GitHub Actions upload-artifact v4: "already exists" — the matrix artifact name collision that became a hard failure

verbatim errorError: Failed to CreateArtifact: Received non-retryable error returned from GitHub: Request failed with status 422: an artifact with this name already exists on the workflow run (artifact: build-output, matrix job: test (ubuntu-latest, node 20))

Problem

A matrix workflow that ran fine for a year starts failing on every job after the first:

Error: Failed to CreateArtifact: Received non-retryable error returned from GitHub: Request failed
  with status 422: an artifact with this name already exists on the workflow run
  (artifact: build-output, matrix job: test (ubuntu-latest, node 20))

The trigger was invisible in the diff: the action version went from upload-artifact@v3 to @v4.

Root cause

upload-artifact@v3 allowed matrix jobs to upload artifacts with the same name, silently overwriting each other — the classic bug that only showed up later when a download fetched the wrong variant. v4 made each artifact an immutable feed-stored entry with a unique-name requirement: the second matrix job uploading build-output to the same run gets a hard 422, because the old overwrite behavior was the bug being fixed.

So this error is the new collision detection working as designed — the workflow's artifact names were never unique, and v3 was hiding that. The related downstream break: any download-artifact step that fetched by the old shared name now finds nothing or gets one job's variant, since v4 stores each name separately.

fix preview — first 3 of 21 lines (yaml), truncated:

jobs:
  build:
    strategy:
… 18 more lines in the fix

🔒 the fix — including 2 code blocks — is members-only. $1/mo unlocks everything.