fix(sandbox): return actionable hint when read_file hits a binary file (#3624)

read_file decodes with UTF-8. Binary uploads (.xlsx, images, ...) raise
UnicodeDecodeError in the sandbox layer. UnicodeDecodeError is a ValueError
subclass, not an OSError, so it bypassed the typed handlers and fell through
to the generic except, surfacing a vague "Unexpected error reading file"
message. The model could not tell the file was binary, so it retried
read_file instead of switching to bash + pandas/openpyxl, burning LLM
round-trips and bloating context with repeated failures.

Add a dedicated UnicodeDecodeError handler that tells the model the file is
binary and to use bash with a suitable library (or view_image for images).

backend/packages/harness/deerflow/sandbox/tools.py

@@ -1665,6 +1665,12 @@ def read_file_tool(
         return f"Error: Permission denied reading file: {requested_path}"
     except IsADirectoryError:
         return f"Error: Path is a directory, not a file: {requested_path}"
+    except UnicodeDecodeError:
+        return (
+            f"Error: cannot read '{requested_path}' as text — it appears to be a binary file "
+            "(e.g. .xlsx, .pdf, or an image). read_file only supports UTF-8 text. Use bash with a "
+            "suitable library instead (pandas/openpyxl for spreadsheets), or view_image for images."
+        )
     except Exception as e:
         return f"Error: Unexpected error reading file: {_sanitize_error(e, runtime)}"
 

backend/tests/test_read_file_tool_binary.py

@@ -0,0 +1,65 @@
+"""read_file tool behaviour on binary files.
+
+``read_file`` decodes with UTF-8. Binary uploads (``.xlsx``, images, ...) raise
+``UnicodeDecodeError`` deep in the sandbox layer, which previously surfaced to
+the model as a vague ``Unexpected error reading file`` message. The model could
+not tell that the file was binary, so it retried ``read_file`` instead of
+switching to ``bash`` + pandas/openpyxl — burning LLM round-trips. These tests
+pin the actionable error contract and guard the normal text path.
+"""
+
+from pathlib import Path
+from types import SimpleNamespace
+
+from deerflow.sandbox.local.local_sandbox import LocalSandbox
+from deerflow.sandbox.tools import read_file_tool
+
+
+def _local_runtime(tmp_path: Path) -> SimpleNamespace:
+    for sub in ("workspace", "uploads", "outputs"):
+        (tmp_path / sub).mkdir(parents=True, exist_ok=True)
+    thread_data = {
+        "workspace_path": str(tmp_path / "workspace"),
+        "uploads_path": str(tmp_path / "uploads"),
+        "outputs_path": str(tmp_path / "outputs"),
+    }
+    return SimpleNamespace(
+        state={"sandbox": {"sandbox_id": "local:t1"}, "thread_data": thread_data},
+        context={"thread_id": "t1"},
+    )
+
+
+def test_read_file_tool_binary_file_returns_actionable_hint(tmp_path, monkeypatch) -> None:
+    runtime = _local_runtime(tmp_path)
+    # .xlsx is a zip container: header bytes PK\x03\x04 plus a non-UTF-8 byte 0x82
+    # that makes strict UTF-8 decoding fail (the exact byte seen in the field logs).
+    (tmp_path / "uploads" / "data.xlsx").write_bytes(b"PK\x03\x04\x14\x00\x00\x00\x08\x00\x82\x6a\xb1\x55")
+    monkeypatch.setattr("deerflow.sandbox.tools.ensure_sandbox_initialized", lambda runtime: LocalSandbox("t1"))
+    monkeypatch.setattr("deerflow.sandbox.tools.ensure_thread_directories_exist", lambda runtime: None)
+
+    result = read_file_tool.func(
+        runtime=runtime,
+        description="read uploaded excel",
+        path="/mnt/user-data/uploads/data.xlsx",
+    )
+
+    assert "Unexpected error" not in result, result
+    assert "binary" in result.lower(), result
+    # The model must be steered to bash + pandas/openpyxl, not another read_file.
+    assert "bash" in result.lower(), result
+
+
+def test_read_file_tool_text_file_unaffected(tmp_path, monkeypatch) -> None:
+    runtime = _local_runtime(tmp_path)
+    (tmp_path / "uploads" / "notes.txt").write_text("hello 你好\nsecond line", encoding="utf-8")
+    monkeypatch.setattr("deerflow.sandbox.tools.ensure_sandbox_initialized", lambda runtime: LocalSandbox("t1"))
+    monkeypatch.setattr("deerflow.sandbox.tools.ensure_thread_directories_exist", lambda runtime: None)
+
+    result = read_file_tool.func(
+        runtime=runtime,
+        description="read notes",
+        path="/mnt/user-data/uploads/notes.txt",
+    )
+
+    assert "hello 你好" in result, result
+    assert "binary" not in result.lower(), result