diff --git a/docs/environments.md b/docs/environments.md
index 36e5df4..2d46919 100644
--- a/docs/environments.md
+++ b/docs/environments.md
@@ -88,9 +88,9 @@ deslicer inventory validate --environment acme-prod
 deslicer inventory validate --environment acme-prod --log-format json
 ```
 
-Checks (fail closed): YAML shape and required `inventory_group`; no duplicate groups; no duplicate `source_path`+`dest_dir`; `source_path` exists on disk unless `state: absent`; thin `dest_dir` authoring allowlist (`apps`, `deployment-apps`, `manager-apps`, `shcluster/apps`, `peer-apps`, `slave-apps`, `users` — not `master-apps`); live host-group allowlist via auth → Observer `GET /api/v1/groups` (not a hardcoded list).
+Checks (fail closed): YAML shape and required `inventory_group`; no duplicate groups; no duplicate `source_path`+`dest_dir`; `source_path` exists on disk unless `state: absent`; thin `dest_dir` authoring allowlist (`apps`, `deployment-apps`, `manager-apps`, `shcluster/apps` — not `master-apps`, `peer-apps`, `slave-apps`, or `users`); live host-group allowlist via auth → Observer `GET /api/v1/groups` (not a hardcoded list).
 
-Role-based `dest_dir` defaults (when YAML omits `dest_dir`) and identity alias `master-apps` → `manager-apps` are owned by DAP / Observer at compile time — the CLI does not invent defaults.
+Role-based `dest_dir` defaults (when YAML omits `dest_dir`) and identity alias `master-apps` → `manager-apps` are owned by DAP / Observer at compile time — the CLI does not invent defaults. `peer-apps` / `slave-apps` / `users` remain Observer observation scopes, not env YAML deploy targets.
 
 `--force` on `init` overwrites workflow templates only — it does not wipe operator `apps:` lists.
 
diff --git a/src/environment_yaml/validate/checks.rs b/src/environment_yaml/validate/checks.rs
index b643712..f1129bb 100644
--- a/src/environment_yaml/validate/checks.rs
+++ b/src/environment_yaml/validate/checks.rs
@@ -301,6 +301,29 @@ fn validate_common_fields(map: &serde_yml::Mapping, obj_path: &str, ctx: &mut Va
             )),
         }
     }
+
+    // Phase 1b: thin authoring check for optional target_host override.
+    // DAP enforces that the host is a member of the destination inventory_group.
+    if let Some(target_host) = map.get("target_host") {
+        match scalar_string(Some(target_host)) {
+            Some(value) if !value.trim().is_empty() => {}
+            Some(_) => ctx.issues.push(issue(
+                ctx.file_label,
+                &format!("{obj_path}.target_host"),
+                Severity::Error,
+                "target_host must be a non-empty string".into(),
+                "Set target_host to a hostname that is a member of this destination's \
+                 inventory_group (DAP verifies membership at compile)",
+            )),
+            None => ctx.issues.push(issue(
+                ctx.file_label,
+                &format!("{obj_path}.target_host"),
+                Severity::Error,
+                "target_host must be a string".into(),
+                "Set target_host to a hostname string, or omit the field",
+            )),
+        }
+    }
 }
 
 fn warn_if_not_splunk_app_root(base: &str, ctx: &mut ValidationCtx<'_>) {
diff --git a/src/environment_yaml/validate/mod.rs b/src/environment_yaml/validate/mod.rs
index 07a2996..150ce17 100644
--- a/src/environment_yaml/validate/mod.rs
+++ b/src/environment_yaml/validate/mod.rs
@@ -17,21 +17,13 @@ pub const VALID_STATES: &[&str] = &["present", "absent"];
 
 /// Thin authoring allowlist for env YAML `dest_dir` (fail-fast in GH Actions / local validate).
 ///
-/// DAP / Observer owns the full scope set, role-based defaults when `dest_dir` is
-/// omitted, and identity alias `master-apps` → `manager-apps`. Do not put that
-/// logic in the CLI.
+/// Deploy targets only (D4 role defaults): DS → `deployment-apps`, CM → `manager-apps`,
+/// SH deployer → `shcluster/apps`, else `apps` (on-disk `etc/apps`).
 ///
-/// `apps` maps on-disk to `etc/apps`. Legacy `master-apps` is rejected here —
-/// authors must use `manager-apps`.
-pub const VALID_DEST_DIRS: &[&str] = &[
-    "apps",
-    "deployment-apps",
-    "manager-apps",
-    "shcluster/apps",
-    "peer-apps",
-    "slave-apps",
-    "users",
-];
+/// DAP / Observer still **observes/parses** `peer-apps`, `slave-apps`, `users`, and
+/// legacy `master-apps` — those are not GitOps deploy destinations for env YAML.
+/// Legacy `master-apps` is rejected here; authors must use `manager-apps`.
+pub const VALID_DEST_DIRS: &[&str] = &["apps", "deployment-apps", "manager-apps", "shcluster/apps"];
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize)]
 #[serde(rename_all = "lowercase")]
diff --git a/src/environment_yaml/validate/tests.rs b/src/environment_yaml/validate/tests.rs
index d2bf385..f92952d 100644
--- a/src/environment_yaml/validate/tests.rs
+++ b/src/environment_yaml/validate/tests.rs
@@ -131,7 +131,7 @@ fn rejects_legacy_master_apps_dest_dir() {
 }
 
 #[test]
-fn accepts_peer_apps_dest_dir() {
+fn rejects_peer_apps_dest_dir() {
     let dir = tempdir().unwrap();
     write_app(dir.path(), "apps/ta_nix");
     let yaml = "destinations:\n\
@@ -139,17 +139,65 @@ fn accepts_peer_apps_dest_dir() {
 \x20\x20\x20\x20apps:\n\
 \x20\x20\x20\x20\x20\x20- source_path: apps/ta_nix\n\
 \x20\x20\x20\x20\x20\x20\x20\x20dest_dir: peer-apps\n";
+    let report =
+        validate_environment_yaml(yaml, "prod.yml", dir.path(), Some(&known(&["indexers"])));
+    assert!(report
+        .errors()
+        .any(|issue| issue.message.contains("invalid dest_dir")));
+}
+
+#[test]
+fn rejects_slave_apps_dest_dir() {
+    let dir = tempdir().unwrap();
+    write_app(dir.path(), "apps/ta_nix");
+    let yaml = "destinations:\n\
+\x20\x20- inventory_group: indexers\n\
+\x20\x20\x20\x20apps:\n\
+\x20\x20\x20\x20\x20\x20- source_path: apps/ta_nix\n\
+\x20\x20\x20\x20\x20\x20\x20\x20dest_dir: slave-apps\n";
+    let report =
+        validate_environment_yaml(yaml, "prod.yml", dir.path(), Some(&known(&["indexers"])));
+    assert!(report
+        .errors()
+        .any(|issue| issue.message.contains("invalid dest_dir")));
+}
+
+#[test]
+fn accepts_nonempty_target_host() {
+    let dir = tempdir().unwrap();
+    write_app(dir.path(), "apps/ta_nix");
+    let yaml = "destinations:\n\
+\x20\x20- inventory_group: indexers\n\
+\x20\x20\x20\x20apps:\n\
+\x20\x20\x20\x20\x20\x20- source_path: apps/ta_nix\n\
+\x20\x20\x20\x20\x20\x20\x20\x20target_host: splunk-idx-01.example.com\n";
     let report =
         validate_environment_yaml(yaml, "prod.yml", dir.path(), Some(&known(&["indexers"])));
     assert!(
         report
             .errors()
-            .all(|issue| !issue.message.contains("dest_dir")),
+            .all(|issue| !issue.message.contains("target_host")),
         "{:?}",
         report.issues
     );
 }
 
+#[test]
+fn rejects_empty_target_host() {
+    let dir = tempdir().unwrap();
+    write_app(dir.path(), "apps/ta_nix");
+    let yaml = "destinations:\n\
+\x20\x20- inventory_group: indexers\n\
+\x20\x20\x20\x20apps:\n\
+\x20\x20\x20\x20\x20\x20- source_path: apps/ta_nix\n\
+\x20\x20\x20\x20\x20\x20\x20\x20target_host: '  '\n";
+    let report =
+        validate_environment_yaml(yaml, "prod.yml", dir.path(), Some(&known(&["indexers"])));
+    assert!(report.errors().any(|issue| issue
+        .message
+        .contains("target_host must be a non-empty string")));
+}
+
 #[test]
 fn rejects_unknown_live_group() {
     let dir = tempdir().unwrap();