feat: rename experimental_instructions_file to model_instructions_file (#9555)

A user who has `experimental_instructions_file` set will now see this:

<img width="888" height="660" alt="image"
src="https://github.com/user-attachments/assets/51c98312-eb9b-4881-81f1-bea6677e158d"
/>

And a `codex exec` would include this warning:

<img width="888" height="660" alt="image"
src="https://github.com/user-attachments/assets/a89f62be-1edf-4593-a75e-e0b4a762ed7d"
/>

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

@@ -109,11 +109,11 @@ async fn chat_mode_stream_cli() {
     );
 }
 
-/// Verify that passing `-c experimental_instructions_file=...` to the CLI
+/// Verify that passing `-c model_instructions_file=...` to the CLI
 /// overrides the built-in base instructions by inspecting the request body
 /// received by a mock OpenAI Responses endpoint.
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn exec_cli_applies_experimental_instructions_file() {
+async fn exec_cli_applies_model_instructions_file() {
     skip_if_no_network!();
 
     // Start mock server which will capture the request and return a minimal
@@ -128,7 +128,7 @@ async fn exec_cli_applies_experimental_instructions_file() {
     // Create a temporary instructions file with a unique marker we can assert
     // appears in the outbound request payload.
     let custom = TempDir::new().unwrap();
-    let marker = "cli-experimental-instructions-marker";
+    let marker = "cli-model-instructions-file-marker";
     let custom_path = custom.path().join("instr.md");
     std::fs::write(&custom_path, marker).unwrap();
     let custom_path_str = custom_path.to_string_lossy().replace('\\', "/");
@@ -151,9 +151,7 @@ async fn exec_cli_applies_experimental_instructions_file() {
         .arg("-c")
         .arg("model_provider=\"mock\"")
         .arg("-c")
-        .arg(format!(
-            "experimental_instructions_file=\"{custom_path_str}\""
-        ))
+        .arg(format!("model_instructions_file=\"{custom_path_str}\""))
         .arg("-C")
         .arg(&repo_root)
         .arg("hello?\n");

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

@@ -1,15 +1,22 @@
 #![cfg(not(target_os = "windows"))]
 
 use anyhow::Ok;
+use codex_app_server_protocol::ConfigLayerSource;
+use codex_core::config_loader::ConfigLayerEntry;
+use codex_core::config_loader::ConfigLayerStack;
+use codex_core::config_loader::ConfigRequirements;
+use codex_core::config_loader::ConfigRequirementsToml;
 use codex_core::features::Feature;
 use codex_core::protocol::DeprecationNoticeEvent;
 use codex_core::protocol::EventMsg;
 use core_test_support::responses::start_mock_server;
 use core_test_support::skip_if_no_network;
+use core_test_support::test_absolute_path;
 use core_test_support::test_codex::TestCodex;
 use core_test_support::test_codex::test_codex;
 use core_test_support::wait_for_event_match;
 use pretty_assertions::assert_eq;
+use toml::Value as TomlValue;
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn emits_deprecation_notice_for_legacy_feature_flag() -> anyhow::Result<()> {
@@ -48,3 +55,58 @@ async fn emits_deprecation_notice_for_legacy_feature_flag() -> anyhow::Result<()
 
     Ok(())
 }
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn emits_deprecation_notice_for_experimental_instructions_file() -> anyhow::Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+
+    let mut builder = test_codex().with_config(|config| {
+        let mut table = toml::map::Map::new();
+        table.insert(
+            "experimental_instructions_file".to_string(),
+            TomlValue::String("legacy.md".to_string()),
+        );
+        let config_layer = ConfigLayerEntry::new(
+            ConfigLayerSource::User {
+                file: test_absolute_path("/tmp/config.toml"),
+            },
+            TomlValue::Table(table),
+        );
+        let config_layer_stack = ConfigLayerStack::new(
+            vec![config_layer],
+            ConfigRequirements::default(),
+            ConfigRequirementsToml::default(),
+        )
+        .expect("build config layer stack");
+        config.config_layer_stack = config_layer_stack;
+    });
+
+    let TestCodex { codex, .. } = builder.build(&server).await?;
+
+    let notice = wait_for_event_match(&codex, |event| match event {
+        EventMsg::DeprecationNotice(ev)
+            if ev.summary.contains("experimental_instructions_file") =>
+        {
+            Some(ev.clone())
+        }
+        _ => None,
+    })
+    .await;
+
+    let DeprecationNoticeEvent { summary, details } = notice;
+    assert_eq!(
+        summary,
+        "`experimental_instructions_file` is deprecated and ignored. Use `model_instructions_file` instead."
+            .to_string(),
+    );
+    assert_eq!(
+        details.as_deref(),
+        Some(
+            "Move the setting to `model_instructions_file` in config.toml (or under a profile) to load instructions from a file."
+        ),
+    );
+
+    Ok(())
+}