【Unreal Engine】Using Blueprint Debugger: Breakpoints and Step Execution

When developing games with Unreal Engine (UE), it's common for Blueprint node graphs to become complex, leading to problems where "things don't work as intended." Many developers first turn to the Print String node, but as logic becomes more intricate, the flood of log output can actually make it harder to identify problems.

This article thoroughly explains Blueprint Debugger, a powerful debugging tool built into Unreal Engine. Mastering this tool allows you to pause execution, examine variable states in detail at that moment, and dramatically improve debugging efficiency.

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Blueprint Debugger Overview

Blueprint Debugger is a dedicated window for pausing running Blueprint execution and visually examining variable values and execution paths at that point. It provides functionality similar to debuggers in traditional programming languages and is especially powerful for tracing Blueprint logic.

How to Launch

Blueprint Debugger can be opened through either of these methods:

1. From Main Menu: Window → Developer Tools → Blueprint Debugger
2. From Blueprint Editor: The Debug dropdown menu in the toolbar

Setting and Using Breakpoints

The first step in debugging is setting breakpoints on nodes where you want to pause execution.

Setting Breakpoints

In the Blueprint editor, right-click on the node where you want to pause execution and select Toggle Breakpoint. When a red circle appears in the node's top-left, the breakpoint is set.

Starting Debug

With breakpoints set, play the game in the editor (PIE: Play In Editor). When execution reaches a breakpoint node, game execution pauses and the Blueprint Debugger window becomes active.

Tracing Execution with Step Execution

Once execution is paused, you can trace the execution flow node by node using these step execution features.

Feature	Purpose
Step Into	Execute the current node and proceed to the next. For function calls, enter that function.
Step Over	Execute the current node and proceed to the next. For function calls, don't enter the function, proceed to the node after the function completes.
Step Out	Exit the current function and return to the caller.
Continue	Resume execution until the next breakpoint or until processing ends.

Using the Call Stack

The Blueprint Debugger window has a Call Stack panel showing the history of function calls leading to the current execution position. This is useful for understanding "where did I get here from" when called from deeply nested functions or events.

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Variable Watch and Debug Target Selection

Blueprint Debugger's true value lies in examining variable states in real-time.

Watch Variables

When execution is paused at a breakpoint, you can check the values of accessible variables at that moment in the Blueprint Debugger window's "Watch" tab.

Particularly important is that you can check values flowing through input and output pins simply by hovering over them. This lets you instantly determine whether calculation results or references are as expected.

Debugging Multiple Actors

When multiple Actors with the same Blueprint class exist in a level, you need to specify which instance's execution to track.

1. Start PIE and when execution pauses at a breakpoint, check the "Debug Filter" dropdown in the Blueprint editor toolbar.
2. This dropdown shows a list of Blueprint instances currently in the level.
3. By selecting the specific Actor you want to debug (e.g., ThirdPersonCharacter_C_0), you can track only that instance's execution.

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Common Mistakes and Best Practices

Here are tips to further improve debugging efficiency and common beginner mistakes.

Common Mistakes

Mistake	Solution
Breakpoint isn't being hit	Check if the target Actor is selected in the Debug Filter. Especially for Actors controlled by gamepads or AI, an unintended instance may be selected.
Trying to debug outside PIE	Blueprint Debugger basically only works during PIE (Play In Editor) or Standalone Game execution. Editor operations (like Construction Scripts) are not debug targets.
Missing debug settings after packaging	To debug after packaging, you need to enable the Include Debug Files setting in project settings.

Best Practices

1. Use Auto-Break on None Reference Exceptions: One of the most frequent Blueprint errors is None reference exceptions—trying to access invalid object references (None). Unreal Engine can automatically break the debugger when this exception occurs. This feature can be toggled in Editor Preferences (Edit → Editor Preferences → General → Debugging).
2. Disable/Remove Breakpoints After Debugging: After debugging is complete, always delete or disable breakpoints (from the right-click menu). Remaining breakpoints can stop the game at unintended times, reducing development efficiency.
3. Watch Variables Before and After Complex Calculations: Make it a habit to place breakpoints before and after particularly complex mathematical calculations or nodes with multiple conditionals, and use the watch feature to verify that variable values are changing as expected.

Key Points for Blueprint Debugger

Blueprint Debugger is an essential tool for Unreal Engine developers. Moving beyond Print String log output, by utilizing breakpoints, step execution, and variable watch features, you can accurately understand complex Blueprint logic and quickly identify bugs.