diff --git a/crates/adapters/activitypub/src/instance_actor.rs b/crates/adapters/activitypub/src/instance_actor.rs
new file mode 100644
index 0000000..77192f0
--- /dev/null
+++ b/crates/adapters/activitypub/src/instance_actor.rs
@@ -0,0 +1,57 @@
+use std::sync::Arc;
+
+use async_trait::async_trait;
+use k_ap::{ApActorType, ApUser, ApUserRepository};
+
+pub const INSTANCE_ACTOR_ID: uuid::Uuid =
+    uuid::Uuid::from_bytes([0, 0, 0, 0, 0, 0, 0x40, 0, 0x80, 0, 0, 0, 0, 0, 0, 0]);
+
+pub struct InstanceActorUserRepo {
+    inner: Arc<dyn ApUserRepository>,
+    base_url: String,
+}
+
+impl InstanceActorUserRepo {
+    pub fn new(inner: Arc<dyn ApUserRepository>, base_url: String) -> Self {
+        Self { inner, base_url }
+    }
+}
+
+fn instance_ap_user(base_url: &str) -> ApUser {
+    ApUser {
+        id: INSTANCE_ACTOR_ID,
+        username: "instance".to_string(),
+        display_name: None,
+        bio: None,
+        avatar_url: None,
+        banner_url: None,
+        also_known_as: vec![],
+        profile_url: url::Url::parse(base_url).ok(),
+        attachment: vec![],
+        manually_approves_followers: false,
+        discoverable: false,
+        actor_type: ApActorType::Service,
+        featured_url: None,
+    }
+}
+
+#[async_trait]
+impl ApUserRepository for InstanceActorUserRepo {
+    async fn find_by_id(&self, id: uuid::Uuid) -> anyhow::Result<Option<ApUser>> {
+        if id == INSTANCE_ACTOR_ID {
+            return Ok(Some(instance_ap_user(&self.base_url)));
+        }
+        self.inner.find_by_id(id).await
+    }
+
+    async fn find_by_username(&self, username: &str) -> anyhow::Result<Option<ApUser>> {
+        if username == "instance" {
+            return Ok(Some(instance_ap_user(&self.base_url)));
+        }
+        self.inner.find_by_username(username).await
+    }
+
+    async fn count_users(&self) -> anyhow::Result<usize> {
+        self.inner.count_users().await
+    }
+}
diff --git a/crates/adapters/activitypub/src/lib.rs b/crates/adapters/activitypub/src/lib.rs
index 9aa10d0..fe32db1 100644
--- a/crates/adapters/activitypub/src/lib.rs
+++ b/crates/adapters/activitypub/src/lib.rs
@@ -1,4 +1,5 @@
 pub mod handler;
+pub mod instance_actor;
 pub mod note;
 pub mod port;
 pub mod service;
@@ -33,17 +34,22 @@ pub struct ApServiceConfig {
 pub async fn build_ap_service(
     cfg: ApServiceConfig,
 ) -> (Arc<ActivityPubService>, Arc<ApFederationAdapter>) {
+    let user_repo = Arc::new(instance_actor::InstanceActorUserRepo::new(
+        cfg.user_repo,
+        cfg.base_url.clone(),
+    ));
     let mut builder = ActivityPubService::builder(cfg.base_url)
         .activity_repo(cfg.activity_repo)
         .follow_repo(cfg.follow_repo)
         .actor_repo(cfg.actor_repo)
         .blocklist_repo(cfg.blocklist_repo)
-        .user_repo(cfg.user_repo)
+        .user_repo(user_repo)
         .content_reader(cfg.ap_handler.clone())
         .object_handler(cfg.ap_handler)
         .allow_registration(cfg.allow_registration)
         .software_name("thoughts")
-        .debug(cfg.debug);
+        .debug(cfg.debug)
+        .signed_fetch_actor_id(instance_actor::INSTANCE_ACTOR_ID);
     if let Some(publisher) = cfg.event_publisher {
         builder = builder.event_publisher(publisher);
     }
diff --git a/crates/adapters/activitypub/src/service.rs b/crates/adapters/activitypub/src/service.rs
index 374e2fe..aaa7d69 100644
--- a/crates/adapters/activitypub/src/service.rs
+++ b/crates/adapters/activitypub/src/service.rs
@@ -136,8 +136,6 @@ fn k_ap_actor_to_domain(a: k_ap::RemoteActor) -> DomainRemoteActor {
     }
 }
 
-// TODO: these fetches are unsigned — fails on instances with authorized-fetch (Secure Mode).
-// Fix requires exposing k-ap's signed HTTP client.
 async fn resolve_actor_profiles_from_urls(
     urls: Vec<String>,
 ) -> Vec<domain::models::actor_connection_summary::ActorConnectionSummary> {