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/app-server/tests/suite/v2/turn_start.rs

@@ -152,6 +152,103 @@ async fn turn_start_sends_originator_header() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn turn_start_honors_explicit_null_thread_instructions() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = responses::start_mock_server().await;
+    let body = responses::sse(vec![
+        responses::ev_response_created("resp-1"),
+        responses::ev_assistant_message("msg-1", "Done"),
+        responses::ev_completed("resp-1"),
+    ]);
+    let response_mock = responses::mount_sse_sequence(&server, vec![body.clone(), body]).await;
+
+    let codex_home = TempDir::new()?;
+    create_config_toml(codex_home.path(), &server.uri(), "never", &BTreeMap::new())?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let disabled_instruction_config = json!({
+        "include_permissions_instructions": false,
+        "include_apps_instructions": false,
+        "include_environment_context": false,
+        "features.apps": false,
+        "features.plugins": false,
+        "features.codex_hooks": false,
+        "skills.bundled.enabled": false,
+    });
+
+    let thread_start_params = [
+        (
+            json!({
+                "model": "mock-model",
+                "config": disabled_instruction_config.clone(),
+            }),
+            /*expect_instructions*/ true,
+        ),
+        (
+            json!({
+                "model": "mock-model",
+                "config": disabled_instruction_config.clone(),
+                "baseInstructions": null,
+                "developerInstructions": null,
+            }),
+            /*expect_instructions*/ false,
+        ),
+    ];
+
+    for (params, _expect_instructions) in thread_start_params {
+        let thread_req = mcp.send_raw_request("thread/start", Some(params)).await?;
+        let thread_resp: JSONRPCResponse = timeout(
+            DEFAULT_READ_TIMEOUT,
+            mcp.read_stream_until_response_message(RequestId::Integer(thread_req)),
+        )
+        .await??;
+        let ThreadStartResponse { thread, .. } = to_response::<ThreadStartResponse>(thread_resp)?;
+
+        let turn_req = mcp
+            .send_turn_start_request(TurnStartParams {
+                thread_id: thread.id,
+                input: vec![V2UserInput::Text {
+                    text: "Hello".to_string(),
+                    text_elements: Vec::new(),
+                }],
+                ..Default::default()
+            })
+            .await?;
+        timeout(
+            DEFAULT_READ_TIMEOUT,
+            mcp.read_stream_until_response_message(RequestId::Integer(turn_req)),
+        )
+        .await??;
+        timeout(
+            DEFAULT_READ_TIMEOUT,
+            mcp.read_stream_until_notification_message("turn/completed"),
+        )
+        .await??;
+    }
+
+    let requests = response_mock.requests();
+    assert_eq!(requests.len(), 2);
+    for (request, expect_instructions) in requests.into_iter().zip([true, false]) {
+        let payload = request.body_json();
+        assert_eq!(
+            payload.get("instructions").is_some(),
+            expect_instructions,
+            "unexpected instructions field in payload: {payload:?}"
+        );
+        let developer_texts = request.message_input_texts("developer");
+        assert!(
+            developer_texts.iter().all(|text| !text.is_empty()),
+            "did not expect empty developer instruction messages: {developer_texts:?}"
+        );
+    }
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn turn_start_emits_user_message_item_with_text_elements() -> Result<()> {
     let responses = vec![create_final_assistant_message_sse_response("Done")?];