core: enable remote compaction v2 by default (#27573)

## Why

Remote compaction v2 is ready to become the default for providers that
already support remote compaction. Leaving it behind an
under-development opt-in keeps eligible sessions on the legacy
remote-compaction path.

This does not broaden provider eligibility: OpenAI and Azure move to v2,
while Bedrock and OSS providers retain their existing local-compaction
behavior.

## What changed

- Mark `remote_compaction_v2` stable and enable it by default.
- Make tests that intentionally cover legacy remote compaction
explicitly disable v2.
- Update parity coverage so v2 exercises the production default and only
legacy mode opts out.

## Verification

- `just test -p codex-core
auto_compact_runs_after_resume_when_token_usage_is_over_limit
auto_compact_counts_encrypted_reasoning_before_last_user
auto_compact_runs_when_reasoning_header_clears_between_turns
responses_lite_compact_request_uses_lite_transport_contract`

codex-rs/app-server/tests/suite/v2/compaction.rs

@@ -29,6 +29,7 @@ use codex_app_server_protocol::TurnStartParams;
 use codex_app_server_protocol::TurnStartResponse;
 use codex_app_server_protocol::UserInput as V2UserInput;
 use codex_config::types::AuthCredentialsStoreMode;
+use codex_features::Feature;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::ResponseItem;
 use core_test_support::responses;
@@ -150,7 +151,7 @@ async fn auto_compaction_remote_emits_started_and_completed_items() -> Result<()
     write_mock_responses_config_toml(
         codex_home.path(),
         &server.uri(),
-        &BTreeMap::default(),
+        &BTreeMap::from([(Feature::RemoteCompactionV2, false)]),
         REMOTE_AUTO_COMPACT_LIMIT,
         Some(true),
         "mock_provider",

codex-rs/core/tests/suite/compact.rs

@@ -2015,6 +2015,7 @@ async fn auto_compact_runs_after_resume_when_token_usage_is_over_limit() {
     let mut builder = test_codex().with_config(move |config| {
         set_test_compact_prompt(config);
         config.model_auto_compact_token_limit = Some(limit);
+        let _ = config.features.disable(Feature::RemoteCompactionV2);
     });
     let initial = builder.build(&server).await.unwrap();
     let home = initial.home.clone();
@@ -2043,6 +2044,7 @@ async fn auto_compact_runs_after_resume_when_token_usage_is_over_limit() {
     let mut resume_builder = test_codex().with_config(move |config| {
         set_test_compact_prompt(config);
         config.model_auto_compact_token_limit = Some(limit);
+        let _ = config.features.disable(Feature::RemoteCompactionV2);
     });
     let resumed = resume_builder
         .resume(&server, home, rollout_path)
@@ -4023,6 +4025,7 @@ async fn auto_compact_counts_encrypted_reasoning_before_last_user() {
             config.chatgpt_base_url = chatgpt_base_url;
             set_test_compact_prompt(config);
             config.model_auto_compact_token_limit = Some(300);
+            let _ = config.features.disable(Feature::RemoteCompactionV2);
         })
         .build(&server)
         .await
@@ -4145,6 +4148,7 @@ async fn auto_compact_runs_when_reasoning_header_clears_between_turns() {
         .with_config(|config| {
             set_test_compact_prompt(config);
             config.model_auto_compact_token_limit = Some(300);
+            let _ = config.features.disable(Feature::RemoteCompactionV2);
         })
         .build(&server)
         .await

codex-rs/core/tests/suite/compact_remote.rs

@@ -36,7 +36,7 @@ use core_test_support::responses::start_websocket_server;
 use core_test_support::skip_if_no_network;
 use core_test_support::test_codex::TestCodexBuilder;
 use core_test_support::test_codex::TestCodexHarness;
-use core_test_support::test_codex::test_codex;
+use core_test_support::test_codex::test_codex as base_test_codex;
 use core_test_support::test_path_buf;
 use core_test_support::wait_for_event;
 use core_test_support::wait_for_event_match;
@@ -160,6 +160,12 @@ fn compacted_summary_only_output(summary: &str) -> Vec<ResponseItem> {
     }]
 }
 
+fn test_codex() -> TestCodexBuilder {
+    base_test_codex().with_config(|config| {
+        let _ = config.features.disable(Feature::RemoteCompactionV2);
+    })
+}
+
 fn remote_realtime_test_codex_builder(
     realtime_server: &responses::WebSocketTestServer,
 ) -> TestCodexBuilder {

codex-rs/core/tests/suite/compact_remote_parity.rs

@@ -529,8 +529,8 @@ async fn build_harness_inner(
         if hooks {
             trust_discovered_hooks(config);
         }
-        if mode == Mode::V2 {
-            let _ = config.features.enable(Feature::RemoteCompactionV2);
+        if mode == Mode::Legacy {
+            let _ = config.features.disable(Feature::RemoteCompactionV2);
         }
     }))
     .await

codex-rs/core/tests/suite/hooks.rs

@@ -34,7 +34,6 @@ use core_test_support::responses::ev_function_call;
 use core_test_support::responses::ev_message_item_added;
 use core_test_support::responses::ev_output_text_delta;
 use core_test_support::responses::ev_response_created;
-use core_test_support::responses::mount_compact_json_once;
 use core_test_support::responses::mount_sse_once;
 use core_test_support::responses::mount_sse_sequence;
 use core_test_support::responses::sse;
@@ -1441,21 +1440,32 @@ async fn resumed_thread_runs_resume_then_compact_session_start_hooks() -> Result
     let remote_summary = "remote compact summary";
     let resume_context = "remember the resumed reef";
     let compact_context = "remember the compacted reef";
-    let compacted_history = vec![
-        ResponseItem::Message {
-            id: None,
-            role: "assistant".to_string(),
-            content: vec![ContentItem::OutputText {
-                text: remote_summary.to_string(),
-            }],
-            phase: None,
-        },
-        ResponseItem::Compaction {
-            encrypted_content: "encrypted compact summary".to_string(),
-        },
-    ];
-    let compact_mock =
-        mount_compact_json_once(&server, serde_json::json!({ "output": compacted_history })).await;
+    let responses_mock = mount_sse_sequence(
+        &server,
+        vec![
+            sse(vec![
+                ev_response_created("resp-1"),
+                ev_assistant_message("msg-1", "hello before resume"),
+                ev_completed_with_tokens("resp-1", over_limit_tokens),
+            ]),
+            sse(vec![
+                serde_json::json!({
+                    "type": "response.output_item.done",
+                    "item": {
+                        "type": "compaction",
+                        "encrypted_content": remote_summary,
+                    }
+                }),
+                ev_completed("resp-compact"),
+            ]),
+            sse(vec![
+                ev_response_created("resp-2"),
+                ev_assistant_message("msg-2", "hello after resume"),
+                ev_completed("resp-2"),
+            ]),
+        ],
+    )
+    .await;
 
     let mut builder = test_codex()
         .with_pre_build_hook(move |home| {
@@ -1479,37 +1489,19 @@ async fn resumed_thread_runs_resume_then_compact_session_start_hooks() -> Result
         .clone()
         .context("rollout path")?;
 
-    mount_sse_once(
-        &server,
-        sse(vec![
-            ev_response_created("resp-1"),
-            ev_assistant_message("msg-1", "hello before resume"),
-            ev_completed_with_tokens("resp-1", over_limit_tokens),
-        ]),
-    )
-    .await;
     initial.submit_turn("hello before resume").await?;
-    assert!(compact_mock.requests().is_empty());
+    assert_eq!(responses_mock.requests().len(), 1);
 
     let mut resume_builder = test_codex().with_config(move |config| {
         config.model_auto_compact_token_limit = Some(limit);
         trust_discovered_hooks(config);
     });
     let resumed = resume_builder.resume(&server, home, rollout_path).await?;
-    let follow_up = mount_sse_once(
-        &server,
-        sse(vec![
-            ev_response_created("resp-2"),
-            ev_assistant_message("msg-2", "hello after resume"),
-            ev_completed("resp-2"),
-        ]),
-    )
-    .await;
-
     resumed.submit_turn("hello after resume").await?;
 
-    assert_eq!(compact_mock.requests().len(), 1);
-    let developer_messages = follow_up.single_request().message_input_texts("developer");
+    let requests = responses_mock.requests();
+    assert_eq!(requests.len(), 3);
+    let developer_messages = requests[2].message_input_texts("developer");
     assert!(
         developer_messages
             .iter()

codex-rs/core/tests/suite/responses_lite.rs

@@ -177,10 +177,14 @@ async fn responses_lite_compact_request_uses_lite_transport_contract() -> Result
     let compact_mock =
         responses::mount_compact_json_once(&server, serde_json::json!({ "output": [] })).await;
 
-    let mut builder = test_codex().with_model_info_override("gpt-5.4", |model_info| {
-        model_info.use_responses_lite = true;
-        model_info.supports_parallel_tool_calls = true;
-    });
+    let mut builder = test_codex()
+        .with_model_info_override("gpt-5.4", |model_info| {
+            model_info.use_responses_lite = true;
+            model_info.supports_parallel_tool_calls = true;
+        })
+        .with_config(|config| {
+            let _ = config.features.disable(Feature::RemoteCompactionV2);
+        });
     let test = builder.build(&server).await?;
 
     test.submit_turn("Compact this conversation").await?;

codex-rs/features/src/lib.rs

@@ -1249,8 +1249,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::RemoteCompactionV2,
         key: "remote_compaction_v2",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
+        stage: Stage::Stable,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::WorkspaceDependencies,

codex-rs/features/src/tests.rs

@@ -150,16 +150,6 @@ fn request_permissions_tool_is_under_development() {
     assert_eq!(Feature::RequestPermissionsTool.default_enabled(), false);
 }
 
-#[test]
-fn remote_compaction_v2_is_under_development() {
-    assert_eq!(Feature::RemoteCompactionV2.stage(), Stage::UnderDevelopment);
-    assert_eq!(Feature::RemoteCompactionV2.default_enabled(), false);
-    assert_eq!(
-        feature_for_key("remote_compaction_v2"),
-        Some(Feature::RemoteCompactionV2)
-    );
-}
-
 #[test]
 fn terminal_resize_reflow_is_experimental_and_enabled_by_default() {
     assert_eq!(