test CreatePost use case

7 cases: null + whitespace title -> BadRequest; null +
whitespace body -> BadRequest; valid request returns Post with
correct userId/title/body and createdAt = clock.now(); the post
is findable via the repo afterwards; title and body get trimmed
of leading/trailing whitespace. fails red - CreatePost class
absent.

backend/tests/Unit/Post/UseCases/CreatePostTest.php

@@ -0,0 +1,118 @@
+<?php
+
+namespace Tests\Unit\Post\UseCases;
+
+use App\Exceptions\BadRequestException;
+use App\Post\Post;
+use App\Post\UseCases\CreatePost\CreatePost;
+use App\Post\UseCases\CreatePost\CreatePostRequest;
+use DateTimeImmutable;
+use DateTimeZone;
+use Tests\Fakes\FakeClock;
+use Tests\Fakes\FakePostRepository;
+use Tests\TestCase;
+
+class CreatePostTest extends TestCase
+{
+    private FakePostRepository $postRepo;
+
+    private FakeClock $clock;
+
+    private DateTimeImmutable $now;
+
+    private CreatePost $useCase;
+
+    protected function setUp(): void
+    {
+        $this->now = new DateTimeImmutable(
+            '2026-05-06T12:00:00',
+            new DateTimeZone('UTC')
+        );
+        $this->postRepo = new FakePostRepository;
+        $this->clock = new FakeClock($this->now);
+        $this->useCase = new CreatePost(
+            $this->postRepo,
+            $this->clock,
+        );
+    }
+
+    public function test_null_title_throws_bad_request(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new CreatePostRequest(
+            userId: 7,
+            title: null,
+            body: 'some body content',
+        ));
+    }
+
+    public function test_empty_title_throws_bad_request(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new CreatePostRequest(
+            userId: 7,
+            title: '   ',
+            body: 'some body content',
+        ));
+    }
+
+    public function test_null_body_throws_bad_request(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new CreatePostRequest(
+            userId: 7,
+            title: 'My Post',
+            body: null,
+        ));
+    }
+
+    public function test_empty_body_throws_bad_request(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new CreatePostRequest(
+            userId: 7,
+            title: 'My Post',
+            body: '   ',
+        ));
+    }
+
+    public function test_valid_request_returns_post(): void
+    {
+        $post = $this->useCase->execute(new CreatePostRequest(
+            userId: 7,
+            title: 'My Post',
+            body: 'Some body content here.',
+        ));
+
+        $this->assertInstanceOf(Post::class, $post);
+        $this->assertSame(7, $post->getUserId());
+        $this->assertSame('My Post', $post->getTitle());
+        $this->assertSame('Some body content here.', $post->getBody());
+        $this->assertEquals($this->now, $post->getCreatedAt());
+    }
+
+    public function test_post_is_findable_after_creation(): void
+    {
+        $created = $this->useCase->execute(new CreatePostRequest(
+            userId: 7,
+            title: 'My Post',
+            body: 'Some body content here.',
+        ));
+
+        $found = $this->postRepo->find($created->getId());
+        $this->assertNotNull($found);
+        $this->assertSame('My Post', $found->getTitle());
+    }
+
+    public function test_title_and_body_are_trimmed(): void
+    {
+        $post = $this->useCase->execute(new CreatePostRequest(
+            userId: 7,
+            title: '  Padded Title  ',
+            body: "  Padded body  \n",
+        ));
+
+        $this->assertSame('Padded Title', $post->getTitle());
+        $this->assertSame('Padded body', $post->getBody());
+    }
+}