Allow models to enable token budgeting by default (#41803)

## What changed

- Add model metadata flags for enabling token budgeting and its history-notes extension.
- Apply those activation defaults once at thread startup when the user has not explicitly configured token budgeting, while respecting managed feature requirements.
- Continue resolving model-owned prompts for the active model so model switches can use their own token-budget guidance.

## Testing

- Update the token-budget integration test to verify that model metadata activates the feature without manual configuration.

GitOrigin-RevId: 73a32623eaeabb9243e240c43da7a5e7e93e9a19

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

@@ -668,6 +668,10 @@ impl Session {
         let model_info = models_manager
             .get_model_info(model.as_str(), &config.to_models_manager_config())
             .await;
+        // Intentionally resolve `enabled` and `use_history_notes_extension` only at
+        // thread startup. Both activation flags stay fixed for this thread runtime,
+        // even if the selected model changes later.
+        token_budget::apply_model_defaults(Arc::make_mut(&mut config), &model_info);
         let configured_config = Arc::clone(&config);
         let multi_agent_version = config.multi_agent_version_override().or_else(|| {
             resolve_multi_agent_version(&conversation_history, inherited_multi_agent_version)

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

@@ -277,6 +277,8 @@ async fn submitted_sparse_updates_preserve_captured_steps_and_ordering() {
             .as_mut()
             .expect("model messages")
             .token_budget = Some(ModelTokenBudgetConfig {
+            enabled: false,
+            use_history_notes_extension: false,
             reminder_threshold_tokens: 2_000,
             reminder_message_template: "{n_remaining} tokens remain.".to_string(),
             guidance_message: format!("Guidance for {}.", model.slug),

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

@@ -68,6 +68,45 @@ pub(super) fn resolve_token_budget(
     Some(token_budget)
 }
 
+/// Applies model activation defaults before thread extensions are initialized.
+pub(super) fn apply_model_defaults(config: &mut Config, model_info: &ModelInfo) {
+    let Some(model_defaults) = model_info
+        .model_messages
+        .as_ref()
+        .and_then(|messages| messages.token_budget.as_ref())
+    else {
+        return;
+    };
+    if !model_defaults.enabled {
+        return;
+    }
+
+    let has_explicit_config = config.token_budget.is_some()
+        || config
+            .config_layer_stack
+            .effective_config()
+            .get("features")
+            .and_then(|features| features.get("token_budget"))
+            .is_some();
+    if has_explicit_config {
+        return;
+    }
+
+    if config.features.enable(Feature::TokenBudget).is_err() {
+        return;
+    }
+    // Managed requirements can pin the feature off even when enable() succeeds.
+    if !config.features.enabled(Feature::TokenBudget) {
+        return;
+    }
+
+    // Keep prompts unresolved so later turns and model switches use their own defaults.
+    config.token_budget = Some(TokenBudgetConfig {
+        use_history_notes_extension: model_defaults.use_history_notes_extension,
+        ..TokenBudgetConfig::default()
+    });
+}
+
 pub(super) async fn maybe_record(
     sess: &Session,
     turn_context: &TurnContext,