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-protocol/src/protocol/v2.rs

@@ -2603,10 +2603,22 @@ pub struct ThreadStartParams {
     pub config: Option<HashMap<String, JsonValue>>,
     #[ts(optional = nullable)]
     pub service_name: Option<String>,
+    #[serde(
+        default,
+        deserialize_with = "super::serde_helpers::deserialize_double_option",
+        serialize_with = "super::serde_helpers::serialize_double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
     #[ts(optional = nullable)]
-    pub base_instructions: Option<String>,
+    pub base_instructions: Option<Option<String>>,
+    #[serde(
+        default,
+        deserialize_with = "super::serde_helpers::deserialize_double_option",
+        serialize_with = "super::serde_helpers::serialize_double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
     #[ts(optional = nullable)]
-    pub developer_instructions: Option<String>,
+    pub developer_instructions: Option<Option<String>>,
     #[ts(optional = nullable)]
     pub personality: Option<Personality>,
     #[ts(optional = nullable)]
@@ -2721,10 +2733,22 @@ pub struct ThreadResumeParams {
     pub sandbox: Option<SandboxMode>,
     #[ts(optional = nullable)]
     pub config: Option<HashMap<String, serde_json::Value>>,
+    #[serde(
+        default,
+        deserialize_with = "super::serde_helpers::deserialize_double_option",
+        serialize_with = "super::serde_helpers::serialize_double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
     #[ts(optional = nullable)]
-    pub base_instructions: Option<String>,
+    pub base_instructions: Option<Option<String>>,
+    #[serde(
+        default,
+        deserialize_with = "super::serde_helpers::deserialize_double_option",
+        serialize_with = "super::serde_helpers::serialize_double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
     #[ts(optional = nullable)]
-    pub developer_instructions: Option<String>,
+    pub developer_instructions: Option<Option<String>>,
     #[ts(optional = nullable)]
     pub personality: Option<Personality>,
     /// If true, persist additional rollout EventMsg variants required to
@@ -2798,10 +2822,22 @@ pub struct ThreadForkParams {
     pub sandbox: Option<SandboxMode>,
     #[ts(optional = nullable)]
     pub config: Option<HashMap<String, serde_json::Value>>,
+    #[serde(
+        default,
+        deserialize_with = "super::serde_helpers::deserialize_double_option",
+        serialize_with = "super::serde_helpers::serialize_double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
     #[ts(optional = nullable)]
-    pub base_instructions: Option<String>,
+    pub base_instructions: Option<Option<String>>,
+    #[serde(
+        default,
+        deserialize_with = "super::serde_helpers::deserialize_double_option",
+        serialize_with = "super::serde_helpers::serialize_double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
     #[ts(optional = nullable)]
-    pub developer_instructions: Option<String>,
+    pub developer_instructions: Option<Option<String>>,
     #[serde(default, skip_serializing_if = "std::ops::Not::not")]
     pub ephemeral: bool,
     /// If true, persist additional rollout EventMsg variants required to
@@ -8325,6 +8361,35 @@ mod tests {
         assert_eq!(serialized_without_override.get("serviceTier"), None);
     }
 
+    #[test]
+    fn thread_start_params_preserve_explicit_null_instructions() {
+        let params: ThreadStartParams = serde_json::from_value(json!({
+            "baseInstructions": null,
+            "developerInstructions": null,
+        }))
+        .expect("params should deserialize");
+        assert_eq!(params.base_instructions, Some(None));
+        assert_eq!(params.developer_instructions, Some(None));
+
+        let serialized = serde_json::to_value(&params).expect("params should serialize");
+        assert_eq!(
+            serialized.get("baseInstructions"),
+            Some(&serde_json::Value::Null)
+        );
+        assert_eq!(
+            serialized.get("developerInstructions"),
+            Some(&serde_json::Value::Null)
+        );
+
+        let serialized_without_override =
+            serde_json::to_value(ThreadStartParams::default()).expect("params should serialize");
+        assert_eq!(serialized_without_override.get("baseInstructions"), None);
+        assert_eq!(
+            serialized_without_override.get("developerInstructions"),
+            None
+        );
+    }
+
     #[test]
     fn turn_start_params_preserve_explicit_null_service_tier() {
         let params: TurnStartParams = serde_json::from_value(json!({