#851 autoGc emits auto_gc_completed event on --json stdout, breaking single-parse consumers

First — thank you for shipping auto-GC (#823), it's exactly the inline-cleanup mechanism I'd hoped for. I hit one side effect worth flagging.

What happens

With autoGc: true enabled, swamp model method run <model> <method> --json emits a second JSON object on stdout after the result: the method's result object (pretty-printed), immediately followed by a line like:

{"event":"auto_gc_completed","versionsDeleted":2,"bytesReclaimed":7508}

So the --json stdout is now two concatenated JSON values. Any consumer that does a single parse of the output breaks:

• python3 -c "import sys,json; json.load(sys.stdin)" → json.decoder.JSONDecodeError: Extra data: line N column 1
• Ruby JSON.parse(stdout) → raises on the trailing object.

In my case this broke several poller bridge scripts and an internal Ruby shellout helper that all consume --json with a single parse — they'd worked fine until autoGc was enabled, then started failing on every run that triggered a collection. Disabling autoGc immediately restored clean single-object --json output and everything parsed again.

Why it's worth fixing

The whole point of --json is machine-readable, single-document output for piping into a parser. An auto-GC lifecycle event appended to that stream means --json is no longer reliably parseable by the most common consumer pattern (one parse of stdout). It also means enabling autoGc silently changes the output contract of every method run for every downstream tool.

A few directions that could work

• Keep the auto_gc_completed event off stdout under --json — emit it to stderr, or to the log stream, or fold it into the single result object (e.g. a gc: field on the existing result) rather than as a second top-level document.
• Or gate the event line behind a verbose/non-json mode so --json stays single-document.

Whichever fits — the key property is that --json stdout stays exactly one JSON value regardless of whether auto-GC fired.

Repro

1. Set autoGc: true in .swamp.yaml on an @swamp/s3-datastore repo with some collectable (expired/excess-version) data.
2. swamp model method run <model> <method> --json | python3 -c "import sys,json;json.load(sys.stdin)"
3. On a run where auto-GC collects something → JSONDecodeError: Extra data. Disabling autoGc → clean parse.

Environment

• @swamp/s3-datastore@2026.06.26.1
• swamp 20260626.202321.0

Thanks again — happy to share fuller sample output if useful.