Will This File Sync? Name and Path Checker

When every file in a folder syncs and one does not, the cause is usually sitting in plain sight. The name.

This is the least satisfying explanation in computing, and the most common. There is no corruption, no account problem, nothing to reinstall. The file contains a character the platform has refused since before OneDrive existed, or its path crossed a limit you were never shown.

Why the failure is silent

What makes this hard is not the rule. It is that nothing tells you.

On macOS, Microsoft notes that the restricted characters are supported. So a Mac will create Client: final.docx without complaint. Finder shows it. You can open it, edit it, and put it in a OneDrive folder.

The name only becomes illegal when it reaches the service. And at that point the failure is one file quietly not appearing on another device, which nobody notices until they need it.

The 400 is not the 260

Almost everyone who has heard of a path limit has heard of 260 characters, the old Windows MAX_PATH. That is a different number for a different thing.

Microsoft's figure for OneDrive and SharePoint is that the entire decoded file path, including the file name, cannot contain more than 400 characters. There is a separate 255-character cap on any single segment, from the operating system rather than from OneDrive. And on Windows, the OneDrive root folder plus the relative path cannot exceed 520 characters.

Three limits, three different numbers, and the one people quote is none of them.

The practical consequence: a file with a perfectly short name can breach the limit because of where it lives. notes.txt is eight characters and completely safe until it sits eleven folders deep under a SharePoint site with a long name.

Fix it high in the tree

This is the part worth taking away.

When a path is too long, the instinct is to rename the file. That gains you a handful of characters and fixes one file.

Rename a folder near the top instead. Shortening 2026 Client Projects and Proposals to 2026 Clients removes 26 characters from every single path beneath it — hundreds of files fixed by one edit, and the ones that were nearly over the line stop being fragile.

The names that are not really yours

Two entries on the reserved list are worth recognising rather than avoiding.

Anything starting with ~$ is an Office lock file. You did not create it and you do not want it synced. If you are seeing these, a document is open somewhere — possibly on a machine you have forgotten about.

.tmp files are not blocked, they are skipped. Microsoft states temporary TMP files will not be synced. That distinction matters: a blocked file produces an error, a skipped file produces nothing at all. If your application writes its working file as .tmp and you are relying on cloud sync to back it up, it is not being backed up.

And CON, PRN, AUX, NUL, COM1, LPT1 — these are device names from MS-DOS, still reserved four decades later. CON.txt fails for the same reason CON does.

What this checker cannot tell you

It checks names and paths against documented rules. It does not know whether your account is signed in, whether the file is open in another application, whether you are out of storage, or whether the sync client has stalled.

If the name comes back clean, the cause is elsewhere — and the full fix report for this symptom works through the other causes in order, each with its own check.