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/core/src/session/mod.rs

@@ -1314,6 +1314,26 @@ impl Session {
         }
     }
 
+    /// Render the request copy without changing instructions persisted or inherited by forks.
+    pub(crate) async fn get_prompt_base_instructions(&self) -> BaseInstructions {
+        let config = self.get_config().await;
+        let instructions = self.get_base_instructions().await;
+        if !config.update_plan_enabled
+            && config.model_catalog.is_none()
+            && matches!(
+                instructions.provenance,
+                Some(BaseInstructionsProvenance::Model { .. })
+            )
+        {
+            BaseInstructions {
+                text: crate::context::without_update_plan_instructions(&instructions.text),
+                ..instructions
+            }
+        } else {
+            instructions
+        }
+    }
+
     // Merges connector IDs into the session-level explicit connector selection.
     #[tracing::instrument(
         level = "trace",

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

@@ -78,7 +78,11 @@ pub(super) fn usage_hint_text(
         .as_ref()
         .and_then(|messages| messages.multi_agent.as_ref())
         .and_then(|messages| messages.role.as_ref());
-    let snapshot = resolve_usage_hints(&turn_context.config.multi_agent_v2, catalog);
+    let snapshot = resolve_usage_hints(
+        &turn_context.config.multi_agent_v2,
+        catalog,
+        !turn_context.config.update_plan_enabled && turn_context.config.model_catalog.is_none(),
+    );
     match session_source {
         SessionSource::SubAgent(SubAgentSource::ThreadSpawn { .. }) => snapshot.subagent,
         SessionSource::Cli
@@ -94,6 +98,7 @@ pub(super) fn usage_hint_text(
 pub(crate) fn resolve_usage_hints(
     config: &MultiAgentV2Config,
     catalog: Option<&MultiAgentRoleMessages>,
+    omit_update_plan_instructions: bool,
 ) -> ResolvedMultiAgentV2UsageHints {
     let resolve_role = |configured: Option<&str>, catalog: Option<&str>, bundled: &str| {
         // Configured roles take precedence; empty configured or catalog roles suppress fallback.
@@ -106,6 +111,11 @@ pub(crate) fn resolve_usage_hints(
         if base.is_empty() {
             return None;
         }
+        let base = if omit_update_plan_instructions {
+            crate::context::without_update_plan_instructions(base)
+        } else {
+            base.to_string()
+        };
 
         let max_concurrency = config.max_concurrent_threads_per_session;
         let wait_agent_guidance = if config.wait_agent_enabled {

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

@@ -1579,7 +1579,7 @@ impl Session {
             )
             .await?;
             sess.start_mcp_prewarm_worker(mcp_prewarm_rx, mcp_auth_changes);
-            sess.schedule_startup_prewarm(session_configuration.base_instructions.clone())
+            sess.schedule_startup_prewarm(sess.get_prompt_base_instructions().await.text)
                 .await;
             let session_start_source = match &initial_history {
                 InitialHistory::Resumed(_) => codex_hooks::SessionStartSource::Resume,

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

@@ -1370,7 +1370,7 @@ async fn run_sampling_request(
     cancellation_token: CancellationToken,
 ) -> CodexResult<(SamplingRequestResult, Vec<ResponseItem>)> {
     let turn_context = Arc::clone(&step_context.turn);
-    let base_instructions = sess.get_base_instructions().await;
+    let base_instructions = sess.get_prompt_base_instructions().await;
 
     let tool_runtime = ToolCallRuntime::new(
         Arc::clone(&sess),

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

@@ -44,9 +44,20 @@ impl Session {
         let model_instructions = turn_context
             .model_info()
             .get_model_instructions(turn_context.personality());
+        let model_instructions = if !turn_context.config.update_plan_enabled
+            && turn_context.config.model_catalog.is_none()
+            && (turn_context.config.base_instructions.is_none()
+                || matches!(
+                    turn_context.config.base_instructions_provenance,
+                    Some(BaseInstructionsProvenance::Model { .. })
+                )) {
+            crate::context::without_update_plan_instructions(&model_instructions)
+        } else {
+            model_instructions
+        };
+        let base_instructions = self.get_prompt_base_instructions().await.text;
         let (previous_model, previous_context, base_instructions) = {
             let state = self.state.lock().await;
-            let base_instructions = state.session_configuration.base_instructions.clone();
             (
                 state
                     .previous_turn_settings()
@@ -189,6 +200,8 @@ impl Session {
                     .model_messages
                     .as_ref()
                     .and_then(|messages| messages.collaboration_modes.as_ref()),
+                turn_context.config.update_plan_enabled,
+                turn_context.config.model_catalog.is_some(),
             ));
         }
         if !crate::guardian::is_basic_session_source(&turn_context.session_source) {