Reported in #743: with `parallel_relay > 1`, a single POST (e.g. submitting a comment) reached the destination as N concurrent requests, so the comment got posted twice. Root cause is unfixable from the Rust side: `select_ok` cancels only OUR futures, but Apps Script has no way to learn the cancellation, so every fan-out call still runs to completion and each `UrlFetchApp.fetch()` still hits the destination.
Fan-out now only triggers for idempotent methods (GET / HEAD / OPTIONS); POST / PUT / PATCH / DELETE always go sequential. Same pattern as `SAFE_REPLAY_METHODS` in Code.gs `_doBatch` fallback — safe methods are idempotent so re-firing is at worst wasteful, unsafe methods can have side effects so re-firing is incorrect.
New regression test locks down `is_method_safe_for_fanout` predicate. Tests: 180 lib + 35 tunnel-node green.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
therealaleph
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