diff --git a/SPMS.API/Extensions/ApplicationBuilderExtensions.cs b/SPMS.API/Extensions/ApplicationBuilderExtensions.cs
index 0d5f0ae..409b319 100644
--- a/SPMS.API/Extensions/ApplicationBuilderExtensions.cs
+++ b/SPMS.API/Extensions/ApplicationBuilderExtensions.cs
@@ -62,8 +62,8 @@ public static class ApplicationBuilderExtensions
         // -- 13. X-API-KEY 검증 (SDK/디바이스 엔드포인트용) --
         app.UseMiddleware<ApiKeyMiddleware>();
 
-        // -- 14. X-SPMS-TEST 샌드박스 모드 (미구현 — Issue #14) --
-        // app.UseMiddleware<SandboxMiddleware>();
+        // -- 14. X-SPMS-TEST 샌드박스 모드 --
+        app.UseMiddleware<SandboxMiddleware>();
 
         // -- 15. Swagger UI (개발 환경만) --
         if (app.Environment.IsDevelopment())
diff --git a/SPMS.API/Middlewares/SandboxMiddleware.cs b/SPMS.API/Middlewares/SandboxMiddleware.cs
new file mode 100644
index 0000000..29fca01
--- /dev/null
+++ b/SPMS.API/Middlewares/SandboxMiddleware.cs
@@ -0,0 +1,16 @@
+namespace SPMS.API.Middlewares;
+
+public class SandboxMiddleware
+{
+    private readonly RequestDelegate _next;
+
+    public SandboxMiddleware(RequestDelegate next) => _next = next;
+
+    public async Task InvokeAsync(HttpContext context)
+    {
+        var isTest = context.Request.Headers["X-SPMS-TEST"].FirstOrDefault();
+        context.Items["IsSandbox"] = string.Equals(isTest, "true", StringComparison.OrdinalIgnoreCase);
+
+        await _next(context);
+    }
+}