Make the update_plan tool opt-in (#41744)

## What changed

- Default `tools.update_plan.enabled` to `false`; users can explicitly enable it to expose `update_plan`.
- Remove bundled `update_plan` guidance from model, collaboration-mode, multi-agent, compaction, prewarm, and goal-continuation prompts when the tool is disabled.
- Preserve custom base instructions, model catalog instructions, collaboration policies, and user goal text even when they mention planning or `update_plan`.

## Testing

- Cover default and explicitly enabled tool registration, prompt consistency across request paths, and preservation of custom instructions.

GitOrigin-RevId: a53964e6e72f98e2557dd3090fdb7caccb956527

codex-rs/ext/goal/src/runtime.rs

@@ -448,7 +448,10 @@ impl GoalRuntimeHandle {
             .get::<TurnStartOptions>()
             .map(|options| options.as_ref().clone())
             .unwrap_or_default();
-        let item = continuation_steering_item(&protocol_goal_from_state(goal));
+        let item = continuation_steering_item(
+            &protocol_goal_from_state(goal),
+            thread.config().await.update_plan_enabled,
+        );
 
         match thread
             .start_turn_if_idle(

codex-rs/ext/goal/src/steering.rs

@@ -1,6 +1,7 @@
 use codex_core::context::ContextualUserFragment;
 use codex_core::context::InternalContextSource;
 use codex_core::context::InternalModelContextFragment;
+use codex_core::context::without_update_plan_instructions;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::protocol::ThreadGoal;
 use codex_utils_template::Template;
@@ -13,6 +14,13 @@ static CONTINUATION_PROMPT_TEMPLATE: LazyLock<Template> = LazyLock::new(|| {
     )
 });
 
+static CONTINUATION_PROMPT_WITHOUT_UPDATE_PLAN: LazyLock<Template> = LazyLock::new(|| {
+    parse_embedded_template(
+        &without_update_plan_instructions(include_str!("../templates/goals/continuation.md")),
+        "goals/continuation.md",
+    )
+});
+
 static BUDGET_LIMIT_PROMPT_TEMPLATE: LazyLock<Template> = LazyLock::new(|| {
     parse_embedded_template(
         include_str!("../templates/goals/budget_limit.md"),
@@ -27,7 +35,7 @@ static OBJECTIVE_UPDATED_PROMPT_TEMPLATE: LazyLock<Template> = LazyLock::new(||
     )
 });
 
-fn parse_embedded_template(source: &'static str, template_name: &str) -> Template {
+fn parse_embedded_template(source: &str, template_name: &str) -> Template {
     match Template::parse(source) {
         Ok(template) => template,
         Err(err) => panic!("embedded template {template_name} is invalid: {err}"),
@@ -42,8 +50,11 @@ pub(crate) fn objective_updated_steering_item(goal: &ThreadGoal) -> ResponseItem
     goal_context_input_item(objective_updated_prompt(goal))
 }
 
-pub(crate) fn continuation_steering_item(goal: &ThreadGoal) -> ResponseItem {
-    goal_context_input_item(continuation_prompt(goal))
+pub(crate) fn continuation_steering_item(
+    goal: &ThreadGoal,
+    update_plan_enabled: bool,
+) -> ResponseItem {
+    goal_context_input_item(continuation_prompt(goal, update_plan_enabled))
 }
 
 fn goal_context_input_item(prompt: String) -> ResponseItem {
@@ -53,7 +64,7 @@ fn goal_context_input_item(prompt: String) -> ResponseItem {
     ))
 }
 
-fn continuation_prompt(goal: &ThreadGoal) -> String {
+fn continuation_prompt(goal: &ThreadGoal, update_plan_enabled: bool) -> String {
     let objective = escape_xml_text(&goal.objective);
     let tokens_used = goal.tokens_used.to_string();
     let token_budget = goal
@@ -65,7 +76,12 @@ fn continuation_prompt(goal: &ThreadGoal) -> String {
         .map(|budget| (budget - goal.tokens_used).max(0).to_string())
         .unwrap_or_else(|| "unbounded".to_string());
 
-    CONTINUATION_PROMPT_TEMPLATE
+    let template = if update_plan_enabled {
+        &*CONTINUATION_PROMPT_TEMPLATE
+    } else {
+        &*CONTINUATION_PROMPT_WITHOUT_UPDATE_PLAN
+    };
+    template
         .render([
             ("objective", objective.as_str()),
             ("tokens_used", tokens_used.as_str()),