Now stable is the CI line you can promise

So Buildkite Agent shipped v4.0.0 this morning. We flagged it Important as a breaking change. The sentence that matters is v4 is now stable. Not systemd watchdog, and not the Ubuntu 26.04 image. That's the status word. Once it's stable you can talk about it on a call as the line people run. The breaks in the same note are the ones that hit the account.

I keep treating a v4 like a version bump. Kind of the wrong habit. Most of the v4 work was already backported to v3, so this changelog is the leftover behavior that still differs. They tell you to review the v3 to v4 upgrade guide before you upgrade. To the best of my understanding that's the document, not the tag. v3 is still the line a lot of agents are on. v4 being stable does not mean anyone flipped overnight. It means you can start talking about v4 as the current agent.

The Important flag caught the breaks. The deprecated built-in Docker integration is gone. The docker and docker-compose plugins still work. Cancelled Windows jobs now return exit status 1 instead of 0, so a cancel that used to look green is a fail. Pipeline uploads reject interpolated secrets by default. OpenTracing and direct DogStatsD are out. Use OpenTelemetry instead. ANSI timestamps are always on. That's the kind of list an AE has to carry if anyone in the account runs the agent.

Recency still wants you looking at Next.js canaries. Fine. Recency is not the upgrade guide.

We pull it into a uniform entry shape at /sources/buildkite-agent-updates. Same JSON as everything else. The talking point is now stable, then Docker is a plugin, Windows cancel is exit 1, and secrets are rejected unless you allow them. The watchdog and the Ubuntu image are decoration.