diff --git a/crates/application/src/use_cases/api_keys.rs b/crates/application/src/use_cases/api_keys.rs
index 2f8e63c..1f0ef56 100644
--- a/crates/application/src/use_cases/api_keys.rs
+++ b/crates/application/src/use_cases/api_keys.rs
@@ -27,3 +27,52 @@ fn sha256_hex(s: &str) -> String {
     let hash = Sha256::digest(s.as_bytes());
     hex::encode(hash)
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use domain::{testing::TestStore, value_objects::UserId};
+
+    #[tokio::test]
+    async fn create_key_saves_hashed_not_raw() {
+        let store = TestStore::default();
+        let uid = UserId::new();
+        let (key, raw) = create_api_key(&store, &uid, "my-key".to_string()).await.unwrap();
+        assert_ne!(key.key_hash, raw, "stored hash must differ from raw key");
+        assert!(!key.key_hash.is_empty());
+        assert_eq!(key.name, "my-key");
+        assert_eq!(key.user_id, uid);
+        assert_eq!(store.api_keys.lock().unwrap().len(), 1);
+    }
+
+    #[tokio::test]
+    async fn raw_key_verifies_against_stored_hash() {
+        use sha2::{Digest, Sha256};
+        let store = TestStore::default();
+        let uid = UserId::new();
+        let (key, raw) = create_api_key(&store, &uid, "test".to_string()).await.unwrap();
+        let expected_hash = hex::encode(Sha256::digest(raw.as_bytes()));
+        assert_eq!(key.key_hash, expected_hash);
+    }
+
+    #[tokio::test]
+    async fn delete_key_removes_it() {
+        let store = TestStore::default();
+        let uid = UserId::new();
+        let (key, _) = create_api_key(&store, &uid, "k".to_string()).await.unwrap();
+        delete_api_key(&store, &uid, &key.id).await.unwrap();
+        assert!(store.api_keys.lock().unwrap().is_empty());
+    }
+
+    #[tokio::test]
+    async fn list_keys_returns_only_own_keys() {
+        let store = TestStore::default();
+        let alice = UserId::new();
+        let bob = UserId::new();
+        create_api_key(&store, &alice, "a".to_string()).await.unwrap();
+        create_api_key(&store, &bob, "b".to_string()).await.unwrap();
+        let alice_keys = list_api_keys(&store, &alice).await.unwrap();
+        assert_eq!(alice_keys.len(), 1);
+        assert_eq!(alice_keys[0].user_id, alice);
+    }
+}
diff --git a/crates/application/src/use_cases/auth.rs b/crates/application/src/use_cases/auth.rs
index d57caa7..9244c52 100644
--- a/crates/application/src/use_cases/auth.rs
+++ b/crates/application/src/use_cases/auth.rs
@@ -124,4 +124,19 @@ mod tests {
         assert_eq!(events.len(), 1);
         assert!(matches!(events[0], DomainEvent::UserRegistered { .. }));
     }
+
+    #[tokio::test]
+    async fn login_fails_for_nonexistent_user() {
+        let store = TestStore::default();
+        let err = login(&store, &FakeHasher, &FakeAuth, LoginInput { email: "ghost@ex.com".into(), password: "pass".into() }).await.unwrap_err();
+        assert!(matches!(err, DomainError::Unauthorized));
+    }
+
+    #[tokio::test]
+    async fn register_rejects_duplicate_email() {
+        let store = TestStore::default();
+        register(&store, &FakeHasher, &FakeAuth, &NoOpEventPublisher, input()).await.unwrap();
+        let err = register(&store, &FakeHasher, &FakeAuth, &NoOpEventPublisher, RegisterInput { username: "alice2".into(), email: "alice@ex.com".into(), password: "pass2".into() }).await.unwrap_err();
+        assert!(matches!(err, DomainError::Conflict(_)));
+    }
 }
diff --git a/crates/application/src/use_cases/profile.rs b/crates/application/src/use_cases/profile.rs
index 006ba54..3f52803 100644
--- a/crates/application/src/use_cases/profile.rs
+++ b/crates/application/src/use_cases/profile.rs
@@ -35,3 +35,63 @@ pub async fn set_top_friends(top_friends: &dyn TopFriendRepository, user_id: &Us
     let friends: Vec<(UserId, i16)> = friend_ids.into_iter().enumerate().map(|(i, id)| (id, (i + 1) as i16)).collect();
     top_friends.set_top_friends(user_id, friends).await
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use domain::{
+        errors::DomainError,
+        models::user::User,
+        testing::TestStore,
+        value_objects::{Email, PasswordHash, UserId, Username},
+    };
+
+    fn make_user() -> User {
+        User::new_local(
+            UserId::new(),
+            Username::new("alice").unwrap(),
+            Email::new("alice@ex.com").unwrap(),
+            PasswordHash("h".into()),
+        )
+    }
+
+    #[tokio::test]
+    async fn set_top_friends_rejects_more_than_eight() {
+        let store = TestStore::default();
+        let uid = UserId::new();
+        let friends: Vec<UserId> = (0..9).map(|_| UserId::new()).collect();
+        let err = set_top_friends(&store, &uid, friends).await.unwrap_err();
+        assert!(matches!(err, DomainError::InvalidInput(_)));
+    }
+
+    #[tokio::test]
+    async fn set_top_friends_assigns_sequential_positions() {
+        let store = TestStore::default();
+        let uid = UserId::new();
+        let f1 = UserId::new();
+        let f2 = UserId::new();
+        let f3 = UserId::new();
+        set_top_friends(&store, &uid, vec![f1.clone(), f2.clone(), f3.clone()]).await.unwrap();
+        let tf = store.top_friends.lock().unwrap();
+        assert_eq!(tf.len(), 3);
+        let pos_f1 = tf.iter().find(|t| t.friend_id == f1).map(|t| t.position).unwrap();
+        let pos_f2 = tf.iter().find(|t| t.friend_id == f2).map(|t| t.position).unwrap();
+        assert!(pos_f1 < pos_f2, "f1 should come before f2");
+    }
+
+    #[tokio::test]
+    async fn get_user_by_username_returns_not_found_for_missing_user() {
+        let store = TestStore::default();
+        let err = get_user_by_username(&store, "nobody").await.unwrap_err();
+        assert!(matches!(err, DomainError::NotFound));
+    }
+
+    #[tokio::test]
+    async fn get_user_by_username_returns_correct_user() {
+        let store = TestStore::default();
+        let user = make_user();
+        store.users.lock().unwrap().push(user.clone());
+        let found = get_user_by_username(&store, "alice").await.unwrap();
+        assert_eq!(found.id, user.id);
+    }
+}
diff --git a/crates/application/src/use_cases/social.rs b/crates/application/src/use_cases/social.rs
index 2a467a8..0486896 100644
--- a/crates/application/src/use_cases/social.rs
+++ b/crates/application/src/use_cases/social.rs
@@ -136,4 +136,37 @@ mod tests {
         assert_eq!(events.len(), 1);
         assert!(matches!(events[0], DomainEvent::UserUnblocked { .. }));
     }
+
+    #[tokio::test]
+    async fn block_user_saves_block_and_publishes_event() {
+        let store = TestStore::default();
+        let alice = user("alice");
+        let bob = user("bob");
+        block_user(&store, &store, &alice.id, &bob.id).await.unwrap();
+        assert_eq!(store.blocks.lock().unwrap().len(), 1);
+        let events = store.events.lock().unwrap();
+        assert!(events.iter().any(|e| matches!(e, DomainEvent::UserBlocked { blocker_id, .. } if blocker_id == &alice.id)));
+    }
+
+    #[tokio::test]
+    async fn cannot_block_self() {
+        let store = TestStore::default();
+        let alice = user("alice");
+        let err = block_user(&store, &store, &alice.id, &alice.id).await.unwrap_err();
+        assert!(matches!(err, DomainError::InvalidInput(_)));
+    }
+
+    #[tokio::test]
+    async fn boost_and_unboost() {
+        let store = TestStore::default();
+        let alice = user("alice");
+        let tid = ThoughtId::new();
+        boost_thought(&store, &store, &alice.id, &tid).await.unwrap();
+        assert_eq!(store.boosts.lock().unwrap().len(), 1);
+        unboost_thought(&store, &store, &alice.id, &tid).await.unwrap();
+        assert!(store.boosts.lock().unwrap().is_empty());
+        let events = store.events.lock().unwrap();
+        assert!(events.iter().any(|e| matches!(e, DomainEvent::BoostAdded { .. })));
+        assert!(events.iter().any(|e| matches!(e, DomainEvent::BoostRemoved { .. })));
+    }
 }
diff --git a/crates/application/src/use_cases/thoughts.rs b/crates/application/src/use_cases/thoughts.rs
index 48d0470..2d7f098 100644
--- a/crates/application/src/use_cases/thoughts.rs
+++ b/crates/application/src/use_cases/thoughts.rs
@@ -119,4 +119,42 @@ mod tests {
         let err = delete_thought(&store, &NoOpEventPublisher, &out.thought.id, &bob.id).await.unwrap_err();
         assert!(matches!(err, DomainError::NotFound));
     }
+
+    #[tokio::test]
+    async fn edit_thought_changes_content_and_emits_event() {
+        let store = TestStore::default();
+        let alice = user();
+        store.users.lock().unwrap().push(alice.clone());
+        let out = create_thought(&store, &store, &store, input(alice.id.clone())).await.unwrap();
+        let tid = out.thought.id.clone();
+
+        edit_thought(&store, &store, &tid, &alice.id, "updated".to_string()).await.unwrap();
+
+        let saved = store.thoughts.lock().unwrap().iter().find(|t| t.id == tid).unwrap().clone();
+        assert_eq!(saved.content.as_str(), "updated");
+
+        let events = store.events.lock().unwrap();
+        assert!(events.iter().any(|e| matches!(e, DomainEvent::ThoughtUpdated { thought_id, .. } if thought_id == &tid)));
+    }
+
+    #[tokio::test]
+    async fn create_reply_sets_in_reply_to_id() {
+        let store = TestStore::default();
+        let alice = user();
+        store.users.lock().unwrap().push(alice.clone());
+        let original = create_thought(&store, &store, &NoOpEventPublisher, input(alice.id.clone())).await.unwrap().thought;
+
+        create_thought(&store, &store, &NoOpEventPublisher, CreateThoughtInput {
+            user_id: alice.id.clone(),
+            content: "reply".into(),
+            in_reply_to_id: Some(original.id.clone()),
+            visibility: None,
+            content_warning: None,
+            sensitive: false,
+        }).await.unwrap();
+
+        let thoughts = store.thoughts.lock().unwrap();
+        let reply = thoughts.iter().find(|t| t.content.as_str() == "reply").unwrap();
+        assert_eq!(reply.in_reply_to_id, Some(original.id.clone()));
+    }
 }