Fix: Unity Reflection Probe Not Updating at Runtime

Day-night cycle ticking past noon, but the chrome car in your scene still reflects an orange sunset? Reflection probes are baked by default, and a baked probe will never see a light change again until you re-bake it.

The Symptom

Lights animate, the skybox rotates, the player opens a door — but reflective surfaces still mirror the scene as it looked when the level was last baked. The static reflection is correct for the original lighting and slowly drifts more wrong as the scene changes.

What Causes This

Reflection Probes have three Type options: Baked, Custom, and Realtime. Baked stores a cubemap on disk and never re-renders. Custom uses a manually authored cubemap. Only Realtime captures the live scene at runtime.

The Fix

Step 1: Type = Realtime. Inspector → Reflection Probe → Type → Realtime. The Bake button disappears; new fields appear for Refresh Mode and Time Slicing.

Step 2: Refresh Mode = Via Scripting. Do not use Every Frame — rendering six cubemap faces every frame is brutal even on desktop. Via Scripting gives you control over when the cost happens.

Step 3: Time Slicing = All Faces At Once or Individual Faces. All Faces is one big spike per refresh. Individual Faces spreads the cost across six frames at the cost of a 6-frame seam if reflections change mid-update. For most games, Individual Faces is the better tradeoff.

Step 4: Render when needed.

public class DayNightProbe : MonoBehaviour
{
    public ReflectionProbe probe;
    private float _lastSunAngle;

    void Update()
    {
        var sunAngle = SunDirector.CurrentAngle;
        if (Mathf.Abs(sunAngle - _lastSunAngle) > 5f)
        {
            probe.RenderProbe();
            _lastSunAngle = sunAngle;
        }
    }
}

Refresh only when the lighting has visibly changed by some threshold. For static-lit environments with one moving event (a door, an explosion), trigger a refresh on that event and never otherwise.

Box Projection for Indoor Scenes

If your probe is inside a room and reflections look like the cubemap is at infinity (everything looks tiny and far away), enable Box Projection on the probe. The probe’s Box Size becomes the assumed reflection geometry, which makes wall reflections look correct.

Resolution Tradeoff

Probe Resolution defaults to 128 — fine for matte materials, blurry on chrome. Bump to 256 or 512 for visibly metallic surfaces. The cost scales quadratically with resolution; profile before going past 512.

Understanding the issue

This bug class falls into a pattern that's worth understanding beyond the specific case. In Unity Engine, the underlying behavior is shaped by how the engine layers its abstractions - the public API you call, the runtime systems that respond, and the platform-specific implementations underneath. A bug at any layer can produce symptoms that look like they originate at a different layer. Triaging effectively means recognizing which layer the symptom belongs to, even when the gameplay code is what's visible.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

Bugs of this class are particularly easy to ship past internal QA because they often depend on specific runtime conditions - hardware combinations, network states, or asset configurations that QA didn't reproduce. Players hit them in the wild, file reports that are hard to repro, and the bug accumulates negative reviews while engineering tries to recreate the failure mode.

At the engine level, the behavior comes from a deliberate design decision in Unity. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

After applying the fix, the verification step has three parts: confirm the original repro is resolved, confirm no obvious regressions in adjacent functionality, and (for shipping titles) deploy to a small player cohort first and watch the crash and report rates. Each step catches something the others miss.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

Related bug classes often share the same root cause. If you find yourself fixing this issue, look for cousins: similar symptoms in adjacent systems, the same data flow but a different value, or the same fix pattern in another module. The catalog of 'we've seen this before' becomes valuable institutional knowledge.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

In shipping builds, this issue may interact with other production-only behavior. Stripping, encryption, asset bundling, and platform-specific code paths can each modify the symptoms. When players report a related issue, capture build SHA, platform, and any feature flags - those three fields cover most of the production-only variations.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

Performance implications matter when this bug class scales with player count or asset count. A bug that fires once per session is annoying; a bug that fires once per frame compounds. After fixing, profile the affected code path under realistic load. The fix that's correct for one entity may be too slow for ten thousand.

Diagnostic approach

The diagnostic tools available depend on your engine and platform. Use the engine's native profilers and debug overlays before reaching for external tools. The native tools have context that external tools lack - they know which subsystem owns the code, which assets are loaded, and what state the engine is in.

For Unity-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

Third-party plugins often provide better diagnostics for their own behavior than the engine does. If the affected code is in a plugin, check the plugin's documentation for debug modes, verbose logging, or inspector tools - these can save hours of investigation when they exist.

Within Unity, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Platform-specific edge cases are worth enumerating explicitly. iOS handles backgrounding differently than Android; Windows handles focus changes differently than macOS. A fix that works on the development platform may not work on every target. Test on each shipping platform deliberately.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“Realtime type, Via Scripting refresh, RenderProbe on real change. The mirror finally sees what is in front of it.”

Related Issues

For light probes failing on dynamic objects, see light probe seams. For PBR materials looking flat, see PBR specular.

Realtime. Via scripting. RenderProbe on change.