diff --git a/acp_adapter/tools.py b/acp_adapter/tools.py
index 22b181650c1..8de0b6b1ac1 100644
--- a/acp_adapter/tools.py
+++ b/acp_adapter/tools.py
@@ -202,7 +202,7 @@ def _json_loads_maybe(value: Optional[str]) -> Any:
         return None
 
 
-def _tool_result_failed(result: Optional[str]) -> bool:
+def _tool_result_failed(result: Optional[str], tool_name: str | None = None) -> bool:
     """Return True when a structured Hermes tool result clearly failed.
 
     Keep this deliberately conservative. Plain text can contain words like
@@ -221,6 +221,13 @@ def _tool_result_failed(result: Optional[str]) -> bool:
     if isinstance(exit_code, int) and exit_code != 0:
         return True
 
+    # Hermes core/polished tools commonly report tool-level failures as a
+    # structured {"error": "..."} payload without an explicit success flag.
+    # Keep generic plugin/unknown tool payloads conservative to avoid marking
+    # optional diagnostic messages as failed.
+    if tool_name in _POLISHED_TOOLS and data.get("error") and not data.get("content"):
+        return True
+
     return False
 
 
@@ -1318,7 +1325,7 @@ def build_tool_complete(
     return acp.update_tool_call(
         tool_call_id,
         kind=kind,
-        status="failed" if _tool_result_failed(result) else "completed",
+        status="failed" if _tool_result_failed(result, tool_name) else "completed",
         content=content,
         raw_output=None if tool_name in _POLISHED_TOOLS or _is_structured_json_result(result) else result,
     )
diff --git a/tests/acp/test_tools.py b/tests/acp/test_tools.py
index efce0d24bfd..a077160b1f0 100644
--- a/tests/acp/test_tools.py
+++ b/tests/acp/test_tools.py
@@ -365,6 +365,10 @@ class TestBuildToolComplete:
         result = build_tool_complete("tc-ok", "terminal", "tests failed: 1 assertion error")
         assert result.status == "completed"
 
+    def test_build_tool_complete_marks_structured_polished_tool_error_as_failed(self):
+        result = build_tool_complete("tc-fail", "read_file", '{"error": "File not found"}')
+        assert result.status == "failed"
+
     def test_build_tool_complete_keeps_json_error_without_failure_flag_completed(self):
         result = build_tool_complete("tc-ok", "some_tool", '{"error": "timeout while reading optional source"}')
         assert result.status == "completed"