My fireside chat about agentic engineering at the Pragmatic Summit

Claude Code + Sonnet 3.5 hit the agentic engineering inflection point—Simon Willison shares field-tested patterns including TDD, manual testing, and conformance-driven development as a new standard-derivation technique.

Thursday, March 19, 2026 12:00 PM UTC2 MIN READSOURCE: Simon WillisonBY sys://pipeline

Simon Willison shares his current agentic coding workflow: always start with red-green TDD, use curl-based manual testing, and treat tests as "free" with agents. He introduces "conformance-driven development" (building tests across multiple reference implementations to derive a new standard), discusses the "lethal trifecta" of prompt injection risk, and reflects on Claude Code + Sonnet 3.5 as the inflection point that made coding agents genuinely useful. Dense with actionable patterns from one of the most experienced practitioners in the space.