diff --git a/src/test/java/net/javahippie/fitpub/service/InboxProcessorTest.java b/src/test/java/net/javahippie/fitpub/service/InboxProcessorTest.java
new file mode 100644
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+++ b/src/test/java/net/javahippie/fitpub/service/InboxProcessorTest.java
@@ -0,0 +1,133 @@
+package net.javahippie.fitpub.service;
+
+import net.javahippie.fitpub.model.entity.Follow;
+import net.javahippie.fitpub.model.entity.RemoteActor;
+import net.javahippie.fitpub.model.entity.User;
+import net.javahippie.fitpub.repository.ActivityRepository;
+import net.javahippie.fitpub.repository.CommentRepository;
+import net.javahippie.fitpub.repository.FollowRepository;
+import net.javahippie.fitpub.repository.LikeRepository;
+import net.javahippie.fitpub.repository.RemoteActivityRepository;
+import net.javahippie.fitpub.repository.RemoteActorRepository;
+import net.javahippie.fitpub.repository.UserRepository;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.ArgumentCaptor;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.test.util.ReflectionTestUtils;
+
+import java.time.Instant;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.UUID;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+@ExtendWith(MockitoExtension.class)
+@DisplayName("InboxProcessor Tests")
+class InboxProcessorTest {
+
+    @Mock
+    private UserRepository userRepository;
+
+    @Mock
+    private FollowRepository followRepository;
+
+    @Mock
+    private FederationService federationService;
+
+    @Mock
+    private ActivityRepository activityRepository;
+
+    @Mock
+    private LikeRepository likeRepository;
+
+    @Mock
+    private CommentRepository commentRepository;
+
+    @Mock
+    private NotificationService notificationService;
+
+    @Mock
+    private RemoteActivityRepository remoteActivityRepository;
+
+    @Mock
+    private RemoteActorRepository remoteActorRepository;
+
+    @InjectMocks
+    private InboxProcessor inboxProcessor;
+
+    private User localUser;
+    private String remoteActorUri;
+
+    @BeforeEach
+    void setUp() {
+        localUser = User.builder()
+            .id(UUID.randomUUID())
+            .username("JaneDoe")
+            .email("janedoe@example.com")
+            .passwordHash("irrelevant")
+            .publicKey("public-key")
+            .privateKey("private-key")
+            .build();
+
+        remoteActorUri = "https://fitpub.example.com/users/JohnDoe";
+
+        ReflectionTestUtils.setField(inboxProcessor, "baseUrl", "https://fitpub.example");
+    }
+
+    @Test
+    @DisplayName("Should persist remote activity when published timestamp has fractional seconds but no timezone")
+    void processCreateRemoteActivity_WithPublishedTimestampWithoutTimezone_ShouldPersistRemoteActivity() {
+        when(remoteActivityRepository.existsByActivityUri("https://fitpub.example.com/activities/123"))
+            .thenReturn(false);
+        when(federationService.fetchRemoteActor(remoteActorUri)).thenReturn(RemoteActor.builder()
+            .actorUri(remoteActorUri)
+            .username("JohnDoe")
+            .domain("fitpub.example.com")
+            .inboxUrl("https://fitpub.example.com/users/JohnDoe/inbox")
+            .publicKey("public-key")
+            .build());
+        when(userRepository.findByUsername("JaneDoe")).thenReturn(Optional.of(localUser));
+        when(followRepository.findByFollowerIdAndFollowingActorUri(localUser.getId(), remoteActorUri))
+            .thenReturn(Optional.of(Follow.builder()
+                .followerId(localUser.getId())
+                .followingActorUri(remoteActorUri)
+                .status(Follow.FollowStatus.ACCEPTED)
+                .build()));
+
+        Map<String, Object> note = Map.of(
+            "id", "https://fitpub.example.com/activities/123",
+            "type", "Note",
+            "name", "Lunch Run",
+            "content", "<p>Sunny run</p>",
+            "published", "2026-05-02T09:24:50.921241",
+            "to", List.of("https://www.w3.org/ns/activitystreams#Public")
+        );
+
+        Map<String, Object> activity = Map.of(
+            "type", "Create",
+            "actor", remoteActorUri,
+            "object", note
+        );
+
+        ArgumentCaptor<net.javahippie.fitpub.model.entity.RemoteActivity> remoteActivityCaptor =
+            ArgumentCaptor.forClass(net.javahippie.fitpub.model.entity.RemoteActivity.class);
+
+        inboxProcessor.processActivity("JaneDoe", activity);
+
+        verify(remoteActivityRepository).existsByActivityUri("https://fitpub.example.com/activities/123");
+        verify(federationService).fetchRemoteActor(remoteActorUri);
+        verify(remoteActivityRepository).save(remoteActivityCaptor.capture());
+
+        assertThat(remoteActivityCaptor.getValue().getPublishedAt())
+            .isEqualTo(Instant.parse("2026-05-02T09:24:50.921241Z"));
+    }
+}