I believe 8848.5 under modern rules (with the 75-move rule, as in FIDE rules since 2014). There's a reasonably rigorous demonstration here:

https://wismuth.com/chess/longest-game.html

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LegionMammal978 6 hours ago | parent | next [-]

Thanks for that link! I was looking for that answer a few years ago, but I couldn't find anyone who had carried it out all the way through (with the cost of "switching control" fully accounted for), nor many people who were even aware of the 75-move rule.

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sim7c00 5 hours ago | parent | prev [-]

how can the longest game have half a move???

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InitialLastName 5 hours ago | parent | next [-]

"Moves" are counted for both players. Half a move means White moved but Black hasn't yet.

	▲	sim7c00 4 hours ago \| parent [-]
		thank you!

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pxx 2 hours ago | parent | prev [-]

An A press is an A press. You can't say it's only a half.