test(streaming): pin partial-stream-stub finish_reason + continuation contract

Three test classes lock in the #30963 fix:

1. TestPartialStreamStubFinishReason — drives _interruptible_streaming_api_call
   through the two recovery branches and asserts:
     - text-only partial → finish_reason="length" (the new behaviour),
     - mid-tool-call partial → finish_reason="stop" (unchanged on purpose).

2. TestLengthContinuationPromptBranching — pure-Python check on the branch
   that picks the continuation prompt by response.id. Locks the network
   error wording for partial-stream-stub vs. the output-length wording
   for everything else.

3. TestConversationLoopPartialStreamContinuation — feeds a stub +
   continuation pair into run_conversation, verifies the loop makes a
   second API call (instead of exiting with text_response(stop)),
   confirms the network-error continuation prompt actually reaches the
   model on call #2, and that final_response stitches both halves.

Refs: NousResearch/hermes-agent#30963

tests/run_agent/test_partial_stream_finish_reason.py

@@ -0,0 +1,258 @@
+"""Regression tests for issue #30963 — partial-stream stub finish_reason.
+
+Pins the contract:
+
+- text-only partial stream → stub.finish_reason == "length" so the
+  conversation loop's existing length-continuation path can keep the
+  agent moving against an unfinished goal.
+- partial mid-tool-call → stub.finish_reason == "stop" so the loop
+  hands control back to the user (matches the user-visible warning
+  "Ask me to retry if you want to continue").
+- conversation_loop's length-continuation prompt distinguishes a real
+  output-length truncation from a partial-stream-stub network error
+  via response.id.
+"""
+
+from __future__ import annotations
+
+from types import SimpleNamespace
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+
+# ── Helpers (mirrors test_streaming.py) ────────────────────────────────────
+
+def _make_stream_chunk(content=None, tool_calls=None, finish_reason=None):
+    delta = SimpleNamespace(
+        content=content, tool_calls=tool_calls,
+        reasoning_content=None, reasoning=None,
+    )
+    choice = SimpleNamespace(index=0, delta=delta, finish_reason=finish_reason)
+    return SimpleNamespace(choices=[choice], model=None, usage=None)
+
+
+def _make_tool_call_delta(index=0, tc_id=None, name=None, arguments=None):
+    func = SimpleNamespace(name=name, arguments=arguments)
+    return SimpleNamespace(index=index, id=tc_id, function=func)
+
+
+def _make_agent():
+    from run_agent import AIAgent
+    agent = AIAgent(
+        api_key="test-key",
+        base_url="https://example.com/v1",
+        model="test/model",
+        quiet_mode=True,
+        skip_context_files=True,
+        skip_memory=True,
+    )
+    agent.api_mode = "chat_completions"
+    agent._interrupt_requested = False
+    return agent
+
+
+# ── Stub finish_reason ────────────────────────────────────────────────────
+
+class TestPartialStreamStubFinishReason:
+    """The stub returned by interruptible_streaming_api_call when the
+    upstream connection dies mid-flight."""
+
+    @patch("run_agent.AIAgent._create_request_openai_client")
+    @patch("run_agent.AIAgent._close_request_openai_client")
+    def test_text_only_partial_returns_length(self, _mock_close, mock_create, monkeypatch):
+        """#30963: text-only partials must classify as length so the loop
+        keeps continuing instead of exiting with budget remaining."""
+
+        def _stalling_stream():
+            yield _make_stream_chunk(content="Here's my answer so far")
+            raise RuntimeError("simulated upstream stall")
+
+        mock_client = MagicMock()
+        mock_client.chat.completions.create.side_effect = lambda *a, **kw: _stalling_stream()
+        mock_create.return_value = mock_client
+
+        agent = _make_agent()
+        agent._current_streamed_assistant_text = "Here's my answer so far"
+
+        monkeypatch.setenv("HERMES_STREAM_RETRIES", "0")
+        response = agent._interruptible_streaming_api_call({})
+
+        assert response.id == "partial-stream-stub"
+        assert response.choices[0].finish_reason == "length", (
+            "Text-only partial streams must use finish_reason=length so the "
+            "conversation loop continues from where the network died "
+            "(issue #30963)."
+        )
+        assert response.choices[0].message.content == "Here's my answer so far"
+        assert response.choices[0].message.tool_calls is None
+
+    @patch("run_agent.AIAgent._create_request_openai_client")
+    @patch("run_agent.AIAgent._close_request_openai_client")
+    def test_partial_tool_call_keeps_stop(self, _mock_close, mock_create, monkeypatch):
+        """Mid-tool-call partials keep finish_reason=stop on purpose — the
+        warning text asks the user to drive the retry, not the agent."""
+
+        def _stalling_stream():
+            yield _make_stream_chunk(content="Let me write the audit: ")
+            yield _make_stream_chunk(tool_calls=[
+                _make_tool_call_delta(index=0, tc_id="call_1", name="write_file"),
+            ])
+            yield _make_stream_chunk(tool_calls=[
+                _make_tool_call_delta(index=0, arguments='{"path": "/tmp/x", '),
+            ])
+            raise RuntimeError("simulated upstream stall")
+
+        mock_client = MagicMock()
+        mock_client.chat.completions.create.side_effect = lambda *a, **kw: _stalling_stream()
+        mock_create.return_value = mock_client
+
+        agent = _make_agent()
+        agent._fire_stream_delta = lambda text: None
+        agent._current_streamed_assistant_text = "Let me write the audit: "
+
+        monkeypatch.setenv("HERMES_STREAM_RETRIES", "0")
+        response = agent._interruptible_streaming_api_call({})
+
+        assert response.id == "partial-stream-stub"
+        assert response.choices[0].finish_reason == "stop", (
+            "Partial mid-tool-call must keep finish_reason=stop — the warning "
+            "appended to content asks the user to retry, so the agent must "
+            "not auto-replay a tool call with possible side-effects."
+        )
+        content = response.choices[0].message.content or ""
+        assert "Stream stalled mid tool-call" in content
+        assert "write_file" in content
+
+
+# ── Length-continuation prompt branching ──────────────────────────────────
+
+class TestLengthContinuationPromptBranching:
+    """When finish_reason=length, the continuation prompt that reaches the
+    model has to tell the truth: real truncation vs. network interruption.
+    Lying ("you were truncated") on a partial-stream stub leads the model
+    to no-op ("I wasn't truncated, I'm done"), defeating recovery."""
+
+    def _simulate_branch(self, response_id: str) -> str:
+        """Return the continuation prompt text the loop would inject for
+        a `finish_reason=length` response with the given id. Mirrors the
+        exact branch in agent/conversation_loop.py."""
+        response = SimpleNamespace(id=response_id)
+        if getattr(response, "id", "") == "partial-stream-stub":
+            return (
+                "[System: The previous response was cut off by a "
+                "network error mid-stream. Continue exactly where "
+                "you left off. Do not restart or repeat prior text. "
+                "Finish the answer directly.]"
+            )
+        return (
+            "[System: Your previous response was truncated by the output "
+            "length limit. Continue exactly where you left off. Do not "
+            "restart or repeat prior text. Finish the answer directly.]"
+        )
+
+    def test_partial_stream_stub_uses_network_prompt(self):
+        prompt = self._simulate_branch("partial-stream-stub")
+        assert "network error mid-stream" in prompt
+        assert "output length limit" not in prompt
+
+    def test_real_truncation_uses_length_prompt(self):
+        prompt = self._simulate_branch("chatcmpl-abc123")
+        assert "output length limit" in prompt
+        assert "network error" not in prompt
+
+    def test_no_id_falls_through_to_length_prompt(self):
+        prompt = self._simulate_branch("")
+        assert "output length limit" in prompt
+
+
+# ── Integration: live conversation loop ───────────────────────────────────
+
+@pytest.fixture()
+def loop_agent():
+    """AIAgent with a mocked OpenAI client (mirrors test_run_agent's fixture)
+    so we can stage a stub + continuation pair on .chat.completions.create."""
+    from run_agent import AIAgent
+    with (
+        patch("run_agent.get_tool_definitions", return_value=[]),
+        patch("run_agent.check_toolset_requirements", return_value={}),
+        patch("run_agent.OpenAI"),
+    ):
+        a = AIAgent(
+            api_key="test-key-1234567890",
+            base_url="https://openrouter.ai/api/v1",
+            quiet_mode=True,
+            skip_context_files=True,
+            skip_memory=True,
+        )
+        a.client = MagicMock()
+        a._cached_system_prompt = "You are helpful."
+        a._use_prompt_caching = False
+        a.tool_delay = 0
+        a.compression_enabled = False
+        a.save_trajectories = False
+        return a
+
+
+class TestConversationLoopPartialStreamContinuation:
+    """End-to-end: a partial-stream stub feeds the loop and the loop
+    asks for continuation instead of exiting with finish_reason=stop."""
+
+    def test_partial_stream_stub_does_not_exit_loop_immediately(self, loop_agent):
+        """The stub from chat_completion_helpers used to exit the loop with
+        text_response(finish_reason=stop). Now finish_reason=length routes
+        through length_continue_retries — the loop persists the partial
+        content and asks the model to continue."""
+
+        from tests.run_agent.test_run_agent import _mock_response, _mock_assistant_msg
+
+        # First API call: the partial-stream stub (length on partial-stream-stub id).
+        partial_stub = SimpleNamespace(
+            id="partial-stream-stub",
+            model="test/model",
+            choices=[SimpleNamespace(
+                index=0,
+                message=_mock_assistant_msg(content="The first half of "),
+                finish_reason="length",
+            )],
+            usage=None,
+        )
+        # Second API call: model continues with the rest, clean stop.
+        continuation = _mock_response(
+            content="the answer is forty-two.", finish_reason="stop",
+        )
+
+        loop_agent.client.chat.completions.create.side_effect = [
+            partial_stub, continuation,
+        ]
+
+        with (
+            patch.object(loop_agent, "_persist_session"),
+            patch.object(loop_agent, "_save_trajectory"),
+            patch.object(loop_agent, "_cleanup_task_resources"),
+        ):
+            result = loop_agent.run_conversation("ask me something")
+
+        # The loop made TWO API calls (stub + continuation), not one.
+        assert loop_agent.client.chat.completions.create.call_count == 2, (
+            "Partial-stream-stub must trigger a continuation API call, not "
+            "exit the loop after one call."
+        )
+        # The continuation prompt the loop appended must be the network-error
+        # variant, not the "output length limit" lie — otherwise the model
+        # no-ops with "I wasn't truncated, I'm done."
+        # We assert it indirectly by inspecting the second-call kwargs.
+        second_call_kwargs = loop_agent.client.chat.completions.create.call_args_list[1]
+        msgs = second_call_kwargs.kwargs.get("messages") or second_call_kwargs.args[0].get("messages")
+        last_user = next(
+            (m for m in reversed(msgs) if m.get("role") == "user"), None,
+        )
+        assert last_user is not None
+        assert "network error mid-stream" in (last_user.get("content") or ""), (
+            "Continuation prompt for partial-stream-stub must mention the "
+            "network error, not the 'output length limit'."
+        )
+
+        # And the final response stitches both halves together.
+        assert "first half of" in result["final_response"]
+        assert "forty-two" in result["final_response"]