diff --git a/cmux-tui/crates/cmux-tui/src/plugin_manager.rs b/cmux-tui/crates/cmux-tui/src/plugin_manager.rs
index 8c9c584aec74..19febe0915ee 100644
--- a/cmux-tui/crates/cmux-tui/src/plugin_manager.rs
+++ b/cmux-tui/crates/cmux-tui/src/plugin_manager.rs
@@ -55,6 +55,15 @@ impl PluginKind {
             Self::Agent => "agent_plugin_",
         }
     }
+
+    /// Resource noun named by `validation.invalid` errors about the selected
+    /// plugin, matching the `sidebar_plugin.*` operations in the resource spec.
+    fn resource_field(self) -> &'static str {
+        match self {
+            Self::Sidebar => "sidebar_plugin",
+            Self::Agent => "agent_plugin",
+        }
+    }
 }
 
 /// A userland plugin build must not hold the CLI forever.
@@ -353,12 +362,13 @@ fn update_command(
     let mut plugin = resolve_installed_plugin(&positionals[1], kind)?;
     let source = git_text(&plugin.dir, ["remote", "get-url", "origin"]).ok_or_else(|| {
         ManagerError::validation(
-            Some("plugin"),
+            Some(kind.resource_field()),
             format!("plugin {} has no readable origin remote", plugin.name),
         )
     })?;
-    validate_git_source(&source)
-        .map_err(|error| ManagerError::validation(Some("plugin"), error.to_string()))?;
+    validate_git_source(&source).map_err(|error| {
+        ManagerError::validation(Some(kind.resource_field()), error.to_string())
+    })?;
 
     // Build and validate a fresh clone before touching the active install.
     // Updating in place would let a failed pull or build leave the selected
@@ -630,18 +640,20 @@ fn resolve_installed_plugin(
     let selector = forced_name.unwrap_or(selector);
     let by_id = forced_name.is_none() && selector.starts_with(kind.id_prefix());
     if by_id {
-        validate_plugin_id_for(selector, kind)
-            .map_err(|error| ManagerError::validation(Some("plugin"), error.to_string()))?;
+        validate_plugin_id_for(selector, kind).map_err(|error| {
+            ManagerError::validation(Some(kind.resource_field()), error.to_string())
+        })?;
     } else {
-        validate_plugin_name(selector)
-            .map_err(|error| ManagerError::validation(Some("plugin"), error.to_string()))?;
+        validate_plugin_name(selector).map_err(|error| {
+            ManagerError::validation(Some(kind.resource_field()), error.to_string())
+        })?;
     }
     installed_plugins(kind)?
         .into_iter()
         .find(|plugin| if by_id { plugin.id == selector } else { plugin.name == selector })
         .ok_or_else(|| {
             ManagerError::validation(
-                Some("plugin"),
+                Some(kind.resource_field()),
                 format!("plugin {selector:?} is not installed"),
             )
         })
diff --git a/cmux-tui/crates/cmux-tui/tests/resource_cli_v2.rs b/cmux-tui/crates/cmux-tui/tests/resource_cli_v2.rs
index 46e13976cebe..67e1f6b14218 100644
--- a/cmux-tui/crates/cmux-tui/tests/resource_cli_v2.rs
+++ b/cmux-tui/crates/cmux-tui/tests/resource_cli_v2.rs
@@ -826,6 +826,21 @@ fn local_plugin_jsonl_never_connects_to_the_session_socket() {
     assert_eq!(invalid["retryable"], false);
     assert_eq!(invalid["details"]["field"], "sidebar_plugin");
     assert!(invalid["details"]["reason"].is_string());
+
+    // Agent plugins share the manager but name their own resource noun.
+    let invalid_agent = Command::new(bin())
+        .args(["--json", "--socket"])
+        .arg(dir.join("missing.sock"))
+        .args(["agent", "plugin", "use", "Bad"])
+        .env("XDG_DATA_HOME", &data)
+        .env("XDG_CONFIG_HOME", &config)
+        .env_remove("CMUX_TUI_SOCKET")
+        .output()
+        .unwrap();
+    assert_eq!(invalid_agent.status.code(), Some(1));
+    let invalid_agent = parse_single_json(&invalid_agent.stderr);
+    assert_eq!(invalid_agent["code"], "validation.invalid");
+    assert_eq!(invalid_agent["details"]["field"], "agent_plugin");
     fs::remove_dir_all(dir).unwrap();
 }