diff --git a/crates/manager-core/src/apply_service.rs b/crates/manager-core/src/apply_service.rs
index e00665e..e4d03bc 100644
--- a/crates/manager-core/src/apply_service.rs
+++ b/crates/manager-core/src/apply_service.rs
@@ -576,6 +576,19 @@ impl ApplyService {
                 ));
             }
         }
+        // §11.6: shared collections are owned by GROUPS. Reject them on an app
+        // node — the inverse of the group route/trigger guard above. The CLI
+        // already prevents this (`ManifestApp` has no `collections` field), so
+        // this is defense-in-depth against a hand-rolled wire `Bundle`; without
+        // it the reconcile would insert an inert `app_id`-owned marker row that
+        // no resolver ever reads.
+        if matches!(owner, ApplyOwner::App(_)) && !bundle.collections.is_empty() {
+            return Err(ApplyError::Invalid(
+                "an app manifest cannot declare shared collections — \
+                 they are owned by a group"
+                    .into(),
+            ));
+        }
         self.validate_bundle(bundle, inherited_endpoints)
     }
 
diff --git a/crates/picloud-cli/tests/collections.rs b/crates/picloud-cli/tests/collections.rs
index e30bdc3..2cb228e 100644
--- a/crates/picloud-cli/tests/collections.rs
+++ b/crates/picloud-cli/tests/collections.rs
@@ -158,6 +158,111 @@ fn shared_collection_is_read_write_across_the_subtree() {
     );
 }
 
+/// Nearest-ancestor-group-wins (§11.6, the CoW-shadowing guarantee). A parent
+/// AND a child group both declare `catalog`; an app under the child must bind
+/// the CHILD's store, not the parent's. Discriminated by a second app directly
+/// under the parent: it resolves to the PARENT's store, so it must NOT see what
+/// the deep app wrote. If the resolver's `ORDER BY depth LIMIT 1` regressed to
+/// the farther group, the shallow app would see the deep app's value.
+#[ignore = "needs DATABASE_URL pointing at a running Postgres"]
+#[test]
+fn nearest_declaring_group_wins() {
+    let Some(fx) = common::fixture_or_skip() else {
+        return;
+    };
+    let env = common::admin_env(fx);
+    let root = common::unique_slug("nw-root");
+    let team = common::unique_slug("nw-team");
+    let deep = common::unique_slug("nw-deep");
+    let shallow = common::unique_slug("nw-shallow");
+
+    let _gr = GroupGuard::new(&env.url, &env.token, &root);
+    let _gt = GroupGuard::new(&env.url, &env.token, &team);
+    common::pic_as(&env)
+        .args(["groups", "create", &root])
+        .assert()
+        .success();
+    common::pic_as(&env)
+        .args(["groups", "create", &team, "--parent", &root])
+        .assert()
+        .success();
+
+    // BOTH groups declare `catalog` (two distinct stores, same name).
+    let dir = manifest_dir();
+    for g in [&root, &team] {
+        let m = format!("[group]\nslug = \"{g}\"\nname = \"NW\"\n\ncollections = [\"catalog\"]\n");
+        let p = dir.path().join(format!("{g}.toml"));
+        fs::write(&p, &m).unwrap();
+        common::pic_as(&env)
+            .args(["apply", "--file"])
+            .arg(&p)
+            .assert()
+            .success();
+    }
+
+    // `deep` under team, `shallow` directly under root.
+    let _ad = AppGuard::new(&env.url, &env.token, &deep);
+    let _as = AppGuard::new(&env.url, &env.token, &shallow);
+    common::pic_as(&env)
+        .args(["apps", "create", &deep, "--group", &team])
+        .assert()
+        .success();
+    common::pic_as(&env)
+        .args(["apps", "create", &shallow, "--group", &root])
+        .assert()
+        .success();
+
+    // The deep app writes — must land in TEAM's store (nearest declarer).
+    fs::write(
+        dir.path().join("scripts/w.rhai"),
+        r#"kv::shared_collection("catalog").set("k", "team-value"); "ok""#,
+    )
+    .unwrap();
+    common::pic_as(&env)
+        .args(["scripts", "deploy"])
+        .arg(dir.path().join("scripts/w.rhai"))
+        .args(["--app", &deep, "--name", "w"])
+        .assert()
+        .success();
+    common::pic_as(&env)
+        .args(["scripts", "invoke", &app_script_id(&env, &deep, "w")])
+        .assert()
+        .success();
+
+    // The deep app reads its own write back (team's store).
+    fs::write(
+        dir.path().join("scripts/r.rhai"),
+        r#"kv::shared_collection("catalog").get("k")"#,
+    )
+    .unwrap();
+    common::pic_as(&env)
+        .args(["scripts", "deploy"])
+        .arg(dir.path().join("scripts/r.rhai"))
+        .args(["--app", &deep, "--name", "r"])
+        .assert()
+        .success();
+    assert_eq!(
+        invoke_body(&env, &app_script_id(&env, &deep, "r")),
+        serde_json::json!("team-value"),
+        "the deep app reads back its own write to the nearest (team) store"
+    );
+
+    // The shallow app reads catalog → ROOT's store, which is empty. Seeing
+    // `team-value` here would mean the write leaked to the farther group.
+    common::pic_as(&env)
+        .args(["scripts", "deploy"])
+        .arg(dir.path().join("scripts/r.rhai"))
+        .args(["--app", &shallow, "--name", "r"])
+        .assert()
+        .success();
+    assert_eq!(
+        invoke_body(&env, &app_script_id(&env, &shallow, "r")),
+        serde_json::Value::Null,
+        "the shallow app resolves the ROOT store (empty) — nearest-wins kept the \
+         deep write in the team store"
+    );
+}
+
 /// Id of the named script in an app (`pic scripts ls --app <a>`).
 fn app_script_id(env: &common::TestEnv, app: &str, name: &str) -> String {
     let ls = common::pic_as(env)