diff --git a/backend/packages/harness/deerflow/runtime/AGENTS.md b/backend/packages/harness/deerflow/runtime/AGENTS.md
index 4da7ab93e..d77bc9f23 100644
--- a/backend/packages/harness/deerflow/runtime/AGENTS.md
+++ b/backend/packages/harness/deerflow/runtime/AGENTS.md
@@ -61,6 +61,25 @@ fetch-and-decode of every message row's tool outputs on long threads.
 client input, because a welded-in seq goes stale when a fork re-seeds the feed
 (#4380).
 
+**LLM response callback coalescing** (`runtime/journal.py`): a provider may fire
+`on_llm_end` twice for one LangChain run id, first without usage (or with all token
+counts zero) and immediately again with usage populated. The first callback's generation
+set is always canonical: `RunJournal` stages only its response events and immutable
+message-summary fields while retaining the first caller, and applies that callback's
+fallback state and tool-call bookkeeping immediately; those effects remain canonical.
+It must not retain provider-owned message objects because a provider may mutate and
+reuse the same response for the usage replay. Usage metadata is deep-snapshotted,
+including nested token-detail mappings, before it enters a staged or buffered event.
+An adjacent same-id positive-usage replay may enrich only each corresponding staged
+event's metadata/content usage fields. Replay
+generation-count differences never add, remove, or replace canonical messages. The next
+unrelated event, an effective buffer size (committed plus pending events) reaching the
+flush threshold, or an explicit flush commits the staged unit and updates the message
+summary. Once that ordering boundary is crossed, a late usage replay can still update the
+authoritative run token summary, but it cannot mutate the append-only message event,
+caller attribution, fallback state, or tool-call bookkeeping. Closed journals return
+from `on_llm_end` before inspecting the response or touching any run state.
+
 **Run delivery receipts** (`runtime/journal.py` + `runs/worker.py`):
 `RunJournal` records each non-empty artifact update once per tool `Command` for
 the terminal `run.delivery` event. When a command contains multiple messages, a
diff --git a/backend/packages/harness/deerflow/runtime/journal.py b/backend/packages/harness/deerflow/runtime/journal.py
index 1a5945889..3aa4ff446 100644
--- a/backend/packages/harness/deerflow/runtime/journal.py
+++ b/backend/packages/harness/deerflow/runtime/journal.py
@@ -22,6 +22,8 @@ import logging
 import threading
 import time
 from collections.abc import Awaitable, Callable, Iterable, Mapping, Sequence
+from copy import deepcopy
+from dataclasses import dataclass
 from datetime import UTC, datetime
 from typing import TYPE_CHECKING, Any, cast
 from uuid import UUID
@@ -53,6 +55,14 @@ _LEGACY_SUMMARY_MESSAGE_NAME = "summary"
 _PERSISTED_HIDDEN_HUMAN_INPUT_RESPONSE_SOURCES = frozenset({"ask_clarification", "sandbox_network"})
 
 
+@dataclass
+class _PendingLlmResponse:
+    llm_run_id: str
+    events: list[dict]
+    message_count: int
+    last_ai_message: str | None
+
+
 def _should_persist_human_input_message(message: BaseMessage) -> bool:
     if not isinstance(message, HumanMessage):
         return False
@@ -243,6 +253,7 @@ class RunJournal(BaseCallbackHandler):
 
         # Write buffer
         self._buffer: list[dict] = []
+        self._pending_llm_response: _PendingLlmResponse | None = None
         self._pending_flush_tasks: set[asyncio.Task[None]] = set()
         self._pending_progress_task: asyncio.Task[None] | None = None
         self._pending_progress_delayed = False
@@ -267,6 +278,7 @@ class RunJournal(BaseCallbackHandler):
         self._counted_llm_run_ids: set[str] = set()
         self._counted_external_source_ids: set[str] = set()
         self._counted_message_llm_run_ids: set[str] = set()
+        self._llm_response_callers: dict[str, str] = {}
         self._memory_context_recorded = False
         self._tool_promotion_claim_lock = threading.Lock()
         self._claimed_tool_promotions: set[str] = set()
@@ -306,6 +318,14 @@ class RunJournal(BaseCallbackHandler):
         """Extract displayable text from a message's mixed content shape."""
         return message_to_text(message, text_attribute_fallback=True)
 
+    def _message_summary_text(self, message: BaseMessage, *, caller: str | None = None) -> str | None:
+        """Return the bounded user-facing AI summary text for one message."""
+        is_ai_message = isinstance(message, AIMessage) or getattr(message, "type", None) == "ai"
+        if not is_ai_message or (caller is not None and caller != "lead_agent"):
+            return None
+        text = self._message_text(message).strip()
+        return text[:2000] if text else None
+
     def _record_message_summary(self, message: BaseMessage, *, caller: str | None = None) -> None:
         """Update run-level convenience fields for persisted run rows."""
         self._msg_count += 1
@@ -313,11 +333,9 @@ class RunJournal(BaseCallbackHandler):
         # ``last_ai_message`` should represent the lead agent's user-facing
         # answer. Middleware/subagent model calls and empty tool-call-only
         # AI messages must not overwrite the last useful assistant text.
-        is_ai_message = isinstance(message, AIMessage) or getattr(message, "type", None) == "ai"
-        if is_ai_message and (caller is None or caller == "lead_agent"):
-            text = self._message_text(message).strip()
-            if text:
-                self._last_ai_msg = text[:2000]
+        summary_text = self._message_summary_text(message, caller=caller)
+        if summary_text is not None:
+            self._last_ai_msg = summary_text
 
     def on_chain_start(
         self,
@@ -433,7 +451,16 @@ class RunJournal(BaseCallbackHandler):
         tags: list[str] | None = None,
         **kwargs: Any,
     ) -> None:
+        if self._closed:
+            return
+
         messages: list[AnyMessage] = []
+        response_events: list[dict] = []
+        should_schedule_progress = False
+        rid = str(run_id)
+        callback_caller = self._identify_caller(tags)
+        is_canonical_callback = rid not in self._counted_message_llm_run_ids
+        caller = self._llm_response_callers.get(rid, callback_caller)
         logger.debug("on_llm_end %s: tags=%s", run_id, tags)
         for generation in response.generations:
             for gen in generation:
@@ -443,19 +470,20 @@ class RunJournal(BaseCallbackHandler):
                     logger.warning(f"on_llm_end {run_id}: generation has no message attribute: {gen}")
 
         for message in messages:
-            caller = self._identify_caller(tags)
-            self._remember_current_run_tool_calls(message, caller=caller)
+            if is_canonical_callback:
+                self._remember_current_run_tool_calls(message, caller=caller)
 
             # Latency
-            rid = str(run_id)
             start = self._llm_start_times.pop(rid, None)
             latency_ms = int((time.monotonic() - start) * 1000) if start else None
 
             # Token usage from message
             usage = getattr(message, "usage_metadata", None)
-            usage_dict = dict(usage) if usage else {}
+            # Providers may mutate and reuse the same response object after the
+            # callback returns, including nested token-detail mappings.
+            usage_dict = deepcopy(dict(usage)) if usage else {}
             additional_kwargs = getattr(message, "additional_kwargs", None) or {}
-            if isinstance(additional_kwargs, dict) and additional_kwargs.get("deerflow_error_fallback"):
+            if is_canonical_callback and isinstance(additional_kwargs, dict) and additional_kwargs.get("deerflow_error_fallback"):
                 self._had_llm_error_fallback = True
                 detail = additional_kwargs.get("error_detail")
                 reason = additional_kwargs.get("error_reason")
@@ -475,20 +503,19 @@ class RunJournal(BaseCallbackHandler):
                 call_index = self._llm_call_index
                 self._seen_llm_starts.add(rid)
 
-            # Message event: checkpoint-aligned llm.ai.response payload.
-            self._put(
-                event_type=LLM_AI_RESPONSE_EVENT.event_type,
-                category=LLM_AI_RESPONSE_EVENT.category,
-                content=message.model_dump(),
-                metadata={
-                    "caller": caller,
-                    "usage": usage_dict,
-                    "latency_ms": latency_ms,
-                    "llm_call_index": call_index,
-                },
+            response_events.append(
+                self._make_event(
+                    event_type=LLM_AI_RESPONSE_EVENT.event_type,
+                    category=LLM_AI_RESPONSE_EVENT.category,
+                    content=message.model_dump(),
+                    metadata={
+                        "caller": caller,
+                        "usage": usage_dict,
+                        "latency_ms": latency_ms,
+                        "llm_call_index": call_index,
+                    },
+                )
             )
-            if rid not in self._counted_message_llm_run_ids:
-                self._record_message_summary(message, caller=caller)
 
             # Token accumulation (dedup by langchain run_id to avoid double-counting
             # when the callback fires more than once for the same response)
@@ -519,10 +546,18 @@ class RunJournal(BaseCallbackHandler):
                         per_call_model = response_metadata.get("model_name") or response_metadata.get("model")
                     self._record_model_usage(per_call_model, input_tk, output_tk, total_tk, self._extract_cache_read(usage_dict))
 
-                    self._schedule_progress_flush()
+                    should_schedule_progress = True
 
         if messages:
-            self._counted_message_llm_run_ids.add(str(run_id))
+            self._queue_llm_response_events(
+                str(run_id),
+                response_events,
+                messages,
+                caller=caller,
+            )
+
+        if should_schedule_progress:
+            self._schedule_progress_flush()
 
     def on_llm_error(self, error: BaseException, *, run_id: UUID, **kwargs: Any) -> None:
         self._llm_start_times.pop(str(run_id), None)
@@ -661,20 +696,123 @@ class RunJournal(BaseCallbackHandler):
             if self._should_reconcile_tool_message(message):
                 self._persist_tool_result_message(message)
 
+    def _make_event(self, *, event_type: str, category: str, content: str | dict = "", metadata: dict | None = None) -> dict:
+        return {
+            "thread_id": self.thread_id,
+            "run_id": self.run_id,
+            "event_type": event_type,
+            "category": category,
+            "content": content,
+            "metadata": metadata or {},
+            "created_at": datetime.now(UTC).isoformat(),
+        }
+
+    def _commit_pending_llm_response(self) -> None:
+        pending = self._pending_llm_response
+        if pending is None:
+            return
+        self._pending_llm_response = None
+        self._buffer.extend(pending.events)
+        self._msg_count += pending.message_count
+        if pending.last_ai_message is not None:
+            self._last_ai_msg = pending.last_ai_message
+
+    def _snapshot_message_summary(self, messages: Sequence[AnyMessage], *, caller: str) -> tuple[int, str | None]:
+        """Freeze summary fields before a provider can mutate replayed messages."""
+        last_ai_message: str | None = None
+        for message in messages:
+            summary_text = self._message_summary_text(message, caller=caller)
+            if summary_text is not None:
+                last_ai_message = summary_text
+        return len(messages), last_ai_message
+
+    @staticmethod
+    def _has_positive_usage(events: list[dict]) -> bool:
+        for event in events:
+            usage = event["metadata"].get("usage")
+            if not isinstance(usage, Mapping):
+                continue
+            for key in ("input_tokens", "output_tokens", "total_tokens"):
+                try:
+                    if int(usage.get(key) or 0) > 0:
+                        return True
+                except (TypeError, ValueError):
+                    continue
+        return False
+
+    @staticmethod
+    def _merge_response_event_usage(canonical_events: list[dict], replay_events: list[dict]) -> None:
+        """Enrich canonical generation events with only replayed usage fields."""
+        for canonical, replay in zip(canonical_events, replay_events, strict=False):
+            replay_metadata = replay.get("metadata")
+            if isinstance(replay_metadata, Mapping):
+                replay_usage = replay_metadata.get("usage")
+                if isinstance(replay_usage, Mapping):
+                    canonical["metadata"]["usage"] = deepcopy(dict(replay_usage))
+
+            canonical_content = canonical.get("content")
+            replay_content = replay.get("content")
+            if isinstance(canonical_content, dict) and isinstance(replay_content, Mapping) and "usage_metadata" in replay_content:
+                replay_content_usage = replay_content.get("usage_metadata")
+                canonical_content["usage_metadata"] = deepcopy(dict(replay_content_usage)) if isinstance(replay_content_usage, Mapping) else replay_content_usage
+
+    def _flush_if_threshold_reached(self) -> None:
+        pending_count = len(self._pending_llm_response.events) if self._pending_llm_response is not None else 0
+        if len(self._buffer) + pending_count >= self._flush_threshold:
+            self._flush_sync()
+
+    def _queue_llm_response_events(
+        self,
+        llm_run_id: str,
+        events: list[dict],
+        messages: list[AnyMessage],
+        *,
+        caller: str,
+    ) -> None:
+        """Queue one logical response and merge usage into its canonical callback."""
+        if self._closed:
+            return
+
+        has_usage = self._has_positive_usage(events)
+        pending = self._pending_llm_response
+        if pending is not None and pending.llm_run_id == llm_run_id:
+            if has_usage:
+                # The first callback's generation set, immutable summary,
+                # caller, and non-usage payload are canonical. A provider's
+                # immediate replay may enrich only corresponding usage fields.
+                self._merge_response_event_usage(pending.events, events)
+                self._commit_pending_llm_response()
+                self._flush_if_threshold_reached()
+            return
+        if llm_run_id in self._counted_message_llm_run_ids:
+            return
+
+        # A different event is the ordering boundary for an earlier no-usage
+        # callback. Commit it before accepting this response.
+        self._commit_pending_llm_response()
+        self._flush_if_threshold_reached()
+
+        message_count, last_ai_message = self._snapshot_message_summary(messages, caller=caller)
+        pending_response = _PendingLlmResponse(
+            llm_run_id=llm_run_id,
+            events=events,
+            message_count=message_count,
+            last_ai_message=last_ai_message,
+        )
+        self._counted_message_llm_run_ids.add(llm_run_id)
+        self._llm_response_callers[llm_run_id] = caller
+        self._pending_llm_response = pending_response
+        if has_usage:
+            self._commit_pending_llm_response()
+        self._flush_if_threshold_reached()
+        # Some providers immediately re-fire on_llm_end with usage filled in.
+        # Defer an incomplete copy until the next event or flush.
+
     def _put(self, *, event_type: str, category: str, content: str | dict = "", metadata: dict | None = None) -> None:
         if self._closed:
             return
-        self._buffer.append(
-            {
-                "thread_id": self.thread_id,
-                "run_id": self.run_id,
-                "event_type": event_type,
-                "category": category,
-                "content": content,
-                "metadata": metadata or {},
-                "created_at": datetime.now(UTC).isoformat(),
-            }
-        )
+        self._commit_pending_llm_response()
+        self._buffer.append(self._make_event(event_type=event_type, category=category, content=content, metadata=metadata))
         if len(self._buffer) >= self._flush_threshold:
             self._flush_sync()
 
@@ -686,6 +824,7 @@ class RunJournal(BaseCallbackHandler):
         stay in the buffer and are flushed later by the async ``flush()``
         call in the worker's ``finally`` block.
         """
+        self._commit_pending_llm_response()
         if not self._buffer:
             return
         # Skip if a flush is already in flight — avoids concurrent writes
@@ -929,6 +1068,7 @@ class RunJournal(BaseCallbackHandler):
         """Force flush remaining buffer. Called in worker's finally block."""
         if self._closed:
             return
+        self._commit_pending_llm_response()
         if self._pending_flush_tasks:
             await asyncio.gather(*tuple(self._pending_flush_tasks), return_exceptions=True)
         while self._pending_progress_task is not None:
@@ -968,6 +1108,7 @@ class RunJournal(BaseCallbackHandler):
         self._store = None
         self._progress_reporter = None
         self._buffer.clear()
+        self._pending_llm_response = None
         self._pending_flush_tasks.clear()
         self._pending_progress_task = None
         self._pending_progress_delayed = False
@@ -976,6 +1117,7 @@ class RunJournal(BaseCallbackHandler):
         self._counted_llm_run_ids.clear()
         self._counted_external_source_ids.clear()
         self._counted_message_llm_run_ids.clear()
+        self._llm_response_callers.clear()
         self._llm_start_times.clear()
         self._seen_llm_starts.clear()
         self._current_run_tool_call_names.clear()
diff --git a/backend/tests/test_run_journal.py b/backend/tests/test_run_journal.py
index 061eba45e..c7115e2a0 100644
--- a/backend/tests/test_run_journal.py
+++ b/backend/tests/test_run_journal.py
@@ -11,7 +11,8 @@ from unittest.mock import MagicMock
 from uuid import uuid4
 
 import pytest
-from langchain_core.messages import HumanMessage
+from langchain_core.messages import AIMessage, HumanMessage
+from langchain_core.outputs import ChatGeneration, LLMResult
 
 from deerflow.runtime.events.store.memory import MemoryRunEventStore
 from deerflow.runtime.journal import RunJournal
@@ -68,6 +69,24 @@ async def test_close_flushes_and_detaches_runtime_dependencies():
     assert reporter_ref() is None
 
 
+@pytest.mark.anyio
+async def test_closed_on_llm_end_returns_before_touching_response_or_state():
+    store = MemoryRunEventStore()
+    journal = RunJournal("r-closed-callback", "t-closed-callback", store)
+    await journal.close()
+    completion_before = journal.get_completion_data()
+
+    # A plain object has no generations attribute, so this also pins the
+    # early return ahead of response inspection.
+    journal.on_llm_end(object(), run_id=uuid4(), tags=["lead_agent"])
+
+    assert journal.get_completion_data() == completion_before
+    assert journal._pending_llm_response is None
+    assert journal._buffer == []
+    assert journal._counted_message_llm_run_ids == set()
+    assert journal._counted_llm_run_ids == set()
+
+
 @pytest.mark.anyio
 async def test_close_preserves_buffer_and_dependencies_when_flush_fails():
     class FailOnceRunEventStore(MemoryRunEventStore):
@@ -101,6 +120,73 @@ async def test_close_preserves_buffer_and_dependencies_when_flush_fails():
     assert [event["event_type"] for event in events] == ["middleware:test"]
 
 
+@pytest.mark.anyio
+async def test_close_retries_pending_no_usage_response_without_duplication():
+    class FailOnceRunEventStore(MemoryRunEventStore):
+        def __init__(self) -> None:
+            super().__init__()
+            self.put_batch_calls = 0
+
+        async def put_batch(self, events):
+            self.put_batch_calls += 1
+            if self.put_batch_calls == 1:
+                raise RuntimeError("transient store failure")
+            return await super().put_batch(events)
+
+    async def progress_reporter(snapshot):
+        del snapshot
+
+    store = FailOnceRunEventStore()
+    journal = RunJournal(
+        "r-close-pending-retry",
+        "t-close-pending-retry",
+        store,
+        flush_threshold=100,
+        progress_reporter=progress_reporter,
+    )
+    journal.record_middleware("before", name="test", hook="after", action="record", changes={})
+    journal.on_llm_end(
+        _make_llm_response("Canonical without usage"),
+        run_id=uuid4(),
+        parent_run_id=None,
+        tags=["lead_agent"],
+    )
+
+    assert journal._pending_llm_response is not None
+    assert journal.get_completion_data()["message_count"] == 0
+
+    with pytest.raises(RuntimeError, match="transient store failure"):
+        await journal.close()
+
+    assert journal._closed is False
+    assert journal._store is store
+    assert journal._progress_reporter is progress_reporter
+    assert journal._pending_llm_response is None
+    assert [event["event_type"] for event in journal._buffer] == [
+        "middleware:before",
+        "llm.ai.response",
+    ]
+    assert journal.get_completion_data()["message_count"] == 1
+    assert journal.get_completion_data()["last_ai_message"] == "Canonical without usage"
+
+    await journal.close()
+
+    events = await store.list_events("t-close-pending-retry", "r-close-pending-retry")
+    assert [event["event_type"] for event in events] == [
+        "middleware:before",
+        "llm.ai.response",
+    ]
+    responses = [event for event in events if event["event_type"] == "llm.ai.response"]
+    assert len(responses) == 1
+    assert responses[0]["content"]["content"] == "Canonical without usage"
+    assert responses[0]["content"]["usage_metadata"] is None
+    assert responses[0]["metadata"]["usage"] == {}
+    assert journal.get_completion_data()["message_count"] == 1
+    assert journal._closed is True
+    assert journal._store is None
+    assert journal._progress_reporter is None
+
+
 @pytest.mark.anyio
 async def test_close_without_flush_discards_buffer_and_detaches_runtime_dependencies():
     class TrackingRunEventStore(MemoryRunEventStore):
@@ -197,6 +283,12 @@ def _make_llm_response(content="Hello", usage=None, tool_calls=None, additional_
     return response
 
 
+def _combine_llm_responses(*responses):
+    response = MagicMock()
+    response.generations = [generation for item in responses for generation in item.generations]
+    return response
+
+
 class TestLlmCallbacks:
     @pytest.mark.anyio
     async def test_on_chat_model_start_persists_original_user_input_without_mutating_model_message(self, journal_setup):
@@ -568,14 +660,28 @@ class TestBufferFlush:
         j, store = journal_setup
         j._flush_threshold = 2
         # Each on_llm_end emits 1 event
-        j.on_llm_end(_make_llm_response("A"), run_id=uuid4(), parent_run_id=None, tags=["lead_agent"])
+        usage = {"input_tokens": 1, "output_tokens": 1, "total_tokens": 2}
+        j.on_llm_end(_make_llm_response("A", usage=usage), run_id=uuid4(), parent_run_id=None, tags=["lead_agent"])
         assert len(j._buffer) == 1
-        j.on_llm_end(_make_llm_response("B"), run_id=uuid4(), parent_run_id=None, tags=["lead_agent"])
+        j.on_llm_end(_make_llm_response("B", usage=usage), run_id=uuid4(), parent_run_id=None, tags=["lead_agent"])
         # At threshold the buffer should have been flushed asynchronously
         await asyncio.sleep(0.1)
         events = await store.list_events("t1", "r1")
         assert len(events) >= 2
 
+    @pytest.mark.anyio
+    async def test_pending_response_counts_toward_flush_threshold(self, journal_setup):
+        j, store = journal_setup
+        j._flush_threshold = 2
+        j.record_middleware("before", name="BeforeMiddleware", hook="after_model", action="record", changes={})
+
+        j.on_llm_end(_make_llm_response("Pending"), run_id=uuid4(), parent_run_id=None, tags=["lead_agent"])
+        await asyncio.sleep(0.1)
+
+        assert j._pending_llm_response is None
+        events = await store.list_events("t1", "r1")
+        assert [event["event_type"] for event in events] == ["middleware:before", "llm.ai.response"]
+
     @pytest.mark.anyio
     async def test_events_retained_when_no_loop(self, journal_setup):
         """Events buffered in a sync (no-loop) context should survive
@@ -611,11 +717,12 @@ class TestFeedGeneration:
     """
 
     @pytest.mark.anyio
-    async def test_buffering_alone_does_not_advance_it(self, journal_setup):
+    async def test_pending_response_alone_does_not_advance_it(self, journal_setup):
         j, _store = journal_setup
         j.on_llm_end(_make_llm_response("A"), run_id=uuid4(), parent_run_id=None, tags=["lead_agent"])
 
-        assert len(j._buffer) == 1
+        assert j._buffer == []
+        assert j._pending_llm_response is not None
         assert j.feed_generation == 0
 
     @pytest.mark.anyio
@@ -623,7 +730,8 @@ class TestFeedGeneration:
         j, _store = journal_setup
         j._flush_threshold = 1
 
-        j.on_llm_end(_make_llm_response("A"), run_id=uuid4(), parent_run_id=None, tags=["lead_agent"])
+        usage = {"input_tokens": 1, "output_tokens": 1, "total_tokens": 2}
+        j.on_llm_end(_make_llm_response("A", usage=usage), run_id=uuid4(), parent_run_id=None, tags=["lead_agent"])
         await asyncio.sleep(0.1)
 
         assert j.feed_generation == 1
@@ -734,6 +842,7 @@ class TestConvenienceFields:
             parent_run_id=None,
             tags=["lead_agent"],
         )
+        await j.flush()
 
         data = j.get_completion_data()
 
@@ -756,6 +865,7 @@ class TestConvenienceFields:
             parent_run_id=None,
             tags=["lead_agent"],
         )
+        await j.flush()
 
         data = j.get_completion_data()
 
@@ -766,6 +876,7 @@ class TestConvenienceFields:
     async def test_last_ai_message_extracts_mapping_content(self, journal_setup):
         j, _ = journal_setup
         j.on_llm_end(_make_llm_response({"content": "Nested answer"}), run_id=uuid4(), parent_run_id=None, tags=["lead_agent"])
+        await j.flush()
 
         data = j.get_completion_data()
 
@@ -796,6 +907,7 @@ class TestConvenienceFields:
         j, _ = journal_setup
         j.on_llm_end(_make_llm_response("Lead answer"), run_id=uuid4(), parent_run_id=None, tags=["lead_agent"])
         j.on_llm_end(_make_llm_response("Subagent detail"), run_id=uuid4(), parent_run_id=None, tags=["subagent:research"])
+        await j.flush()
 
         data = j.get_completion_data()
 
@@ -950,17 +1062,317 @@ class TestCallerBucketing:
         assert j._lead_agent_tokens == 15
         assert j._llm_call_count == 1
 
-    def test_first_no_usage_second_with_usage(self, journal_setup):
-        """First callback with no usage must not block second callback with usage for same run_id."""
-        j, _ = journal_setup
+    @pytest.mark.anyio
+    async def test_dedup_same_run_id_persists_single_message(self, journal_setup):
+        """A re-fired on_llm_end for one run_id must persist the message once.
+
+        LangChain can deliver on_llm_end more than once for the same run_id.
+        Token accounting already dedups on that; the durable llm.ai.response
+        row must be deduped on the same premise, or count_messages and message
+        pagination (which read append-only rows without dedup) inflate.
+        """
+        j, store = journal_setup
+        run_id = uuid4()
+        response = _make_llm_response("Answer")
+        j.on_llm_end(response, run_id=run_id, parent_run_id=None, tags=["lead_agent"])
+        j.on_llm_end(response, run_id=run_id, parent_run_id=None, tags=["lead_agent"])
+        await j.flush()
+        messages = await store.list_messages("t1")
+        assert [m["event_type"] for m in messages] == ["llm.ai.response"]
+        assert await store.count_messages("t1") == 1
+        # The run summary counts the message exactly once as well.
+        assert j._msg_count == 1
+
+    @pytest.mark.anyio
+    async def test_adjacent_late_usage_enriches_canonical_response_only(self, journal_setup):
+        j, store = journal_setup
+        run_id = uuid4()
+        usage = {"input_tokens": 10, "output_tokens": 5, "total_tokens": 15}
+        original_tool_calls = [{"id": "call-original", "name": "search", "args": {}}]
+        replay_tool_calls = [{"id": "call-replay", "name": "write_file", "args": {}}]
+
+        j.on_llm_end(
+            _make_llm_response(
+                "Canonical",
+                tool_calls=original_tool_calls,
+                additional_kwargs={
+                    "deerflow_error_fallback": True,
+                    "error_detail": "canonical fallback",
+                },
+            ),
+            run_id=run_id,
+            parent_run_id=None,
+            tags=["lead_agent"],
+        )
+        j.on_llm_end(
+            _make_llm_response(
+                "Replay",
+                usage=usage,
+                tool_calls=replay_tool_calls,
+                additional_kwargs={
+                    "deerflow_error_fallback": True,
+                    "error_detail": "replay fallback",
+                },
+            ),
+            run_id=run_id,
+            parent_run_id=None,
+            tags=["subagent:research"],
+        )
+        await j.flush()
+
+        messages = await store.list_messages("t1")
+        assert len(messages) == 1
+        assert messages[0]["content"]["content"] == "Canonical"
+        assert messages[0]["content"]["tool_calls"] == original_tool_calls
+        assert messages[0]["content"]["additional_kwargs"]["error_detail"] == "canonical fallback"
+        assert messages[0]["content"]["usage_metadata"] == usage
+        assert messages[0]["metadata"]["caller"] == "lead_agent"
+        assert messages[0]["metadata"]["usage"] == usage
+        assert j._current_run_tool_call_names == {"call-original": "search"}
+        assert j.had_llm_error_fallback is True
+        assert j.llm_error_fallback_message == "canonical fallback"
+        assert j.get_completion_data()["last_ai_message"] == "Canonical"
+        assert j.get_completion_data()["lead_agent_tokens"] == 15
+        assert j.get_completion_data()["subagent_tokens"] == 0
+
+    @pytest.mark.anyio
+    async def test_same_message_object_replay_cannot_mutate_canonical_summary(self, journal_setup):
+        j, store = journal_setup
+        run_id = uuid4()
+        message = AIMessage(content="Canonical answer")
+        response = LLMResult(generations=[[ChatGeneration(message=message)]])
+
+        j.on_llm_end(response, run_id=run_id, parent_run_id=None, tags=["lead_agent"])
+        message.content = "Replay answer"
+        message.usage_metadata = {
+            "input_tokens": 10,
+            "output_tokens": 5,
+            "total_tokens": 15,
+            "input_token_details": {"cache_read": 3},
+        }
+        j.on_llm_end(response, run_id=run_id, parent_run_id=None, tags=["lead_agent"])
+        message.usage_metadata["input_token_details"]["cache_read"] = 999
+        await j.flush()
+
+        messages = await store.list_messages("t1")
+        assert len(messages) == 1
+        assert messages[0]["content"]["content"] == "Canonical answer"
+        expected_usage = {
+            "input_tokens": 10,
+            "output_tokens": 5,
+            "total_tokens": 15,
+            "input_token_details": {"cache_read": 3},
+        }
+        assert messages[0]["metadata"]["usage"] == expected_usage
+        assert messages[0]["content"]["usage_metadata"] == expected_usage
+        assert j.get_completion_data()["message_count"] == 1
+        assert j.get_completion_data()["last_ai_message"] == "Canonical answer"
+
+    @pytest.mark.anyio
+    async def test_positive_usage_event_does_not_retain_nested_provider_metadata(self, journal_setup):
+        j, store = journal_setup
+        usage = {
+            "input_tokens": 8,
+            "output_tokens": 3,
+            "total_tokens": 11,
+            "output_token_details": {"reasoning": 2},
+        }
+        message = AIMessage(content="Canonical with usage", usage_metadata=usage)
+        response = LLMResult(generations=[[ChatGeneration(message=message)]])
+
+        j.on_llm_end(response, run_id=uuid4(), parent_run_id=None, tags=["lead_agent"])
+        message.usage_metadata["output_token_details"]["reasoning"] = 999
+        await j.flush()
+
+        messages = await store.list_messages("t1")
+        assert len(messages) == 1
+        assert messages[0]["metadata"]["usage"]["output_token_details"] == {"reasoning": 2}
+        assert messages[0]["content"]["usage_metadata"]["output_token_details"] == {"reasoning": 2}
+
+    @pytest.mark.anyio
+    async def test_mutating_staged_message_before_flush_cannot_mutate_canonical_summary(self, journal_setup):
+        j, store = journal_setup
+        message = AIMessage(content="Canonical before flush")
+        response = LLMResult(generations=[[ChatGeneration(message=message)]])
+
+        j.on_llm_end(response, run_id=uuid4(), parent_run_id=None, tags=["lead_agent"])
+        message.content = "Mutation before flush"
+        await j.flush()
+
+        messages = await store.list_messages("t1")
+        assert len(messages) == 1
+        assert messages[0]["content"]["content"] == "Canonical before flush"
+        assert j.get_completion_data()["message_count"] == 1
+        assert j.get_completion_data()["last_ai_message"] == "Canonical before flush"
+
+    @pytest.mark.anyio
+    async def test_nested_same_message_object_replay_cannot_mutate_canonical_summary(self, journal_setup):
+        j, store = journal_setup
+        run_id = uuid4()
+        message = AIMessage(content=[{"type": "text", "text": "Canonical nested answer"}])
+        response = LLMResult(generations=[[ChatGeneration(message=message)]])
+
+        j.on_llm_end(response, run_id=run_id, parent_run_id=None, tags=["lead_agent"])
+        message.content[0]["text"] = "Replay nested answer"
+        message.usage_metadata = {"input_tokens": 8, "output_tokens": 3, "total_tokens": 11}
+        j.on_llm_end(response, run_id=run_id, parent_run_id=None, tags=["lead_agent"])
+        await j.flush()
+
+        messages = await store.list_messages("t1")
+        assert len(messages) == 1
+        assert messages[0]["content"]["content"] == [{"type": "text", "text": "Canonical nested answer"}]
+        assert messages[0]["content"]["usage_metadata"] == message.usage_metadata
+        assert j.get_completion_data()["message_count"] == 1
+        assert j.get_completion_data()["last_ai_message"] == "Canonical nested answer"
+
+    @pytest.mark.anyio
+    async def test_all_zero_usage_remains_pending_and_positive_usage_enriches_it(self, journal_setup):
+        j, store = journal_setup
+        run_id = uuid4()
+        zero_usage = {"input_tokens": 0, "output_tokens": 0, "total_tokens": 0}
+        positive_usage = {"input_tokens": 4, "output_tokens": 2, "total_tokens": 6}
+
+        j.on_llm_end(_make_llm_response("Zero usage", usage=zero_usage), run_id=run_id, parent_run_id=None, tags=["lead_agent"])
+        assert j._buffer == []
+        assert j._pending_llm_response is not None
+        assert j.get_completion_data()["message_count"] == 0
+
+        j.on_llm_end(_make_llm_response("Replay payload", usage=positive_usage), run_id=run_id, parent_run_id=None, tags=["lead_agent"])
+        await j.flush()
+
+        messages = await store.list_messages("t1")
+        assert len(messages) == 1
+        assert messages[0]["content"]["content"] == "Zero usage"
+        assert messages[0]["content"]["usage_metadata"] == positive_usage
+        assert messages[0]["metadata"]["usage"] == positive_usage
+        assert j.get_completion_data()["message_count"] == 1
+        assert j.get_completion_data()["last_ai_message"] == "Zero usage"
+
+    @pytest.mark.anyio
+    async def test_replay_generation_length_cannot_change_canonical_set(self, journal_setup):
+        j, store = journal_setup
+        short_usage = {"input_tokens": 8, "output_tokens": 3, "total_tokens": 11}
+        extra_usage = {"input_tokens": 1, "output_tokens": 1, "total_tokens": 2}
+        first_run_id = uuid4()
+        second_run_id = uuid4()
+
+        j.on_llm_end(
+            _combine_llm_responses(_make_llm_response("Canonical one"), _make_llm_response("Canonical two")),
+            run_id=first_run_id,
+            parent_run_id=None,
+            tags=["lead_agent"],
+        )
+        j.on_llm_end(
+            _make_llm_response("Short replay", usage=short_usage),
+            run_id=first_run_id,
+            parent_run_id=None,
+            tags=["lead_agent"],
+        )
+        j.on_llm_end(
+            _make_llm_response("Single canonical"),
+            run_id=second_run_id,
+            parent_run_id=None,
+            tags=["lead_agent"],
+        )
+        j.on_llm_end(
+            _combine_llm_responses(
+                _make_llm_response("Long replay one", usage=extra_usage),
+                _make_llm_response("Long replay two"),
+            ),
+            run_id=second_run_id,
+            parent_run_id=None,
+            tags=["lead_agent"],
+        )
+        await j.flush()
+
+        messages = await store.list_messages("t1")
+        assert [message["content"]["content"] for message in messages] == [
+            "Canonical one",
+            "Canonical two",
+            "Single canonical",
+        ]
+        assert messages[0]["metadata"]["usage"] == short_usage
+        assert messages[0]["content"]["usage_metadata"] == short_usage
+        assert messages[1]["metadata"]["usage"] == {}
+        assert messages[1]["content"]["usage_metadata"] is None
+        assert messages[2]["metadata"]["usage"] == extra_usage
+        assert messages[2]["content"]["usage_metadata"] == extra_usage
+        assert j.get_completion_data()["message_count"] == 3
+        assert j.get_completion_data()["last_ai_message"] == "Single canonical"
+
+    @pytest.mark.anyio
+    async def test_interleaved_late_usage_updates_summary_only(self, journal_setup):
+        j, store = journal_setup
+        first_run_id = uuid4()
+        second_run_id = uuid4()
+        usage = {"input_tokens": 9, "output_tokens": 4, "total_tokens": 13}
+
+        j.on_llm_end(_make_llm_response("First canonical"), run_id=first_run_id, parent_run_id=None, tags=["lead_agent"])
+        j.on_llm_end(_make_llm_response("Second canonical"), run_id=second_run_id, parent_run_id=None, tags=["lead_agent"])
+        j.on_llm_end(
+            _make_llm_response(
+                "Late replay",
+                usage=usage,
+                tool_calls=[{"id": "late-call", "name": "write_file", "args": {}}],
+                additional_kwargs={"deerflow_error_fallback": True, "error_detail": "late fallback"},
+            ),
+            run_id=first_run_id,
+            parent_run_id=None,
+            tags=["subagent:research"],
+        )
+        await j.flush()
+
+        messages = await store.list_messages("t1")
+        assert [message["content"]["content"] for message in messages] == ["First canonical", "Second canonical"]
+        assert messages[0]["metadata"]["usage"] == {}
+        assert messages[0]["content"]["usage_metadata"] is None
+        assert j.get_completion_data()["total_tokens"] == 13
+        assert j.get_completion_data()["lead_agent_tokens"] == 13
+        assert j.get_completion_data()["subagent_tokens"] == 0
+        assert j.get_completion_data()["message_count"] == 2
+        assert j.get_completion_data()["last_ai_message"] == "Second canonical"
+        assert "late-call" not in j._current_run_tool_call_names
+        assert j.had_llm_error_fallback is False
+
+    @pytest.mark.anyio
+    async def test_single_no_usage_response_persists_once_at_flush(self, journal_setup):
+        j, store = journal_setup
+
+        j.on_llm_end(_make_llm_response("No usage"), run_id=uuid4(), parent_run_id=None, tags=["lead_agent"])
+        assert j._buffer == []
+        assert j._pending_llm_response is not None
+
+        await j.flush()
+
+        messages = await store.list_messages("t1")
+        assert len(messages) == 1
+        assert messages[0]["content"]["content"] == "No usage"
+        assert messages[0]["metadata"]["usage"] == {}
+
+    @pytest.mark.anyio
+    async def test_distinct_run_ids_each_persist_a_message(self, journal_setup):
+        """The dedup guard is per run_id and must not drop distinct responses."""
+        j, store = journal_setup
+        j.on_llm_end(_make_llm_response("First"), run_id=uuid4(), parent_run_id=None, tags=["lead_agent"])
+        j.on_llm_end(_make_llm_response("Second"), run_id=uuid4(), parent_run_id=None, tags=["lead_agent"])
+        await j.flush()
+        assert await store.count_messages("t1") == 2
+
+    @pytest.mark.anyio
+    async def test_first_no_usage_second_with_usage(self, journal_setup):
+        """Late usage enriches the single canonical event and the run summary."""
+        j, store = journal_setup
         run_id = uuid4()
         j.on_llm_end(_make_llm_response("A", usage=None), run_id=run_id, parent_run_id=None, tags=["lead_agent"])
-        assert str(run_id) not in j._counted_llm_run_ids
-        # Second callback for the same run_id with actual usage must still count
         usage = {"input_tokens": 10, "output_tokens": 5, "total_tokens": 15}
         j.on_llm_end(_make_llm_response("A", usage=usage), run_id=run_id, parent_run_id=None, tags=["lead_agent"])
-        assert j._total_tokens == 15
-        assert j._lead_agent_tokens == 15
+        await j.flush()
+
+        messages = await store.list_messages("t1")
+        assert len(messages) == 1
+        assert messages[0]["metadata"]["usage"] == usage
+        assert messages[0]["content"]["usage_metadata"] == usage
+        assert j.get_completion_data()["total_tokens"] == 15
 
     def test_track_token_usage_false_skips_buckets(self):
         """When token tracking is disabled, caller buckets stay at 0."""