Search selected plugin apps before falling back (#38484)

## What changed

- When an explicitly selected plugin has apps available, instruct the model to use `tool_search` to discover relevant tools before falling back to unrelated or built-in tools.
- Scope the guidance to the request that selected the plugin and omit it for plugins without available apps.

## Testing

- Cover app-specific rendering, non-app plugins, bounded instruction output, and app enablement in dual-surface plugin requests.

GitOrigin-RevId: 1b9501cc403d2817334a9307e683e99c140a794a

codex-rs/core/src/plugins/render.rs

@@ -24,6 +24,17 @@ pub(crate) fn render_explicit_plugin_instructions(
         plugin.display_name
     )];
 
+    if !available_apps.is_empty() {
+        lines.push(
+            concat!(
+                "- For the user request that explicitly selected this plugin, and only for that ",
+                "request, if `tool_search` is available and an app from this plugin may help, ",
+                "search for its tools before falling back to unrelated or built-in tools."
+            )
+            .to_string(),
+        );
+    }
+
     if plugin.has_skills {
         let skill_namespace = plugin
             .plugin_namespace
@@ -34,17 +45,6 @@ pub(crate) fn render_explicit_plugin_instructions(
         ));
     }
 
-    if !available_mcp_servers.is_empty() {
-        lines.push(format!(
-            "- MCP servers from this plugin available in this session: {}.",
-            available_mcp_servers
-                .iter()
-                .map(|server| format!("`{server}`"))
-                .collect::<Vec<_>>()
-                .join(", ")
-        ));
-    }
-
     if !available_apps.is_empty() {
         lines.push(format!(
             "- Apps from this plugin available in this session: {}.",
@@ -56,6 +56,17 @@ pub(crate) fn render_explicit_plugin_instructions(
         ));
     }
 
+    if !available_mcp_servers.is_empty() {
+        lines.push(format!(
+            "- MCP servers from this plugin available in this session: {}.",
+            available_mcp_servers
+                .iter()
+                .map(|server| format!("`{server}`"))
+                .collect::<Vec<_>>()
+                .join(", ")
+        ));
+    }
+
     if lines.len() == 1 {
         return None;
     }

codex-rs/core/src/plugins/render_tests.rs

@@ -40,6 +40,31 @@ fn explicit_plugin_instructions_use_manifest_namespace_for_skills() {
 
     assert!(rendered.contains("`acme.tools:`"));
     assert!(!rendered.contains("`Acme Developer Tools:`"));
+    assert!(!rendered.contains("tool_search"));
+}
+
+#[test]
+fn explicit_plugin_instructions_search_available_apps_before_fallback() {
+    let rendered = render_explicit_plugin_instructions(
+        &PluginCapabilitySummary {
+            config_name: "app-adobe@openai-curated-remote".to_string(),
+            display_name: "Adobe".to_string(),
+            ..PluginCapabilitySummary::default()
+        },
+        &[],
+        &["Adobe".to_string()],
+    )
+    .expect("app capability should render");
+
+    assert_eq!(
+        rendered,
+        "Capabilities from the `Adobe` plugin:\n\
+         - For the user request that explicitly selected this plugin, and only for that request, \
+         if `tool_search` is available and an app from this plugin may help, search for its \
+         tools before falling back to unrelated or built-in tools.\n\
+         - Apps from this plugin available in this session: `Adobe`.\n\
+         Use these plugin-associated capabilities to help solve the task."
+    );
 }
 
 #[test]
@@ -47,18 +72,26 @@ fn explicit_plugin_instructions_are_bounded() {
     let servers = (0..1_024)
         .map(|index| format!("server-{index}"))
         .collect::<Vec<_>>();
+    let apps = (0..1_024)
+        .map(|index| format!("app-{index}"))
+        .collect::<Vec<_>>();
 
     let rendered = render_explicit_plugin_instructions(
         &PluginCapabilitySummary {
             config_name: "sample@test".to_string(),
             display_name: "sample".to_string(),
+            has_skills: true,
             ..PluginCapabilitySummary::default()
         },
         &servers,
-        &[],
+        &apps,
     )
     .expect("MCP capability should render");
 
     assert!(rendered.len() <= MAX_EXPLICIT_PLUGIN_INSTRUCTIONS_BYTES);
+    assert!(rendered.contains("only for that request"));
+    assert!(rendered.contains("if `tool_search` is available"));
+    assert!(rendered.contains("Skills from this plugin"));
+    assert!(rendered.contains("`app-0`"));
     assert!(rendered.ends_with(TRUNCATED_PLUGIN_INSTRUCTIONS_SUFFIX));
 }

codex-rs/core/tests/suite/plugins.rs

@@ -1134,6 +1134,13 @@ async fn explicit_plugin_mentions_use_apps_for_chatgpt_dual_surface_plugins(
         app_enabled,
         "plugin app guidance should match app enablement: {developer_messages:?}"
     );
+    assert_eq!(
+        developer_messages
+            .iter()
+            .any(|text| text.contains("if `tool_search` is available")),
+        app_enabled,
+        "plugin app search guidance should match app enablement: {developer_messages:?}"
+    );
     assert!(
         request
             .tool_by_name(SAMPLE_PLUGIN_MCP_NAMESPACE, "echo")