This is the most concrete thing I read today on why agent architecture matters in production, and the framing belongs in the harness debate alongside Anthropic's and Glean's. The frame: agent codebases that survive past six months don't survive because the team has more discipline. They survive because the architecture made the bad shape harder to write than the right one. That's a sharper claim than the "harness is the backend" version. It says the production failures are reproducible across teams because the abstractions allow them. The four canonical month-six failures Mike lists are worth memorizing: → Class-level mutable defaults shared between agents the moment a second user shows up → Tool functions that accept any string and return None on every kind of failure → Session memory mutated by an LLM-extracted string, silently poisoning every subsequent action → Multi-agent setups passing the parent's full conversation history to a sub-agent because it was the easiest wire-up Every team I've talked to in the last year has shipped at least two of these to production. The fix posts always say the same thing: validate inputs, isolate state, propagate spans, and bound your loops. Discipline. It gets forgotten in approximately every codebase. What Mike's arguing is the React-of-2013 move. jQuery apps scattered DOM state across whichever closure was handy. The discipline of "keep state in one place" was well understood and ignored everywhere. React made the discipline structural: the bug class went away because the framework stopped allowing the bad shape. Worker / Function / Trigger does the same thing one layer up. Class-level mutable state stops being expressible because worker invocations are stateless and persistent state lives in a memory worker addressed by namespace. Two agents in two processes can't share Python state because there is no shared Python state. Tool functions returning None on every failure stop being expressible because every function has a typed input/output schema the engine validates at the boundary. Wrong-shaped input gets rejected before the worker code runs. Failures return typed events with status and error type. Cross-agent history leakage stops being expressible because sub-agents are workers with their own context, called by function ID. The orchestrator passes a payload, not a conversation buffer. Agent Loops without step bounds stop being expressible because step bounds and timeouts are engine-level config, not something the agent author remembers to wrap. The Claude Code April 2026 postmortem is the cleanest evidence anyone's produced for why this matters. Three runtime changes, no model change, dropped median thinking length 73% and pushed retry rates up 80x. The community had to surface this from sampled session logs because most production systems don't ship that level of runtime telemetry by default. Making it default is the lever.