fix(run): preserve last known response_id on conversation resume (#3245)

src/agents/run_internal/oai_conversation.py

@@ -189,8 +189,14 @@ class OpenAIServerConversationTracker:
         self.sent_initial_input = True
         self.remaining_initial_input = None
 
-        latest_response = model_responses[-1] if model_responses else None
+        # Pick the most recent response that actually carries an id; live runs preserve the
+        # last-known id via track_server_items, so resume must mirror that behavior instead of
+        # blindly using model_responses[-1] (which may have response_id=None for non-Responses
+        # providers and would silently break the chain).
+        latest_response_id: str | None = None
         for response in model_responses:
+            if response.response_id is not None:
+                latest_response_id = response.response_id
             for output_item in response.output:
                 if output_item is None:
                     continue
@@ -207,8 +213,8 @@ class OpenAIServerConversationTracker:
                 if isinstance(call_id, str) and has_output_payload:
                     self.server_tool_call_ids.add(call_id)
 
-        if self.conversation_id is None and latest_response and latest_response.response_id:
-            self.previous_response_id = latest_response.response_id
+        if self.conversation_id is None and latest_response_id is not None:
+            self.previous_response_id = latest_response_id
 
         if session_items:
             for item in session_items:

tests/test_oaiconv_resume_response_id.py

@@ -0,0 +1,44 @@
+"""Tests for OpenAIServerConversationTracker.hydrate_from_state response_id seeding."""
+
+from typing import Any
+
+from agents.items import ModelResponse
+from agents.run_internal.oai_conversation import OpenAIServerConversationTracker
+from agents.usage import Usage
+
+
+def _make_response(response_id: str | None) -> ModelResponse:
+    response = object.__new__(ModelResponse)
+    response.output = []
+    response.usage = Usage()
+    response.response_id = response_id
+    return response
+
+
+def test_hydrate_from_state_uses_latest_non_none_response_id() -> None:
+    """Resume should chain to the most recent response_id, not None when last response lacks id.
+
+    A run might produce model responses across providers where some have no `response_id`
+    (e.g., a non-Responses fallback). `track_server_items` skips updates when response_id is
+    None, so live runs preserve the last known id. Resume hydration should match that
+    behavior — falling back to the last id-bearing response instead of forgetting the chain.
+    """
+    tracker = OpenAIServerConversationTracker(
+        conversation_id=None,
+        previous_response_id=None,
+        auto_previous_response_id=True,
+    )
+
+    responses: list[Any] = [
+        _make_response("resp_first"),
+        _make_response("resp_second"),
+        _make_response(None),
+    ]
+
+    tracker.hydrate_from_state(
+        original_input=[],
+        generated_items=[],
+        model_responses=responses,
+    )
+
+    assert tracker.previous_response_id == "resp_second"