attempt to yield across metamethod/C-call boundary

What the message means

attempt to yield across metamethod/C-call boundary

A Roblox script can pause (yield) and continue later. That works only when every function between the pause and the script's start can be paused too. Some functions are run by Luau itself, in C: table.sort calling your comparator, string.gsub calling your replacement function, and the engine calling a metamethod. Those cannot be paused halfway, so a yield inside them stops with this error.

pcall is not one of them in current Luau: you can wait inside a pcall. The message itself may not carry a line number: read the stack lines under it in Output, which name the script and the line of the sort, gsub or lookup that started it.

Causes and fixes

Three things cause this error in practice. Each one below has the smallest code that triggers it, then the fix.

• The table.sort comparator waits
• An __index metamethod waits
• A string.gsub callback waits

The table.sort comparator waits

Sorting a leaderboard by player name, and fetching each name inside the comparator, is the classic case. GetNameFromUserIdAsync waits for a web request, and table.sort cannot wait.

local Players = game:GetService("Players")

local userIds = { 156, 1, 261 }

table.sort(userIds, function(a, b)
	-- yields inside the comparator: attempt to yield across metamethod/C-call boundary
	return Players:GetNameFromUserIdAsync(a) < Players:GetNameFromUserIdAsync(b)
end)

Fetch everything the comparator needs before you sort, and keep the comparator to plain comparisons. As a bonus each name is fetched once instead of on every comparison.

local Players = game:GetService("Players")

local userIds = { 156, 1, 261 }

local names: { [number]: string } = {}
for _, userId in userIds do
	local ok, name = pcall(Players.GetNameFromUserIdAsync, Players, userId)
	names[userId] = if ok then name else tostring(userId)
end

table.sort(userIds, function(a, b)
	return names[a] < names[b]
end)
print(userIds)

An __index metamethod waits

A "lazy" table that finds objects the first time you ask for them often calls WaitForChild in its __index. The first lookup of a missing key yields inside the metamethod.

local ReplicatedStorage = game:GetService("ReplicatedStorage")

local Remotes = setmetatable({}, {
	__index = function(self, name)
		-- WaitForChild yields: attempt to yield across metamethod/C-call boundary
		local remote = ReplicatedStorage:WaitForChild(name)
		rawset(self, name, remote)
		return remote
	end,
})

local buy = Remotes.BuyItem
print(buy)

Use a normal function for anything that may wait, and keep metamethods instant. Code that calls Remotes.get("BuyItem") reads just as clearly and can wait safely.

local ReplicatedStorage = game:GetService("ReplicatedStorage")

local cache: { [string]: Instance } = {}

local Remotes = {}

function Remotes.get(name: string): Instance
	local found = cache[name]
	if not found then
		found = ReplicatedStorage:WaitForChild(name)
		cache[name] = found
	end
	return found
end

local buy = Remotes.get("BuyItem")
print(buy)

A string.gsub callback waits

Filling placeholders in a message with values that take a request to get, such as {name} replaced by a user name, puts the yield inside the function gsub calls.

local Players = game:GetService("Players")

local template = "Top player: {156}"

local text = string.gsub(template, "{(%d+)}", function(id)
	-- yields inside gsub: attempt to yield across metamethod/C-call boundary
	return Players:GetNameFromUserIdAsync(tonumber(id) or 0)
end)
print(text)

Collect the placeholders first with string.gmatch, fetch their values in a normal loop, then pass gsub a table of ready answers. gsub accepts a table as the replacement and looks each capture up in it.

local Players = game:GetService("Players")

local template = "Top player: {156}"

local values: { [string]: string } = {}
for id in string.gmatch(template, "{(%d+)}") do
	local ok, name = pcall(Players.GetNameFromUserIdAsync, Players, tonumber(id) or 0)
	values[id] = if ok then name else "someone"
end

local text = string.gsub(template, "{(%d+)}", values)
print(text)

How to find the line that causes it

1. Open Output (Window menu, or the Script tab), clear it with Ctrl+K (⌘K on Mac), and press Play.
2. Find the first red line. It starts with the script's place in the Explorer and the line number, such as ServerScriptService.Shop:12:.
3. Click the red line. Studio opens that script with the cursor on the failing line.
4. Read the lines between Stack Begin and Stack End under it: they list the calls that led there, newest first, so you can see which function called the broken one.
5. Still not sure which value is wrong? Add print(typeof(x), x) just above the line, or set a breakpoint on it and read the values in the Watch window.

Questions

Sources and further reading

• Roblox Creator Docs: table.sort
• Roblox Creator Docs: metatables
• Roblox Creator Docs: Players:GetNameFromUserIdAsync

How the code was checked: every block marked Fixed passes Luau's strict type checker against Roblox's API definitions (luau-lsp, 4 November 2026), and every block marked Breaks compiles and fails only when it runs. The message wording was checked against the message strings in Roblox Studio itself (macOS build installed on 24 October 2026) on 4 November 2026, and each cause was reproduced with the Luau command line tool the same day: a yield inside a table.sort comparator, a string.gsub callback and an __index metamethod each print this message. A type check catches misspelt APIs and wrong types, not game logic, so playtest in Studio before you publish.