Honor null thread instructions (#16964)

- Treat explicit null thread instructions as a blank-slate override
while preserving omitted-field fallback behavior.
- Preserve null through rollout resume/fork and keep explicit empty
strings distinct.
- Add app-server v2 start/fork coverage for the tri-state instruction
params.

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

@@ -535,7 +535,7 @@ async fn build_runner_options(
     let max_concurrency =
         normalize_concurrency(requested_concurrency, turn.config.agent_max_threads);
     let base_instructions = session.get_base_instructions().await;
-    let spawn_config = build_agent_spawn_config(&base_instructions, turn.as_ref())?;
+    let spawn_config = build_agent_spawn_config(base_instructions.as_ref(), turn.as_ref())?;
     Ok(JobRunnerOptions {
         max_concurrency,
         spawn_config,

codex-rs/core/src/tools/handlers/multi_agents/spawn.rs

@@ -59,8 +59,10 @@ impl ToolHandler for Handler {
                 .into(),
             )
             .await;
-        let mut config =
-            build_agent_spawn_config(&session.get_base_instructions().await, turn.as_ref())?;
+        let mut config = build_agent_spawn_config(
+            session.get_base_instructions().await.as_ref(),
+            turn.as_ref(),
+        )?;
         apply_requested_spawn_agent_model_overrides(
             &session,
             turn.as_ref(),

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

@@ -201,11 +201,12 @@ pub(crate) fn parse_collab_input(
 /// skipping this helper and cloning stale config state directly can send the child agent out with
 /// the wrong provider or runtime policy.
 pub(crate) fn build_agent_spawn_config(
-    base_instructions: &BaseInstructions,
+    base_instructions: Option<&BaseInstructions>,
     turn: &TurnContext,
 ) -> Result<Config, FunctionCallError> {
     let mut config = build_agent_shared_config(turn)?;
-    config.base_instructions = Some(base_instructions.text.clone());
+    config.base_instructions =
+        Some(base_instructions.map(|base_instructions| base_instructions.text.clone()));
     Ok(config)
 }
 

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

@@ -3209,9 +3209,9 @@ async fn build_agent_spawn_config_uses_turn_context_values() {
         .set(AskForApproval::OnRequest)
         .expect("approval policy set");
 
-    let config = build_agent_spawn_config(&base_instructions, &turn).expect("spawn config");
+    let config = build_agent_spawn_config(Some(&base_instructions), &turn).expect("spawn config");
     let mut expected = (*turn.config).clone();
-    expected.base_instructions = Some(base_instructions.text);
+    expected.base_instructions = Some(Some(base_instructions.text));
     expected.model = Some(turn.model_info.slug.clone());
     expected.model_provider = turn.provider.clone();
     expected.model_reasoning_effort = turn.reasoning_effort;
@@ -3247,7 +3247,7 @@ async fn build_agent_spawn_config_preserves_base_user_instructions() {
         text: "base".to_string(),
     };
 
-    let config = build_agent_spawn_config(&base_instructions, &turn).expect("spawn config");
+    let config = build_agent_spawn_config(Some(&base_instructions), &turn).expect("spawn config");
 
     assert_eq!(config.user_instructions, base_config.user_instructions);
 }
@@ -3256,7 +3256,7 @@ async fn build_agent_spawn_config_preserves_base_user_instructions() {
 async fn build_agent_resume_config_clears_base_instructions() {
     let (_session, mut turn) = make_session_and_context().await;
     let mut base_config = (*turn.config).clone();
-    base_config.base_instructions = Some("caller-base".to_string());
+    base_config.base_instructions = Some(Some("caller-base".to_string()));
     turn.config = Arc::new(base_config);
     turn.approval_policy
         .set(AskForApproval::OnRequest)

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

@@ -68,8 +68,10 @@ impl ToolHandler for Handler {
                 .into(),
             )
             .await;
-        let mut config =
-            build_agent_spawn_config(&session.get_base_instructions().await, turn.as_ref())?;
+        let mut config = build_agent_spawn_config(
+            session.get_base_instructions().await.as_ref(),
+            turn.as_ref(),
+        )?;
         apply_requested_spawn_agent_model_overrides(
             &session,
             turn.as_ref(),