Debugging

Things will break. Here's how to find out why.

Printing to the Output Log

Your first debugging tool is the Output Log (Window > Output Log in HELIX Studio).

Blueprint

Blueprint— Print String Debugging

For formatted output, use Format String or Append nodes to build your message before passing it to Print String.

C++ equivalent (for reference)

// Use the "Print String" node
// Connect your value to the In String pin
// Check "Print to Log" to see it in Output Log
// Check "Print to Screen" to see it as an overlay

Lua

-- Basic print (goes to Output Log)
print("Hello from Lua!")

-- Print variables
local health = 75
local name = "Player1"
print(string.format("Player %s has %d HP", name, health))

-- UE log with categories
UE.UKismetSystemLibrary.PrintString(
    nil,                    -- world context
    "Debug message",        -- message
    true,                   -- print to screen
    true,                   -- print to log
    UE.FLinearColor(1, 1, 0, 1),  -- yellow
    5.0                     -- screen duration
)

JavaScript

// Standard console methods (all go to Output Log)
console.log("Info message");
console.warn("Warning message");    // yellow in the log
console.error("Error message");     // red in the log

// Structured data
const playerData = { name: "Player1", health: 75, position: { x: 100, y: 200 } };
console.log("Player data:", JSON.stringify(playerData, null, 2));

// Quick timing
console.time("SpawnEnemies");
spawnAllEnemies();
console.timeEnd("SpawnEnemies");  // prints "SpawnEnemies: 12.34ms"

Blueprint Debugging in HELIX Studio

HELIX Studio (built on UE5) has a powerful visual debugger for Blueprints:

1. Breakpoints -- Click the left edge of any Blueprint node to set a breakpoint (red circle). Execution pauses there during Play-in-Editor.
2. Watch values -- While paused, hover over any pin to see its current value.
3. Step controls -- Use the toolbar buttons to Step Into, Step Over, or Resume.
4. Blueprint Profiler -- Window > Developer Tools > Blueprint Profiler shows execution time per node.

Enabling debug flow

When you hit Play with breakpoints set, the editor highlights the execution path with animated wires. This makes it easy to see which branch of an If/Branch node was taken.

Common Errors and Fixes

Error	What it means	Fix
Accessed None	You're using a variable that's null	Add an IsValid check before using the reference
Cast Failed	The object isn't the type you expected	Use the Cast Failed pin to handle this case
Infinite loop detected	A loop ran too many iterations	Check your loop exit condition; use a counter as a safety net
Module not found (Lua)	require can't find your file	Check the module path matches your folder structure
Cannot read property of undefined (JS)	Accessing a field on null/undefined	Add null checks or use optional chaining (?.)

Debugging Network Issues

Multiplayer bugs are the trickiest. Some tips:

• Use HasAuthority to check if code is running on the server or client. Print which side you're on.
• Watch the Net Mode in HELIX Studio's play settings. Test with "Play As Client" and multiple player windows.
• Log on both sides. Add prints at the start of RPCs (Remote Procedure Calls) to confirm they're being received.

Blueprint

Blueprint— Network Authority Check

C++ equivalent (for reference)

// Has Authority -> Branch
//   True  -> Print "Running on SERVER"
//   False -> Print "Running on CLIENT"

Lua

function M:OnPlayerAction(player, action)
    if self:HasAuthority() then
        print("[SERVER] OnPlayerAction:", player:GetName(), action)
    else
        print("[CLIENT] OnPlayerAction:", player:GetName(), action)
    end
end

JavaScript

OnPlayerAction(player, action) {
    const side = this.HasAuthority() ? "SERVER" : "CLIENT";
    console.log(`[${side}] OnPlayerAction: ${player.GetName()} - ${action}`);
}

Console Commands

Open the console in HELIX Studio with the ` (backtick) key:

Command	What it does
stat fps	Show FPS counter
stat unit	Show frame time breakdown
UnLua.HotReload	Reload all Lua scripts
ShowDebug	Toggle debug info overlay
log LogBlueprintUserMessages Verbose	See all Blueprint print messages

Tips

• Remove or disable debug prints before publishing. Heavy logging tanks performance.
• Use screen-printed debug text (Print to Screen) for real-time values you need to watch while playing.
• When something works in single-player but breaks in multiplayer, it's almost always a replication or authority issue. Check the Client-Server Model docs.