From b7f7e7f7e85a1c63e520f27cac42b836f242d9f7 Mon Sep 17 00:00:00 2001
From: Benjamin Carlsson <bc@openai.com>
Date: Tue, 27 Oct 2026 04:01:42 +0000
Subject: [PATCH] Separate TUI session resume preparation from activation
 (#57342)

## What changed

Split saved-thread resume into preparation and activation. Preparation resolves configuration and attaches to the destination thread before activation replaces the displayed conversation.

Keep stored-backend configuration and local settings separate from active settings during preparation, while retaining the existing eager settings refresh for active-backend resumes. Use the destination backend for working-directory, permission, and directory-trust checks.

Carry the backend identity in the prepared result and reject activation through a different backend before changing the active conversation or composer.

## Testing

Extend coverage for preserving active settings and composer drafts during preparation, rejecting activation through another backend, applying destination remote-permission checks, and retaining active-backend settings refresh behavior.

GitOrigin-RevId: 7171f1abcf4bdd62793c21232a8b895bb809cf70
---
 codex-rs/tui/src/app.rs                       |   1 +
 codex-rs/tui/src/app/agents_overview.rs       |  13 +-
 codex-rs/tui/src/app/config_persistence.rs    |  90 ++++++++--
 codex-rs/tui/src/app/resume_config.rs         | 156 +++++++++++++-----
 codex-rs/tui/src/app/resume_preparation.rs    | 122 ++++++++++++++
 codex-rs/tui/src/app/session_lifecycle.rs     |  79 ++++-----
 codex-rs/tui/src/app/tests.rs                 |  68 ++++++--
 .../src/app/tests/backend_ownership_tests.rs  |  49 ++++++
 .../tests/terminal_size_telemetry_tests.rs    |   8 +-
 9 files changed, 463 insertions(+), 123 deletions(-)
 create mode 100644 codex-rs/tui/src/app/resume_preparation.rs

diff --git a/codex-rs/tui/src/app.rs b/codex-rs/tui/src/app.rs
index d2bb38e890..ca996b0d27 100644
--- a/codex-rs/tui/src/app.rs
+++ b/codex-rs/tui/src/app.rs
@@ -260,6 +260,7 @@ mod reconnect;
 mod replay_filter;
 mod resize_reflow;
 mod resume_config;
+mod resume_preparation;
 mod right_click_paste;
 pub(crate) use right_click_paste::PasteEnvironment;
 mod safety_buffering;
diff --git a/codex-rs/tui/src/app/agents_overview.rs b/codex-rs/tui/src/app/agents_overview.rs
index 82160d24d9..3cb5a425a9 100644
--- a/codex-rs/tui/src/app/agents_overview.rs
+++ b/codex-rs/tui/src/app/agents_overview.rs
@@ -558,7 +558,12 @@ impl App {
                     history_mode: Some(target_thread.history_mode),
                 };
                 match self
-                    .resume_config_for_target(tui, app_server, &target_session)
+                    .resume_config_for_target(
+                        tui,
+                        app_server,
+                        &target_session,
+                        super::resume_preparation::ResumeSource::Active,
+                    )
                     .await
                 {
                     Ok(config) => config,
@@ -572,7 +577,11 @@ impl App {
                 };
                 let current_cwd = self.config.cwd.to_path_buf();
                 match self
-                    .rebuild_config_for_resume_or_fallback(&current_cwd, config_cwd)
+                    .rebuild_config_for_resume_or_fallback(
+                        &current_cwd,
+                        config_cwd,
+                        (&self.config, &self.local_settings),
+                    )
                     .await
                 {
                     Ok(config) => config,
diff --git a/codex-rs/tui/src/app/config_persistence.rs b/codex-rs/tui/src/app/config_persistence.rs
index ef5b912785..8a04d72452 100644
--- a/codex-rs/tui/src/app/config_persistence.rs
+++ b/codex-rs/tui/src/app/config_persistence.rs
@@ -96,18 +96,26 @@ impl App {
     }
 
     pub(super) async fn rebuild_config_for_cwd(&self, cwd: PathBuf) -> Result<Config> {
+        self.rebuild_config_for_cwd_from(&self.config, cwd).await
+    }
+
+    pub(super) async fn rebuild_config_for_cwd_from(
+        &self,
+        base: &Config,
+        cwd: PathBuf,
+    ) -> Result<Config> {
         let mut overrides = self.harness_overrides.clone();
         overrides.cwd = Some(cwd.clone());
         let cwd_display = cwd.display().to_string();
         let builder = ConfigBuilder::default()
-            .codex_home(self.config.codex_home.to_path_buf())
+            .codex_home(base.codex_home.to_path_buf())
             .cli_overrides(self.cli_kv_overrides.clone())
             .harness_overrides(overrides)
             .loader_overrides(self.loader_overrides.clone())
             .cloud_config_bundle(self.cloud_config_bundle.clone());
         build_config_on_runtime_worker(
             builder,
-            self.config.application_network_policy.clone(),
+            base.application_network_policy.clone(),
             format!("Failed to rebuild config for cwd {cwd_display}"),
         )
         .await
@@ -525,13 +533,17 @@ impl App {
     }
 
     pub(super) async fn rebuild_config_for_resume_or_fallback(
-        &mut self,
+        &self,
         current_cwd: &Path,
         resume_cwd: PathBuf,
+        fallback: (&Config, &crate::local_settings::LocalSettings),
     ) -> Result<(Config, crate::local_settings::LocalSettings)> {
-        match self.rebuild_config_for_cwd(resume_cwd.clone()).await {
+        match self
+            .rebuild_config_for_cwd_from(fallback.0, resume_cwd.clone())
+            .await
+        {
             Ok(config) => {
-                let local_settings = self.local_settings.reloaded(&config);
+                let local_settings = fallback.1.reloaded(&config);
                 Ok((config, local_settings))
             }
             Err(err) => {
@@ -544,7 +556,7 @@ impl App {
                         cwd = %resume_cwd_display,
                         "failed to rebuild config for same-cwd resume; using current in-memory config"
                     );
-                    Ok((self.config.clone(), self.local_settings.clone()))
+                    Ok((fallback.0.clone(), fallback.1.clone()))
                 }
             }
         }
@@ -554,6 +566,19 @@ impl App {
         &mut self,
         config: &mut Config,
         scope: RuntimePolicyOverrideScope,
+    ) -> Result<()> {
+        self.apply_runtime_policy_overrides_with_reviewer(
+            config,
+            scope,
+            self.config.approvals_reviewer,
+        )
+    }
+
+    pub(super) fn apply_runtime_policy_overrides_with_reviewer(
+        &mut self,
+        config: &mut Config,
+        scope: RuntimePolicyOverrideScope,
+        restored_reviewer: ApprovalsReviewer,
     ) -> Result<()> {
         if let Some(policy) = self.runtime_approval_policy_override
             && (scope == RuntimePolicyOverrideScope::All
@@ -584,7 +609,7 @@ impl App {
                 .filter(|profile| {
                     profile.turn_override == RuntimePermissionProfileTurnOverride::Preserve
                 })
-                .map(|_| self.config.approvals_reviewer)
+                .map(|_| restored_reviewer)
         }) {
             config
                 .config_layer_stack
@@ -1137,10 +1162,12 @@ impl App {
         resume_model_settings_for_overrides(&self.config, &self.harness_overrides)
     }
 
-    pub(super) fn reject_remote_resume_permission_override(&mut self, config: &Config) -> bool {
-        if self.app_server_target.thread_params_mode()
-            != crate::app_server_session::ThreadParamsMode::Remote
-        {
+    pub(super) fn reject_remote_resume_permission_override(
+        &mut self,
+        config: &Config,
+        target: &AppServerTarget,
+    ) -> bool {
+        if target.thread_params_mode() != crate::app_server_session::ThreadParamsMode::Remote {
             return false;
         }
         let explicitly_selected =
@@ -1925,6 +1952,35 @@ enabled = false
         assert_eq!(app.chat_widget.config_ref().cwd.to_path_buf(), next_cwd);
         assert_eq!(app.config.cwd, original_cwd);
 
+        std::fs::write(app.config.codex_home.join("config.toml"), "")?;
+        crate::legacy_core::config::set_project_trust_level(
+            &app.config.codex_home,
+            &next_cwd,
+            codex_protocol::config_types::TrustLevel::Trusted,
+        )
+        .map_err(std::io::Error::other)?;
+        let mut server = crate::start_embedded_app_server_for_picker(&app.config).await?;
+        let target = SessionTarget {
+            backend_id: server.backend_id(),
+            thread_id: ThreadId::new(),
+            path: None,
+            cwd: None,
+            history_mode: None,
+        };
+        let mut tui = crate::tui::test_support::make_test_tui()?;
+        let candidate = app
+            .resume_config_for_target(
+                &mut tui,
+                &mut server,
+                &target,
+                super::resume_preparation::ResumeSource::Active,
+            )
+            .await
+            .map_err(|_| color_eyre::eyre::eyre!("resume configuration should succeed"))?;
+        assert_eq!(candidate.0.cwd.to_path_buf(), next_cwd);
+        assert_eq!(app.config.cwd.to_path_buf(), next_cwd);
+        server.shutdown().await?;
+
         app.refresh_in_memory_config_from_disk().await?;
 
         assert_eq!(app.config.cwd, app.chat_widget.config_ref().cwd);
@@ -2038,7 +2094,11 @@ theme = "dracula"
         let current_cwd = current_config.cwd.clone();
 
         let resume_config = app
-            .rebuild_config_for_resume_or_fallback(&current_cwd, current_cwd.to_path_buf())
+            .rebuild_config_for_resume_or_fallback(
+                &current_cwd,
+                current_cwd.to_path_buf(),
+                (&app.config, &app.local_settings),
+            )
             .await?;
 
         assert_eq!(resume_config, (current_config, app.local_settings.clone()));
@@ -2056,7 +2116,11 @@ theme = "dracula"
         let next_cwd = next_cwd_tmp.path().to_path_buf();
 
         let result = app
-            .rebuild_config_for_resume_or_fallback(&current_cwd, next_cwd)
+            .rebuild_config_for_resume_or_fallback(
+                &current_cwd,
+                next_cwd,
+                (&app.config, &app.local_settings),
+            )
             .await;
 
         assert!(result.is_err());
diff --git a/codex-rs/tui/src/app/resume_config.rs b/codex-rs/tui/src/app/resume_config.rs
index 5d6dc7f6e4..b28eb82f91 100644
--- a/codex-rs/tui/src/app/resume_config.rs
+++ b/codex-rs/tui/src/app/resume_config.rs
@@ -1,6 +1,6 @@
 //! Folder-entry consent and configuration for command-center dispatch and resume.
 //! Keeps CLI/runtime cwd precedence, remote-workspace checks, and interactive prompts aligned.
-//! Carries local preferences alongside the resolved configuration for session replacement.
+//! Stored-backend settings stay with the candidate; active-backend resume refreshes preferences.
 
 use super::*;
 use crate::onboarding::onboarding_screen::check_directory_trust;
@@ -31,13 +31,53 @@ impl App {
         tui: &mut tui::Tui,
         app_server: &mut AppServerSession,
         target_session: &SessionTarget,
+        source: super::resume_preparation::ResumeSource<'_>,
     ) -> std::result::Result<(Config, crate::local_settings::LocalSettings), AppRunControl> {
         if let Err(error) = target_session.ensure_backend(app_server.backend_id()) {
             self.add_session_picker_error(error.to_string());
             return Err(AppRunControl::Continue);
         }
-        self.refresh_in_memory_config_from_disk_best_effort("resuming a thread")
-            .await;
+        // Active-backend resume keeps its existing eager settings refresh. A stored backend must
+        // not replace the displayed conversation's settings before attachment succeeds.
+        let (target, current_config, local_settings) = match source {
+            super::resume_preparation::ResumeSource::Active => {
+                self.refresh_in_memory_config_from_disk_best_effort("resuming a thread")
+                    .await;
+                (
+                    self.app_server_target.clone(),
+                    self.config.clone(),
+                    self.local_settings.clone(),
+                )
+            }
+            super::resume_preparation::ResumeSource::Backend(settings) => {
+                let refreshed = async {
+                    let mut config = self
+                        .rebuild_config_for_cwd_from(
+                            &settings.config,
+                            settings.config.cwd.to_path_buf(),
+                        )
+                        .await?;
+                    self.apply_runtime_policy_overrides_with_reviewer(
+                        &mut config,
+                        RuntimePolicyOverrideScope::All,
+                        settings.config.approvals_reviewer,
+                    )?;
+                    Ok::<_, color_eyre::Report>(config)
+                }
+                .await;
+                let (config, local_settings) = match refreshed {
+                    Ok(config) => {
+                        let local_settings = settings.local_settings.reloaded(&config);
+                        (config, local_settings)
+                    }
+                    Err(error) => {
+                        tracing::warn!(%error, "failed to refresh resume settings; using stored backend configuration");
+                        (settings.config.clone(), settings.local_settings.clone())
+                    }
+                };
+                (settings.target.clone(), config, local_settings)
+            }
+        };
         let cwd_override = self
             .runtime_working_directory_override
             .as_deref()
@@ -46,19 +86,17 @@ impl App {
             .map(Path::to_path_buf);
         let cwd_override = cwd_override.as_deref();
         let resume_cwd_mode = crate::session_resume::effective_resume_cwd_mode(
-            self.local_settings.tui.resume_cwd,
+            local_settings.tui.resume_cwd,
             cwd_override,
         );
         let remembered_current_cwd = cwd_override.unwrap_or(self.launch_cwd.as_path());
         let current_cwd = if matches!(resume_cwd_mode, Some(ResumeCwdMode::Current)) {
             remembered_current_cwd.to_path_buf()
         } else {
-            self.config.cwd.to_path_buf()
+            current_config.cwd.to_path_buf()
         };
-        let uses_remote_workspace_or_environment = crate::uses_remote_workspace_or_environment(
-            &self.app_server_target,
-            &self.environment_manager,
-        );
+        let uses_remote_workspace_or_environment =
+            crate::uses_remote_workspace_or_environment(&target, &self.environment_manager);
         if uses_remote_workspace_or_environment
             && self.harness_overrides.cwd.is_none()
             && app_server.remote_cwd_override().is_none()
@@ -70,7 +108,7 @@ impl App {
             );
             return Err(AppRunControl::Continue);
         }
-        let resume_cwd = if self.app_server_target.uses_remote_workspace() {
+        let resume_cwd = if target.uses_remote_workspace() {
             current_cwd.clone()
         } else {
             let history_cwd = if matches!(resume_cwd_mode, Some(ResumeCwdMode::Current)) {
@@ -82,7 +120,7 @@ impl App {
             };
             let outcome = crate::session_resume::resolve_cwd_for_resume_or_fork(
                 tui,
-                &self.config,
+                &current_config,
                 history_cwd,
                 CwdPromptAction::Resume,
                 crate::session_resume::ResumeCwdContext {
@@ -110,15 +148,18 @@ impl App {
             }
         };
 
-        let (config_current_cwd, config_resume_cwd) =
-            if self.app_server_target.uses_remote_workspace() {
-                let local_config_cwd = self.config.cwd.to_path_buf();
-                (local_config_cwd.clone(), local_config_cwd)
-            } else {
-                (current_cwd, resume_cwd)
-            };
+        let (config_current_cwd, config_resume_cwd) = if target.uses_remote_workspace() {
+            let local_config_cwd = current_config.cwd.to_path_buf();
+            (local_config_cwd.clone(), local_config_cwd)
+        } else {
+            (current_cwd, resume_cwd)
+        };
         let mut resume_config = match self
-            .rebuild_config_for_resume_or_fallback(&config_current_cwd, config_resume_cwd)
+            .rebuild_config_for_resume_or_fallback(
+                &config_current_cwd,
+                config_resume_cwd,
+                (&current_config, &local_settings),
+            )
             .await
         {
             Ok(cfg) => cfg,
@@ -129,30 +170,31 @@ impl App {
                 return Err(AppRunControl::Continue);
             }
         };
-        if self.reject_remote_resume_permission_override(&resume_config.0) {
+        if self.reject_remote_resume_permission_override(&resume_config.0, &target) {
             return Err(AppRunControl::Continue);
         }
-        let resumed_thread =
-            if matches!(self.app_server_target, AppServerTarget::LocalDaemon { .. }) {
-                Some(
-                    app_server
-                        .thread_read(target_session.thread_id, /*include_turns*/ false)
-                        .await
-                        .map_err(|error| {
-                            self.add_session_picker_error(format!(
-                                "Unable to check resumed folder: {error}"
-                            ));
-                            AppRunControl::Continue
-                        })?,
-                )
-            } else {
-                None
-            };
+        let resumed_thread = if matches!(target, AppServerTarget::LocalDaemon { .. }) {
+            Some(
+                app_server
+                    .thread_read(target_session.thread_id, /*include_turns*/ false)
+                    .await
+                    .map_err(|error| {
+                        self.add_session_picker_error(format!(
+                            "Unable to check resumed folder: {error}"
+                        ));
+                        AppRunControl::Continue
+                    })?,
+            )
+        } else {
+            None
+        };
         let trust_cwd = resume_config.0.cwd.to_path_buf();
         if let Some(local_settings) = self
-            .confirm_directory_trust(
+            .confirm_directory_trust_for_target(
                 tui,
                 app_server,
+                &target,
+                &local_settings,
                 &mut resume_config.0,
                 &trust_cwd,
                 crate::onboarding::DirectoryTrustOptions {
@@ -175,11 +217,41 @@ impl App {
         config: &mut Config,
         cwd: &Path,
         options: crate::onboarding::DirectoryTrustOptions<'_>,
+        startup_draft: Option<&mut StartupDraftPump>,
+    ) -> std::result::Result<Option<crate::local_settings::LocalSettings>, AppRunControl> {
+        let target = self.app_server_target.clone();
+        let local_settings = self.local_settings.clone();
+        self.confirm_directory_trust_for_target(
+            tui,
+            app_server,
+            &target,
+            &local_settings,
+            config,
+            cwd,
+            options,
+            startup_draft,
+        )
+        .await
+    }
+
+    #[expect(
+        clippy::too_many_arguments,
+        reason = "The destination target is explicit alongside the existing trust interaction inputs."
+    )]
+    async fn confirm_directory_trust_for_target(
+        &mut self,
+        tui: &mut tui::Tui,
+        app_server: &mut AppServerSession,
+        target: &AppServerTarget,
+        local_settings: &crate::local_settings::LocalSettings,
+        config: &mut Config,
+        cwd: &Path,
+        options: crate::onboarding::DirectoryTrustOptions<'_>,
         mut startup_draft: Option<&mut StartupDraftPump>,
     ) -> std::result::Result<Option<crate::local_settings::LocalSettings>, AppRunControl> {
         // Keep the existing explicit remote --cd gate, including retries after cancellation.
         // Other remote destinations await authoritative trust-root metadata.
-        let cwd = if self.app_server_target.uses_remote_workspace() {
+        let cwd = if target.uses_remote_workspace() {
             let Some(cwd) = app_server.remote_cwd_override() else {
                 return Ok(None);
             };
@@ -191,7 +263,7 @@ impl App {
             tui,
             app_server,
             config,
-            &self.app_server_target,
+            target,
             cwd,
             options,
             startup_draft.as_deref_mut(),
@@ -205,7 +277,7 @@ impl App {
             if options.cancel == Some(crate::onboarding::TrustCancelAction::CurrentTask) {
                 return Err(AppRunControl::Continue);
             }
-            if !self.app_server_target.supports_command_center() {
+            if !target.supports_command_center() {
                 return Err(AppRunControl::Exit(ExitReason::UserRequested));
             }
             if self
@@ -224,7 +296,7 @@ impl App {
             *config = StartupDraftPump::run_with_optional_draft(
                 startup_draft.as_deref_mut(),
                 tui,
-                self.rebuild_config_for_cwd(config.cwd.to_path_buf()),
+                self.rebuild_config_for_cwd_from(config, config.cwd.to_path_buf()),
             )
             .await
             .map_err(|error| {
@@ -287,7 +359,7 @@ impl App {
                     }
                 }
             }
-            return Ok(Some(self.local_settings.reloaded(config)));
+            return Ok(Some(local_settings.reloaded(config)));
         }
         Ok(None)
     }
diff --git a/codex-rs/tui/src/app/resume_preparation.rs b/codex-rs/tui/src/app/resume_preparation.rs
new file mode 100644
index 0000000000..086329b5c7
--- /dev/null
+++ b/codex-rs/tui/src/app/resume_preparation.rs
@@ -0,0 +1,122 @@
+//! Prepare saved-thread attachment before replacing the active conversation UI.
+//!
+//! Stored-backend preparation leaves active settings unchanged; active-backend resume retains its
+//! eager preference refresh. Neither detaches the active thread or replaces its composer. The
+//! prepared result retains the connection identity.
+
+use super::*;
+
+/// Settings retained for a backend that is not currently displayed.
+#[derive(Clone)]
+pub(super) struct BackendSettings {
+    pub(super) config: Config,
+    pub(super) local_settings: crate::local_settings::LocalSettings,
+    pub(super) target: AppServerTarget,
+}
+
+/// The settings source for a resume attempt.
+///
+/// The active source may refresh the displayed conversation's settings eagerly. A stored backend
+/// is refreshed without adopting its settings until attachment succeeds.
+#[derive(Clone, Copy)]
+pub(super) enum ResumeSource<'a> {
+    Active,
+    #[cfg_attr(not(test), allow(dead_code, reason = "Used by backend handoff."))]
+    Backend(&'a BackendSettings),
+}
+
+pub(super) struct PreparedResume {
+    pub(super) backend_id: crate::backend_id::BackendId,
+    pub(super) config: Config,
+    pub(super) local_settings: crate::local_settings::LocalSettings,
+    pub(super) thread: AppServerStartedThread,
+    pub(super) read_only: bool,
+    pub(super) history_notice: Option<&'static str>,
+}
+
+impl App {
+    #[cfg_attr(not(test), allow(dead_code, reason = "Used by backend handoff."))]
+    pub(super) fn backend_settings(&self) -> BackendSettings {
+        BackendSettings {
+            config: self.config.clone(),
+            local_settings: self.local_settings.clone(),
+            target: self.app_server_target.clone(),
+        }
+    }
+
+    pub(super) async fn prepare_target_session(
+        &mut self,
+        tui: &mut tui::Tui,
+        app_server: &mut AppServerSession,
+        target_session: &SessionTarget,
+        source: ResumeSource<'_>,
+    ) -> std::result::Result<PreparedResume, AppRunControl> {
+        let (mut resume_config, local_settings) = self
+            .resume_config_for_target(tui, app_server, target_session, source)
+            .await?;
+        if let Err(error) = self.apply_runtime_policy_overrides(
+            &mut resume_config,
+            RuntimePolicyOverrideScope::ExplicitOnly,
+        ) {
+            self.add_session_picker_error(format!("{error:#}"));
+            return Err(AppRunControl::Continue);
+        }
+        let permission_overrides = self.resume_permission_overrides(&resume_config);
+        let model_settings = match source {
+            ResumeSource::Active => self.resume_model_settings(),
+            ResumeSource::Backend(_) => {
+                super::config_persistence::resume_model_settings_for_overrides(
+                    &resume_config,
+                    &self.harness_overrides,
+                )
+            }
+        };
+
+        if let Some(history_mode) = target_session.history_mode {
+            app_server.remember_thread_history_mode(target_session.thread_id, history_mode);
+        }
+        let resumed = app_server
+            .resume_thread_with_permission_overrides(
+                &local_settings,
+                resume_config.clone(),
+                target_session.thread_id,
+                model_settings,
+                permission_overrides,
+            )
+            .await;
+        let mut history_notice = None;
+        let (resumed, read_only) = match resumed {
+            Ok(resumed) => (resumed, false),
+            Err(err) if crate::app_server_session::is_active_writer_error(&err) => match app_server
+                .read_thread_for_viewing(&resume_config, &local_settings, target_session.thread_id)
+                .await
+            {
+                Ok((thread, notice)) => {
+                    history_notice = notice;
+                    (thread, true)
+                }
+                Err(read_err) => {
+                    self.add_session_picker_error(format!(
+                        "Failed to view thread open elsewhere: {read_err}"
+                    ));
+                    return Err(AppRunControl::Continue);
+                }
+            },
+            Err(err) => {
+                let path_display = target_session.display_label();
+                self.add_session_picker_error(format!(
+                    "Failed to resume session from {path_display}: {err}"
+                ));
+                return Err(AppRunControl::Continue);
+            }
+        };
+        Ok(PreparedResume {
+            backend_id: app_server.backend_id(),
+            config: resume_config,
+            local_settings,
+            thread: resumed,
+            read_only,
+            history_notice,
+        })
+    }
+}
diff --git a/codex-rs/tui/src/app/session_lifecycle.rs b/codex-rs/tui/src/app/session_lifecycle.rs
index 0e4ced1cde..c7dc18d544 100644
--- a/codex-rs/tui/src/app/session_lifecycle.rs
+++ b/codex-rs/tui/src/app/session_lifecycle.rs
@@ -1362,60 +1362,43 @@ impl App {
         if self.reject_pending_permission_root_switch() {
             return Ok(AppRunControl::Continue);
         }
-        let (mut resume_config, local_settings) = match self
-            .resume_config_for_target(tui, app_server, &target_session)
+        let prepared = match self
+            .prepare_target_session(
+                tui,
+                app_server,
+                &target_session,
+                super::resume_preparation::ResumeSource::Active,
+            )
             .await
         {
-            Ok(config) => config,
+            Ok(prepared) => prepared,
             Err(control) => return Ok(control),
         };
-        if let Err(error) = self.apply_runtime_policy_overrides(
-            &mut resume_config,
-            RuntimePolicyOverrideScope::ExplicitOnly,
-        ) {
-            self.add_session_picker_error(format!("{error:#}"));
+        self.activate_prepared_session(tui, app_server, prepared)
+            .await
+    }
+
+    pub(super) async fn activate_prepared_session(
+        &mut self,
+        tui: &mut tui::Tui,
+        app_server: &mut AppServerSession,
+        prepared: super::resume_preparation::PreparedResume,
+    ) -> Result<AppRunControl> {
+        let super::resume_preparation::PreparedResume {
+            backend_id,
+            config: resume_config,
+            local_settings,
+            thread: resumed,
+            read_only,
+            history_notice,
+        } = prepared;
+        if backend_id != app_server.backend_id() {
+            self.add_session_picker_error(
+                "This session belongs to a different backend. Open the session picker again."
+                    .to_string(),
+            );
             return Ok(AppRunControl::Continue);
         }
-        let permission_overrides = self.resume_permission_overrides(&resume_config);
-
-        if let Some(history_mode) = target_session.history_mode {
-            app_server.remember_thread_history_mode(target_session.thread_id, history_mode);
-        }
-        let resumed = app_server
-            .resume_thread_with_permission_overrides(
-                &local_settings,
-                resume_config.clone(),
-                target_session.thread_id,
-                self.resume_model_settings(),
-                permission_overrides,
-            )
-            .await;
-        let mut history_notice = None;
-        let (resumed, read_only) = match resumed {
-            Ok(resumed) => (resumed, false),
-            Err(err) if crate::app_server_session::is_active_writer_error(&err) => match app_server
-                .read_thread_for_viewing(&resume_config, &local_settings, target_session.thread_id)
-                .await
-            {
-                Ok((thread, notice)) => {
-                    history_notice = notice;
-                    (thread, true)
-                }
-                Err(read_err) => {
-                    self.add_session_picker_error(format!(
-                        "Failed to view thread open elsewhere: {read_err}"
-                    ));
-                    return Ok(AppRunControl::Continue);
-                }
-            },
-            Err(err) => {
-                let path_display = target_session.display_label();
-                self.add_session_picker_error(format!(
-                    "Failed to resume session from {path_display}: {err}"
-                ));
-                return Ok(AppRunControl::Continue);
-            }
-        };
         let resumed_thread_id = resumed.session.thread_id;
         let retained_input = (self.chat_widget.is_external_writer_view()
             && self.chat_widget.thread_id() == Some(resumed_thread_id))
diff --git a/codex-rs/tui/src/app/tests.rs b/codex-rs/tui/src/app/tests.rs
index 87f5bceb68..cbad3bf375 100644
--- a/codex-rs/tui/src/app/tests.rs
+++ b/codex-rs/tui/src/app/tests.rs
@@ -3610,7 +3610,7 @@ async fn server_only_profile_selection_keeps_turns_on_the_selected_profile() ->
 async fn remote_resume_rejects_explicit_permission_override() -> Result<()> {
     let (mut app, mut events, _ops) = make_test_app_with_channels().await;
     let endpoint = crate::resolve_remote_addr("ws://127.0.0.1:8765")?;
-    app.app_server_target = crate::AppServerTarget::Remote { endpoint };
+
     let home = tempdir()?;
     std::fs::write(
         home.path().join("config.toml"),
@@ -3635,18 +3635,23 @@ async fn remote_resume_rejects_explicit_permission_override() -> Result<()> {
         &app.harness_overrides
     ));
     let mut tui = crate::tui::test_support::make_test_tui()?;
-    app.resume_target_session(
-        &mut tui,
-        &mut server,
-        SessionTarget {
-            backend_id,
-            thread_id: ThreadId::new(),
-            path: None,
-            cwd: None,
-            history_mode: None,
-        },
-    )
-    .await?;
+    let mut settings = app.backend_settings();
+    settings.target = crate::AppServerTarget::Remote { endpoint };
+    let result = app
+        .prepare_target_session(
+            &mut tui,
+            &mut server,
+            &SessionTarget {
+                backend_id,
+                thread_id: ThreadId::new(),
+                path: None,
+                cwd: None,
+                history_mode: None,
+            },
+            super::resume_preparation::ResumeSource::Backend(&settings),
+        )
+        .await;
+    assert!(matches!(result, Err(AppRunControl::Continue)));
     assert_eq!(app.active_thread_id, None);
     let cell = std::iter::from_fn(|| events.try_recv().ok())
         .find_map(|event| match event {
@@ -8235,6 +8240,7 @@ async fn in_app_resume_session_cwd_without_metadata_is_non_fatal() -> Result<()>
     )?;
     app.state_db = None;
     let active_thread_id = app.chat_widget.thread_id();
+    app.chat_widget.insert_str("keep this draft");
     let mut app_server = Box::pin(crate::start_embedded_app_server_for_picker(&app.config)).await?;
     let backend_id = app_server.backend_id();
     let mut tui = crate::tui::test_support::make_test_tui()?;
@@ -8255,6 +8261,10 @@ async fn in_app_resume_session_cwd_without_metadata_is_non_fatal() -> Result<()>
 
     assert!(matches!(control, AppRunControl::Continue));
     assert_eq!(app.chat_widget.thread_id(), active_thread_id);
+    assert_eq!(
+        app.chat_widget.composer_text_with_pending(),
+        "keep this draft"
+    );
     let cell = match app_event_rx.try_recv() {
         Ok(AppEvent::InsertHistoryCell(cell)) => cell,
         other => panic!("expected InsertHistoryCell event, got {other:?}"),
@@ -8481,18 +8491,44 @@ async fn in_app_resume_uses_configured_or_explicit_cwd() -> Result<()> {
             .handle_thread_session_quiet(test_thread_session(ThreadId::new(), active_cwd.clone()));
         let mut tui = crate::tui::test_support::make_test_tui()?;
 
-        let control = app
-            .resume_target_session(
+        let settings = app.backend_settings();
+        // Prepare a destination while another backend owns the displayed configuration.
+        let other_home = tempdir()?;
+        app.config.codex_home = other_home.path().to_path_buf().abs();
+        let active_settings = (app.config.clone(), app.local_settings.clone());
+        let active_thread_id = app.chat_widget.thread_id();
+        app.chat_widget
+            .insert_str("keep this draft until attachment");
+        let prepared = match app
+            .prepare_target_session(
                 &mut tui,
                 app_server,
-                crate::resume_picker::SessionTarget {
+                &crate::resume_picker::SessionTarget {
                     backend_id,
                     path: Some(rollout_path),
                     thread_id,
                     cwd: None,
                     history_mode: None,
                 },
+                super::resume_preparation::ResumeSource::Backend(&settings),
             )
+            .await
+        {
+            Ok(prepared) => prepared,
+            Err(_) => panic!("saved thread should be ready for attachment"),
+        };
+        assert_eq!(
+            (&app.config, &app.local_settings),
+            (&active_settings.0, &active_settings.1)
+        );
+        assert_eq!(app.chat_widget.thread_id(), active_thread_id);
+        assert_eq!(
+            app.chat_widget.composer_text_with_pending(),
+            "keep this draft until attachment"
+        );
+        assert_eq!(app.chat_widget.config_ref().cwd.as_path(), active_cwd);
+        let control = app
+            .activate_prepared_session(&mut tui, app_server, prepared)
             .await?;
 
         assert!(matches!(control, AppRunControl::Continue));
diff --git a/codex-rs/tui/src/app/tests/backend_ownership_tests.rs b/codex-rs/tui/src/app/tests/backend_ownership_tests.rs
index b9ccf3fced..587a63f7d6 100644
--- a/codex-rs/tui/src/app/tests/backend_ownership_tests.rs
+++ b/codex-rs/tui/src/app/tests/backend_ownership_tests.rs
@@ -273,3 +273,52 @@ async fn startup_rejects_foreign_resume_and_fork_before_requests() -> Result<()>
     }
     Ok(())
 }
+
+#[tokio::test]
+async fn prepared_resume_cannot_activate_through_another_backend() -> Result<()> {
+    let (mut app, mut events, _) = make_test_app_with_channels().await;
+    let mut tui = crate::tui::test_support::make_test_tui()?;
+    let (mut server, requests, proxy) = start_recording_app_server(
+        &app.config,
+        /*blocked_thread_list*/ None,
+        /*failed_thread_name*/ None,
+    )
+    .await?;
+    let active_thread = ThreadId::new();
+    app.enqueue_primary_thread_session(
+        test_thread_session(active_thread, app.config.cwd.to_path_buf()),
+        Vec::new(),
+    )
+    .await?;
+    app.chat_widget.insert_str("keep this draft");
+    let prepared = super::super::resume_preparation::PreparedResume {
+        backend_id: BackendId::new(),
+        config: app.config.clone(),
+        local_settings: app.local_settings.clone(),
+        thread: AppServerStartedThread {
+            session: test_thread_session(ThreadId::new(), app.config.cwd.to_path_buf()),
+            turns: Vec::new(),
+            blocks_direct_input: false,
+            task_tools_available: true,
+        },
+        read_only: false,
+        history_notice: None,
+    };
+    requests.lock().unwrap().clear();
+    while events.try_recv().is_ok() {}
+    app.activate_prepared_session(&mut tui, &mut server, prepared)
+        .await?;
+    assert_eq!(app.current_displayed_thread_id(), Some(active_thread));
+    assert_eq!(
+        app.chat_widget.composer_text_with_pending(),
+        "keep this draft"
+    );
+    assert!(requests.lock().unwrap().is_empty());
+    let Some(AppEvent::InsertHistoryCell(cell)) = events.recv().await else {
+        panic!("expected backend mismatch error");
+    };
+    insta::assert_snapshot!(lines_to_single_string(&cell.display_lines(/*width*/ 100)), @"■ This session belongs to a different backend. Open the session picker again.");
+    server.shutdown().await?;
+    proxy.abort();
+    Ok(())
+}
diff --git a/codex-rs/tui/src/app/tests/terminal_size_telemetry_tests.rs b/codex-rs/tui/src/app/tests/terminal_size_telemetry_tests.rs
index 9542f599f2..5075ba1149 100644
--- a/codex-rs/tui/src/app/tests/terminal_size_telemetry_tests.rs
+++ b/codex-rs/tui/src/app/tests/terminal_size_telemetry_tests.rs
@@ -15,8 +15,12 @@ async fn terminal_size_telemetry_respects_reloaded_exporter_and_consent() -> Res
         "analytics.enabled = false\n[otel.metrics_exporter.otlp-http]\nendpoint = \"http://127.0.0.1:4318/v1/metrics\"\nprotocol = \"json\"\n",
     ] {
         std::fs::write(app.config.codex_home.join("config.toml"), config)?;
-        let (config, local_settings) =
-            Box::pin(app.rebuild_config_for_resume_or_fallback(&cwd, cwd.clone())).await?;
+        let (config, local_settings) = Box::pin(app.rebuild_config_for_resume_or_fallback(
+            &cwd,
+            cwd.clone(),
+            (&app.config, &app.local_settings),
+        ))
+        .await?;
         app.config = config;
         app.local_settings = local_settings;
         enabled.push(crate::terminal_size_telemetry::enabled(&app.config));