diff --git a/src/test/java/com/preponderous/parpt/repo/ProjectRepositoryTest.java b/src/test/java/com/preponderous/parpt/repo/ProjectRepositoryTest.java
new file mode 100644
index 0000000..124b6e2
--- /dev/null
+++ b/src/test/java/com/preponderous/parpt/repo/ProjectRepositoryTest.java
@@ -0,0 +1,122 @@
+package com.preponderous.parpt.repo;
+
+import com.preponderous.parpt.domain.Project;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.mockito.Mockito.*;
+
+@ExtendWith(MockitoExtension.class)
+class ProjectRepositoryTest {
+
+    @Mock
+    private ProjectJsonReaderWriter projectJsonReaderWriter;
+
+    private ProjectRepository repository;
+
+    // writeJson receives the repository's own mutable list, so each call's contents are snapshotted here
+    private final List<List<String>> writtenNames = new ArrayList<>();
+
+    private Project alpha;
+    private Project beta;
+
+    @BeforeEach
+    void setUp() {
+        alpha = Project.builder().name("Alpha").description("First").impact(1).confidence(2).ease(3).reach(4).effort(5).build();
+        beta = Project.builder().name("Beta").description("Second").impact(5).confidence(4).ease(3).reach(2).effort(1).build();
+
+        when(projectJsonReaderWriter.readJson()).thenReturn(new ArrayList<>(List.of(alpha)));
+        lenient().doAnswer(invocation -> {
+            List<Project> written = invocation.getArgument(0);
+            writtenNames.add(written.stream().map(Project::getName).toList());
+            return null;
+        }).when(projectJsonReaderWriter).writeJson(anyList());
+
+        repository = new ProjectRepository(projectJsonReaderWriter);
+    }
+
+    @Test
+    void constructor_ShouldSeedProjectsFromReadJson() {
+        assertThat(repository.findAll()).containsExactly(alpha);
+        verify(projectJsonReaderWriter, times(1)).readJson();
+        verify(projectJsonReaderWriter, never()).writeJson(anyList());
+    }
+
+    @Test
+    void findAll_ShouldReturnDefensiveCopy() {
+        List<Project> returned = repository.findAll();
+        returned.clear();
+        returned.add(beta);
+
+        assertThat(repository.findAll()).containsExactly(alpha);
+    }
+
+    @Test
+    void add_ShouldAppendProjectAndPersistOnce() throws Exception {
+        repository.add(beta);
+
+        assertThat(repository.findAll()).containsExactly(alpha, beta);
+        verify(projectJsonReaderWriter, times(1)).writeJson(anyList());
+        assertThat(writtenNames).containsExactly(List.of("Alpha", "Beta"));
+    }
+
+    @Test
+    void add_WithDuplicateName_ShouldThrowAndLeaveStateUnchanged() {
+        Project duplicate = Project.builder().name("Alpha").description("Other").build();
+
+        assertThrows(ProjectRepository.NameTakenException.class, () -> repository.add(duplicate));
+
+        assertThat(repository.findAll()).containsExactly(alpha);
+        verify(projectJsonReaderWriter, never()).writeJson(anyList());
+    }
+
+    @Test
+    void remove_ShouldDeleteProjectAndPersistOnce() throws Exception {
+        repository.add(beta);
+        writtenNames.clear();
+        clearInvocations(projectJsonReaderWriter);
+
+        repository.remove("Alpha");
+
+        assertThat(repository.findAll()).containsExactly(beta);
+        verify(projectJsonReaderWriter, times(1)).writeJson(anyList());
+        assertThat(writtenNames).containsExactly(List.of("Beta"));
+    }
+
+    @Test
+    void remove_WithUnknownName_ShouldThrowAndNotPersist() {
+        ProjectRepository.ProjectNotFoundException exception = assertThrows(
+                ProjectRepository.ProjectNotFoundException.class, () -> repository.remove("Missing"));
+
+        assertThat(exception.getMessage()).isEqualTo("Project not found: Missing");
+        assertThat(repository.findAll()).containsExactly(alpha);
+        verify(projectJsonReaderWriter, never()).writeJson(anyList());
+    }
+
+    @Test
+    void clear_ShouldEmptyProjectsAndPersistOnce() {
+        repository.clear();
+
+        assertThat(repository.findAll()).isEmpty();
+        verify(projectJsonReaderWriter, times(1)).writeJson(anyList());
+        assertThat(writtenNames).containsExactly(List.of());
+    }
+
+    @Test
+    void findByName_ShouldReturnMatchingProject() throws Exception {
+        assertThat(repository.findByName("Alpha")).isSameAs(alpha);
+    }
+
+    @Test
+    void findByName_WithUnknownName_ShouldThrow() {
+        assertThrows(ProjectRepository.ProjectNotFoundException.class, () -> repository.findByName("Missing"));
+    }
+}