stack overflow

What the message means

ServerScriptService.Respawn:4: stack overflow

Other forms of the same message:

• '__index' chain too long; possible loop

The line in the message is where the stack ran out, which is inside the repeating function. The stack under the error repeats the same lines many times: those lines are the loop.

If you meant something to repeat forever, such as a game loop, use while true do ... task.wait() end instead of a function calling itself.

Causes and fixes

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

• A function calls itself with no way to stop
• A repeating task was written as a function calling itself
• A metatable sends a lookup back to itself

A function calls itself with no way to stop

Recursion is fine when each call gets closer to an end. A countdown that calls itself again without checking for zero never ends.

local function countdown(n)
	print(n)
	countdown(n - 1) -- no stopping point: stack overflow
end

countdown(10)

Add the stopping condition before the call. For a simple count, a loop is clearer and cannot overflow.

local function countdown(n: number)
	if n < 0 then
		return
	end
	print(n)
	countdown(n - 1)
end

countdown(10)

-- the same as a loop
for i = 10, 0, -1 do
	print(i)
end

A repeating task was written as a function calling itself

A round loop or a spawner that ends by calling itself again looks fine, because each round waits. But every call stays on the stack until the game ends, so the script fails after about 20,000 waves (the depth the Luau command line tool reached in our test). That can be hours into a server, which makes it hard to reproduce.

local function spawnWave(wave)
	print("wave", wave)
	task.wait(1)
	spawnWave(wave + 1) -- grows the stack every wave
end

spawnWave(1)

Use a loop for repeating work. Each pass finishes before the next starts, so nothing builds up.

local wave = 1
while true do
	print("wave", wave)
	task.wait(1)
	wave += 1
end

A metatable sends a lookup back to itself

In class-style code, __index pointing at the table itself (instead of the class) sends every missing key round in a circle. Luau stops it with '__index' chain too long; possible loop. A __index function that reads self[key] again gives a plain stack overflow.

local Pet = {}

local pet = setmetatable({}, {})
getmetatable(pet).__index = pet -- points at itself, not at Pet

-- '__index' chain too long; possible loop
print(pet.Name)

Point __index at the class table, which holds the shared methods, and give each object its own fields.

local Pet = {}
Pet.__index = Pet

function Pet.new(name: string)
	local self = setmetatable({}, Pet)
	self.Name = name
	return self
end

function Pet.greet(self: typeof(Pet.new("")))
	print("Hi, I am " .. self.Name)
end

local pet = Pet.new("Rex")
pet:greet()

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: functions
• Roblox Creator Docs: metatables
• Roblox Creator Docs: control structures (loops)

How the code was checked: every block marked Fixed passes Luau's strict type checker against Roblox's API definitions (luau-lsp, 23 October 2026), and every block marked Breaks compiles and fails only when it runs. The message wording was checked by running the broken code with the Luau command line tool and against the message strings in Roblox Studio itself on 23 October 2026. A type check catches misspelt APIs and wrong types, not game logic, so playtest in Studio before you publish.