app-server: ignore persist_extended_history param (#21225)

## Why

Taking a step to removing the `persistExtendedHistory` field. It's not
scalable to be persisting so much data in the rollout file and returning
it in the thread history.

When a client explicitly sends `true`, the server now tells that client
the parameter is deprecated and ignored so the caller has a clear
migration signal via the `deprecationNotice` notification.

## What changed

- Keep the `persist_extended_history` / `persistExtendedHistory` field
in the v2 protocol for compatibility, but document it as deprecated and
ignored.
- Ignore the parameter in app-server `thread/start`, `thread/resume`,
and `thread/fork`; those paths always use limited history persistence
now.
- Stop treating `persistExtendedHistory` as a running-thread resume
override mismatch.
- Emit a connection-scoped `deprecationNotice` when a request explicitly
sets `persist_extended_history: true`.

## Verification

- Added `thread_start_deprecates_persist_extended_history_true` to cover
the deprecation notice.
- `cargo test -p codex-app-server`
- `cargo test -p codex-app-server-protocol`

codex-rs/app-server/src/request_processors/thread_processor.rs

@@ -2,6 +2,10 @@ use super::*;
 
 const THREAD_LIST_DEFAULT_LIMIT: usize = 25;
 const THREAD_LIST_MAX_LIMIT: usize = 100;
+const PERSIST_EXTENDED_HISTORY_DEPRECATION_SUMMARY: &str =
+    "persistExtendedHistory is deprecated and ignored";
+const PERSIST_EXTENDED_HISTORY_DEPRECATION_DETAILS: &str =
+    "Remove this parameter. App-server always uses limited history persistence.";
 
 struct ThreadListFilters {
     model_providers: Option<Vec<String>>,
@@ -121,12 +125,6 @@ fn collect_resume_override_mismatches(
             "developerInstructions override was provided and ignored while running".to_string(),
         );
     }
-    if request.persist_extended_history {
-        mismatch_details.push(
-            "persistExtendedHistory override was provided and ignored while running".to_string(),
-        );
-    }
-
     mismatch_details
 }
 
@@ -750,6 +748,10 @@ impl ThreadRequestProcessor {
                 "`permissions` cannot be combined with `sandbox`",
             ));
         }
+        if persist_extended_history {
+            self.send_persist_extended_history_deprecation_notice(request_id.connection_id)
+                .await;
+        }
         let environment_selections = self.parse_environment_selections(environments)?;
         let mut typesafe_overrides = self.build_thread_config_overrides(
             model,
@@ -792,7 +794,6 @@ impl ThreadRequestProcessor {
                 dynamic_tools,
                 session_start_source,
                 environment_selections,
-                persist_extended_history,
                 service_name,
                 experimental_raw_events,
                 request_trace,
@@ -841,6 +842,18 @@ impl ThreadRequestProcessor {
         self.outgoing.request_trace_context(request_id).await
     }
 
+    async fn send_persist_extended_history_deprecation_notice(&self, connection_id: ConnectionId) {
+        self.outgoing
+            .send_server_notification_to_connections(
+                &[connection_id],
+                ServerNotification::DeprecationNotice(DeprecationNoticeNotification {
+                    summary: PERSIST_EXTENDED_HISTORY_DEPRECATION_SUMMARY.to_string(),
+                    details: Some(PERSIST_EXTENDED_HISTORY_DEPRECATION_DETAILS.to_string()),
+                }),
+            )
+            .await;
+    }
+
     async fn submit_core_op(
         &self,
         request_id: &ConnectionRequestId,
@@ -864,7 +877,6 @@ impl ThreadRequestProcessor {
         dynamic_tools: Option<Vec<ApiDynamicToolSpec>>,
         session_start_source: Option<codex_app_server_protocol::ThreadStartSource>,
         environments: Option<Vec<TurnEnvironmentSelection>>,
-        persist_extended_history: bool,
         service_name: Option<String>,
         experimental_raw_events: bool,
         request_trace: Option<W3cTraceContext>,
@@ -981,7 +993,7 @@ impl ThreadRequestProcessor {
                 },
                 session_source: None,
                 dynamic_tools: core_dynamic_tools,
-                persist_extended_history,
+                persist_extended_history: false,
                 metrics_service_name: service_name,
                 parent_trace: request_trace,
                 environments,
@@ -990,7 +1002,7 @@ impl ThreadRequestProcessor {
                 "app_server.thread_start.create_thread",
                 otel.name = "app_server.thread_start.create_thread",
                 thread_start.dynamic_tool_count = core_dynamic_tool_count,
-                thread_start.persist_extended_history = persist_extended_history,
+                thread_start.persist_extended_history = false,
             ))
             .await
             .map_err(|err| match err {
@@ -2202,6 +2214,10 @@ impl ThreadRequestProcessor {
                 .await;
             return Ok(());
         }
+        if params.persist_extended_history {
+            self.send_persist_extended_history_deprecation_notice(request_id.connection_id)
+                .await;
+        }
 
         let _thread_list_state_permit = match self.acquire_thread_list_state_permit().await {
             Ok(permit) => permit,
@@ -2236,7 +2252,7 @@ impl ThreadRequestProcessor {
             developer_instructions,
             personality,
             exclude_turns,
-            persist_extended_history,
+            persist_extended_history: _persist_extended_history,
         } = params;
         let include_turns = !exclude_turns;
 
@@ -2300,7 +2316,7 @@ impl ThreadRequestProcessor {
                 config.clone(),
                 thread_history,
                 self.auth_manager.clone(),
-                persist_extended_history,
+                /*persist_extended_history*/ false,
                 self.request_trace_context(&request_id).await,
             )
             .await
@@ -2836,6 +2852,10 @@ impl ThreadRequestProcessor {
                 "`permissions` cannot be combined with `sandbox`",
             ));
         }
+        if persist_extended_history {
+            self.send_persist_extended_history_deprecation_notice(request_id.connection_id)
+                .await;
+        }
 
         let source_thread = self
             .read_stored_thread_for_resume(&thread_id, path.as_ref(), /*include_history*/ true)
@@ -2913,7 +2933,7 @@ impl ThreadRequestProcessor {
                     history: history_items.clone(),
                     rollout_path: source_thread.rollout_path.clone(),
                 }),
-                persist_extended_history,
+                /*persist_extended_history*/ false,
                 self.request_trace_context(&request_id).await,
             )
             .await