core/agent: classify empty/truncated/missing stream terminals as errors

src/plyngent/agent/loop.py

@@ -114,6 +114,66 @@ async def _execute_tool_calls(
         yield ToolResultEvent(message=tool_msg)
 
 
+def _assistant_has_payload(assistant: AssistantChatMessage) -> bool:
+    """True if the model produced text, reasoning, or tool calls."""
+    if assistant.tool_calls is not UNSET and assistant.tool_calls:
+        return True
+    if isinstance(assistant.content, str) and assistant.content.strip():
+        return True
+    reasoning = assistant.reasoning_content
+    return bool(isinstance(reasoning, str) and reasoning.strip())
+
+
+def _finish_reason_value(finish: object) -> str | None:
+    if finish is UNSET or finish is None:
+        return None
+    if isinstance(finish, str) and finish.strip():
+        return finish.strip()
+    return None
+
+
+def _validate_assistant_terminal(
+    assistant: AssistantChatMessage,
+    *,
+    finish_reason: str | None,
+    stream_terminal: bool,
+) -> None:
+    """Raise when the round is not a usable agent stop.
+
+    Distinguishes empty generation, truncated/filtered stops, and missing
+    stream terminals (network/client glitch) from a normal stop/tool_calls end.
+    """
+    reason = (finish_reason or "").lower() or None
+
+    if reason in {"length", "content_filter"}:
+        label = "truncated (max tokens)" if reason == "length" else "content filter"
+        detail = ""
+        if isinstance(assistant.content, str) and assistant.content.strip():
+            detail = f"; partial text kept ({len(assistant.content)} chars)"
+        msg = f"model stopped early: {label}{detail}"
+        raise RuntimeError(msg)
+
+    if reason in {"failed", "incomplete", "cancelled"}:
+        msg = f"model response status: {reason}"
+        raise RuntimeError(msg)
+
+    if _assistant_has_payload(assistant):
+        return
+
+    if not stream_terminal and reason is None:
+        msg = (
+            "stream ended without a terminal signal (no finish_reason / response.completed) and empty assistant output"
+        )
+        raise RuntimeError(msg)
+
+    if reason in {"stop", "tool_calls", "function_call"} or reason is None:
+        msg = "empty model completion (no text, reasoning, or tool calls)"
+        raise RuntimeError(msg)
+
+    msg = f"empty model completion (finish_reason={reason})"
+    raise RuntimeError(msg)
+
+
 async def _non_stream_round(
     client: ChatClient,
     param: ChatCompletionsParam,
@@ -122,7 +182,14 @@ async def _non_stream_round(
     if not response.choices:
         msg = "chat completion response contained no choices"
         raise RuntimeError(msg)
-    assistant = response.choices[0].message
+    choice = response.choices[0]
+    assistant = choice.message
+    finish = _finish_reason_value(choice.finish_reason)
+    _validate_assistant_terminal(
+        assistant,
+        finish_reason=finish,
+        stream_terminal=True,
+    )
     reasoning = assistant.reasoning_content
     if isinstance(reasoning, str) and reasoning:
         yield ReasoningDeltaEvent(content=reasoning)
@@ -154,6 +221,8 @@ async def _stream_round(
     reasoning_parts: list[str] = []
     tool_deltas: list[StreamToolCallDelta] = []
     last_api_usage: object = UNSET
+    finish_reason: str | None = None
+    saw_terminal = False
 
     async for chunk in stream:
         parsed = token_usage_from_api(chunk.usage)
@@ -162,6 +231,10 @@ async def _stream_round(
         if not chunk.choices:
             continue
         choice = chunk.choices[0]
+        fr = _finish_reason_value(choice.finish_reason)
+        if fr is not None:
+            finish_reason = fr
+            saw_terminal = True
         delta = choice.delta
         if isinstance(delta.reasoning_content, str) and delta.reasoning_content:
             reasoning_parts.append(delta.reasoning_content)
@@ -185,6 +258,16 @@ async def _stream_round(
         tool_calls=tool_calls,
         reasoning_content=full_reasoning or UNSET,
     )
+    # Usage-only final chunks leave choices empty; a non-empty usage after
+    # content still is not a finish_reason. Prefer explicit finish_reason.
+    # If we got payload but no finish_reason, treat as terminal (some providers
+    # omit it); if empty and no finish_reason, flag as missing terminal.
+    stream_terminal = saw_terminal or _assistant_has_payload(assistant)
+    _validate_assistant_terminal(
+        assistant,
+        finish_reason=finish_reason,
+        stream_terminal=stream_terminal,
+    )
     yield AssistantMessageEvent(message=assistant)
     yield UsageEvent(
         usage=resolve_round_usage(last_api_usage, param.messages, assistant),

src/plyngent/agent/responses_bridge.py

@@ -158,10 +158,36 @@ def _reasoning_summary_text(response: Response) -> str:
     return "".join(parts)
 
 
+def responses_status_to_finish_reason(
+    response: Response,
+    *,
+    has_tool_calls: bool,
+) -> str:
+    """Map Responses ``status`` to a chat-style finish_reason for the agent loop."""
+    status = response.status
+    status_s = status if isinstance(status, str) else None
+    if status_s == "incomplete":
+        details = response.incomplete_details
+        reason = None
+        if details is not UNSET and details is not None:
+            raw_reason = details.reason
+            if raw_reason is not UNSET and isinstance(raw_reason, str):
+                reason = raw_reason
+        if reason == "content_filter":
+            return "content_filter"
+        return "length"
+    if status_s in {"failed", "cancelled"}:
+        return status_s
+    if has_tool_calls:
+        return "tool_calls"
+    return "stop"
+
+
 def response_to_chat_completion(response: Response) -> ChatCompletionResponse:
     """Wrap Responses result as a synthetic chat completion for the agent loop."""
     assistant = response_to_assistant_message(response)
-    finish: str | None = "tool_calls" if assistant.tool_calls is not UNSET else "stop"
+    has_tools = assistant.tool_calls is not UNSET and bool(assistant.tool_calls)
+    finish = responses_status_to_finish_reason(response, has_tool_calls=has_tools)
     usage = response.usage if response.usage is not UNSET else UNSET
     created = int(response.created_at)
     return ChatCompletionResponse(
@@ -180,6 +206,28 @@ def response_to_chat_completion(response: Response) -> ChatCompletionResponse:
     )
 
 
+def finish_reason_chunk(
+    *,
+    model: str,
+    finish_reason: str,
+    created: int = 0,
+) -> ChatCompletionChunk:
+    """Terminal stream chunk carrying only ``finish_reason`` (no delta text)."""
+    return ChatCompletionChunk(
+        id="resp-stream",
+        object="chat.completion.chunk",
+        created=created,
+        model=model,
+        choices=[
+            ChunkChoice(
+                index=0,
+                delta=DeltaMessage(),
+                finish_reason=cast("Any", finish_reason),
+            )
+        ],
+    )
+
+
 def text_delta_chunk(*, model: str, content: str, created: int = 0) -> ChatCompletionChunk:
     return ChatCompletionChunk(
         id="resp-stream",

src/plyngent/agent/responses_client.py

@@ -8,8 +8,11 @@ from msgspec import UNSET
 
 from plyngent.agent.responses_bridge import (
     chat_param_to_responses_kwargs,
+    finish_reason_chunk,
     reasoning_delta_chunk,
+    response_to_assistant_message,
     response_to_chat_completion,
+    responses_status_to_finish_reason,
     text_delta_chunk,
     tool_call_chunks_from_response,
     usage_chunk_from_response,
@@ -109,12 +112,20 @@ class ResponsesChatClient:
                 except TypeError, ValueError, msgspec.ValidationError:
                     final = None
 
-        if final is not None:
+        if final is None:
-            for chunk in tool_call_chunks_from_response(final, model=model):
+            # Stream ended without response.completed — signal missing terminal.
-                yield chunk
+            # Loop will treat empty + no finish_reason as a glitch.
-            usage = usage_chunk_from_response(final, model=model)
+            return
-            if usage is not None:
+
-                yield usage
+        assistant = response_to_assistant_message(final)
+        has_tools = assistant.tool_calls is not UNSET and bool(assistant.tool_calls)
+        finish = responses_status_to_finish_reason(final, has_tool_calls=has_tools)
+        for chunk in tool_call_chunks_from_response(final, model=model):
+            yield chunk
+        yield finish_reason_chunk(model=model, finish_reason=finish)
+        usage = usage_chunk_from_response(final, model=model)
+        if usage is not None:
+            yield usage
 
 
 def wrap_openai_for_agent(

tests/test_agent/test_loop.py

@@ -61,6 +61,28 @@ def _chunks_from_response(response: ChatCompletionResponse) -> list[ChatCompleti
                 ],
             )
         )
+    # Emit terminal finish_reason when content was streamed without one.
+    fr = response.choices[0].finish_reason
+    if (
+        fr
+        and (isinstance(message.content, str) and message.content)
+        and (message.tool_calls is UNSET or not message.tool_calls)
+    ):
+        chunks.append(
+            ChatCompletionChunk(
+                id=response.id,
+                object="chat.completion.chunk",
+                created=response.created,
+                model=response.model,
+                choices=[
+                    ChunkChoice(
+                        index=0,
+                        delta=DeltaMessage(),
+                        finish_reason=fr,
+                    )
+                ],
+            )
+        )
     tool_calls = message.tool_calls
     if tool_calls is not UNSET and tool_calls:
         deltas: list[StreamToolCallDelta] = []
@@ -153,6 +175,7 @@ def _response(
     message: AssistantChatMessage,
     *,
     usage: dict[str, int] | None = None,
+    finish_reason: str | None = "stop",
 ) -> ChatCompletionResponse:
     return ChatCompletionResponse(
         id="1",
@@ -164,7 +187,7 @@ def _response(
                 index=0,
                 message=message,
                 logprobs={},
-                finish_reason="stop",
+                finish_reason=finish_reason,
             )
         ],
         system_fingerprint="",
@@ -825,3 +848,118 @@ async def test_tool_result_char_budget() -> None:
     assert len(tool_msgs) == 1
     assert tool_msgs[0].content.startswith("x" * 20)
     assert "truncated" in tool_msgs[0].content
+
+
+async def test_empty_completion_raises() -> None:
+    client = ScriptedClient([_response(AssistantChatMessage(content=None))])
+    messages: list[AnyChatMessage] = [UserChatMessage(content="hi")]
+    with pytest.raises(RuntimeError, match="empty model completion"):
+        _ = [e async for e in run_chat_loop(client, messages, model="m", stream=False)]
+
+
+async def test_length_finish_raises() -> None:
+    client = ScriptedClient(
+        [
+            _response(
+                AssistantChatMessage(content="partial"),
+                finish_reason="length",
+            )
+        ]
+    )
+    messages: list[AnyChatMessage] = [UserChatMessage(content="hi")]
+    with pytest.raises(RuntimeError, match="truncated"):
+        _ = [e async for e in run_chat_loop(client, messages, model="m", stream=False)]
+
+
+async def test_content_filter_finish_raises() -> None:
+    client = ScriptedClient(
+        [
+            _response(
+                AssistantChatMessage(content=""),
+                finish_reason="content_filter",
+            )
+        ]
+    )
+    messages: list[AnyChatMessage] = [UserChatMessage(content="hi")]
+    with pytest.raises(RuntimeError, match="content filter"):
+        _ = [e async for e in run_chat_loop(client, messages, model="m", stream=False)]
+
+
+async def test_stream_missing_terminal_empty_raises() -> None:
+    class EmptyStreamClient:
+        @overload
+        async def chat_completions(
+            self, param: ChatCompletionsParam, *, stream: Literal[False] = False
+        ) -> ChatCompletionResponse: ...
+
+        @overload
+        async def chat_completions(
+            self, param: ChatCompletionsParam, *, stream: Literal[True]
+        ) -> AsyncIterator[ChatCompletionChunk]: ...
+
+        async def chat_completions(
+            self, param: ChatCompletionsParam, *, stream: bool = False
+        ) -> ChatCompletionResponse | AsyncIterator[ChatCompletionChunk]:
+            del param
+            if not stream:
+                return _response(AssistantChatMessage(content="x"))
+
+            async def chunks() -> AsyncIterator[ChatCompletionChunk]:
+                # Usage-only style chunk with no finish_reason and no content.
+                yield ChatCompletionChunk(
+                    id="1",
+                    object="chat.completion.chunk",
+                    created=0,
+                    model="t",
+                    choices=[],
+                    usage={"prompt_tokens": 1, "completion_tokens": 0, "total_tokens": 1},
+                )
+
+            return chunks()
+
+    messages: list[AnyChatMessage] = [UserChatMessage(content="hi")]
+    with pytest.raises(RuntimeError, match="stream ended without a terminal"):
+        _ = [e async for e in run_chat_loop(EmptyStreamClient(), messages, model="m", stream=True)]
+
+
+async def test_stream_payload_without_finish_reason_ok() -> None:
+    """Some providers omit finish_reason; non-empty text still counts as terminal."""
+
+    class PayloadNoFinishClient:
+        @overload
+        async def chat_completions(
+            self, param: ChatCompletionsParam, *, stream: Literal[False] = False
+        ) -> ChatCompletionResponse: ...
+
+        @overload
+        async def chat_completions(
+            self, param: ChatCompletionsParam, *, stream: Literal[True]
+        ) -> AsyncIterator[ChatCompletionChunk]: ...
+
+        async def chat_completions(
+            self, param: ChatCompletionsParam, *, stream: bool = False
+        ) -> ChatCompletionResponse | AsyncIterator[ChatCompletionChunk]:
+            del param
+            if not stream:
+                return _response(AssistantChatMessage(content="ok"))
+
+            async def chunks() -> AsyncIterator[ChatCompletionChunk]:
+                yield ChatCompletionChunk(
+                    id="1",
+                    object="chat.completion.chunk",
+                    created=0,
+                    model="t",
+                    choices=[
+                        ChunkChoice(
+                            index=0,
+                            delta=DeltaMessage(content="ok"),
+                            finish_reason=None,
+                        )
+                    ],
+                )
+
+            return chunks()
+
+    messages: list[AnyChatMessage] = [UserChatMessage(content="hi")]
+    events = [e async for e in run_chat_loop(PayloadNoFinishClient(), messages, model="m", stream=True)]
+    assert any(isinstance(e, TextDeltaEvent) and e.content == "ok" for e in events)

tests/test_agent/test_responses_bridge.py

@@ -8,6 +8,7 @@ from plyngent.agent.responses_bridge import (
     chat_param_to_responses_kwargs,
     response_to_assistant_message,
     response_to_chat_completion,
+    responses_status_to_finish_reason,
     tool_items_to_response_tools,
 )
 from plyngent.agent.responses_client import ResponsesChatClient
@@ -100,6 +101,31 @@ def test_response_to_assistant_with_tools() -> None:
     assert completion.choices[0].message.content == "done"
     assert isinstance(completion.usage, dict)
     assert completion.usage["input_tokens"] == 10
+    assert completion.choices[0].finish_reason == "tool_calls"
+
+
+def test_responses_status_to_finish_reason_incomplete() -> None:
+    body = {
+        "id": "resp_i",
+        "object": "response",
+        "created_at": 1,
+        "model": "gpt-test",
+        "status": "incomplete",
+        "incomplete_details": {"reason": "max_output_tokens"},
+        "output": [
+            {
+                "id": "msg_1",
+                "type": "message",
+                "role": "assistant",
+                "status": "incomplete",
+                "content": [{"type": "output_text", "text": "partial", "annotations": []}],
+            }
+        ],
+    }
+    resp = msgspec.convert(body, Response)
+    assert responses_status_to_finish_reason(resp, has_tool_calls=False) == "length"
+    completion = response_to_chat_completion(resp)
+    assert completion.choices[0].finish_reason == "length"
 
 
 def test_chat_param_to_responses_kwargs() -> None: