Quick answer: Burst defaults to FloatMode.Fast which allows FMA fusion and reassociation. Annotate your job with [BurstCompile(FloatPrecision.High, FloatMode.Strict)] to match the managed result bit-for-bit.

You ran a Burst job that computes player physics and compared its output against a managed C# reference for unit testing. The two agreed on most values but diverged on a few by 1e−7. Multiplied across a few thousand frames of integration, the discrepancy compounded into noticeable positional drift — enough to break lockstep multiplayer.

Why Burst and Managed Diverge

Burst is an LLVM-based ahead-of-time compiler. By default, it enables aggressive floating-point optimizations:

The managed (Mono/IL2CPP) pipeline does none of these. It emits the literal IL operations defined in the source, so a * b + c performs a multiply and an add with two rounding steps. The two results differ in the lowest mantissa bits — below the threshold of single-step visibility but above the threshold of cumulative integration error.

The Fix: Strict FloatMode

using Unity.Burst;
using Unity.Jobs;
using Unity.Collections;

[BurstCompile(FloatPrecision.High, FloatMode.Strict)]
public struct PhysicsStepJob : IJobParallelFor
{
    [ReadOnly] public NativeArray<float3> velocities;
    public NativeArray<float3> positions;
    public float deltaTime;

    public void Execute(int index)
    {
        positions[index] += velocities[index] * deltaTime;
    }
}

FloatPrecision.High disables fast reciprocals and trigonometric approximations; FloatMode.Strict disables FMA fusion and reassociation. Performance drops, typically 10–30% on a math-bound job, but the output matches managed C# exactly.

Per-Operation Control

If only a few operations need strict semantics, you can wrap them locally instead of changing the whole job:

[BurstCompile(FloatMode.Fast)]
public struct MixedJob : IJob
{
    public NativeArray<float> data;

    public void Execute()
    {
        // Fast for bulk math
        for (int i = 0; i < data.Length; i++)
            data[i] = math.sin(data[i]);

        // Strict for a critical hash step
        data[0] = StrictMath.PreciseAdd(data[0], 1.0f);
    }
}

[BurstCompile(FloatMode.Strict)]
public static class StrictMath
{
    public static float PreciseAdd(float a, float b) => a + b;
}

Burst applies the attribute on the calling function’s static class, so StrictMath.PreciseAdd compiles under strict rules even when called from a Fast-mode job.

Audit for Other Determinism Issues

Strict FloatMode is necessary but not sufficient for cross-platform determinism. Also check:

Verifying

Run a unit test that executes the same operation managed and Burst-compiled, then compares with bitwise equality:

[Test]
public void PhysicsStep_ManagedAndBurst_Match()
{
    var managed = ManagedReference(input);
    var burst = RunBurstJob(input);
    Assert.That(BitConverter.DoubleToInt64Bits(managed),
                Is.EqualTo(BitConverter.DoubleToInt64Bits(burst)));
}

Bitwise comparison rules out cases where values are “close” but not identical.

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

The triage path for this kind of bug is long. The symptom appears in gameplay, but the cause is in a different system. The reporter describes the gameplay effect; the engineer has to translate that into a hypothesis about the underlying cause. Misdirection is common.

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

For shipping games, the safest verification is a staged rollout. Apply the fix to 1% of players for 24 hours; watch the affected metric; expand if green. Skipping the staged rollout means the verification is the entire player base, which is too high a stakes for most fixes.

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

For shipping titles with a long support window, watch for this issue resurfacing after dependency updates. Engine upgrades, driver updates, OS releases - each one can resurface a bug class you thought you'd fixed because the underlying behavior changed slightly. Regression tests catch the obvious ones; player reports catch the rest.

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

If this issue manifests under high load (many actors, many particles, many network connections), profile the post-fix code path with realistic counts. The original cost was a bug; the new cost is real work, and real work has a budget.

Diagnostic approach

Before applying any fix, gather enough context to be confident you're addressing the actual cause and not a similar-looking symptom. The cheapest diagnostic step is reproducing the bug deterministically - if you can't get the same failure twice in a row, your fix attempts will be hard to evaluate. Lock down the reproduction first.

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

The tooling around this bug class matters as much as the fix itself. Good logging, accessible profilers, and clear error messages turn 30-minute investigations into 5-minute ones. If your project doesn't have visibility into this code path, the first fix should add the visibility - the second fix uses it.

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

Edge cases for this class of issue often involve specific timing: the first frame after a state change, the last frame before a transition, frames where multiple subsystems update simultaneously. Reproducing these reliably is part of what makes the bug class hard to test.

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.

“Fast Burst gives you speed; Strict Burst gives you determinism. Pick the one that matches your needs.”

Lockstep multiplayer demands Strict mode from day one. Cosmetic effects can stay Fast.