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/tests/suite/client.rs

@@ -921,7 +921,7 @@ async fn includes_base_instructions_override_in_request() {
     let mut builder = test_codex()
         .with_auth(CodexAuth::from_api_key("Test API Key"))
         .with_config(|config| {
-            config.base_instructions = Some("test instructions".to_string());
+            config.base_instructions = Some(Some("test instructions".to_string()));
         });
     let codex = builder
         .build(&server)
@@ -953,6 +953,47 @@ async fn includes_base_instructions_override_in_request() {
     );
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn omits_explicit_null_base_instructions_from_request() {
+    skip_if_no_network!();
+    // Mock server
+    let server = MockServer::start().await;
+    let resp_mock = mount_sse_once(
+        &server,
+        sse(vec![ev_response_created("resp1"), ev_completed("resp1")]),
+    )
+    .await;
+
+    let mut builder = test_codex()
+        .with_auth(CodexAuth::from_api_key("Test API Key"))
+        .with_config(|config| {
+            config.base_instructions = Some(None);
+        });
+    let codex = builder
+        .build(&server)
+        .await
+        .expect("create new conversation")
+        .codex;
+
+    codex
+        .submit(Op::UserInput {
+            items: vec![UserInput::Text {
+                text: "hello".into(),
+                text_elements: Vec::new(),
+            }],
+            final_output_json_schema: None,
+        })
+        .await
+        .unwrap();
+
+    wait_for_event(&codex, |ev| matches!(ev, EventMsg::TurnComplete(_))).await;
+
+    let request = resp_mock.single_request();
+    let request_body = request.body_json();
+
+    assert_eq!(request_body.get("instructions"), None);
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn chatgpt_auth_sends_correct_request() {
     skip_if_no_network!();

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

@@ -1654,9 +1654,9 @@ fn prompt_with_input(input: Vec<ResponseItem>) -> Prompt {
 
 fn prompt_with_input_and_instructions(input: Vec<ResponseItem>, instructions: &str) -> Prompt {
     let mut prompt = prompt_with_input(input);
-    prompt.base_instructions = BaseInstructions {
+    prompt.base_instructions = Some(BaseInstructions {
         text: instructions.to_string(),
-    };
+    });
     prompt
 }
 

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

@@ -869,7 +869,7 @@ async fn remote_compact_trim_estimate_uses_session_base_instructions() -> Result
                 let override_base_instructions = override_base_instructions.clone();
                 move |config| {
                     config.model_context_window = Some(override_context_window);
-                    config.base_instructions = Some(override_base_instructions);
+                    config.base_instructions = Some(Some(override_base_instructions));
                 }
             }),
     )

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

@@ -66,7 +66,7 @@ async fn base_instructions_override_disables_personality_template() {
         .enable(Feature::Personality)
         .expect("test config should allow feature update");
     config.personality = Some(Personality::Friendly);
-    config.base_instructions = Some("override instructions".to_string());
+    config.base_instructions = Some(Some("override instructions".to_string()));
 
     let model_info =
         codex_core::test_support::construct_model_info_offline("gpt-5.2-codex", &config);

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

@@ -173,7 +173,7 @@ async fn find_locates_rollout_file_written_by_recorder() -> std::io::Result<()>
             thread_id,
             /*forked_from_id*/ None,
             SessionSource::Exec,
-            BaseInstructions::default(),
+            Some(BaseInstructions::default()),
             Vec::new(),
             EventPersistenceMode::Limited,
         ),

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

@@ -84,7 +84,7 @@ async fn continue_after_stream_error() {
 
     let TestCodex { codex, .. } = test_codex()
         .with_config(move |config| {
-            config.base_instructions = Some("You are a helpful assistant".to_string());
+            config.base_instructions = Some(Some("You are a helpful assistant".to_string()));
             config.model_provider = provider;
         })
         .build(&server)