https://21xventures.com/blog/ Short notes from 21xVentures on AI, building, and the work in front of us. A small AI-first tech and consulting practice, from Brazil. en Mon, 11 May 2026 12:00:00 +0000 https://21xventures.com/blog/the-demo-trap/ https://21xventures.com/blog/the-demo-trap/ Mon, 11 May 2026 16:00:00 +0000 Most AI demos optimize for convincing the room. The ones that translate to production optimize for telling the truth. Four sins of demos that lie, and the test that catches them. https://21xventures.com/blog/when-evals-stop-telling-truth/ https://21xventures.com/blog/when-evals-stop-telling-truth/ Mon, 11 May 2026 12:00:00 +0000 Your eval set is supposed to be the truth. It can quietly stop being it. Four ways evals lie, the quarterly audit that catches them, and the discipline of killing examples. https://21xventures.com/blog/latency-budgets/ https://21xventures.com/blog/latency-budgets/ Mon, 04 May 2026 12:00:00 +0000 Cost gets a ceiling. Latency rarely does — until users churn. The interactive bar, the budget hierarchy, and the four levers when you're over the line. https://21xventures.com/blog/structured-outputs-validation-trap/ https://21xventures.com/blog/structured-outputs-validation-trap/ Tue, 07 Apr 2026 12:00:00 +0000 How to get a model to return clean JSON without paying the retry tax. The minimum schema design, the validation pattern, and the three schema failures that quietly leak budget. https://21xventures.com/blog/picking-a-model-size/ https://21xventures.com/blog/picking-a-model-size/ Tue, 17 Mar 2026 12:00:00 +0000 Smaller models do most of the work — when given the right work. A five-step process for sizing models to tasks, and the three signals you've picked wrong. https://21xventures.com/blog/tool-use-vs-agents/ https://21xventures.com/blog/tool-use-vs-agents/ Mon, 23 Feb 2026 12:00:00 +0000 Most "we need an agent" problems are tool-use problems, and most tool-use problems are prompt problems. The hierarchy of complexity — and the cost of skipping a rung. https://21xventures.com/blog/retrieval-that-earns-its-keep/ https://21xventures.com/blog/retrieval-that-earns-its-keep/ Mon, 02 Feb 2026 12:00:00 +0000 Most RAG isn't worth it. A four-question test for when to add retrieval, the three failure modes that turn it into a debugging burden, and what to try first. https://21xventures.com/blog/prompt-versioning/ https://21xventures.com/blog/prompt-versioning/ Mon, 12 Jan 2026 12:00:00 +0000 How to keep a prompt from becoming 30 untracked variants in 30 places. A four-rule discipline that scales from one prompt to a hundred. https://21xventures.com/blog/cost-ceilings/ https://21xventures.com/blog/cost-ceilings/ Mon, 01 Dec 2025 12:00:00 +0000 Most AI features die for cost, not quality. Set the unit-economics ceiling before you ship, watch the four cost vectors, and know the three levers when you're over budget. https://21xventures.com/blog/logging-for-llm-systems/ https://21xventures.com/blog/logging-for-llm-systems/ Mon, 10 Nov 2025 12:00:00 +0000 What to capture before you regret not capturing it. The minimum log schema for any AI feature in production — and the three questions it should let you answer in under five minutes. https://21xventures.com/blog/where-humans-belong/ https://21xventures.com/blog/where-humans-belong/ Mon, 20 Oct 2025 12:00:00 +0000 Every AI feature has humans somewhere in the loop. Most teams put them in the wrong place. Four placement modes — and a four-question test for picking the right one. https://21xventures.com/blog/evals-before-features/ https://21xventures.com/blog/evals-before-features/ Mon, 29 Sep 2025 12:00:00 +0000 The unit-test playbook for LLM systems. How to build your first 50-example eval set in a week — and why every team that skips this step pays for it later. https://21xventures.com/blog/the-ai-ambition-gap/ https://21xventures.com/blog/the-ai-ambition-gap/ Mon, 08 Sep 2025 12:00:00 +0000 Almost every team has shipped an AI demo. Almost none have shipped an AI feature their users rely on every day. A diagnostic — and a three-question filter to get unstuck.