diff --git a/codex-rs/Cargo.lock b/codex-rs/Cargo.lock
index df6766fda2..b93b4109fe 100644
--- a/codex-rs/Cargo.lock
+++ b/codex-rs/Cargo.lock
@@ -2121,6 +2121,7 @@ dependencies = [
  "codex-model-provider",
  "codex-model-provider-info",
  "codex-models-manager",
+ "codex-multi-env-copy-file",
  "codex-network-proxy",
  "codex-otel",
  "codex-plugin",
@@ -2547,6 +2548,7 @@ dependencies = [
  "codex-memories-write",
  "codex-model-provider",
  "codex-models-manager",
+ "codex-multi-env-copy-file",
  "codex-plugin",
  "codex-protocol",
  "codex-responses-api-proxy",
diff --git a/codex-rs/Cargo.toml b/codex-rs/Cargo.toml
index ee3d8c493a..b5a8ec45c6 100644
--- a/codex-rs/Cargo.toml
+++ b/codex-rs/Cargo.toml
@@ -225,6 +225,7 @@ codex-exec-server-test-support = { path = "exec-server/tests/support" }
 codex-execpolicy = { path = "execpolicy" }
 codex-extension-api = { path = "ext/extension-api" }
 codex-extension-items = { path = "ext/items" }
+codex-multi-env-copy-file = { path = "ext/multi-env-copy-file" }
 codex-goal-extension = { path = "ext/goal" }
 codex-git-attribution = { path = "ext/git-attribution" }
 codex-guardian-reviewer = { path = "ext/guardian-reviewer" }
diff --git a/codex-rs/app-server/Cargo.toml b/codex-rs/app-server/Cargo.toml
index 0ef30505cc..b205a05ffb 100644
--- a/codex-rs/app-server/Cargo.toml
+++ b/codex-rs/app-server/Cargo.toml
@@ -51,6 +51,7 @@ codex-exec-server = { workspace = true }
 codex-extension-api = { workspace = true }
 codex-external-agent-migration = { workspace = true }
 codex-features = { workspace = true }
+codex-multi-env-copy-file = { workspace = true }
 codex-goal-extension = { workspace = true }
 codex-git-attribution = { workspace = true }
 codex-guardian-v2 = { workspace = true }
diff --git a/codex-rs/app-server/src/extensions.rs b/codex-rs/app-server/src/extensions.rs
index c2d6d6d0a2..aa704bae81 100644
--- a/codex-rs/app-server/src/extensions.rs
+++ b/codex-rs/app-server/src/extensions.rs
@@ -48,6 +48,7 @@ pub(crate) struct ThreadExtensionDependencies {
 }
 
 pub(crate) fn thread_extensions(
+    config: &Config,
     dependencies: ThreadExtensionDependencies,
 ) -> Arc<ExtensionRegistry<Config>> {
     let ThreadExtensionDependencies {
@@ -65,6 +66,12 @@ pub(crate) fn thread_extensions(
         turn_start_admission,
     } = dependencies;
     let mut builder = ExtensionRegistryBuilder::<Config>::with_event_sink(Arc::clone(&event_sink));
+    if config
+        .features
+        .enabled(codex_features::Feature::MultiEnvCopyFile)
+    {
+        codex_multi_env_copy_file::install(&mut builder);
+    }
     if let Some(admission) = turn_start_admission {
         builder.turn_start_admission(admission);
     }
diff --git a/codex-rs/app-server/src/mcp_refresh.rs b/codex-rs/app-server/src/mcp_refresh.rs
index ac965523fb..3611ab3992 100644
--- a/codex-rs/app-server/src/mcp_refresh.rs
+++ b/codex-rs/app-server/src/mcp_refresh.rs
@@ -476,20 +476,23 @@ enabled = false
                 codex_core::CodexAppsToolsCache::default(),
                 SessionSource::Exec,
                 Arc::clone(&environment_manager),
-                thread_extensions(ThreadExtensionDependencies {
-                    event_sink: Arc::new(NoopExtensionEventSink),
-                    auth_manager: auth_manager.clone(),
-                    state_db: Some(state_db.clone()),
-                    analytics_events_client: codex_analytics::AnalyticsEventsClient::disabled(),
-                    thread_manager: thread_manager.clone(),
-                    goal_service: Arc::new(codex_goal_extension::GoalService::new()),
-                    environment_manager: Arc::clone(&environment_manager),
-                    executor_skill_provider: Arc::clone(&executor_skill_provider),
-                    git_attribution_base_url: good_config.chatgpt_base_url.clone(),
-                    http_client_factory: good_config.http_client_factory(),
-                    queue_service: None,
-                    turn_start_admission: None,
-                }),
+                thread_extensions(
+                    &good_config,
+                    ThreadExtensionDependencies {
+                        event_sink: Arc::new(NoopExtensionEventSink),
+                        auth_manager: auth_manager.clone(),
+                        state_db: Some(state_db.clone()),
+                        analytics_events_client: codex_analytics::AnalyticsEventsClient::disabled(),
+                        thread_manager: thread_manager.clone(),
+                        goal_service: Arc::new(codex_goal_extension::GoalService::new()),
+                        environment_manager: Arc::clone(&environment_manager),
+                        executor_skill_provider: Arc::clone(&executor_skill_provider),
+                        git_attribution_base_url: good_config.chatgpt_base_url.clone(),
+                        http_client_factory: good_config.http_client_factory(),
+                        queue_service: None,
+                        turn_start_admission: None,
+                    },
+                ),
                 Arc::new(CodexHomeUserInstructionsProvider::new(
                     good_config.codex_home.clone(),
                 )),
diff --git a/codex-rs/app-server/src/message_processor.rs b/codex-rs/app-server/src/message_processor.rs
index 767c600e07..09d5f1762f 100644
--- a/codex-rs/app-server/src/message_processor.rs
+++ b/codex-rs/app-server/src/message_processor.rs
@@ -344,20 +344,23 @@ impl MessageProcessor {
                 codex_core::CodexAppsToolsCache::default(),
                 session_source,
                 environment_manager,
-                thread_extensions(ThreadExtensionDependencies {
-                    event_sink: Arc::clone(&extension_event_sink),
-                    auth_manager: auth_manager.clone(),
-                    state_db: state_db.clone(),
-                    analytics_events_client: analytics_events_client.clone(),
-                    thread_manager: thread_manager.clone(),
-                    goal_service: Arc::clone(&goal_service),
-                    environment_manager: Arc::clone(&environment_manager_for_extensions),
-                    executor_skill_provider: Arc::clone(&executor_skill_provider),
-                    git_attribution_base_url: config.chatgpt_base_url.clone(),
-                    http_client_factory: config.http_client_factory(),
-                    queue_service: queue_service.clone(),
-                    turn_start_admission: Some(Arc::clone(&turn_start_admission)),
-                }),
+                thread_extensions(
+                    config.as_ref(),
+                    ThreadExtensionDependencies {
+                        event_sink: Arc::clone(&extension_event_sink),
+                        auth_manager: auth_manager.clone(),
+                        state_db: state_db.clone(),
+                        analytics_events_client: analytics_events_client.clone(),
+                        thread_manager: thread_manager.clone(),
+                        goal_service: Arc::clone(&goal_service),
+                        environment_manager: Arc::clone(&environment_manager_for_extensions),
+                        executor_skill_provider: Arc::clone(&executor_skill_provider),
+                        git_attribution_base_url: config.chatgpt_base_url.clone(),
+                        http_client_factory: config.http_client_factory(),
+                        queue_service: queue_service.clone(),
+                        turn_start_admission: Some(Arc::clone(&turn_start_admission)),
+                    },
+                ),
                 Arc::new(CodexHomeUserInstructionsProvider::new(
                     config.codex_home.clone(),
                 )),
diff --git a/codex-rs/cli/Cargo.toml b/codex-rs/cli/Cargo.toml
index 9443824741..26dee62ece 100644
--- a/codex-rs/cli/Cargo.toml
+++ b/codex-rs/cli/Cargo.toml
@@ -55,6 +55,7 @@ codex-exec-server = { workspace = true }
 codex-execpolicy = { workspace = true }
 codex-extension-api = { workspace = true }
 codex-features = { workspace = true }
+codex-multi-env-copy-file = { workspace = true }
 codex-git-attribution = { workspace = true }
 codex-git-utils = { workspace = true }
 codex-install-context = { workspace = true }
diff --git a/codex-rs/cli/src/main.rs b/codex-rs/cli/src/main.rs
index 616c3f0f51..c3beb4f7c0 100644
--- a/codex-rs/cli/src/main.rs
+++ b/codex-rs/cli/src/main.rs
@@ -2099,6 +2099,13 @@ async fn run_debug_prompt_input_command(
     let auth_manager =
         AuthManager::shared_from_config(&config, /*enable_codex_api_key_env*/ false).await?;
     let mut extensions = codex_extension_api::ExtensionRegistryBuilder::new();
+    // Debug prompt construction bypasses app-server and needs its own extension registry.
+    if config
+        .features
+        .enabled(codex_features::Feature::MultiEnvCopyFile)
+    {
+        codex_multi_env_copy_file::install(&mut extensions);
+    }
     codex_git_attribution::install(
         &mut extensions,
         auth_manager,
diff --git a/codex-rs/core/config.schema.json b/codex-rs/core/config.schema.json
index 2331f64ed6..80c0539b5b 100644
--- a/codex-rs/core/config.schema.json
+++ b/codex-rs/core/config.schema.json
@@ -1066,6 +1066,9 @@
             "multi_agent_v2_dynamic_tools": {
               "type": "boolean"
             },
+            "multi_env_copy_file": {
+              "type": "boolean"
+            },
             "network_proxy": {
               "$ref": "#/definitions/FeatureToml_for_NetworkProxyConfigToml"
             },
@@ -7452,6 +7455,9 @@
         "multi_agent_v2_dynamic_tools": {
           "type": "boolean"
         },
+        "multi_env_copy_file": {
+          "type": "boolean"
+        },
         "network_proxy": {
           "$ref": "#/definitions/FeatureToml_for_NetworkProxyConfigToml"
         },
diff --git a/codex-rs/features/src/lib.rs b/codex-rs/features/src/lib.rs
index edcaa6ffd7..df12fcddf4 100644
--- a/codex-rs/features/src/lib.rs
+++ b/codex-rs/features/src/lib.rs
@@ -122,6 +122,8 @@ pub enum Feature {
     /// Advertise enabled environment-backed tools and their shell parameters before the
     /// executor is ready. Actual execution still requires a usable environment and its policy.
     StableEnvironmentTools,
+    /// Copy files between explicitly selected execution environments.
+    MultiEnvCopyFile,
     /// Send per-content-entry classifications in internal Responses metadata.
     ContentItemKinds,
     /// Record model-attempted tool calls in internal Responses metadata.
@@ -1060,6 +1062,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::MultiEnvCopyFile,
+        key: "multi_env_copy_file",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::SleepTool,
         key: "sleep_tool",