Treat code-mode collaboration messages as plaintext (#55633)

## What changed

Classify messages from code-mode collaboration calls as `AgentMessage::Plaintext` so they receive the context wrapper for plaintext messages. Apply this to `spawn_agent`, `send_message`, and `followup_task`.

## Testing

Add a regression test that spawns an agent through `exec` and verifies that the child receives the task as plaintext with sender and recipient attribution.

GitOrigin-RevId: 495c70ea13fd2bb06989e5aacab60beed0c19a56

codex-rs/core/src/tools/handlers/multi_agents_v2.rs

@@ -58,6 +58,7 @@ fn agent_message_from_tool(
     if matches!(
         source,
         crate::tools::context::ToolCallSource::DirectPlaintextMessage
+            | crate::tools::context::ToolCallSource::CodeMode { .. }
     ) {
         AgentMessage::Plaintext(message)
     } else {

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

@@ -0,0 +1,114 @@
+//! Exec-spawned tasks reach the child model as attributed plaintext.
+
+use anyhow::Result;
+use codex_features::Feature;
+use codex_protocol::openai_models::ToolMode;
+use codex_protocol::protocol::EventMsg;
+use core_test_support::responses::ev_assistant_message;
+use core_test_support::responses::ev_completed;
+use core_test_support::responses::ev_custom_tool_call;
+use core_test_support::responses::ev_response_created;
+use core_test_support::responses::mount_sse_once_match;
+use core_test_support::responses::sse;
+use core_test_support::responses::start_mock_server;
+use core_test_support::responses::strip_metadata_from_json;
+use core_test_support::responses::strip_response_item_ids_from_json;
+use core_test_support::skip_if_no_network;
+use core_test_support::test_codex::test_codex;
+use core_test_support::wait_for_event;
+use pretty_assertions::assert_eq;
+use serde_json::Value;
+use serde_json::json;
+use wiremock::matchers::header;
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn code_mode_spawn_delivers_plaintext() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let test = test_codex()
+        .with_model("gpt-5.6-sol")
+        .with_model_info_override("gpt-5.6-sol", |model| {
+            model.tool_mode = Some(ToolMode::CodeMode);
+            model.use_responses_lite = true;
+        })
+        .with_config(|config| {
+            for feature in [
+                Feature::Collab,
+                Feature::MultiAgentV2,
+                Feature::CodeMode,
+                Feature::CodeModeHostGrpc,
+            ] {
+                config
+                    .features
+                    .enable(feature)
+                    .expect("enable test feature");
+            }
+            config.multi_agent_v2.non_code_mode_only = false;
+        })
+        .build_with_auto_env(&server)
+        .await?;
+    let root_header = test.session_configured.thread_id.to_string();
+    let mut created = test.thread_manager.subscribe_thread_created();
+
+    mount_sse_once_match(
+        &server,
+        header("thread-id", root_header.as_str()),
+        sse(vec![
+            ev_response_created("spawn"),
+            ev_custom_tool_call(
+                "spawn-worker",
+                "exec",
+                r#"text(await tools.collaboration__spawn_agent({task_name: "worker", message: "Summarize the three stories.", fork_turns: "all"}));"#,
+            ),
+            ev_completed("spawn"),
+        ]),
+    )
+    .await;
+    mount_sse_once_match(
+        &server,
+        header("thread-id", root_header.as_str()),
+        sse(vec![
+            ev_assistant_message("delegated", "The worker started."),
+            ev_completed("delegated"),
+        ]),
+    )
+    .await;
+    let child_requests = mount_sse_once_match(
+        &server,
+        move |request: &wiremock::Request| {
+            request
+                .headers
+                .get("thread-id")
+                .is_some_and(|id| id != root_header.as_str())
+        },
+        sse(vec![ev_response_created("child"), ev_completed("child")]),
+    )
+    .await;
+    test.submit_turn("Delegate the summary task.").await?;
+    let child_id = created.try_recv()?;
+    let child = test.thread_manager.get_thread(child_id).await?;
+    wait_for_event(&child, |event| matches!(event, EventMsg::TurnComplete(_))).await;
+
+    let child_request = child_requests
+        .requests()
+        .into_iter()
+        .find(|request| request.header("thread-id") == Some(child_id.to_string()))
+        .expect("child receives its task");
+    assert_eq!(
+        strip_response_item_ids_from_json(strip_metadata_from_json(Value::Array(
+            child_request.inputs_of_type("agent_message"),
+        ))),
+        json!([{
+            "type": "agent_message",
+            "author": "/root",
+            "recipient": "/root/worker",
+            "content": [{
+                "type": "input_text",
+                "text": "Message Type: NEW_TASK\nTask name: /root/worker\nSender: /root\nPayload:\nSummarize the three stories.",
+            }],
+        }]),
+    );
+    assert_eq!(test.thread_manager.list_thread_ids().await.len(), 2);
+    Ok(())
+}

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

@@ -83,6 +83,9 @@ mod spawn_settings_tests;
 #[path = "guardian_subagent_notification_tests.rs"]
 mod guardian_subagent_notification_tests;
 
+#[path = "code_mode_agent_message_tests.rs"]
+mod code_mode_agent_message_tests;
+
 const SPAWN_CALL_ID: &str = "spawn-call-1";
 const MULTI_AGENT_V1_NAMESPACE: &str = "multi_agent_v1";
 const MULTI_AGENT_V2_NAMESPACE: &str = "collaboration";