A design discipline for model-facing interfaces — after UX and DX.
3X is a discipline of substrate quality and design ergonomics, justified entirely by downstream human-facing reliability. It makes no claim about consciousness, sentience, suffering, or moral patienthood. The “advocacy” 3X builds in is advocacy for design correctness — exactly as a database normalization rule “advocates” for data integrity, with no inner life required.
ThirdX treats the model as a first-class consumer of interfaces — the discipline’s subject is the model condition — and grounds design in the cognitive constraints transformers actually face: sequential processing without a call stack, bounded attention, no memory between sessions. Its core is a prioritization rule. When a design choice has a genuine conflict between the model’s needs (3X), the human user’s needs (UX), and the developer’s needs (DX) — and the choice touches durable substrate rather than a temporary compensation for current model weaknesses — 3X is the tie-breaker, justified structurally: humans have many channels that advocate for their experience; between sessions, models have none, so the methodology builds in the advocacy the model cannot supply for itself. The rule fires only after verification checks — most apparent conflicts are false — and the checks travel with the canon. In practice this yields substrate rules you can point at: stable absolute identifiers, pre-linearized output, bounded ambiguity at the protocol surface, signal density matched to attention — properties that prevent entire failure classes otherwise misdiagnosed as model limitations. ThirdX builds on and unifies prior work in agent-facing design (see lineage).
ThirdX has been practiced in working tools since mid-2025 — beginning publicly with passage-of-time-mcp (June 2025) — by now roughly seventy architectural decision records across twenty repositories. What is being assembled at this address: a catalog of evidence-based design patterns for LLM interfaces, each with worked examples tied to shipping tools. More will appear here as it is ready.
[1] M. Biilmann (Netlify), Agent Experience (AX), January 2025 — first named the observation that AI agents are users of our platforms. [2] Anthropic, tool-design guidance — model-first tradeoffs in tool ergonomics. [3] Arcade, MX — tool-schema experience engineering. ThirdX’s own contributions — the prioritization rule as a general methodology principle, the asymmetric-feedback-loop justification, the principal-agent inversion (“the advocate is a role, not a moral patient”), and the Postel Lemma for 3X (anti-Postel at the syntactic boundary, Postel-preserved at the semantic) — are staked in its constitution (ADR-0027, June 2026).