diff --git a/README.md b/README.md
index ea1bcae..3504efd 100644
--- a/README.md
+++ b/README.md
@@ -290,6 +290,24 @@ awkward (FastAPI middleware, Celery tasks).
 The SDK uses `contextvars`, so concurrent `asyncio.gather` tasks each
 keep their own span stack. No cross-task `parent_id` contamination.
 
+### Mid-conversation resets
+
+When a user clears chat history or a planner restarts from a checkpoint
+mid-trace, call `wikitrace.session_reset()` to mark the boundary.
+Spans before and after share the same `session_id` (cost rollups and
+user attribution stay grouped) but carry distinct `session_segment`
+integers so the dashboard renders them as separate threads.
+
+```python
+with wikitrace.session(id="conv-1", user="alice"):
+    chain.invoke({"input": q1})              # session_segment=0
+    wikitrace.session_reset()                # → 1
+    chain.invoke({"input": "start over"})    # session_segment=1
+```
+
+Outside an active session it's a no-op — safe to call from library
+code that doesn't know whether tracing is active.
+
 ---
 
 ## Custom Properties
diff --git a/tests/test_sdk.py b/tests/test_sdk.py
index 9d9232d..3cfa45a 100644
--- a/tests/test_sdk.py
+++ b/tests/test_sdk.py
@@ -135,6 +135,42 @@ def test_set_session_imperative(trace_dir: Path):
     assert "session_id" not in spans[1]["attrs"]
 
 
+def test_session_reset_segments_under_same_session_id(trace_dir: Path):
+    """Mid-conversation reset: spans before and after share session_id
+    but carry distinct session_segment ids so the dashboard can render
+    them as separate threads while cost rollups still group them."""
+    wt.init(pipeline="t", trace_dir=trace_dir)
+    with wt.session(id="conv-1", user="alice"):
+        with wt.span("turn-A"):
+            pass
+        new_seg = wt.session_reset()
+        assert new_seg == 1
+        with wt.span("turn-B"):
+            pass
+        new_seg = wt.session_reset()
+        assert new_seg == 2
+        with wt.span("turn-C"):
+            pass
+    wt.end()
+
+    spans = {s["name"]: s for s in _read_spans(trace_dir)}
+    # All three turns share the same session_id.
+    assert spans["turn-A"]["attrs"]["session_id"] == "conv-1"
+    assert spans["turn-B"]["attrs"]["session_id"] == "conv-1"
+    assert spans["turn-C"]["attrs"]["session_id"] == "conv-1"
+    # First turn has no segment attr (segment 0 = original); subsequent
+    # turns carry incrementing segment ids.
+    assert spans["turn-A"]["attrs"].get("session_segment") in (None, 0)
+    assert spans["turn-B"]["attrs"]["session_segment"] == 1
+    assert spans["turn-C"]["attrs"]["session_segment"] == 2
+
+
+def test_session_reset_outside_session_is_noop():
+    """Calling session_reset() with no active session returns 0 and
+    does not raise — so library code can call it defensively."""
+    assert wt.session_reset() == 0
+
+
 def test_async_gather_no_parent_id_contamination(trace_dir: Path):
     """5 concurrent async tasks each open + close their own span. No
     span should see another task's span as parent."""
diff --git a/wikitrace/__init__.py b/wikitrace/__init__.py
index aa6ee28..7dfde80 100644
--- a/wikitrace/__init__.py
+++ b/wikitrace/__init__.py
@@ -18,7 +18,7 @@
 from .sdk import (
     init, span, step, cite, end, current_trace_id,
     span_open, span_event, span_close,
-    session, set_session, clear_session,
+    session, set_session, clear_session, session_reset,
     register_span_start_hook, register_span_end_hook,
     register_span_event_hook, clear_hooks,
 )
@@ -32,7 +32,7 @@
 __all__ = [
     "init", "span", "step", "cite", "end", "current_trace_id",
     "span_open", "span_event", "span_close",
-    "session", "set_session", "clear_session",
+    "session", "set_session", "clear_session", "session_reset",
     "register_span_start_hook", "register_span_end_hook",
     "register_span_event_hook", "clear_hooks",
     "trace", "tool", "eval",
diff --git a/wikitrace/sdk.py b/wikitrace/sdk.py
index b6bd6f2..0d588c1 100644
--- a/wikitrace/sdk.py
+++ b/wikitrace/sdk.py
@@ -397,3 +397,39 @@ def set_session(
 
 def clear_session() -> None:
     _session_attrs.set({})
+
+
+def session_reset() -> int:
+    """Close the current conversation segment and start a new one
+    under the same ``session_id``.
+
+    Use this when an agent's conversation history is reset mid-trace
+    (user clears chat, planner restarts from a checkpoint, evaluator
+    rolls back state). Spans before and after the reset still share
+    the same ``session_id`` — so cost rollups, user attribution, and
+    "all activity for this user this hour" queries continue to group
+    them — but they carry distinct ``session_segment`` integers so
+    the dashboard can render them as separate threads.
+
+    Returns the new segment number (starts at 1; increments with each
+    call). Outside an active session this is a no-op and returns 0.
+
+    Example::
+
+        with wikitrace.session(id="conv-1", user="alice"):
+            chain.invoke({"input": q1})            # segment 0
+            wikitrace.session_reset()              # bumps to segment 1
+            chain.invoke({"input": "start over"})  # segment 1
+    """
+    cur = _ambient_session()
+    if not cur.get("session_id"):
+        # No active session_id → nothing to segment. Returning 0
+        # rather than raising so downstream `wikitrace.session_reset()`
+        # calls in shared library code don't crash callers that
+        # forgot to wrap them in `session()`.
+        return 0
+    next_seg = int(cur.get("session_segment") or 0) + 1
+    new = dict(cur)
+    new["session_segment"] = next_seg
+    _session_attrs.set(new)
+    return next_seg