents.
Rensei was built over the summer of 2026 by a founder directing a fleet of coding agents. A spring of hands-on work with agent tooling had shown Mark Kropf what it means when the engineer works for the AI: chat-window-bound workflows, no determinism, no policy frame, no audit. The answer, a deterministic workflow runtime with LLMs as bounded operators inside its steps, is what makes policy, audit, and replay tractable for regulated organizations. The platform as it stands today, the open-source runtime under it, and the record system behind this site were built from June onward, and the git history is the receipt.
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01 · THE NAME
Rensei means refining through practice.
rensei (ren-SEI) - refining through practice
Rensei (錬成) is not a word you hear in everyday Japanese. It belongs to a formal register, the language of craft, martial arts, and serious skill-building. It carries the idea that capability is the product of sustained, deliberate effort, a process you keep practicing.
The kanji read "to forge, temper, refine." Mark grew up around the glow of molten steel at his father's mill, where raw iron was tempered into structural metal under heat and pressure. The anvil-crucible mark is that inheritance: the tool work is shaped against, and the vessel that holds raw material at working temperature. Rensei takes raw agent capability and refines it until it holds under production load.
Donmai (どんまい) sits in the opposite register. The hiragana spelling, the casual phonetic script, signals something approachable and forgiving. Donmai is the thing a teammate says after a mistake on the field: don't worry about it, keep going.
Rensei is the platform. Donmai (どんまい) is the runtime under it, open source and named for its own temperament.
Capability is a process you sustain.
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02 · FOUNDER
Mark Kropf
Mark Kropf
Founder, Rensei
Mark has spent his career on the operational layer above platform shifts. He was Global CTO at Pivotal Software through its $2.7B acquisition by VMware, and he was a founding contributing member of the Cloud Foundry Foundation.
He spent four years in Google's Office of the CTO, responsible for developer experience, then ran global engineering as Group CTO at Genius Sports across eight technology functions after the company listed on the NYSE as GENI. Rensei carries that operating instinct down into the runtime itself.
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03 · METHOD
Agents wrote the platform. The founder directed the fleet.
The platform was written by a fleet of coding agents, Claude, Codex, and Gemini, working end to end across the codebase. I designed the architecture, set the policies, and operated the fleet: dispatching the work, reviewing what came back, and deciding what shipped. The agents did the typing.
That is the operating model Rensei is built to make defensible, so it is the one this company runs on. The platform runs its own fixes through its own SDLC, and the audit rows for those runs exist.
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04 · CONTACT
The operating model is the part to test.
Send the objection and the workflow it comes from.
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