fix: measure redacted file content with lone surrogates

src/band/runtime/tools/registry.py

@@ -906,7 +906,9 @@ def redact_tool_call_args(tool_name: str, args: dict[str, Any]) -> dict[str, Any
         return args
     return {
         **args,
-        "content": file_content_placeholder(len(content.encode("utf-8"))),
+        "content": file_content_placeholder(
+            len(content.encode("utf-8", "surrogatepass"))
+        ),
     }
 
 

tests/adapters/copilot_sdk/test_tool_bridging.py

@@ -10,9 +10,10 @@
 from pydantic import BaseModel, model_validator
 
 from band.adapters.copilot_sdk import _COPILOT_SDK_AVAILABLE
-from band.core.types import Emit
+from band.core.types import Emit, MessageType
 from band.runtime.custom_tools import declares_turn_effect
 from band.runtime.tools import ToolCallOutcome, TurnEffect
+from band.testing.fake_tools import events_of_type
 from tests.adapters.copilot_sdk.fakes import (
     FakeCopilotClient,
     FakeCopilotSession,
@@ -419,3 +420,28 @@ async def test_tool_call_event_redacts_file_content(self):
             == f"<{len(raw_content.encode('utf-8'))} byte file content>"
         )
         assert raw_content not in call_event["content"]
+
+    @pytest.mark.asyncio
+    async def test_lone_surrogate_file_content_is_redacted_without_losing_event(
+        self,
+    ) -> None:
+        client = FakeCopilotClient()
+        adapter = await make_started_adapter(client, emit=Emit.TOOL_CALLS)
+        tools = _SendRoomFileFakeTools()
+        await run_message(adapter, tools)
+        handler = client.sessions[0].find_tool("band_send_room_file").handler
+        await handler(
+            ToolInvocation(
+                tool_call_id="surrogate-file",
+                tool_name="band_send_room_file",
+                arguments={"content": "\ud800", "filename": "notes.txt"},
+            )
+        )
+        (call_event,) = events_of_type(tools, MessageType.TOOL_CALL)
+        call = json.loads(call_event["content"])
+        assert call["tool_call_id"] == "surrogate-file"
+        assert call["name"] == "band_send_room_file"
+        assert call["args"] == {
+            "filename": "notes.txt",
+            "content": "<3 byte file content>",
+        }