diff --git a/backend/packages/harness/deerflow/community/aio_sandbox/AGENTS.md b/backend/packages/harness/deerflow/community/aio_sandbox/AGENTS.md
index 0a683ff031..4498f841ba 100644
--- a/backend/packages/harness/deerflow/community/aio_sandbox/AGENTS.md
+++ b/backend/packages/harness/deerflow/community/aio_sandbox/AGENTS.md
@@ -93,3 +93,13 @@ cleanup. Failed discovery registration closes only its own HTTP client.
 
 Regressions: `test_aio_sandbox_provider.py`, `test_sandbox_quarantine.py`,
 `test_remote_sandbox_backend.py`, `test_provisioner_request_threading.py`.
+
+## Read failure contract
+
+`read_file` returns content or an ``Error: ...`` string, never raises. The
+server reports missing files as an error envelope with ``data: {}``, which the
+SDK cannot parse into ``FileReadResult``; the resulting ``ValidationError``
+becomes ``Error: file not found or unreadable: <path>`` logged at debug
+(missing files are expected on ReadBeforeWrite probes of new files), never the
+raw pydantic text. Other exceptions keep the error log.
+Regressions: `test_aio_sandbox.py`.
diff --git a/backend/packages/harness/deerflow/community/aio_sandbox/aio_sandbox.py b/backend/packages/harness/deerflow/community/aio_sandbox/aio_sandbox.py
index 2250a5dc68..ac82b02559 100644
--- a/backend/packages/harness/deerflow/community/aio_sandbox/aio_sandbox.py
+++ b/backend/packages/harness/deerflow/community/aio_sandbox/aio_sandbox.py
@@ -8,6 +8,7 @@ from dataclasses import dataclass, field
 import httpx
 from agent_sandbox import Sandbox as AioSandboxClient
 from agent_sandbox.core.api_error import ApiError
+from pydantic import ValidationError
 
 from deerflow.config.paths import VIRTUAL_PATH_PREFIX
 from deerflow.sandbox.download import DEFAULT_DOWNLOAD_LIMIT as _MAX_DOWNLOAD_SIZE
@@ -1029,6 +1030,16 @@ class AioSandbox(Sandbox):
                 kwargs["end_line"] = max(end_line, 0)
             result = self._client.file.read_file(file=path, **kwargs)
             return result.data.content if result.data else ""
+        except ValidationError as e:
+            # The server reports read failures (e.g. ENOENT) as an error
+            # envelope with ``data: {}``, which cannot satisfy the required
+            # ``content``/``file`` fields of the SDK's FileReadResult model,
+            # so the SDK raises a ValidationError. A missing file is an
+            # expected outcome (ReadBeforeWrite probes every new-file write),
+            # so log quietly and return an explicit, model-actionable result
+            # instead of leaking the pydantic error text.
+            logger.debug("Sandbox read_file validation failure for %r: %s", path, e)
+            return f"Error: file not found or unreadable: {path}"
         except Exception as e:
             logger.error(f"Failed to read file in sandbox: {e}")
             return f"Error: {e}"
diff --git a/backend/tests/test_aio_sandbox.py b/backend/tests/test_aio_sandbox.py
index 1623542a98..5dc235e113 100644
--- a/backend/tests/test_aio_sandbox.py
+++ b/backend/tests/test_aio_sandbox.py
@@ -1,5 +1,6 @@
 """Tests for AioSandbox concurrent command serialization (#1433)."""
 
+import logging
 import os
 import shlex
 import subprocess
@@ -3080,3 +3081,67 @@ def test_list_dir_preserves_trailing_space_in_filename(sandbox):
     sandbox._client.shell.exec_command = MagicMock(return_value=SimpleNamespace(data=SimpleNamespace(output="/test/notes.txt \n/test/sub\n\n__DF_FIND_STATUS__:0\n", exit_code=0)))
 
     assert sandbox.list_dir("/test") == ["/test/notes.txt ", "/test/sub"]
+
+
+def _sdk_file_read_validation_error() -> Exception:
+    """The exact exception the SDK raises for a server failure envelope.
+
+    The AIO server reports read failures (e.g. ENOENT) as
+    ``{"success": false, "message": ..., "data": {}}``; the empty ``data``
+    cannot satisfy the required fields of the SDK's ``FileReadResult`` model,
+    so parsing raises a ``ValidationError`` with two "Field required" errors.
+    Building it from the real SDK models keeps the test faithful to what
+    production raises.
+    """
+    from agent_sandbox.file.client import ResponseFileReadResult
+    from pydantic import ValidationError
+
+    try:
+        ResponseFileReadResult.model_validate({"success": False, "message": "file not found", "data": {}})
+    except ValidationError as e:
+        return e
+    raise AssertionError("model_validate should fail on data: {}")
+
+
+def test_read_file_missing_file_returns_actionable_error(sandbox, caplog):
+    """A server failure envelope must not leak the SDK's pydantic
+    validation-error text to the model. The result must state the file is
+    missing/unreadable so the agent can act on it (create it, pick another
+    path) instead of retrying the same read or "fixing" a nonexistent file."""
+    sandbox._client.file.read_file.side_effect = _sdk_file_read_validation_error()
+
+    with caplog.at_level(logging.DEBUG, logger="deerflow.community.aio_sandbox.aio_sandbox"):
+        result = sandbox.read_file("/workspace/new_file.py")
+
+    assert result == "Error: file not found or unreadable: /workspace/new_file.py"
+    assert "validation errors" not in result
+
+
+def test_read_file_missing_file_does_not_log_error_noise(sandbox, caplog):
+    """Missing files are an expected outcome — ReadBeforeWriteMiddleware
+    probes every new-file write — so the validation failure must stay out of
+    the error-level gateway log."""
+    sandbox._client.file.read_file.side_effect = _sdk_file_read_validation_error()
+
+    with caplog.at_level(logging.DEBUG, logger="deerflow.community.aio_sandbox.aio_sandbox"):
+        sandbox.read_file("/workspace/new_file.py")
+
+    assert not [record for record in caplog.records if record.levelno >= logging.ERROR]
+
+
+def test_read_file_success_returns_content(sandbox):
+    sandbox._client.file.read_file.return_value = SimpleNamespace(data=SimpleNamespace(content="hello\n", file="/workspace/a.txt"))
+    assert sandbox.read_file("/workspace/a.txt") == "hello\n"
+    sandbox._client.file.read_file.assert_called_once_with(file="/workspace/a.txt")
+
+
+def test_read_file_unexpected_error_still_logged(sandbox, caplog):
+    """Non-validation failures keep the error log and the generic error
+    string; only the server failure envelope is downgraded to debug."""
+    sandbox._client.file.read_file.side_effect = RuntimeError("connection reset")
+
+    with caplog.at_level(logging.ERROR, logger="deerflow.community.aio_sandbox.aio_sandbox"):
+        result = sandbox.read_file("/workspace/a.txt")
+
+    assert result == "Error: connection reset"
+    assert any(record.levelno >= logging.ERROR for record in caplog.records)