diff --git a/acp_adapter/provenance.py b/acp_adapter/provenance.py
new file mode 100644
index 00000000000..58b05daf5af
--- /dev/null
+++ b/acp_adapter/provenance.py
@@ -0,0 +1,127 @@
+"""Derive ACP session-provenance metadata from the existing compression chain.
+
+This is an additive Hermes extension surfaced under ACP ``_meta.hermes`` so
+existing ACP clients ignore it. It carries no new persisted state: everything
+is derived on demand from the ``sessions`` table (``parent_session_id`` /
+``end_reason``), which already models compression-continuation chains.
+
+The ACP/editor ``session_id`` stays the stable public handle. When context
+compression rotates the internal Hermes head, ``build_session_provenance`` lets
+a client see the previous/current internal ids and the lineage root without
+parsing status text, guessing from token drops, or reading ``state.db``.
+"""
+
+from __future__ import annotations
+
+from typing import Any, Dict, Optional
+
+# Bound defensive walks; compression chains this deep are pathological.
+_MAX_WALK = 100
+
+
+def build_session_provenance(
+    db: Any,
+    acp_session_id: str,
+    current_hermes_session_id: str,
+    *,
+    previous_hermes_session_id: Optional[str] = None,
+) -> Optional[Dict[str, Any]]:
+    """Build ``_meta.hermes.sessionProvenance`` for an ACP session.
+
+    Args:
+        db: A ``SessionDB`` (must expose ``get_session``).
+        acp_session_id: The stable ACP/editor-facing session handle.
+        current_hermes_session_id: The live internal Hermes DB session id
+            (``state.agent.session_id``).
+        previous_hermes_session_id: The internal id from before the most recent
+            turn, when known. Supplied by ``prompt()`` to flag a rotation.
+
+    Returns:
+        A dict suitable for ``{"hermes": {"sessionProvenance": <dict>}}`` under
+        ACP ``_meta``, or ``None`` if the session can't be read.
+    """
+    try:
+        row = db.get_session(current_hermes_session_id)
+    except Exception:
+        return None
+    if not row:
+        return None
+
+    parent_id = row.get("parent_session_id")
+    end_reason = row.get("end_reason")
+
+    # Walk parents to the lineage root and count compression depth. Only
+    # compression-split parents (parent.end_reason == 'compression') count
+    # toward depth — delegate/branch children share the parent_session_id
+    # column but are not compaction boundaries.
+    root_id = current_hermes_session_id
+    compression_depth = 0
+    cursor_parent = parent_id
+    seen = {current_hermes_session_id}
+    for _ in range(_MAX_WALK):
+        if not cursor_parent or cursor_parent in seen:
+            break
+        seen.add(cursor_parent)
+        try:
+            prow = db.get_session(cursor_parent)
+        except Exception:
+            prow = None
+        if not prow:
+            break
+        root_id = cursor_parent
+        if prow.get("end_reason") == "compression":
+            compression_depth += 1
+        cursor_parent = prow.get("parent_session_id")
+
+    # A session is a compression continuation when its parent was ended with
+    # end_reason='compression'. Determine that from the immediate parent.
+    is_continuation = False
+    if parent_id:
+        try:
+            immediate_parent = db.get_session(parent_id)
+        except Exception:
+            immediate_parent = None
+        if immediate_parent and immediate_parent.get("end_reason") == "compression":
+            is_continuation = True
+
+    rotated = bool(
+        previous_hermes_session_id
+        and previous_hermes_session_id != current_hermes_session_id
+    )
+
+    provenance: Dict[str, Any] = {
+        "acpSessionId": acp_session_id,
+        "currentHermesSessionId": current_hermes_session_id,
+        "rootHermesSessionId": root_id,
+        "parentHermesSessionId": parent_id,
+        "sessionKind": "continuation" if is_continuation else "root",
+        "compressionDepth": compression_depth,
+    }
+    if previous_hermes_session_id:
+        provenance["previousHermesSessionId"] = previous_hermes_session_id
+    if rotated:
+        # The head moved during the last turn. The only mechanism that rotates
+        # the internal id mid-turn is compression-driven session splitting.
+        provenance["reason"] = "compression"
+        provenance["creatorKind"] = "compression"
+
+    return provenance
+
+
+def session_provenance_meta(
+    db: Any,
+    acp_session_id: str,
+    current_hermes_session_id: str,
+    *,
+    previous_hermes_session_id: Optional[str] = None,
+) -> Optional[Dict[str, Any]]:
+    """Return a ready ``_meta`` payload: ``{"hermes": {"sessionProvenance": ...}}``."""
+    prov = build_session_provenance(
+        db,
+        acp_session_id,
+        current_hermes_session_id,
+        previous_hermes_session_id=previous_hermes_session_id,
+    )
+    if prov is None:
+        return None
+    return {"hermes": {"sessionProvenance": prov}}
diff --git a/acp_adapter/server.py b/acp_adapter/server.py
index b4195af87d8..6901fe28e88 100644
--- a/acp_adapter/server.py
+++ b/acp_adapter/server.py
@@ -71,6 +71,7 @@ from acp_adapter.events import (
     make_tool_progress_cb,
 )
 from acp_adapter.permissions import make_approval_callback
+from acp_adapter.provenance import session_provenance_meta
 from acp_adapter.session import SessionManager, SessionState, _expand_acp_enabled_toolsets
 from acp_adapter.tools import build_tool_complete, build_tool_start
 
@@ -709,8 +710,39 @@ class HermesACPAgent(acp.Agent):
                 exc_info=True,
             )
 
-    async def _send_session_info_update(self, session_id: str) -> None:
-        """Send ACP native session metadata after Hermes changes it."""
+    def _provenance_meta(
+        self,
+        acp_session_id: str,
+        current_hermes_session_id: str,
+        previous_hermes_session_id: Optional[str] = None,
+    ) -> Optional[dict]:
+        """Best-effort ``_meta.hermes.sessionProvenance`` for an ACP session."""
+        try:
+            return session_provenance_meta(
+                self.session_manager._get_db(),
+                acp_session_id,
+                current_hermes_session_id,
+                previous_hermes_session_id=previous_hermes_session_id,
+            )
+        except Exception:
+            logger.debug(
+                "Could not build ACP session provenance for %s", acp_session_id, exc_info=True
+            )
+            return None
+
+    async def _send_session_info_update(
+        self,
+        session_id: str,
+        *,
+        current_hermes_session_id: Optional[str] = None,
+        previous_hermes_session_id: Optional[str] = None,
+    ) -> None:
+        """Send ACP native session metadata after Hermes changes it.
+
+        When the internal Hermes head rotated (e.g. compression-driven session
+        split during a turn), pass ``previous_hermes_session_id`` so the
+        attached ``_meta.hermes.sessionProvenance`` flags the rotation reason.
+        """
         if not self._conn:
             return
         try:
@@ -727,10 +759,16 @@ class HermesACPAgent(acp.Agent):
         # the updated_at since we're emitting this notification precisely
         # because the title was just refreshed.
         updated_at = datetime.now(timezone.utc).isoformat()
+        meta = self._provenance_meta(
+            session_id,
+            current_hermes_session_id or session_id,
+            previous_hermes_session_id,
+        )
         update = SessionInfoUpdate(
             session_update="session_info_update",
             title=title if isinstance(title, str) and title.strip() else None,
             updated_at=updated_at,
+            field_meta=meta,
         )
         try:
             await self._conn.session_update(
@@ -1081,6 +1119,9 @@ class HermesACPAgent(acp.Agent):
             session_id=state.session_id,
             models=self._build_model_state(state),
             modes=self._session_modes(state),
+            field_meta=self._provenance_meta(
+                state.session_id, getattr(state.agent, "session_id", state.session_id)
+            ),
         )
 
     async def load_session(
@@ -1125,6 +1166,9 @@ class HermesACPAgent(acp.Agent):
         return LoadSessionResponse(
             models=self._build_model_state(state),
             modes=self._session_modes(state),
+            field_meta=self._provenance_meta(
+                session_id, getattr(state.agent, "session_id", session_id)
+            ),
         )
 
     async def resume_session(
@@ -1157,6 +1201,9 @@ class HermesACPAgent(acp.Agent):
         return ResumeSessionResponse(
             models=self._build_model_state(state),
             modes=self._session_modes(state),
+            field_meta=self._provenance_meta(
+                state.session_id, getattr(state.agent, "session_id", state.session_id)
+            ),
         )
 
     async def cancel(self, session_id: str, **kwargs: Any) -> None:
@@ -1494,6 +1541,11 @@ class HermesACPAgent(acp.Agent):
                         logger.debug("Could not clear ACP session context", exc_info=True)
 
         try:
+            # Snapshot the internal Hermes DB session id before the turn so we
+            # can detect a compression-driven session rotation afterwards. The
+            # ACP `session_id` stays the stable client handle; agent.session_id
+            # is the live internal head that compression may rotate.
+            pre_turn_hermes_id = getattr(state.agent, "session_id", None)
             # Wrap the executor call in a fresh copy of the current context so
             # concurrent ACP sessions on the shared ThreadPoolExecutor don't
             # stomp on each other's ContextVar writes (HERMES_SESSION_KEY in
@@ -1512,6 +1564,30 @@ class HermesACPAgent(acp.Agent):
             # Persist updated history so sessions survive process restarts.
             self.session_manager.save_session(session_id)
 
+        # Detect a compression-driven internal session rotation. If the agent's
+        # DB head moved during the turn, emit a session_info_update carrying
+        # _meta.hermes.sessionProvenance so ACP clients can render the boundary
+        # and keep old/new ids in lineage. The ACP session_id is unchanged.
+        post_turn_hermes_id = getattr(state.agent, "session_id", None)
+        if (
+            conn
+            and post_turn_hermes_id
+            and pre_turn_hermes_id
+            and post_turn_hermes_id != pre_turn_hermes_id
+        ):
+            try:
+                await self._send_session_info_update(
+                    session_id,
+                    current_hermes_session_id=post_turn_hermes_id,
+                    previous_hermes_session_id=pre_turn_hermes_id,
+                )
+            except Exception:
+                logger.debug(
+                    "Could not emit ACP provenance update after rotation for %s",
+                    session_id,
+                    exc_info=True,
+                )
+
         final_response = result.get("final_response", "")
         cancelled = bool(state.cancel_event and state.cancel_event.is_set())
         interrupted = bool(result.get("interrupted")) or cancelled
diff --git a/tests/acp/test_session_provenance.py b/tests/acp/test_session_provenance.py
new file mode 100644
index 00000000000..b1d80907cf5
--- /dev/null
+++ b/tests/acp/test_session_provenance.py
@@ -0,0 +1,103 @@
+"""Tests for ACP session-provenance derivation (issue #33617).
+
+Exercises acp_adapter.provenance against a real SessionDB — no mocks — covering
+the acceptance-criteria matrix: root session, compression-split continuation,
+multi-depth chains, rotation flagging, and graceful handling of unknown ids.
+"""
+
+import time
+
+import pytest
+
+from acp_adapter.provenance import build_session_provenance, session_provenance_meta
+from hermes_state import SessionDB
+
+
+@pytest.fixture()
+def db(tmp_path):
+    d = SessionDB(db_path=tmp_path / "state.db")
+    yield d
+
+
+def _mk(db, sid, parent=None):
+    db.create_session(session_id=sid, source="acp", parent_session_id=parent)
+
+
+def test_root_session_no_compression(db):
+    _mk(db, "root1")
+    prov = build_session_provenance(db, "acp-1", "root1")
+    assert prov["acpSessionId"] == "acp-1"
+    assert prov["currentHermesSessionId"] == "root1"
+    assert prov["rootHermesSessionId"] == "root1"
+    assert prov["parentHermesSessionId"] is None
+    assert prov["sessionKind"] == "root"
+    assert prov["compressionDepth"] == 0
+    assert "reason" not in prov  # no rotation signalled
+
+
+def test_compression_split_continuation(db):
+    # Parent ended with compression, child created afterwards.
+    _mk(db, "old")
+    db.end_session("old", "compression")
+    time.sleep(0.001)
+    _mk(db, "new", parent="old")
+
+    prov = build_session_provenance(
+        db, "acp-1", "new", previous_hermes_session_id="old"
+    )
+    assert prov["sessionKind"] == "continuation"
+    assert prov["parentHermesSessionId"] == "old"
+    assert prov["rootHermesSessionId"] == "old"
+    assert prov["compressionDepth"] == 1
+    assert prov["previousHermesSessionId"] == "old"
+    # Head rotated this turn → reason/creatorKind flagged.
+    assert prov["reason"] == "compression"
+    assert prov["creatorKind"] == "compression"
+
+
+def test_multi_depth_chain(db):
+    _mk(db, "s0")
+    db.end_session("s0", "compression")
+    _mk(db, "s1", parent="s0")
+    db.end_session("s1", "compression")
+    _mk(db, "s2", parent="s1")
+
+    prov = build_session_provenance(db, "acp-1", "s2")
+    assert prov["rootHermesSessionId"] == "s0"
+    assert prov["compressionDepth"] == 2
+    assert prov["sessionKind"] == "continuation"
+
+
+def test_non_compression_parent_is_root_not_continuation(db):
+    # A child with a parent that did NOT end via compression (e.g. delegate
+    # or branch child) must not be reported as a compression continuation.
+    _mk(db, "p")
+    _mk(db, "c", parent="p")  # parent still live, no end_reason
+    prov = build_session_provenance(db, "acp-1", "c")
+    assert prov["sessionKind"] == "root"
+    assert prov["compressionDepth"] == 0
+    assert prov["rootHermesSessionId"] == "p"  # lineage root still walked
+
+
+def test_no_false_rotation_when_head_unchanged(db):
+    _mk(db, "s")
+    # previous == current → no rotation reason emitted.
+    prov = build_session_provenance(
+        db, "acp-1", "s", previous_hermes_session_id="s"
+    )
+    assert "reason" not in prov
+    assert "creatorKind" not in prov
+    assert prov["previousHermesSessionId"] == "s"
+
+
+def test_unknown_session_returns_none(db):
+    assert build_session_provenance(db, "acp-1", "does-not-exist") is None
+    assert session_provenance_meta(db, "acp-1", "does-not-exist") is None
+
+
+def test_meta_wrapper_shape(db):
+    _mk(db, "root1")
+    meta = session_provenance_meta(db, "acp-1", "root1")
+    assert set(meta.keys()) == {"hermes"}
+    assert "sessionProvenance" in meta["hermes"]
+    assert meta["hermes"]["sessionProvenance"]["currentHermesSessionId"] == "root1"