Battle.net Error BLZBNTAGT00000840, update agent could not modify files

Error BLZBNTAGT00000840 means the Battle.net update agent could not modify the files it needs, usually because they are locked or in use by another program. Blizzard's own description says the agent could not change necessary files on the hard drive, and it most often appears when downloading or updating a game.

The fix, in order

  1. Restart the computer. A restart makes sure no part of the game is still running in the background and holding the files locked.
  2. Run Battle.net as administrator. Right-click the Battle.net shortcut and choose Run as administrator, which gives the agent permission to modify the game files.
  3. Repair the game. In the Battle.net app, open the game page, click the gear icon next to Play, and choose Scan and Repair, then let it fix any damaged files.
  4. Update or temporarily uninstall security software. Antivirus and firewall programs can mistake the Battle.net app for a threat and block it from changing files, so update them or test with them paused briefly.
  5. Clear the Battle.net cache. Close the app fully, delete the Battle.net folder in ProgramData, then reopen the app and let it rebuild the cache.
  6. Check the drive for free space. An update cannot write files to a full drive, so free up space and try the update again.

If it still happens

If the error appears at the same point in every download, the game files may be corrupted beyond repair, so uninstall the game and install it again from Battle.net. If it appears across every Blizzard game, check the drive health and try installing to a different drive, since a failing drive can lock files during writes.

Common questions

What causes error BLZBNTAGT00000840? The update agent could not modify files it needs, usually because they are locked by another program, blocked by security software, or held by a stuck game process. A restart and running as administrator fix most cases.

Is BLZBNTAGT00000840 a server problem? Not usually. It is a local file problem on the PC, which is why Blizzard's fix is to restart, repair the files, and check security software, rather than to wait for a server fix.

Will Scan and Repair delete my game? No. It checks the installed files and replaces only the damaged or missing ones, leaving saves and settings in place.

Related errors

Battle.net Error BLZBNTBGS000003E8 · Battle.net Error BLZBNTBNA00000005

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