diff --git a/tests/TarninaHub.Tests/Controllers/ApplicationsControllerTests.cs b/tests/TarninaHub.Tests/Controllers/ApplicationsControllerTests.cs
new file mode 100644
index 0000000..021ff9b
--- /dev/null
+++ b/tests/TarninaHub.Tests/Controllers/ApplicationsControllerTests.cs
@@ -0,0 +1,87 @@
+using System.Security.Claims;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+using TarninaHub.Application.Abstractions;
+using TarninaHub.Application.Dtos;
+using TarninaHub.Controllers;
+using TarninaHub.Tests.Stubs;
+
+namespace TarninaHub.Tests.Controllers;
+
+public class ApplicationsControllerTests
+{
+    private static ApplicationsController CreateController(
+        IApplicationService applicationService,
+        IIdeaService? ideaService = null)
+    {
+        var controller = new ApplicationsController(applicationService, ideaService ?? new StubIdeaService());
+        var identity = new ClaimsIdentity(
+            [new Claim("sub", "user:tester"), new Claim("role", "user")],
+            authenticationType: "test");
+
+        controller.ControllerContext = new ControllerContext
+        {
+            HttpContext = new DefaultHttpContext { User = new ClaimsPrincipal(identity) },
+        };
+
+        return controller;
+    }
+
+    [Fact]
+    public async Task Create_ReturnsConflictWhenCallNotOpen()
+    {
+        var service = new StubApplicationService(
+            new CreateApplicationResult(CreateApplicationOutcome.CallNotOpen, null));
+        var controller = CreateController(service);
+
+        var result = await controller.Create(
+            new CreateApplicationRequest("grant_call:x", "idea:y", null),
+            CancellationToken.None);
+
+        var conflict = Assert.IsType<ConflictObjectResult>(result.Result);
+        Assert.Equal(StatusCodes.Status409Conflict, conflict.StatusCode);
+    }
+
+    [Fact]
+    public async Task Create_ReturnsCreatedWhenSuccessful()
+    {
+        var application = new ApplicationResponse(
+            "application:new",
+            "grant_call:x",
+            "idea:y",
+            new Dictionary<string, object?>(),
+            "draft",
+            DateTimeOffset.UtcNow);
+        var service = new StubApplicationService(
+            new CreateApplicationResult(CreateApplicationOutcome.Created, application));
+        var controller = CreateController(service);
+
+        var result = await controller.Create(
+            new CreateApplicationRequest("grant_call:x", "idea:y", null),
+            CancellationToken.None);
+
+        Assert.IsType<CreatedAtActionResult>(result.Result);
+    }
+
+    private sealed class StubApplicationService : IApplicationService
+    {
+        private readonly CreateApplicationResult _createResult;
+
+        public StubApplicationService(CreateApplicationResult createResult)
+        {
+            _createResult = createResult;
+        }
+
+        public Task<CreateApplicationResult> CreateAsync(
+            string userId,
+            CreateApplicationRequest request,
+            CancellationToken cancellationToken) => Task.FromResult(_createResult);
+
+        public Task<ApplicationResponse?> GetAsync(string applicationId, CancellationToken cancellationToken) =>
+            Task.FromResult(_createResult.Application);
+
+        public Task<IReadOnlyList<ApplicationResponse>> ListForAuthorAsync(
+            string authorId,
+            CancellationToken cancellationToken) => Task.FromResult<IReadOnlyList<ApplicationResponse>>([]);
+    }
+}
diff --git a/tests/TarninaHub.Tests/Domain/IdeaAndApplicationRulesTests.cs b/tests/TarninaHub.Tests/Domain/IdeaAndApplicationRulesTests.cs
new file mode 100644
index 0000000..4053837
--- /dev/null
+++ b/tests/TarninaHub.Tests/Domain/IdeaAndApplicationRulesTests.cs
@@ -0,0 +1,40 @@
+using TarninaHub.Domain.Enums;
+using TarninaHub.Domain.Rules;
+
+namespace TarninaHub.Tests.Domain;
+
+public class IdeaAndApplicationRulesTests
+{
+    [Theory]
+    [InlineData(IdeaStatus.Draft, IdeaStatus.Submitted, true)]
+    [InlineData(IdeaStatus.Submitted, IdeaStatus.InReview, true)]
+    [InlineData(IdeaStatus.InReview, IdeaStatus.Accepted, true)]
+    [InlineData(IdeaStatus.InReview, IdeaStatus.Rejected, true)]
+    [InlineData(IdeaStatus.Draft, IdeaStatus.Accepted, false)]
+    [InlineData(IdeaStatus.Accepted, IdeaStatus.Draft, false)]
+    public void IdeaWorkflow_EnforcesTransitions(IdeaStatus from, IdeaStatus to, bool expected)
+    {
+        Assert.Equal(expected, IdeaWorkflow.CanTransition(from, to));
+    }
+
+    [Fact]
+    public void IdeaWorkflow_RecognizesTerminalStates()
+    {
+        Assert.True(IdeaWorkflow.IsTerminal(IdeaStatus.Accepted));
+        Assert.True(IdeaWorkflow.IsTerminal(IdeaStatus.Rejected));
+        Assert.False(IdeaWorkflow.IsTerminal(IdeaStatus.Draft));
+    }
+
+    [Fact]
+    public void ApplicationWindow_IsOpenOnlyWithinOpenCallWindow()
+    {
+        var opensAt = new DateTimeOffset(2026, 9, 1, 0, 0, 0, TimeSpan.Zero);
+        var closesAt = new DateTimeOffset(2026, 12, 31, 23, 59, 59, TimeSpan.Zero);
+        var now = new DateTimeOffset(2026, 10, 4, 12, 0, 0, TimeSpan.Zero);
+
+        Assert.True(ApplicationWindow.IsOpen("open", opensAt, closesAt, now));
+        Assert.False(ApplicationWindow.IsOpen("closed", opensAt, closesAt, now));
+        Assert.False(ApplicationWindow.IsOpen("open", opensAt, closesAt, opensAt.AddDays(-1)));
+        Assert.False(ApplicationWindow.IsOpen("open", opensAt, closesAt, closesAt.AddSeconds(1)));
+    }
+}
diff --git a/tests/TarninaHub.Tests/Stubs/StubIdeaService.cs b/tests/TarninaHub.Tests/Stubs/StubIdeaService.cs
new file mode 100644
index 0000000..f20dc62
--- /dev/null
+++ b/tests/TarninaHub.Tests/Stubs/StubIdeaService.cs
@@ -0,0 +1,28 @@
+using TarninaHub.Application.Abstractions;
+using TarninaHub.Application.Dtos;
+
+namespace TarninaHub.Tests.Stubs;
+
+/// <summary>Minimal IIdeaService stub for controller tests.</summary>
+public sealed class StubIdeaService : IIdeaService
+{
+    public IdeaResponse? Idea { get; set; }
+
+    public Task<IdeaResponse> CreateAsync(string authorId, CreateIdeaRequest request, CancellationToken cancellationToken) =>
+        Task.FromResult(Idea ?? throw new InvalidOperationException());
+
+    public Task<IdeaResponse?> GetAsync(string ideaId, CancellationToken cancellationToken) =>
+        Task.FromResult(Idea);
+
+    public Task<IReadOnlyList<IdeaResponse>> ListForAuthorAsync(string authorId, CancellationToken cancellationToken) =>
+        Task.FromResult<IReadOnlyList<IdeaResponse>>(Idea is null ? [] : [Idea]);
+
+    public Task<IdeaResponse?> UpdateAsync(string ideaId, UpdateIdeaRequest request, CancellationToken cancellationToken) =>
+        Task.FromResult(Idea);
+
+    public Task<IdeaCardResponse?> GetCardAsync(string ideaId, CancellationToken cancellationToken) =>
+        Task.FromResult<IdeaCardResponse?>(null);
+
+    public Task<IdeaCardResponse?> UpdateCardAsync(string ideaId, IdeaCardRequest request, CancellationToken cancellationToken) =>
+        Task.FromResult<IdeaCardResponse?>(null);
+}