diff --git a/crates/overslash-api/src/extractors.rs b/crates/overslash-api/src/extractors.rs
index 4cfda184..46303354 100644
--- a/crates/overslash-api/src/extractors.rs
+++ b/crates/overslash-api/src/extractors.rs
@@ -440,6 +440,24 @@ impl FromRequestParts<AppState> for AuthContext {
                     .await?;
                 }
 
+                // Impersonation is the one authentication route that leaves
+                // no trace on the identity it acts as: the touch above stamps
+                // the *key's* identity, and an org service key is never a
+                // sub-agent. Without this, a sub-agent reached only through
+                // this header has `last_active_at` frozen at creation and the
+                // idle sweep archives it however busy it actually is.
+                //
+                // Only the leaf needs stamping — the sweep skips any identity
+                // with a live child, so a live leaf holds its ancestors up.
+                if target.kind == "sub_agent" {
+                    let touch_scope =
+                        OrgScope::new(key_row.org_id, state.db_pool(&parts.extensions));
+                    let target_id = target.identity_id;
+                    tokio::spawn(async move {
+                        let _ = touch_scope.touch_identity_last_active(target_id).await;
+                    });
+                }
+
                 (Some(target.identity_id), Some(key_row.identity_id))
             }
             Some(_) => {
diff --git a/crates/overslash-api/src/impersonation.rs b/crates/overslash-api/src/impersonation.rs
index 3f448382..b4442a3f 100644
--- a/crates/overslash-api/src/impersonation.rs
+++ b/crates/overslash-api/src/impersonation.rs
@@ -37,6 +37,13 @@ use crate::error::AppError;
 pub struct ResolvedTarget {
     /// The effective identity to act as — the leaf of the path.
     pub identity_id: Uuid,
+    /// The kind of the effective identity (`user`, `agent`, `sub_agent`).
+    ///
+    /// Carried out so the extractor can stamp `last_active_at` on a
+    /// `sub_agent` target without a second lookup: impersonation is the one
+    /// authentication route that produces activity on an identity nobody
+    /// holds a key for, and the idle sweep reaps whatever it cannot see.
+    pub kind: String,
     /// The root user whose display name the caller may still refresh, set only
     /// when a name was supplied for a user root that already existed.
     ///
@@ -194,6 +201,7 @@ pub async fn resolve_target(
 
     Ok(ResolvedTarget {
         identity_id: current.id,
+        kind: current.kind,
         renameable_root,
     })
 }
diff --git a/crates/overslash-api/tests/impersonation.rs b/crates/overslash-api/tests/impersonation.rs
index 36e83791..00284238 100644
--- a/crates/overslash-api/tests/impersonation.rs
+++ b/crates/overslash-api/tests/impersonation.rs
@@ -1152,3 +1152,126 @@ async fn name_header_rejects_an_agent_target() {
     .await;
     assert_eq!(status, 400);
 }
+
+// ── Activity tracking ────────────────────────────────────────────────────────
+
+/// Create a `sub_agent` under `parent_id` and return its id.
+async fn create_subagent(
+    base: &str,
+    client: &reqwest::Client,
+    admin_key: &str,
+    parent_id: Uuid,
+    name: &str,
+) -> Uuid {
+    let sub: Value = client
+        .post(format!("{base}/v1/identities"))
+        .header("Authorization", format!("Bearer {admin_key}"))
+        .json(&json!({"name": name, "kind": "sub_agent", "parent_id": parent_id}))
+        .send()
+        .await
+        .unwrap()
+        .json()
+        .await
+        .unwrap();
+    sub["id"].as_str().unwrap().parse().unwrap()
+}
+
+/// The touch is fire-and-forget (`tokio::spawn`), so poll for it rather than
+/// racing it. Returns the observed `last_active_at`.
+async fn await_last_active_after(
+    pool: &PgPool,
+    id: Uuid,
+    floor: time::OffsetDateTime,
+) -> time::OffsetDateTime {
+    for _ in 0..50 {
+        let seen: time::OffsetDateTime =
+            sqlx::query_scalar("SELECT last_active_at FROM identities WHERE id = $1")
+                .bind(id)
+                .fetch_one(pool)
+                .await
+                .unwrap();
+        if seen > floor {
+            return seen;
+        }
+        tokio::time::sleep(std::time::Duration::from_millis(20)).await;
+    }
+    panic!("last_active_at never advanced past {floor} for {id}");
+}
+
+#[tokio::test]
+async fn impersonation_touches_sub_agent_target_last_active() {
+    let (base, client, pool, org_id, admin_key, sa_id, _, target_agent_id) = setup().await;
+    let imp_key = create_impersonation_key(&base, &client, &admin_key, org_id, sa_id).await;
+    let sub_id = create_subagent(&base, &client, &admin_key, target_agent_id, "worker").await;
+
+    // Backdate activity so any advance is unambiguously ours.
+    let floor: time::OffsetDateTime = sqlx::query_scalar(
+        "UPDATE identities SET last_active_at = now() - interval '2 hours'
+         WHERE id = $1 RETURNING last_active_at",
+    )
+    .bind(sub_id)
+    .fetch_one(&pool)
+    .await
+    .unwrap();
+
+    let resp = client
+        .get(format!("{base}/v1/whoami"))
+        .header("Authorization", format!("Bearer {imp_key}"))
+        .header("X-Overslash-As", sub_id.to_string())
+        .send()
+        .await
+        .unwrap();
+    assert!(
+        resp.status().is_success(),
+        "impersonated call should succeed"
+    );
+
+    // Impersonation is the only route that reaches this identity — nobody
+    // holds a key for it — so if the header does not stamp it, nothing does.
+    await_last_active_after(&pool, sub_id, floor).await;
+}
+
+#[tokio::test]
+async fn impersonated_sub_agent_survives_the_idle_sweep() {
+    let (base, client, pool, org_id, admin_key, sa_id, _, target_agent_id) = setup().await;
+    let imp_key = create_impersonation_key(&base, &client, &admin_key, org_id, sa_id).await;
+    let sub_id = create_subagent(&base, &client, &admin_key, target_agent_id, "busy").await;
+
+    sqlx::query("UPDATE orgs SET subagent_idle_timeout_secs = 60 WHERE id = $1")
+        .bind(org_id)
+        .execute(&pool)
+        .await
+        .unwrap();
+    let floor: time::OffsetDateTime = sqlx::query_scalar(
+        "UPDATE identities SET last_active_at = now() - interval '2 hours'
+         WHERE id = $1 RETURNING last_active_at",
+    )
+    .bind(sub_id)
+    .fetch_one(&pool)
+    .await
+    .unwrap();
+
+    let resp = client
+        .get(format!("{base}/v1/whoami"))
+        .header("Authorization", format!("Bearer {imp_key}"))
+        .header("X-Overslash-As", sub_id.to_string())
+        .send()
+        .await
+        .unwrap();
+    assert!(resp.status().is_success());
+    await_last_active_after(&pool, sub_id, floor).await;
+
+    overslash_db::repos::identity::archive_idle_subagents(&pool)
+        .await
+        .unwrap();
+
+    let row = overslash_db::repos::identity::get_by_id(&pool, org_id, sub_id)
+        .await
+        .unwrap()
+        .unwrap();
+    assert!(
+        row.archived_at.is_none(),
+        "a sub-agent used this second must not be reaped as idle (reason: {:?})",
+        row.archived_reason
+    );
+}