fix(v2): retain legacy function_call in dump_message when content is empty

CHANGELOG.md

@@ -10,6 +10,7 @@ Versioning: [Semantic Versioning](https://semver.org/spec/v2.0.0.html)
 ## [Unreleased]
 
 ### Fixed
+- **v2 message handling**: Retain legacy `function_call` payloads in `dump_message` when the assistant message has empty or `None` content, so retry/reask flows no longer drop the model's prior function call. ([#2464](https://github.com/567-labs/instructor/issues/2464))
 - **v2 message handling**: Preserve caller-owned message lists and nested content across request preparation and retries for OpenAI-compatible, Cohere, Mistral, OpenRouter, Writer, and xAI handlers. ([#2417](https://github.com/567-labs/instructor/issues/2417), [#2428](https://github.com/567-labs/instructor/issues/2428))
 - **v2 JSON extraction**: Prefer the final complete top-level JSON value in text responses and retain every JSON object when multiple objects arrive in one streaming chunk.
 - **v2 schemas**: Treat fields with Pydantic `default_factory` values as optional in generated OpenAI tool schemas.

instructor/v2/core/messages.py

@@ -45,11 +45,7 @@ def dump_message(message: ChatCompletionMessage) -> ChatCompletionMessageParam:
     }
     if hasattr(message, "tool_calls") and message.tool_calls is not None:
         ret["tool_calls"] = message.model_dump()["tool_calls"]
-    if (
-        hasattr(message, "function_call")
-        and message.function_call is not None
-        and ret["content"]
-    ):
+    if hasattr(message, "function_call") and message.function_call is not None:
         if not isinstance(ret["content"], str):
             response_message = ""
             for content_message in ret["content"]:

tests/coverage/test_core_helpers_coverage.py

@@ -115,6 +115,18 @@ def test_dump_message_appends_function_call_to_text_content() -> None:
     )
 
 
+def test_dump_message_retains_function_call_when_content_is_empty() -> None:
+    message = ChatCompletionMessage(
+        role="assistant",
+        content=None,
+        function_call=FunctionCall(name="lookup", arguments='{"id":7}'),
+    )
+
+    result = dump_message(message)
+
+    assert result["content"] == json.dumps({"arguments": '{"id":7}', "name": "lookup"})
+
+
 def test_merge_consecutive_messages_checks_tail_for_non_string_content() -> None:
     messages: list[dict[str, Any]] = [
         {"role": "user", "content": f"line {index}"} for index in range(10)