fix(streaming): route mid-tool-call partial-stream-stub through length continuation (#31998) (#32012)

* fix(streaming): route mid-tool-call partial-stream-stub through length continuation (#31998)

When a stream stalls mid-tool-call (e.g. a large write_file), the
partial-stream-stub recovery used finish_reason='stop' which caused the
conversation loop to treat the turn as complete, returning only the
warning text. When users said 'continue', the model retried the same
large tool call, hit the same stale timeout, and looped indefinitely.

Changes:
- chat_completion_helpers.py: change _stub_finish_reason from 'stop' to
  'length' for mid-tool-call partials. The stub still has tool_calls=None
  so no tool auto-executes — the model gets a fresh API call through the
  existing length-continuation machinery (bounded to 3 retries).
  Also attach _dropped_tool_names to the stub for downstream use.
- conversation_loop.py: add a third continuation prompt branch for
  partial-stream-stubs with dropped tool calls. Instead of the generic
  'continue where you left off' (which would retry the same large call),
  tell the model to break the output into smaller tool calls (~8K
  tokens each) to avoid stream timeouts.
- test_partial_stream_finish_reason.py: update existing test from
  finish_reason='stop' to 'length', add _dropped_tool_names assertion,
  add new test_dropped_tool_call_uses_chunking_prompt for the 3-way
  prompt branching.

Safety: tool_calls=None is preserved on the stub, so the conversation
loop enters the text-continuation branch (line 1513), NOT the tool-call
execution branch (line 3246). No tool auto-executes. The model simply
gets another API call with targeted guidance.

* refactor: extract constants and continuation prompt helper

- Move magic strings to hermes_constants.py (PARTIAL_STREAM_STUB_ID,
  FINISH_REASON_LENGTH)
- Extract _get_continuation_prompt() in conversation_loop.py — DRYs the
  3-way prompt branching and lets tests import the real function
- Trim verbose inline comments in chat_completion_helpers.py
- Tests import constants + helper instead of duplicating logic

---------

Co-authored-by: alt-glitch <balyan.sid@gmail.com>

tests/run_agent/test_partial_stream_finish_reason.py

@@ -5,9 +5,9 @@ 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").
+- partial mid-tool-call → stub.finish_reason == "length" so the loop
+  triggers continuation machinery with targeted chunking guidance
+  instead of ending the turn immediately.
 - conversation_loop's length-continuation prompt distinguishes a real
   output-length truncation from a partial-stream-stub network error
   via response.id.
@@ -20,6 +20,9 @@ from unittest.mock import MagicMock, patch
 
 import pytest
 
+from hermes_constants import PARTIAL_STREAM_STUB_ID, FINISH_REASON_LENGTH
+from agent.conversation_loop import _get_continuation_prompt
+
 
 # ── Helpers (mirrors test_streaming.py) ────────────────────────────────────
 
@@ -78,8 +81,8 @@ class TestPartialStreamStubFinishReason:
         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", (
+        assert response.id == PARTIAL_STREAM_STUB_ID
+        assert response.choices[0].finish_reason == FINISH_REASON_LENGTH, (
             "Text-only partial streams must use finish_reason=length so the "
             "conversation loop continues from where the network died "
             "(issue #30963)."
@@ -89,9 +92,11 @@ class TestPartialStreamStubFinishReason:
 
     @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 test_partial_tool_call_uses_length(self, _mock_close, mock_create, monkeypatch):
+        """Mid-tool-call partials now use finish_reason=length so the
+        conversation loop's continuation machinery fires — bounded 3-retry
+        with guidance to break output into smaller chunks (#31998).
+        tool_calls=None is preserved, so no tool auto-executes."""
 
         def _stalling_stream():
             yield _make_stream_chunk(content="Let me write the audit: ")
@@ -114,12 +119,18 @@ class TestPartialStreamStubFinishReason:
         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."
+        assert response.id == PARTIAL_STREAM_STUB_ID
+        assert response.choices[0].finish_reason == FINISH_REASON_LENGTH, (
+            "Partial mid-tool-call must use finish_reason=length so the "
+            "continuation machinery fires instead of ending the turn "
+            "immediately (#31998)."
         )
+        assert response.choices[0].message.tool_calls is None, (
+            "tool_calls must remain None (no auto-execution of side-effectful "
+            "tool calls)."
+        )
+        # The stub should carry dropped tool names for continuation prompt
+        assert getattr(response, "_dropped_tool_names", None) == ["write_file"]
         content = response.choices[0].message.content or ""
         assert "Stream stalled mid tool-call" in content
         assert "write_file" in content
@@ -129,30 +140,17 @@ class TestPartialStreamStubFinishReason:
 
 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."""
+    model has to tell the truth: real truncation vs. network interruption
+    vs. dropped tool call (#31998).  Three distinct prompts now exist."""
 
-    def _simulate_branch(self, response_id: str) -> str:
+    def _simulate_branch(self, response_id: str, dropped_tools=None) -> 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.]"
-        )
+        a `finish_reason=length` response with the given id."""
+        is_partial = response_id == PARTIAL_STREAM_STUB_ID
+        return _get_continuation_prompt(is_partial, dropped_tools)
 
     def test_partial_stream_stub_uses_network_prompt(self):
-        prompt = self._simulate_branch("partial-stream-stub")
+        prompt = self._simulate_branch(PARTIAL_STREAM_STUB_ID)
         assert "network error mid-stream" in prompt
         assert "output length limit" not in prompt
 
@@ -165,6 +163,19 @@ class TestLengthContinuationPromptBranching:
         prompt = self._simulate_branch("")
         assert "output length limit" in prompt
 
+    def test_dropped_tool_call_uses_chunking_prompt(self):
+        """When the stub dropped a tool call, the continuation prompt
+        must guide the model to break its output into smaller chunks
+        instead of retrying the same large tool call (#31998)."""
+        prompt = self._simulate_branch(
+            PARTIAL_STREAM_STUB_ID, dropped_tools=["write_file"],
+        )
+        assert "too large" in prompt
+        assert "break" in prompt.lower()
+        assert "write_file" in prompt
+        assert "network error" not in prompt
+        assert "output length limit" not in prompt
+
 
 # ── Integration: live conversation loop ───────────────────────────────────
 
@@ -208,12 +219,12 @@ class TestConversationLoopPartialStreamContinuation:
 
         # First API call: the partial-stream stub (length on partial-stream-stub id).
         partial_stub = SimpleNamespace(
-            id="partial-stream-stub",
+            id=PARTIAL_STREAM_STUB_ID,
             model="test/model",
             choices=[SimpleNamespace(
                 index=0,
                 message=_mock_assistant_msg(content="The first half of "),
-                finish_reason="length",
+                finish_reason=FINISH_REASON_LENGTH,
             )],
             usage=None,
         )