Code inspection: String.Format(format, ...) only accepts string literals and const strings

This inspection reports string.Format(...) calls in code compiled by the Burst compiler where the format argument is not a string literal, const string, or interpolated string.

Burst only supports a restricted form of string.Format. If the format string comes from a mutable field, variable, or another non-constant source, this inspection warns.

Example

In this example, the format string is stored in a static field. This is not supported in Burst-compiled code.

                using Unity.Burst;
                using Unity.Jobs;

                [BurstCompile]
                public struct ExampleJob : IJob
                {
                    private static string format = "Value: {0}";

                    public void Execute()
                    {
                        // Reported: mutable format string is not supported
                        string.Format(format, 12);
                    }
                }
            

                using Unity.Burst;
                using Unity.Jobs;

                [BurstCompile]
                public struct ExampleJob : IJob
                {
                    public void Execute()
                    {
                        // Correct: Use a string literal directly
                        string.Format("Value: {0}", 12);
                    }
                }
            

Quick-fix

This inspection does not provide a dedicated quick-fix. Fix it manually by replacing the format argument with a string literal or const string.

26 March 2026