unity-input-debugging

name	unity-input-debugging
description	Unity Input Debugging: Phantom Input & Platform Quirks

Unity Input Debugging: Phantom Input & Platform Quirks

Use this skill when diagnosing phantom input values, platform-specific input quirks, or unexpected axis readings in Unity's Legacy Input Manager, especially on Windows.

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Section 1: Windows Phantom Axis Values

The Problem

Unity's Legacy Input Manager reports non-zero values for certain axes even when no controller is connected on Windows:

Axis	Phantom Value	Notes
CombinedTriggers	-1.0 (constant)	Most dangerous — full brake/throttle signal
Horizontal	Small non-zero values	Varies by system
Other joystick axes	Small non-zero values	Hardware-dependent

Root Cause

This is a known Windows platform behavior, not a Unity bug. Windows reports default/resting values for gamepad axes through DirectInput/XInput even when no physical device is present. The Legacy Input Manager passes these values through without filtering.

Key Insight

• This does not happen on all machines or all Windows versions consistently
• The values are constant (no variance frame-to-frame) — this is the key diagnostic signal
• Deadzones alone do NOT fix this: a phantom value of -1.0 blows past any reasonable deadzone
• The New Input System (com.unity.inputsystem) handles device presence detection better, but if you are using the Legacy Input Manager, you must defend against this manually

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Quick Reference: Input Bug Triage

Vehicle moves with no player touching anything?
  |
  v
Attach InputDiagnostics, enter Play mode, hands off controller
  |
  v
Are axis values constant (no variance)?
  YES -> Phantom input. Check:
    1. Is TriggerDetector detecting the stuck axis? (variance < 0.02)
    2. Is mode gating returning 0 for Detecting/None modes?
    3. Are ALL axes gated, not just the one you noticed first?
  NO  -> Real input from a connected device. Check:
    1. Is a controller actually connected? (check Device Manager)
    2. Is the controller drifting? (hardware issue)
    3. Are deadzones too small for the noise level?

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Related Skills

Skill	Relevance
unity-input-system	New Input System (alternative to Legacy Input Manager) — handles device presence natively
unity-testing-debugging-qa	Testing philosophy, TDD cycle, test infrastructure
unity-testing-patterns	Test code examples, parameterized tests, assertions
reverse-engineering	Chain-of-custody debugging methodology
debug-system	Debug overlay and logging architecture
clean-room-qa	Black-box testing from function signatures

Topic Pages

• Section 2: Three-Layer Input Defense