fix(tui): stop empty-id assistant deltas from merging across turns (#4201)

_apply_assistant_delta matched AssistantDelta.id anywhere in the transcript,
which is correct for a genuine per-message id but not for an empty one.
runtime._as_str() coerces a missing/None chunk id to "", and that value is
shared by every id-less chunk from every turn, not just the current one.
Once one assistant row had id="", every later, unrelated AssistantDelta that
also carried id="" (e.g. from a provider that never stamps per-chunk ids)
matched that same stale row instead of starting a fresh one, silently
folding a second turn's answer backward into the first turn's bubble.

Route empty-id deltas to a dedicated path that tracks the current turn's
row by position (streaming_anonymous_row_index, reset on RunStarted/
RunEnded/ClearRows) instead of by id, mirroring the existing empty-id guards
in _apply_tool_started/_apply_tool_result but adapted for assistant text:
unlike a tool call, an id-less assistant delta still needs to be displayed,
so it starts a new row rather than being dropped. Multiple id-less chunks
legitimately arrive within one turn (per-token streaming), so they keep
coalescing into that row -- but only while it is still the transcript tail;
once a tool card is appended after it (the same way a genuine id naturally
changes across a tool round-trip), the next empty-id delta starts fresh
instead of reaching backward past the tool card.

Add regression coverage for the cross-turn merge, same-turn coalescing,
the tool-call-interleaved edge case, and non-interference with the
existing id-keyed path.

backend/packages/harness/deerflow/tui/view_state.py

@@ -140,6 +140,15 @@ class ViewState:
     # Id of the message currently being generated this turn. Only this row renders
     # as plain text while streaming; everything else (history) stays Markdown.
     streaming_id: str | None = None
+    # Row index of the *anonymous* (empty-id) assistant row receiving deltas this
+    # turn, if any. A genuine id is a reliable cross-chunk key (see
+    # `_apply_assistant_delta`'s whole-transcript id scan), but an empty id ("" —
+    # see `runtime._as_str`) is shared by every id-less chunk from every turn, so
+    # it cannot be matched the same way: scanning for `row.id == ""` would fold a
+    # brand new turn's text into whatever earlier turn's row happened to be
+    # id-less too. This index instead pins "this turn's" anonymous row by
+    # position, reset alongside `streaming_id` at the start/end of every turn.
+    streaming_anonymous_row_index: int | None = None
 
 
 def initial_state(rows: tuple[Row, ...] = ()) -> ViewState:
@@ -164,13 +173,14 @@ def reduce(state: ViewState, action: Action) -> ViewState:
     if isinstance(action, RunStarted):
         # New turn: no message is actively streaming yet (the client re-emits
         # prior messages first; those must not be treated as the active one).
-        return replace(state, streaming=True, streaming_id=None)
+        return replace(state, streaming=True, streaming_id=None, streaming_anonymous_row_index=None)
 
     if isinstance(action, RunEnded):
         return replace(
             state,
             streaming=False,
             streaming_id=None,
+            streaming_anonymous_row_index=None,
             usage=action.usage if action.usage is not None else state.usage,
         )
 
@@ -193,20 +203,28 @@ def reduce(state: ViewState, action: Action) -> ViewState:
         return replace(state, title=action.title)
 
     if isinstance(action, ClearRows):
-        return replace(state, rows=(), title=None, streaming_id=None)
+        return replace(state, rows=(), title=None, streaming_id=None, streaming_anonymous_row_index=None)
 
     return state
 
 
 def _apply_assistant_delta(state: ViewState, action: AssistantDelta) -> ViewState:
-    """Update the assistant row with this id (anywhere in the transcript), or
-    start a new one.
+    """Update the assistant row for this delta, or start a new one.
 
-    On a thread with history, the client re-emits every prior message on each
-    new turn (its dedup is per-turn), and a re-emitted *older* message can arrive
-    after a newer one has started — so we must match by id across the whole
-    transcript, not just the most recent assistant row, or prior answers get
-    duplicated.
+    A genuine (non-empty) id is matched anywhere in the transcript, not just
+    the most recent assistant row: on a thread with history, the client
+    re-emits every prior message on each new turn (its dedup is per-turn), and
+    a re-emitted *older* message can arrive after a newer one has started — so
+    matching only the tail row would duplicate prior answers.
+
+    An empty id ("" — some providers/paths never stamp per-chunk ids, see
+    ``runtime._as_str``) is NOT a reliable key for that same scan: unlike a
+    genuine id, it is shared by every id-less chunk from *every* turn, so
+    matching `row.id == ""` across the whole transcript would fold a brand
+    new turn's text into whatever earlier turn's row happened to be id-less
+    too — silently vanishing the new turn's answer into a stale row. Empty-id
+    deltas are therefore routed to `_apply_assistant_delta_anonymous`, which
+    tracks "this turn's" row by position instead of by id.
 
     Updates also merge by content rather than blindly concatenating, to absorb
     full re-sends / cumulative snapshots vs. genuine incremental deltas:
@@ -215,6 +233,8 @@ def _apply_assistant_delta(state: ViewState, action: AssistantDelta) -> ViewStat
     * accumulated starts with new text            -> stale/shorter re-send: keep
     * otherwise                                   -> a real delta: append
     """
+    if not action.id:
+        return _apply_assistant_delta_anonymous(state, action)
 
     rows = list(state.rows)
     for i, row in enumerate(rows):
@@ -234,6 +254,46 @@ def _apply_assistant_delta(state: ViewState, action: AssistantDelta) -> ViewStat
     return _mark_streaming(_append(state, AssistantRow(text=action.text, id=action.id)), action.id)
 
 
+def _apply_assistant_delta_anonymous(state: ViewState, action: AssistantDelta) -> ViewState:
+    """Handle an ``AssistantDelta`` whose id is empty (see `_apply_assistant_delta`).
+
+    Multiple id-less chunks legitimately arrive for a single turn — a provider
+    that never stamps per-chunk ids still streams token by token, e.g.
+    ``"Hel"`` then ``"lo"`` — so the first empty-id delta of a turn starts a
+    new row, and later empty-id deltas keep appending to that row (tracked by
+    ``state.streaming_anonymous_row_index``, reset on every
+    ``RunStarted``/``RunEnded``/``ClearRows``, not by id, so a later turn
+    always starts its own new row instead of matching the previous turn's
+    leftover id-less row — the bug this split exists to avoid).
+
+    The tracked row is only reused while it is still the LAST row in the
+    transcript. A genuine id naturally changes across a tool round-trip
+    (LangGraph gives the post-tool continuation a new AIMessage id), which is
+    why an interleaved ``ToolStarted``/``ToolResult`` already starts a new row
+    in the id-keyed path (see `test_assistant_delta_with_new_id_after_tool_
+    creates_separate_row`). An empty id has no such natural signal — it is
+    always ``""`` before and after the tool call — so this function uses row
+    *position* as the substitute: once anything else has been appended (a
+    tool card, in practice), the anonymous row is no longer the tail, and the
+    next empty-id delta starts a fresh row rather than reaching backward past
+    the tool card into stale text.
+    """
+    index = state.streaming_anonymous_row_index
+    if index is not None and index == len(state.rows) - 1:
+        row = state.rows[index]
+        if isinstance(row, AssistantRow) and not row.error:
+            # Same no-op / merge semantics as the id-keyed path above.
+            if row.text == action.text and len(action.text) > 1:
+                return state
+            rows = list(state.rows)
+            merged = _merge_stream_text(row.text, action.text)
+            rows[index] = replace(row, text=merged)
+            return _mark_streaming_anonymous(replace(state, rows=tuple(rows)), index)
+
+    new_state = _append(state, AssistantRow(text=action.text, id=action.id))
+    return _mark_streaming_anonymous(new_state, len(new_state.rows) - 1)
+
+
 def _mark_streaming(state: ViewState, message_id: str) -> ViewState:
     """Record the actively-streaming message id (only while a run is active)."""
     if state.streaming:
@@ -241,6 +301,24 @@ def _mark_streaming(state: ViewState, message_id: str) -> ViewState:
     return state
 
 
+def _mark_streaming_anonymous(state: ViewState, index: int) -> ViewState:
+    """Record the active turn's anonymous-row index (only while a run is active).
+
+    Deliberately leaves ``streaming_id`` at ``None`` rather than ``""``: unlike
+    a genuine id, ``""`` would be shared by every anonymous row across every
+    turn, so using it as the render layer's "is this the row actively
+    streaming" key (``render.render_transcript``) would flag every past
+    anonymous row as actively streaming too, the moment a new one starts. The
+    cost is purely cosmetic — an anonymous row never gets the
+    raw-text-while-streaming treatment other rows get, only the id-less
+    fallback path is affected — in exchange for not reintroducing a cross-turn
+    ambiguity into the render layer that this fix removes from ``rows``.
+    """
+    if state.streaming:
+        return replace(state, streaming_id=None, streaming_anonymous_row_index=index)
+    return state
+
+
 def _merge_stream_text(existing: str, incoming: str) -> str:
     if not existing:
         return incoming

backend/tests/test_tui_runtime.py

@@ -143,3 +143,36 @@ def test_stream_actions_surfaces_exception_as_error_then_ends():
     actions = list(stream_actions(_BoomClient(), "go"))
     assert any(isinstance(a, AssistantError) and "model down" in a.text for a in actions)
     assert isinstance(actions[-1], RunEnded)
+
+
+def test_stream_actions_two_turns_with_none_ids_produce_separate_rows():
+    """Some providers/paths never stamp per-chunk ids: the raw chunk carries
+    an explicit ``id: None``, which ``_as_str`` coerces to ``""``. Two
+    separate turns from such a provider must not fold into one row -- see
+    ``_apply_assistant_delta_anonymous`` in view_state.py. Drives the real
+    translate()/stream_actions() bridge, not just the reducer directly."""
+    first_turn = _FakeClient(
+        [
+            StreamEvent(type="messages-tuple", data={"type": "ai", "content": "First turn answer.", "id": None}),
+            StreamEvent(type="end", data={"usage": None}),
+        ]
+    )
+    second_turn = _FakeClient(
+        [
+            StreamEvent(type="messages-tuple", data={"type": "ai", "content": "Second turn answer.", "id": None}),
+            StreamEvent(type="end", data={"usage": None}),
+        ]
+    )
+
+    state = initial_state()
+    for action in stream_actions(first_turn, "first question"):
+        state = reduce(state, action)
+    for action in stream_actions(second_turn, "second question"):
+        state = reduce(state, action)
+
+    assistants = [r for r in state.rows if r.kind == "assistant"]
+    # Pre-fix: both turns' AssistantDelta carry id="" and the second turn's
+    # text is folded into the first turn's row instead of starting a new one.
+    assert len(assistants) == 2
+    assert assistants[0].text == "First turn answer."
+    assert assistants[1].text == "Second turn answer."

backend/tests/test_tui_view_state.py

@@ -241,3 +241,135 @@ def test_merge_stream_text_newline_split_across_chunks():
 def test_merge_stream_text_genuine_delta_append():
     """Normal deltas that don't overlap still append."""
     assert _merge_stream_text("Hello ", "world") == "Hello world"
+
+
+# ---------------------------------------------------------------------------
+# Empty/missing-id assistant deltas: some providers/paths never stamp
+# per-chunk ids (runtime._as_str coerces a missing id to ""). Matching by id
+# like the normal path would fold EVERY id-less turn into whichever id-less
+# row happened to exist first, since "" is shared across turns -- unlike a
+# genuine id. These pin the fix: an empty id always keys off the CURRENT
+# turn (never a stale row from an earlier turn), while still coalescing
+# multiple id-less chunks that legitimately arrive within one turn.
+# ---------------------------------------------------------------------------
+
+
+def test_assistant_delta_empty_id_starts_new_row_per_turn_not_merged_with_prior_turn():
+    state = initial_state()
+    state = reduce(state, RunStarted())
+    state = reduce(state, AssistantDelta(id="", text="First turn answer."))
+    state = reduce(state, RunEnded())
+
+    state = reduce(state, UserSubmitted("second question"))
+    state = reduce(state, RunStarted())
+    state = reduce(state, AssistantDelta(id="", text="Second turn answer."))
+    state = reduce(state, RunEnded())
+
+    assistants = [r for r in state.rows if r.kind == "assistant"]
+    # Pre-fix: both turns share id="" so the second folds into the first via
+    # the whole-transcript id scan, losing "First turn answer." entirely.
+    assert len(assistants) == 2
+    assert assistants[0].text == "First turn answer."
+    assert assistants[1].text == "Second turn answer."
+
+
+def test_assistant_delta_empty_id_coalesces_multiple_chunks_within_same_turn():
+    """An id-less provider still streams token by token; chunks within ONE
+    turn must accumulate into a single row, not fragment into many."""
+    state = initial_state()
+    state = reduce(state, RunStarted())
+    state = reduce(state, AssistantDelta(id="", text="Hel"))
+    state = reduce(state, AssistantDelta(id="", text="lo"))
+    state = reduce(state, AssistantDelta(id="", text=" world"))
+    state = reduce(state, RunEnded())
+
+    assistants = [r for r in state.rows if r.kind == "assistant"]
+    assert len(assistants) == 1
+    assert assistants[0].text == "Hello world"
+
+
+def test_assistant_delta_empty_id_starts_fresh_row_after_interleaved_tool_call():
+    """An empty id has no signal to distinguish "same message, paused for a
+    tool call" from "a new message that happens to also be id-less" -- unlike
+    a genuine id, which naturally changes across a tool round-trip (a new
+    AIMessage gets a new id; see
+    test_assistant_delta_with_new_id_after_tool_creates_separate_row). Once a
+    tool card has been appended, the previous anonymous row is no longer the
+    transcript tail, so the next empty-id delta must start a NEW row rather
+    than reach backward past the tool card and silently prepend text that
+    arrived after the tool ran."""
+    state = initial_state()
+    state = reduce(state, RunStarted())
+    state = reduce(state, AssistantDelta(id="", text="Let me check. "))
+    state = reduce(state, ToolStarted(tool_call_id="t1", tool_name="bash", args={}))
+    state = reduce(state, ToolResult(tool_call_id="t1", content="ok", is_error=False))
+    state = reduce(state, AssistantDelta(id="", text="Done."))
+    state = reduce(state, RunEnded())
+
+    kinds = [r.kind for r in state.rows]
+    assert kinds == ["assistant", "tool", "assistant"]
+    assistants = [r for r in state.rows if r.kind == "assistant"]
+    assert [a.text for a in assistants] == ["Let me check. ", "Done."]
+
+
+def test_assistant_delta_empty_id_coalesces_consecutive_chunks_before_a_tool_call():
+    """Multiple id-less chunks with NOTHING interleaved (the realistic
+    per-token streaming case) still coalesce into one row up until a tool
+    card breaks the streak."""
+    state = initial_state()
+    state = reduce(state, RunStarted())
+    state = reduce(state, AssistantDelta(id="", text="Let me "))
+    state = reduce(state, AssistantDelta(id="", text="check. "))
+    state = reduce(state, ToolStarted(tool_call_id="t1", tool_name="bash", args={}))
+    state = reduce(state, ToolResult(tool_call_id="t1", content="ok", is_error=False))
+    state = reduce(state, RunEnded())
+
+    kinds = [r.kind for r in state.rows]
+    assert kinds == ["assistant", "tool"]
+    assistants = [r for r in state.rows if r.kind == "assistant"]
+    assert assistants[0].text == "Let me check. "
+
+
+def test_assistant_delta_empty_id_does_not_disturb_legitimate_id_sequence():
+    """A normal, non-empty id sequence must keep coalescing correctly even
+    after the transcript has already seen an earlier, unrelated empty-id
+    turn (proves the two code paths -- id-keyed vs. anonymous -- don't
+    interfere with each other)."""
+    state = initial_state()
+    state = reduce(state, RunStarted())
+    state = reduce(state, AssistantDelta(id="", text="anonymous turn"))
+    state = reduce(state, RunEnded())
+
+    state = reduce(state, UserSubmitted("question"))
+    state = reduce(state, RunStarted())
+    state = reduce(state, AssistantDelta(id="m1", text="Hel"))
+    state = reduce(state, AssistantDelta(id="m1", text="lo"))
+    state = reduce(state, RunEnded())
+
+    assistants = [r for r in state.rows if r.kind == "assistant"]
+    assert [a.text for a in assistants] == ["anonymous turn", "Hello"]
+
+
+def test_assistant_delta_empty_id_resend_within_turn_is_noop():
+    """Same multi-char no-op re-send semantics apply to the anonymous path."""
+    state = initial_state()
+    state = reduce(state, RunStarted())
+    state = reduce(state, AssistantDelta(id="", text="Hey there!"))
+    state = reduce(state, AssistantDelta(id="", text="Hey there!"))
+    assistants = [r for r in state.rows if r.kind == "assistant"]
+    assert len(assistants) == 1
+    assert assistants[0].text == "Hey there!"
+
+
+def test_clear_rows_resets_anonymous_streaming_index():
+    """A stale anonymous-row index must not resurrect after ClearRows."""
+    state = initial_state()
+    state = reduce(state, RunStarted())
+    state = reduce(state, AssistantDelta(id="", text="before clear"))
+    state = reduce(state, ClearRows())
+    state = reduce(state, RunStarted())
+    state = reduce(state, AssistantDelta(id="", text="after clear"))
+
+    assistants = [r for r in state.rows if r.kind == "assistant"]
+    assert len(assistants) == 1
+    assert assistants[0].text == "after clear"