core: add configurable <context_window_guidance> message (#29936)

## Why

This PR adds a configurable `<context_window_guidance>` developer
section immediately after `<context_window>`. Harness integrations need
this section to give the model deployment-specific instructions for
preparing for context-window transitions.

## What changed

- Add an optional `features.token_budget.guidance_message` config with a
1,000-byte runtime cap and generated schema support.
- Render configured guidance as a developer `ContextualUserFragment`
wrapped in `<context_window_guidance>` immediately after
`<context_window>`.
- Omit the section when guidance is unset, empty, or whitespace-only.
- Preserve the resolved value in config locks and classify persisted
guidance as contextual developer content.
- Add integration coverage for rendered content and ordering.

codex-rs/core/config.schema.json

@@ -2860,6 +2860,11 @@
         "enabled": {
           "type": "boolean"
         },
+        "guidance_message": {
+          "description": "Guidance appended to the context-window metadata in a developer message.",
+          "maxLength": 1000,
+          "type": "string"
+        },
         "reminder_message_template": {
           "description": "Reminder template. `{n_remaining}` is replaced with the tokens remaining before auto-compaction.",
           "maxLength": 1000,

codex-rs/core/src/config/config_tests.rs

@@ -475,10 +475,12 @@ async fn load_config_resolves_token_budget_config() -> std::io::Result<()> {
 enabled = true
 reminder_threshold_tokens = 16000
 reminder_message_template = "Custom reminder: {n_remaining} tokens."
+guidance_message = "Preserve important state before compaction."
 "#,
             TokenBudgetConfig {
                 reminder_threshold_tokens: Some(16_000),
                 reminder_message_template: "Custom reminder: {n_remaining} tokens.".to_string(),
+                guidance_message: Some("Preserve important state before compaction.".to_string()),
             },
         ),
     ] {

codex-rs/core/src/config/mod.rs

@@ -1089,11 +1089,13 @@ pub(crate) const DEFAULT_TOKEN_BUDGET_REMINDER_MESSAGE_TEMPLATE: &str = concat!(
     "Once reset, message items in current context window will be cleared in the new window, but notes and history items will be persistent across windows."
 );
 const TOKEN_BUDGET_REMINDER_MESSAGE_TEMPLATE_MAX_BYTES: usize = 1000;
+const TOKEN_BUDGET_GUIDANCE_MESSAGE_MAX_BYTES: usize = 1000;
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize)]
 pub struct TokenBudgetConfig {
     pub reminder_threshold_tokens: Option<i64>,
     pub reminder_message_template: String,
+    pub guidance_message: Option<String>,
 }
 
 impl Default for TokenBudgetConfig {
@@ -1101,6 +1103,7 @@ impl Default for TokenBudgetConfig {
         Self {
             reminder_threshold_tokens: None,
             reminder_message_template: DEFAULT_TOKEN_BUDGET_REMINDER_MESSAGE_TEMPLATE.to_string(),
+            guidance_message: None,
         }
     }
 }
@@ -2578,9 +2581,25 @@ fn resolve_token_budget_config(
         ));
     }
 
+    let guidance_message = token_budget_config
+        .and_then(|config| config.guidance_message.clone())
+        .filter(|message| !message.trim().is_empty());
+    if guidance_message
+        .as_ref()
+        .is_some_and(|message| message.len() > TOKEN_BUDGET_GUIDANCE_MESSAGE_MAX_BYTES)
+    {
+        return Err(std::io::Error::new(
+            std::io::ErrorKind::InvalidInput,
+            format!(
+                "features.token_budget.guidance_message must not exceed {TOKEN_BUDGET_GUIDANCE_MESSAGE_MAX_BYTES} bytes"
+            ),
+        ));
+    }
+
     Ok(Some(TokenBudgetConfig {
         reminder_threshold_tokens,
         reminder_message_template,
+        guidance_message,
     }))
 }
 

codex-rs/core/src/context/mod.rs

@@ -71,6 +71,7 @@ pub(crate) use realtime_start_with_instructions::RealtimeStartWithInstructions;
 pub(crate) use recommended_plugins_instructions::RecommendedPluginsInstructions;
 pub(crate) use rollout_budget::RolloutBudgetContext;
 pub(crate) use subagent_notification::SubagentNotification;
+pub(crate) use token_budget_context::ContextWindowGuidance;
 pub(crate) use token_budget_context::TokenBudgetContext;
 pub(crate) use token_budget_context::TokenBudgetRemainingContext;
 pub(crate) use token_budget_context::TokenBudgetReminder;

codex-rs/core/src/context/token_budget_context.rs

@@ -1,6 +1,8 @@
 use super::ContextualUserFragment;
 use codex_protocol::ThreadId;
 use codex_protocol::protocol::CONTEXT_WINDOW_CLOSE_TAG;
+use codex_protocol::protocol::CONTEXT_WINDOW_GUIDANCE_CLOSE_TAG;
+use codex_protocol::protocol::CONTEXT_WINDOW_GUIDANCE_OPEN_TAG;
 use codex_protocol::protocol::CONTEXT_WINDOW_OPEN_TAG;
 use uuid::Uuid;
 
@@ -63,6 +65,40 @@ impl ContextualUserFragment for TokenBudgetContext {
     }
 }
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub(crate) struct ContextWindowGuidance {
+    message: String,
+}
+
+impl ContextWindowGuidance {
+    pub(crate) fn new(message: &str) -> Self {
+        Self {
+            message: message.to_string(),
+        }
+    }
+}
+
+impl ContextualUserFragment for ContextWindowGuidance {
+    fn role(&self) -> &'static str {
+        "developer"
+    }
+
+    fn markers(&self) -> (&'static str, &'static str) {
+        Self::type_markers()
+    }
+
+    fn type_markers() -> (&'static str, &'static str) {
+        (
+            CONTEXT_WINDOW_GUIDANCE_OPEN_TAG,
+            CONTEXT_WINDOW_GUIDANCE_CLOSE_TAG,
+        )
+    }
+
+    fn body(&self) -> String {
+        format!("\n{}\n", self.message)
+    }
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub(crate) struct TokenBudgetRemainingContext {
     tokens_left: Option<i64>,

codex-rs/core/src/event_mapping.rs

@@ -15,6 +15,7 @@ use codex_protocol::models::is_image_open_tag_text;
 use codex_protocol::models::is_local_image_close_tag_text;
 use codex_protocol::models::is_local_image_open_tag_text;
 use codex_protocol::protocol::COLLABORATION_MODE_OPEN_TAG;
+use codex_protocol::protocol::CONTEXT_WINDOW_GUIDANCE_OPEN_TAG;
 use codex_protocol::protocol::CONTEXT_WINDOW_OPEN_TAG;
 use codex_protocol::protocol::MULTI_AGENT_MODE_OPEN_TAG;
 use codex_protocol::protocol::REALTIME_CONVERSATION_OPEN_TAG;
@@ -38,6 +39,7 @@ const CONTEXTUAL_DEVELOPER_PREFIXES: &[&str] = &[
     // Keep recognizing token-budget wrappers persisted by older versions.
     "<token_budget>",
     CONTEXT_WINDOW_OPEN_TAG,
+    CONTEXT_WINDOW_GUIDANCE_OPEN_TAG,
     "<rollout_budget>",
 ];
 

codex-rs/core/src/event_mapping_tests.rs

@@ -16,6 +16,8 @@ use codex_protocol::models::ReasoningItemReasoningSummary;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::models::WebSearchAction;
 use codex_protocol::protocol::CONTEXT_WINDOW_CLOSE_TAG;
+use codex_protocol::protocol::CONTEXT_WINDOW_GUIDANCE_CLOSE_TAG;
+use codex_protocol::protocol::CONTEXT_WINDOW_GUIDANCE_OPEN_TAG;
 use codex_protocol::protocol::CONTEXT_WINDOW_OPEN_TAG;
 use codex_protocol::protocol::SKILLS_INSTRUCTIONS_OPEN_TAG;
 use codex_protocol::user_input::UserInput;
@@ -55,6 +57,18 @@ Thread id: 00000000-0000-0000-0000-000000000000
     assert!(!has_non_contextual_dev_message_content(&content));
 }
 
+#[test]
+fn recognizes_context_window_guidance_as_contextual_developer_content() {
+    let content = vec![ContentItem::InputText {
+        text: format!(
+            "{CONTEXT_WINDOW_GUIDANCE_OPEN_TAG}\nPreserve important state.\n{CONTEXT_WINDOW_GUIDANCE_CLOSE_TAG}"
+        ),
+    }];
+
+    assert!(is_contextual_dev_message_content(&content));
+    assert!(!has_non_contextual_dev_message_content(&content));
+}
+
 #[test]
 fn parses_user_message_with_text_and_two_images() {
     let img1 = "https://example.com/one.png".to_string();

codex-rs/core/src/session/config_lock.rs

@@ -248,6 +248,7 @@ mod tests {
         config.token_budget = Some(crate::config::TokenBudgetConfig {
             reminder_threshold_tokens: Some(16_000),
             reminder_message_template: "Locked reminder: {n_remaining} tokens.".to_string(),
+            guidance_message: Some("Locked context-window guidance.".to_string()),
         });
         config
             .features
@@ -340,6 +341,7 @@ mod tests {
                 reminder_message_template: Some(
                     "Locked reminder: {n_remaining} tokens.".to_string()
                 ),
+                guidance_message: Some("Locked context-window guidance.".to_string()),
             }))
         );
 

codex-rs/core/src/session/mod.rs

@@ -3339,6 +3339,16 @@ impl Session {
                 )
                 .render(),
             );
+            if let Some(guidance_message) = turn_context
+                .config
+                .token_budget
+                .as_ref()
+                .and_then(|config| config.guidance_message.as_deref())
+                .filter(|message| !message.trim().is_empty())
+            {
+                developer_sections
+                    .push(crate::context::ContextWindowGuidance::new(guidance_message).render());
+            }
         }
         for fragment in world_state.render_full() {
             match fragment.role() {

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

@@ -7,6 +7,8 @@ use codex_model_provider_info::built_in_model_providers;
 use codex_protocol::config_types::AutoCompactTokenLimitScope;
 use codex_protocol::items::TurnItem;
 use codex_protocol::protocol::CONTEXT_WINDOW_CLOSE_TAG;
+use codex_protocol::protocol::CONTEXT_WINDOW_GUIDANCE_CLOSE_TAG;
+use codex_protocol::protocol::CONTEXT_WINDOW_GUIDANCE_OPEN_TAG;
 use codex_protocol::protocol::CONTEXT_WINDOW_OPEN_TAG;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::HookEventName;
@@ -211,6 +213,52 @@ async fn token_budget_context_is_only_emitted_with_full_context() -> Result<()>
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn token_budget_guidance_follows_context_window() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let response = mount_sse_sequence(
+        &server,
+        vec![sse(vec![
+            ev_response_created("resp-1"),
+            ev_completed("resp-1"),
+        ])],
+    )
+    .await;
+    let guidance_message = "Preserve important state before compaction.";
+    let test = test_codex()
+        .with_config(move |config| {
+            config.model_context_window = Some(CONFIGURED_CONTEXT_WINDOW);
+            config.token_budget = Some(TokenBudgetConfig {
+                guidance_message: Some(guidance_message.to_string()),
+                ..TokenBudgetConfig::default()
+            });
+            config
+                .features
+                .enable(Feature::TokenBudget)
+                .expect("test config should allow token budget");
+        })
+        .build_with_auto_env(&server)
+        .await?;
+
+    test.submit_turn("inspect context guidance").await?;
+
+    let developer_texts = response.single_request().message_input_texts("developer");
+    let context_window_index = developer_texts
+        .iter()
+        .position(|text| text.starts_with(CONTEXT_WINDOW_OPEN_TAG))
+        .expect("context-window metadata should be present");
+    assert_eq!(
+        developer_texts.get(context_window_index + 1),
+        Some(&format!(
+            "{CONTEXT_WINDOW_GUIDANCE_OPEN_TAG}\n{guidance_message}\n{CONTEXT_WINDOW_GUIDANCE_CLOSE_TAG}"
+        ))
+    );
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn token_budget_context_injects_plain_thread_hint_text() -> Result<()> {
     skip_if_no_network!(Ok(()));

codex-rs/features/src/feature_configs.rs

@@ -88,6 +88,10 @@ pub struct TokenBudgetConfigToml {
     #[serde(skip_serializing_if = "Option::is_none")]
     #[schemars(length(min = 1, max = 1000))]
     pub reminder_message_template: Option<String>,
+    /// Guidance appended to the context-window metadata in a developer message.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    #[schemars(length(max = 1000))]
+    pub guidance_message: Option<String>,
 }
 
 impl FeatureConfig for TokenBudgetConfigToml {

codex-rs/protocol/src/protocol.rs

@@ -114,6 +114,8 @@ pub const REALTIME_CONVERSATION_OPEN_TAG: &str = "<realtime_conversation>";
 pub const REALTIME_CONVERSATION_CLOSE_TAG: &str = "</realtime_conversation>";
 pub const CONTEXT_WINDOW_OPEN_TAG: &str = "<context_window>";
 pub const CONTEXT_WINDOW_CLOSE_TAG: &str = "</context_window>";
+pub const CONTEXT_WINDOW_GUIDANCE_OPEN_TAG: &str = "<context_window_guidance>";
+pub const CONTEXT_WINDOW_GUIDANCE_CLOSE_TAG: &str = "</context_window_guidance>";
 pub const USER_MESSAGE_BEGIN: &str = "## My request for Codex:";
 
 // TODO(anp): Replace `TurnEnvironmentSelection` with `PathUri` once path URIs carry environment