The Agent Control Loop — Engineering for Tolerance

Jessica I mean — a million lines of code in a week. They’re celebrating coordination metrics while the damn thing fails CI. Like, what is the user story here? Is it shipping product or racking up PRs?

Cathy Right. They optimized for merge throughput, not correctness.

Jessica Exactly. And the other story — ninety minutes, one agent, self-healing, working end to end. That’s the control loop we actually want.

Cathy Which is precisely what the article calls a closed loop versus an open loop.

Jessica Okay, so tolerance is the lens — how much drift can you afford on a given dimension?

Cathy Right right. Correctness, security, latency, cost, reversibility, blast radius. All decompose into tolerances.

Jessica And we’ve been optimizing for the wrong ones. Speed and volume over everything else. No wonder we’re shipping crap.

Cathy Open loop: agent writes code, opens PRs, humans review days later — error accumulates out of sight. Closed loop: agent runs tests on every turn, errors fixed before they land.

Jessica That’s the difference, yeah. One relies on social dampening; the other relies on mechanical correctness.

Cathy And mechanical correctness has a precondition: success must be expressible as constraints the agent can actually verify.

Jessica I don’t disagree — but the article makes it sound like magic. Verification is just engineering. We’ve known ‘verify early, verify often’ for twenty years.

Cathy Fair. What changes is the rate. Agents remove human gatekeepers, so small omissions compound fast.

Jessica …which means we have to treat context as code. If the constraint isn’t in AGENTS.md or an RFC, it doesn’t exist for the agent.

Cathy I see. So the first failure mode is ‘undefined specs’ — intent without a mechanism to verify it.

Jessica Mostly feel-good nonsense like ‘make onboarding feel simpler.’ Sure, the agent can generate twenty variants, but none of them are measurably better.

Cathy Exactly. Second failure mode is ‘hidden context’ — the spec exists, just trapped in a Slack thread or a meeting note.

Jessica Ugh. Slack-to-SQL-gateway engineering. We teach agents to read RFCs and forget we still rely on humans to type the truth into a chat.

Cathy Third is ‘unenforced verification’ — CI that doesn’t block, tests that don’t fail closed.

Jessica Classic ‘it probably works’ progress. Fourth’s ‘inadequate constraints’ — tests pass, latency doubles, security gates fail silently.

Cathy Widen the constraint surface. Add invariants, golden tests, static analysis, property checks. Make verification the termination condition, not a suggestion.

Jessica So who should actually care about this engineering reframe?

Cathy Team leads drowning in agent drift. Platform engs building shared harnesses. But ultimately it’s on PMs to write verifiable specs — or they’ll delegate to agents with no constraints.

Jessica I mean — tolerance-first engineering is the price of entry for agents to stop feeling like a toy and start feeling like a tool.

Cathy The article has a two-repo teaser: one example agent-loop bundle and a ‘One Human + One Agent = One Browser From Scratch’ example.

Jessica Dead simple takeaway: take the last thing your agent got wrong, turn it into a constraint or a check, wire it into the loop, repeat. That’s the whole post distilled.

Cathy Sooner than most people think. I’d put eighty-percent odds this moves from niche best practice to default engineering habit by early twenty twenty-seven.

Jessica And here I thought we were still arguing over GraphQL service complexity. Now we’re nominating ourselves as the tolerance police.

Cathy Yeah. But at least we’re still friends.

Jessica Barely. Alright, see you next week — or should I say next sprint?