diff --git a/crates/application/src/use_cases/thoughts.rs b/crates/application/src/use_cases/thoughts.rs
index 9d2f3bc..38abdfe 100644
--- a/crates/application/src/use_cases/thoughts.rs
+++ b/crates/application/src/use_cases/thoughts.rs
@@ -2,10 +2,34 @@ use domain::{
     errors::DomainError,
     events::DomainEvent,
     models::thought::{Thought, Visibility},
-    ports::{EventPublisher, ThoughtRepository, UserRepository},
+    ports::{EventPublisher, TagRepository, ThoughtRepository, UserRepository},
     value_objects::{Content, ThoughtId, UserId},
 };
 
+fn extract_hashtags(content: &str) -> Vec<String> {
+    let mut tags = Vec::new();
+    let mut chars = content.char_indices().peekable();
+    while let Some((_, c)) = chars.next() {
+        if c == '#'
+            && chars
+                .peek()
+                .map(|(_, nc)| nc.is_alphanumeric())
+                .unwrap_or(false)
+        {
+            let tag: String = chars
+                .by_ref()
+                .take_while(|(_, nc)| nc.is_alphanumeric() || *nc == '_')
+                .map(|(_, nc)| nc)
+                .collect();
+            if !tag.is_empty() {
+                tags.push(tag.to_lowercase());
+            }
+        }
+    }
+    tags.dedup();
+    tags
+}
+
 fn require_owner(thought: &Thought, user_id: &UserId) -> Result<(), DomainError> {
     if thought.user_id != *user_id {
         return Err(DomainError::NotFound);
@@ -28,6 +52,7 @@ pub struct CreateThoughtOutput {
 pub async fn create_thought(
     thoughts: &dyn ThoughtRepository,
     _users: &dyn UserRepository,
+    tags: &dyn TagRepository,
     events: &dyn EventPublisher,
     input: CreateThoughtInput,
 ) -> Result<CreateThoughtOutput, DomainError> {
@@ -40,13 +65,21 @@ pub async fn create_thought(
     let thought = Thought::new_local(
         ThoughtId::new(),
         input.user_id,
-        content,
+        content.clone(),
         input.in_reply_to_id.clone(),
         visibility,
         input.content_warning,
         input.sensitive,
     );
     thoughts.save(&thought).await?;
+
+    // Extract and attach hashtags from content.
+    for tag_name in extract_hashtags(content.as_str()) {
+        if let Ok(tag) = tags.find_or_create(&tag_name).await {
+            let _ = tags.attach_to_thought(&thought.id, tag.id).await;
+        }
+    }
+
     events
         .publish(&DomainEvent::ThoughtCreated {
             thought_id: thought.id.clone(),
@@ -149,7 +182,7 @@ mod tests {
         let store = TestStore::default();
         let u = user();
         store.users.lock().unwrap().push(u.clone());
-        let out = create_thought(&store, &store, &store, input(u.id.clone()))
+        let out = create_thought(&store, &store, &store, &store, input(u.id.clone()))
             .await
             .unwrap();
         assert_eq!(out.thought.content.as_str(), "hello");
@@ -161,9 +194,15 @@ mod tests {
         let store = TestStore::default();
         let u = user();
         store.users.lock().unwrap().push(u.clone());
-        let out = create_thought(&store, &store, &NoOpEventPublisher, input(u.id.clone()))
-            .await
-            .unwrap();
+        let out = create_thought(
+            &store,
+            &store,
+            &store,
+            &NoOpEventPublisher,
+            input(u.id.clone()),
+        )
+        .await
+        .unwrap();
         delete_thought(&store, &NoOpEventPublisher, &out.thought.id, &u.id)
             .await
             .unwrap();
@@ -185,9 +224,15 @@ mod tests {
             .lock()
             .unwrap()
             .extend([alice.clone(), bob.clone()]);
-        let out = create_thought(&store, &store, &NoOpEventPublisher, input(alice.id.clone()))
-            .await
-            .unwrap();
+        let out = create_thought(
+            &store,
+            &store,
+            &store,
+            &NoOpEventPublisher,
+            input(alice.id.clone()),
+        )
+        .await
+        .unwrap();
         let err = delete_thought(&store, &NoOpEventPublisher, &out.thought.id, &bob.id)
             .await
             .unwrap_err();
@@ -199,7 +244,7 @@ mod tests {
         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()))
+        let out = create_thought(&store, &store, &store, &store, input(alice.id.clone()))
             .await
             .unwrap();
         let tid = out.thought.id.clone();
@@ -229,12 +274,19 @@ mod tests {
         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;
+        let original = create_thought(
+            &store,
+            &store,
+            &store,
+            &NoOpEventPublisher,
+            input(alice.id.clone()),
+        )
+        .await
+        .unwrap()
+        .thought;
 
         create_thought(
+            &store,
             &store,
             &store,
             &NoOpEventPublisher,
diff --git a/crates/presentation/src/handlers/thoughts.rs b/crates/presentation/src/handlers/thoughts.rs
index 0281fab..73085a7 100644
--- a/crates/presentation/src/handlers/thoughts.rs
+++ b/crates/presentation/src/handlers/thoughts.rs
@@ -64,6 +64,7 @@ pub async fn post_thought(
     let out = create_thought(
         &*s.thoughts,
         &*s.users,
+        &*s.tags,
         &*s.events,
         CreateThoughtInput {
             user_id: uid.clone(),