[rnegron]

Problem

• Scheduled Backend CI runs go red with no test findings: a job dies during setup, and a rerun passes.
• The runner fails with Operations that change non-concurrent collections must have exclusive access, or leaves GITHUB_EVENT_PATH empty so the next action crashes on SyntaxError: Unexpected end of JSON input.
• Cause: parallel: step branches that each download an action write the runner's action registry from two threads.
• Sample of 12 scheduled runs: 4 went red on this alone (16:32, 12:40, 11:28, 01:42). The product-test matrix is ~46 jobs, so one bad job is enough.

Changes

• Setup steps that were parallel now run in order, in the 3 workflows whose branches downloaded actions concurrently. Nothing else about them changes: every step keeps its name, uses, with, and if untouched.
• parallel: blocks whose branches are all run: steps keep the speedup. They download nothing.
• cd-sandbox-base-image.yml also keeps it, with a comment saying why: those branches are Depot image builds, and running three multi-platform builds one at a time costs more than the crash has ever cost there.
• Mirrored into .depot/workflows/ci-backend.yml to keep the shadow apples-to-apples.
• New entry in docs/internal/ci-things-already-tried.md, and a warning at each site, so the pattern is not reintroduced.

How did you test this code?

• Parsed both revisions of every touched workflow and compared the step lists with the parallel blocks flattened. Identical in every job, so the change is nesting only.
• bin/hogli lint:workflows, actionlint, and hogli ci:preflight pass.
• The failure is a runner race, so I could not reproduce it locally or prove absence. The check is that CI still passes and the crash stops appearing on scheduled runs.
• The blocks came from #76651, which reported no before/after timings, so there is no measured win to weigh against this.

Automatic notifications

• Publish to changelog?

Docs update

None.

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

• Claude Code (Opus 5), starting from a master-red alert in #alerts-devex.
• Skills invoked: /authoring-ci-workflows, /code-review, /simplify.
• The first draft also serialized cd-sandbox-base-image.yml. Review caught that those branches are image builds rather than setup steps, so they are left alone.
• Considered keeping parallel: and pre-warming the action downloads instead. Rejected: nested uses: inside a composite resolve lazily, so there is no reliable point to pre-warm them.
• Two other reds in the same window are separate and not addressed here: a temporal.download fetch failure, and a warehouse-sources snapshot-pin flake that #93155 already targets.

https://claude.ai/code/session_013SombsNtSjMRnGQPaSF55i

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🤖 CI report

⚠️ Trunk lane — backend Python lane

This PR is assigned to the backend Python lane. It runs backend Python tests and may merge in parallel with PRs in other lanes.

ℹ️ Docs preview — preview build triggered

Docs from this PR will be published at posthog.com.

Project	Preview	Updated (UTC)
posthog.com	Open preview	Sep 2, 2026, 8:35 PM

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