diff --git a/codex-rs/core/config.schema.json b/codex-rs/core/config.schema.json
index 32318cd62..596fe6d2f 100644
--- a/codex-rs/core/config.schema.json
+++ b/codex-rs/core/config.schema.json
@@ -53,14 +53,6 @@
     "experimental_compact_prompt_file": {
       "$ref": "#/definitions/AbsolutePathBuf"
     },
-    "experimental_instructions_file": {
-      "description": "Legacy, now use features",
-      "allOf": [
-        {
-          "$ref": "#/definitions/AbsolutePathBuf"
-        }
-      ]
-    },
     "experimental_use_freeform_apply_patch": {
       "type": "boolean"
     },
@@ -240,6 +232,14 @@
       "type": "integer",
       "format": "int64"
     },
+    "model_instructions_file": {
+      "description": "Optional path to a file containing model instructions that will override the built-in instructions for the selected model. Users are STRONGLY DISCOURAGED from using this field, as deviating from the instructions sanctioned by Codex will likely degrade model performance.",
+      "allOf": [
+        {
+          "$ref": "#/definitions/AbsolutePathBuf"
+        }
+      ]
+    },
     "model_personality": {
       "description": "EXPERIMENTAL Optionally specify a personality for the model",
       "allOf": [
@@ -545,9 +545,6 @@
         "experimental_compact_prompt_file": {
           "$ref": "#/definitions/AbsolutePathBuf"
         },
-        "experimental_instructions_file": {
-          "$ref": "#/definitions/AbsolutePathBuf"
-        },
         "experimental_use_freeform_apply_patch": {
           "type": "boolean"
         },
@@ -640,6 +637,14 @@
         "model": {
           "type": "string"
         },
+        "model_instructions_file": {
+          "description": "Optional path to a file containing model instructions.",
+          "allOf": [
+            {
+              "$ref": "#/definitions/AbsolutePathBuf"
+            }
+          ]
+        },
         "model_personality": {
           "$ref": "#/definitions/Personality"
         },
diff --git a/codex-rs/core/src/codex.rs b/codex-rs/core/src/codex.rs
index b7dbd0796..5bc6156c8 100644
--- a/codex-rs/core/src/codex.rs
+++ b/codex-rs/core/src/codex.rs
@@ -666,6 +666,19 @@ impl Session {
                 msg: EventMsg::DeprecationNotice(DeprecationNoticeEvent { summary, details }),
             });
         }
+        if crate::config::uses_deprecated_instructions_file(&config.config_layer_stack) {
+            post_session_configured_events.push(Event {
+                id: INITIAL_SUBMIT_ID.to_owned(),
+                msg: EventMsg::DeprecationNotice(DeprecationNoticeEvent {
+                    summary: "`experimental_instructions_file` is deprecated and ignored. Use `model_instructions_file` instead."
+                        .to_string(),
+                    details: Some(
+                        "Move the setting to `model_instructions_file` in config.toml (or under a profile) to load instructions from a file."
+                            .to_string(),
+                    ),
+                }),
+            });
+        }
         maybe_push_chat_wire_api_deprecation(&config, &mut post_session_configured_events);
 
         let auth = auth_manager.auth().await;
diff --git a/codex-rs/core/src/config/mod.rs b/codex-rs/core/src/config/mod.rs
index 7d1426e7a..934c54263 100644
--- a/codex-rs/core/src/config/mod.rs
+++ b/codex-rs/core/src/config/mod.rs
@@ -820,6 +820,12 @@ pub struct ConfigToml {
     #[serde(default)]
     pub developer_instructions: Option<String>,
 
+    /// Optional path to a file containing model instructions that will override
+    /// the built-in instructions for the selected model. Users are STRONGLY
+    /// DISCOURAGED from using this field, as deviating from the instructions
+    /// sanctioned by Codex will likely degrade model performance.
+    pub model_instructions_file: Option<AbsolutePathBuf>,
+
     /// Compact prompt used for history compaction.
     pub compact_prompt: Option<String>,
 
@@ -965,6 +971,8 @@ pub struct ConfigToml {
     pub notice: Option<Notice>,
 
     /// Legacy, now use features
+    /// Deprecated: ignored. Use `model_instructions_file`.
+    #[schemars(skip)]
     pub experimental_instructions_file: Option<AbsolutePathBuf>,
     pub experimental_compact_prompt_file: Option<AbsolutePathBuf>,
     pub experimental_use_unified_exec_tool: Option<bool>,
@@ -1432,14 +1440,12 @@ impl Config {
         // Load base instructions override from a file if specified. If the
         // path is relative, resolve it against the effective cwd so the
         // behaviour matches other path-like config values.
-        let experimental_instructions_path = config_profile
-            .experimental_instructions_file
+        let model_instructions_path = config_profile
+            .model_instructions_file
             .as_ref()
-            .or(cfg.experimental_instructions_file.as_ref());
-        let file_base_instructions = Self::try_read_non_empty_file(
-            experimental_instructions_path,
-            "experimental instructions file",
-        )?;
+            .or(cfg.model_instructions_file.as_ref());
+        let file_base_instructions =
+            Self::try_read_non_empty_file(model_instructions_path, "model instructions file")?;
         let base_instructions = base_instructions.or(file_base_instructions);
         let developer_instructions = developer_instructions.or(cfg.developer_instructions);
 
@@ -1682,6 +1688,30 @@ impl Config {
     }
 }
 
+pub(crate) fn uses_deprecated_instructions_file(config_layer_stack: &ConfigLayerStack) -> bool {
+    config_layer_stack
+        .layers_high_to_low()
+        .into_iter()
+        .any(|layer| toml_uses_deprecated_instructions_file(&layer.config))
+}
+
+fn toml_uses_deprecated_instructions_file(value: &TomlValue) -> bool {
+    let Some(table) = value.as_table() else {
+        return false;
+    };
+    if table.contains_key("experimental_instructions_file") {
+        return true;
+    }
+    let Some(profiles) = table.get("profiles").and_then(TomlValue::as_table) else {
+        return false;
+    };
+    profiles.values().any(|profile| {
+        profile.as_table().is_some_and(|profile_table| {
+            profile_table.contains_key("experimental_instructions_file")
+        })
+    })
+}
+
 /// Returns the path to the Codex configuration directory, which can be
 /// specified by the `CODEX_HOME` environment variable. If not set, defaults to
 /// `~/.codex`.
diff --git a/codex-rs/core/src/config/profile.rs b/codex-rs/core/src/config/profile.rs
index 4f1a964eb..e78b02374 100644
--- a/codex-rs/core/src/config/profile.rs
+++ b/codex-rs/core/src/config/profile.rs
@@ -27,6 +27,10 @@ pub struct ConfigProfile {
     pub model_verbosity: Option<Verbosity>,
     pub model_personality: Option<Personality>,
     pub chatgpt_base_url: Option<String>,
+    /// Optional path to a file containing model instructions.
+    pub model_instructions_file: Option<AbsolutePathBuf>,
+    /// Deprecated: ignored. Use `model_instructions_file`.
+    #[schemars(skip)]
     pub experimental_instructions_file: Option<AbsolutePathBuf>,
     pub experimental_compact_prompt_file: Option<AbsolutePathBuf>,
     pub include_apply_patch_tool: Option<bool>,
diff --git a/codex-rs/core/src/config_loader/mod.rs b/codex-rs/core/src/config_loader/mod.rs
index 0460e0157..ffde19f1f 100644
--- a/codex-rs/core/src/config_loader/mod.rs
+++ b/codex-rs/core/src/config_loader/mod.rs
@@ -656,7 +656,7 @@ mod unit_tests {
         let contents = r#"
 # This is a field recognized by config.toml that is an AbsolutePathBuf in
 # the ConfigToml struct.
-experimental_instructions_file = "./some_file.md"
+model_instructions_file = "./some_file.md"
 
 # This is a field recognized by config.toml.
 model = "gpt-1000"
@@ -669,7 +669,7 @@ foo = "xyzzy"
         let normalized_toml_value = resolve_relative_paths_in_config_toml(user_config, base_dir)?;
         let mut expected_toml_value = toml::map::Map::new();
         expected_toml_value.insert(
-            "experimental_instructions_file".to_string(),
+            "model_instructions_file".to_string(),
             TomlValue::String(
                 AbsolutePathBuf::resolve_path_against_base("./some_file.md", base_dir)?
                     .as_path()
diff --git a/codex-rs/core/src/config_loader/tests.rs b/codex-rs/core/src/config_loader/tests.rs
index 773d9951d..365fcdc59 100644
--- a/codex-rs/core/src/config_loader/tests.rs
+++ b/codex-rs/core/src/config_loader/tests.rs
@@ -438,10 +438,10 @@ async fn project_paths_resolve_relative_to_dot_codex_and_override_in_order() ->
     tokio::fs::write(project_root.join(".git"), "gitdir: here").await?;
 
     let root_cfg = r#"
-experimental_instructions_file = "root.txt"
+model_instructions_file = "root.txt"
 "#;
     let nested_cfg = r#"
-experimental_instructions_file = "child.txt"
+model_instructions_file = "child.txt"
 "#;
     tokio::fs::write(project_root.join(".codex").join(CONFIG_TOML_FILE), root_cfg).await?;
     tokio::fs::write(nested.join(".codex").join(CONFIG_TOML_FILE), nested_cfg).await?;
@@ -591,7 +591,7 @@ async fn cli_overrides_with_relative_paths_do_not_break_trust_check() -> std::io
 
     let cwd = AbsolutePathBuf::from_absolute_path(&nested)?;
     let cli_overrides = vec![(
-        "experimental_instructions_file".to_string(),
+        "model_instructions_file".to_string(),
         TomlValue::String("relative.md".to_string()),
     )];
 
diff --git a/codex-rs/core/tests/suite/cli_stream.rs b/codex-rs/core/tests/suite/cli_stream.rs
index 12be98723..57142b07f 100644
--- a/codex-rs/core/tests/suite/cli_stream.rs
+++ b/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");
diff --git a/codex-rs/core/tests/suite/deprecation_notice.rs b/codex-rs/core/tests/suite/deprecation_notice.rs
index bab715ebd..d401a64cd 100644
--- a/codex-rs/core/tests/suite/deprecation_notice.rs
+++ b/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(())
+}