diff --git a/backend/app/gateway/services.py b/backend/app/gateway/services.py
index a371c7b899..46baa35c3c 100644
--- a/backend/app/gateway/services.py
+++ b/backend/app/gateway/services.py
@@ -702,6 +702,33 @@ def _normalize_external_skill_context(value: Any, *, location: str) -> list[dict
     return normalized
 
 
+def _normalize_external_tool_artifacts(value: Any, *, location: str) -> list[dict[str, Any]] | None:
+    """Reject artifact-registry entries the reducer cannot key.
+
+    ``tool_artifacts`` is a checkpointed reducer channel a caller may Overwrite.
+    ``merge_tool_artifacts`` keys each entry by its ``handle``, so an entry whose
+    handle is missing or unhashable raises while the state update merges — a 500
+    instead of a 400 at this boundary. The thread-state route persists the value
+    with ``Overwrite`` before any reducer sees it, so the thread stays broken for
+    every later artifact capture. Require the registry's list-of-objects shape
+    with a non-empty string handle; the reducer also tolerates an already-poisoned
+    checkpoint.
+    """
+    if value is None:
+        return None
+    if not isinstance(value, list):
+        raise HTTPException(status_code=400, detail=f"{location} must be a list of artifact objects")
+    normalized: list[dict[str, Any]] = []
+    for index, entry in enumerate(value):
+        if not isinstance(entry, Mapping):
+            raise HTTPException(status_code=400, detail=f"{location}[{index}] must be an object")
+        handle = entry.get("handle")
+        if not isinstance(handle, str) or not handle:
+            raise HTTPException(status_code=400, detail=f"{location}[{index}].handle must be a non-empty string")
+        normalized.append(dict(entry))
+    return normalized
+
+
 def _normalize_external_goal(value: Any, *, location: str) -> dict[str, Any] | None:
     """Require usable goal text without rebuilding caller-supplied goal state."""
     if value is None:
@@ -759,6 +786,12 @@ def strip_server_owned_state_metadata(values: Mapping[str, Any]) -> dict[str, An
         elif channel == "skill_context":
             normalized_skill_context = _normalize_external_skill_context(value, location="values.skill_context")
             stripped[channel] = None if normalized_skill_context is None else [_strip_external_metadata_from_message_like(item) for item in normalized_skill_context]
+        elif channel == "tool_artifacts":
+            normalized_tool_artifacts = _normalize_external_tool_artifacts(value, location="values.tool_artifacts")
+            # Registry entries are rich dicts, so keep the per-entry metadata
+            # strip the generic list branch applied: a caller-crafted entry must
+            # not smuggle server-owned message metadata into the checkpoint.
+            stripped[channel] = None if normalized_tool_artifacts is None else [_strip_external_metadata_from_message_like(item) for item in normalized_tool_artifacts]
         elif isinstance(value, list):
             stripped[channel] = [_strip_external_metadata_from_message_like(item) for item in value]
         else:
@@ -829,6 +862,8 @@ def normalize_input(raw_input: dict[str, Any] | None, *, trusted_internal: bool
                 result = {**result, "delegations": cleaned}
         if "skill_context" in result:
             result = {**result, "skill_context": _normalize_external_skill_context(result["skill_context"], location="input.skill_context")}
+        if "tool_artifacts" in result:
+            result = {**result, "tool_artifacts": _normalize_external_tool_artifacts(result["tool_artifacts"], location="input.tool_artifacts")}
         # merge_goal ignores None, so only a goal value replaces the goal.
         if result.get("goal") is not None:
             normalized_goal = _normalize_external_goal(result["goal"], location="input.goal")
diff --git a/backend/packages/harness/deerflow/agents/thread_state.py b/backend/packages/harness/deerflow/agents/thread_state.py
index d4656bd923..d213e54485 100644
--- a/backend/packages/harness/deerflow/agents/thread_state.py
+++ b/backend/packages/harness/deerflow/agents/thread_state.py
@@ -398,9 +398,16 @@ def merge_tool_artifacts(existing: list[ArtifactEntry] | None, new: list[Artifac
     by_handle: dict[str, ArtifactEntry] = {}
     order: list[str] = []
     for entry in [*(existing or []), *new]:
-        if not isinstance(entry, dict) or "handle" not in entry:
+        if not isinstance(entry, dict):
+            continue
+        # The state-update route persists reducer values with ``Overwrite``, so
+        # a checkpoint can carry an entry whose handle is missing or unhashable
+        # without this reducer ever seeing it. Skip such an entry instead of
+        # raising on the dict key: one bad write must not fail every later
+        # artifact capture on the thread.
+        handle = entry.get("handle")
+        if not isinstance(handle, str) or not handle:
             continue
-        handle = entry["handle"]
         if handle not in by_handle:
             order.append(handle)
         by_handle[handle] = entry
diff --git a/backend/tests/test_artifact_registry.py b/backend/tests/test_artifact_registry.py
index b41438761f..cfe14c5587 100644
--- a/backend/tests/test_artifact_registry.py
+++ b/backend/tests/test_artifact_registry.py
@@ -629,6 +629,19 @@ def test_merge_tool_artifacts_empty_new_preserves_existing():
     assert merged2 == existing
 
 
+def test_merge_tool_artifacts_skips_an_unhashable_handle():
+    """A checkpoint poisoned through the state route must not fail later captures."""
+    merged = merge_tool_artifacts([{"handle": ["poisoned"]}], [{"handle": "art_1", "display_name": "a.txt"}])
+
+    assert [entry["handle"] for entry in merged] == ["art_1"]
+
+
+def test_merge_tool_artifacts_skips_missing_or_empty_handles():
+    merged = merge_tool_artifacts(None, [{"display_name": "no handle"}, {"handle": ""}, {"handle": "art_2"}])
+
+    assert [entry["handle"] for entry in merged] == ["art_2"]
+
+
 def test_merge_tool_artifacts_trim_directive_slides_window():
     """A trailing trim directive makes the configured cap a sliding window."""
     existing = [_entry(f"art_{i:08x}", i) for i in range(20)]
diff --git a/backend/tests/test_gateway_services.py b/backend/tests/test_gateway_services.py
index 6a1e5c4889..d77a1252aa 100644
--- a/backend/tests/test_gateway_services.py
+++ b/backend/tests/test_gateway_services.py
@@ -861,6 +861,69 @@ def test_normalize_input_rejects_malformed_message_with_400():
     assert "input.messages[1]" in excinfo.value.detail
 
 
+@pytest.mark.parametrize(
+    "tool_artifacts",
+    [
+        "oops",
+        ["oops"],
+        [{"display_name": "report.md"}],
+        [{"handle": 3}],
+        [{"handle": ["h"]}],
+        {"type": "__overwrite__", "value": []},
+    ],
+    ids=["non-list", "non-object-entry", "missing-handle", "non-string-handle", "unhashable-handle", "overwrite-shaped"],
+)
+def test_normalize_input_rejects_unusable_tool_artifacts_with_400(tool_artifacts):
+    """An entry the registry reducer cannot key must be a 400 here.
+
+    ``merge_tool_artifacts`` keys each entry by ``handle``; a missing or
+    unhashable handle raised while the state update merged, and the state route
+    persists reducer values with ``Overwrite``, so the thread stayed broken for
+    every later artifact capture.
+    """
+    from fastapi import HTTPException
+
+    from app.gateway.services import normalize_input
+
+    with pytest.raises(HTTPException) as excinfo:
+        normalize_input({"tool_artifacts": tool_artifacts})
+
+    assert excinfo.value.status_code == 400
+    assert "input.tool_artifacts" in excinfo.value.detail
+
+
+def test_normalize_input_keeps_usable_tool_artifacts():
+    from app.gateway.services import normalize_input
+
+    entry = {"handle": "art_00000001", "display_name": "report.md", "artifact_type": "file"}
+
+    assert normalize_input({"tool_artifacts": [entry]})["tool_artifacts"] == [entry]
+
+
+def test_state_values_reject_unusable_tool_artifacts_with_400():
+    """The thread-state update route shares the artifact-registry boundary."""
+    from fastapi import HTTPException
+
+    from app.gateway.services import strip_server_owned_state_metadata
+
+    with pytest.raises(HTTPException) as excinfo:
+        strip_server_owned_state_metadata({"tool_artifacts": [{"handle": ["h"]}]})
+
+    assert excinfo.value.status_code == 400
+    assert "values.tool_artifacts" in excinfo.value.detail
+
+
+def test_state_values_strip_server_owned_metadata_from_tool_artifacts():
+    """The registry branch keeps the per-entry strip the generic list branch applied."""
+    from app.gateway.services import strip_server_owned_state_metadata
+
+    entry = {"handle": "art_1", "additional_kwargs": {"original_user_content": "forged", "title": "kept"}}
+
+    stripped = strip_server_owned_state_metadata({"tool_artifacts": [entry]})["tool_artifacts"][0]
+
+    assert stripped["additional_kwargs"] == {"title": "kept"}
+
+
 @pytest.mark.parametrize(
     "skill_context",
     [