Update_Manifest.py

from collections import OrderedDict
import re, json, sys, os

def parseObj(tokens):
	if(type(tokens) == str):
		tokens = re.split(r'[\t\r\n]+', tokens)
		tokens.reverse()
	obj = OrderedDict()
	while len(tokens):
		nt = tokens.pop()
		if nt in ["}", ""]:
			return obj
		elif nt.startswith('"'):
			nt = json.loads(nt)
			val = tokens.pop()
			if val.startswith('"'):
				obj[nt] = json.loads(val)
			elif val == "{":
				obj[nt] = parseObj(tokens)
		else:
			raise Exception("nt = %r" % nt)

def dumpObj(obj, indent=0):
	s = ""
	for key in obj:
		s += "\t" * indent + json.dumps(key)
		if type(obj[key]) == OrderedDict:
			s += "\n" + "\t" * indent + "{\n" + dumpObj(obj[key], indent + 1) + "\t" * indent + "}\n"
		else:
			s += "\t\t%s\n" % json.dumps(obj[key])
	return s

with open(sys.argv[1], "r") as f:
	manifestpath = os.path.dirname(f.name)
	data = f.read()

manifest = parseObj(data)

print(dumpObj(manifest))

if "-username" in sys.argv:
	username = sys.argv[sys.argv.index("-username") + 1]
	appstate = manifest['AppState']
	for depotkey in appstate['StagedDepots']:
		cmd = \
			"DepotDownloader.exe -app " + appstate['appid'] + \
			" -depot " + depotkey + " -manifest " + appstate['StagedDepots'][depotkey]['manifest'] + \
			" -username " + username + " -remember-password -dir \"" + \
			manifestpath + "\\common\\" + appstate['installdir'] + "\" -validate"
		print(cmd)
		os.system(cmd)
		installed_depot = appstate['StagedDepots'][depotkey]
		del installed_depot['dlcappid']
		appstate['InstalledDepots'][depotkey] = installed_depot
		del appstate['StagedDepots'][depotkey]
	appstate['StateFlags'] = "4"
	appstate['UpdateResult'] = "0"
	appstate['StagingSize'] = "0"
	appstate['buildid'] = appstate['TargetBuildID']
	appstate['BytesStaged'] = appstate['BytesToStage']
	appstate['BytesDownloaded'] = appstate['BytesToDownload']
	
	print(dumpObj(manifest))
	
	with open(sys.argv[1], "w") as f:
		f.write(dumpObj(manifest))

@metapea Found the reason for that error XD This testing and adding onto things with almost no knowledge has been painfully fun lol The .acf file you feed to this script is intended to have been written by Steam, with details inside of what update it's trying (and failing) to install.

If your game is already updated, or if for any reason Steam has not filled out the StagedDepots details inside the game's .acf file while trying to update it, this script can't help you right now I think. You might wanna have Steam try to update the game, so it fills out the details in the game's .acf file with the details of the update first. Pretty sure that should have happened before it started trying to download or update anything tho idk whats up with that.

You can just open .acf files with your preferred text editor too, to make sure it's the right file/game and verify what's in there at what point.

It doesn't change at all before, in the middle of, or after a corrupt download update.
the .acf should say what it's downloading with "TargetBuildID", "InstalledDepots" or maybe "SharedDepots"

Oh, so you didn't need an answer for your question what is KeyError: 'StagedDepots'? cool :)