Steam Cloud "Unable to sync" — caused by oversized screenshots (>~256 MB), not by brackets in filenames - Oxygen Not Included

Summary

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Steam Cloud intermittently shows "Unable to sync" for ONI. The common advice online

blames brackets/special characters in save names, but in my case that was a red herring.

The real cause was oversized PNG screenshots that ONI itself generated.

Evidence (from Steam's logs/cloud_log.txt)

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The SAME file failed on every sync attempt, while everything else uploaded fine:

[AppID 457140] Upload OK ... Biotrix Cycle 904.sav (10 MB) -> ok

[AppID 457140] Upload OK ... The Super Abyss ..._1623....png (22 MB) -> ok

[AppID 457140] Upload Invalid Parameter ... ..._1560....png (483 MB) -> FAILS

[AppID 457140] Upload complete, result Invalid Parameter

A single "Invalid Parameter" file aborts the whole sync batch -> "Unable to sync".

File sizes in my cloud_save_files screenshot folder:

..._1395....png = 669 MB -> Invalid Parameter

..._1560....png = 483 MB -> Invalid Parameter (this is the one that always failed)

The Super Abyss.png = 150 MB -> uploads OK

..._1623....png = 22 MB -> uploads OK

*.sav = ~18 MB -> upload OK

So the practical per-file upload limit sits somewhere between ~150 MB (ok) and ~480 MB

(fails) — consistent with the ~256 MB single-PUT limit of the cloud storage backend

(uploads go to *.blob.core.windows.net / *.storage.googleapis.com).

Renaming the files to remove brackets did NOT fix it; only removing the >256 MB files did.

Workaround

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Move/delete any PNG > ~250 MB out of the cloud_save_files folder. Sync goes green again.

Suggested fix

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- Don't let a single failed file abort the whole batch (skip + warn instead), and/or

- Cap/compress auto-generated colony screenshots so they never exceed the cloud per-file limit.

Steam Cloud "Unable to sync" — caused by oversized screenshots (>~256 MB), not by brackets in filenames