fix(agent): tolerate explicit null for list fields in streamed events

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - **All languages:** the AI Agent streamed conversation no longer errors mid-run when the server sends an explicit `"outputs": null`. `WorkflowFinishedPayload.outputs`, `NodeToolUseFinishedPayload.outputs`, and `SubagentFinishedPayload.outputs` were annotated `#[serde(default)]`, which only covers a *missing* key, not an explicit `null` — so a `workflow_finished` / `node_tool_use_finished` / `subagent_finished` event carrying `null` outputs failed to deserialize and aborted the whole event stream (`invalid type: null, expected struct WorkflowOutputs`). These fields now map `null` to the type's default
+- **All languages:** likewise, list-typed fields on the streamed AI Agent event payloads no longer error on an explicit `null` (`invalid type: null, expected a sequence`). `tip_chips` (on the node / subagent / agent-tool `*_started` payloads), `WorkflowFinishedPayload.process_data`, and `SubagentStartedPayload.tools` were `#[serde(default)]`, which does not accept an explicit `null`; they now deserialize `null` to an empty list
 
 ## [4.5.0] - 2026-08-14
 

rust/src/agent/types.rs

@@ -433,7 +433,7 @@ pub struct WorkflowFinishedPayload {
     #[serde(default)]
     pub error_args: Option<serde_json::Value>,
     /// Process stages the run went through; for display only
-    #[serde(default)]
+    #[serde(default, deserialize_with = "crate::serde_utils::null_as_default")]
     pub process_data: Vec<serde_json::Value>,
 }
 
@@ -569,7 +569,7 @@ pub struct NodeToolUseStartedPayload {
     #[serde(default)]
     pub tips: String,
     /// Short tags accompanying `tips`; may be omitted
-    #[serde(default)]
+    #[serde(default, deserialize_with = "crate::serde_utils::null_as_default")]
     pub tip_chips: Vec<String>,
     /// Round number. Calls in the same round (same `iteration`) run in
     /// parallel
@@ -639,7 +639,7 @@ pub struct NodeToolUseFinishedPayload {
     #[serde(default)]
     pub tips: String,
     /// Short tags; may be omitted
-    #[serde(default)]
+    #[serde(default, deserialize_with = "crate::serde_utils::null_as_default")]
     pub tip_chips: Vec<String>,
     /// Round number
     #[serde(default)]
@@ -676,7 +676,7 @@ pub struct SubagentStartedPayload {
     #[serde(default)]
     pub subagent_id: String,
     /// Tools granted to the subagent; may be omitted
-    #[serde(default)]
+    #[serde(default, deserialize_with = "crate::serde_utils::null_as_default")]
     pub tools: Vec<serde_json::Value>,
 }
 
@@ -782,7 +782,7 @@ pub struct AgentToolStartedPayload {
     #[serde(default)]
     pub tips: String,
     /// Short tags; may be omitted
-    #[serde(default)]
+    #[serde(default, deserialize_with = "crate::serde_utils::null_as_default")]
     pub tip_chips: Vec<String>,
     /// `true` if called during the thinking phase
     #[serde(default)]
@@ -859,7 +859,7 @@ pub struct AgentToolFinishedPayload {
     #[serde(default)]
     pub tips: String,
     /// Short tags; may be omitted
-    #[serde(default)]
+    #[serde(default, deserialize_with = "crate::serde_utils::null_as_default")]
     pub tip_chips: Vec<String>,
     /// `true` if during the thinking phase
     #[serde(default)]
@@ -1047,6 +1047,23 @@ mod tests {
         assert!(payload.outputs.answer.is_none());
     }
 
+    #[test]
+    fn streamed_vec_fields_tolerate_null_sequences() {
+        // The server can send an explicit `null` for list-typed fields; that
+        // must deserialize to an empty list, not error with
+        // "invalid type: null, expected a sequence" and abort the stream.
+        let payload: WorkflowFinishedPayload =
+            serde_json::from_str(r#"{"status":"succeeded","process_data":null}"#).unwrap();
+        assert!(payload.process_data.is_empty());
+
+        let payload: NodeToolUseStartedPayload =
+            serde_json::from_str(r#"{"tip_chips":null}"#).unwrap();
+        assert!(payload.tip_chips.is_empty());
+
+        let payload: SubagentStartedPayload = serde_json::from_str(r#"{"tools":null}"#).unwrap();
+        assert!(payload.tools.is_empty());
+    }
+
     // The `data` payload of the "Run succeeded" example from
     // https://open.longbridge.com/en/docs/ai/chat/conversation
     const SUCCEEDED_JSON: &str = r#"{