diff --git a/backend/packages/harness/deerflow/community/e2b_sandbox/e2b_sandbox.py b/backend/packages/harness/deerflow/community/e2b_sandbox/e2b_sandbox.py
index 75badf185..e8bee0613 100644
--- a/backend/packages/harness/deerflow/community/e2b_sandbox/e2b_sandbox.py
+++ b/backend/packages/harness/deerflow/community/e2b_sandbox/e2b_sandbox.py
@@ -7,6 +7,7 @@ import shlex
 import threading
 from typing import TYPE_CHECKING
 
+from e2b import FileNotFoundException
 from e2b_code_interpreter import Sandbox as E2BClientSandbox
 
 from deerflow.config.paths import VIRTUAL_PATH_PREFIX
@@ -357,16 +358,24 @@ class E2BSandbox(Sandbox):
             client = self._client
             if client is None:
                 raise RuntimeError("sandbox client has been closed")
-            try:
-                if append:
+            if append:
+                # E2B has no append write. Read-modify-write must treat only
+                # explicit not-found as empty; any other read failure would
+                # otherwise overwrite the original file with just the tail.
+                try:
+                    existing = client.files.read(resolved) or ""
+                except (FileNotFoundException, FileNotFoundError):
                     existing = ""
-                    try:
-                        existing = client.files.read(resolved) or ""
-                        if isinstance(existing, bytes):
-                            existing = existing.decode("utf-8", errors="replace")
-                    except Exception:
-                        existing = ""
-                    content = (existing or "") + content
+                except Exception:
+                    logger.error(
+                        "Append pre-read failed for %s; refusing to overwrite",
+                        resolved,
+                    )
+                    raise
+                if isinstance(existing, bytes):
+                    existing = existing.decode("utf-8", errors="replace")
+                content = existing + content
+            try:
                 client.files.write(resolved, content)
             except Exception as e:
                 logger.error("Failed to write file %s in e2b sandbox: %s", resolved, e)
diff --git a/backend/tests/test_e2b_sandbox_provider.py b/backend/tests/test_e2b_sandbox_provider.py
index 06c4bc8d3..bb5ef817b 100644
--- a/backend/tests/test_e2b_sandbox_provider.py
+++ b/backend/tests/test_e2b_sandbox_provider.py
@@ -18,6 +18,7 @@ from typing import Any
 from unittest.mock import MagicMock
 
 import pytest
+from e2b import FileNotFoundException, TimeoutException
 from pydantic import ValidationError
 
 from deerflow.community.e2b_sandbox.capacity import (
@@ -5213,3 +5214,72 @@ def test_glob_preserves_trailing_space_in_filename():
 
     assert matches == ["/home/user/notes.txt "]
     assert truncated is False
+
+
+@pytest.mark.parametrize("missing_exc", [FileNotFoundError, FileNotFoundException])
+def test_append_creates_file_when_file_does_not_exist(missing_exc):
+    # Append has no native write mode, so a missing file must still create one
+    # containing only the new fragment. Both the e2b SDK exception and the
+    # stdlib one used by FakeFilesAPI / compatible clients count as not-found.
+    class MissingFilesAPI(FakeFilesAPI):
+        def read(self, path: str, *, format: str | None = None):
+            self.read_calls.append((path, format))
+            raise missing_exc(path)
+
+    files = MissingFilesAPI()
+    sb = _make_sandbox(FakeClient(files=files))
+
+    sb.write_file("/mnt/user-data/outputs/report.txt", "conclusion", append=True)
+
+    assert files.write_calls == [("/home/user/outputs/report.txt", "conclusion")]
+
+
+def test_append_does_not_overwrite_when_read_fails(caplog):
+    # If the pre-read fails for any reason other than not-found, we cannot
+    # confirm the existing contents. Continuing would write only the tail and
+    # destroy the original file. Fail closed: raise, and never call write.
+    existing = b"important report body"
+
+    class TimeoutFilesAPI(FakeFilesAPI):
+        def read(self, path: str, *, format: str | None = None):
+            self.read_calls.append((path, format))
+            raise TimeoutException("read timed out")
+
+    files = TimeoutFilesAPI(store={"/home/user/outputs/report.txt": existing})
+    sb = _make_sandbox(FakeClient(files=files))
+
+    with caplog.at_level("ERROR"), pytest.raises(TimeoutException, match="read timed out"):
+        sb.write_file("/mnt/user-data/outputs/report.txt", "conclusion", append=True)
+
+    assert files.write_calls == []
+    assert files.store["/home/user/outputs/report.txt"] == existing
+    assert "refusing to overwrite" in caplog.text
+    assert "Failed to write file" not in caplog.text
+
+
+def test_append_accumulates_existing_content():
+    # The rewrite exists to keep read-modify-write. If someone later drops
+    # `existing` and writes only the tail, the not-found / fail-closed tests
+    # would still pass.
+    files = FakeFilesAPI(store={"/home/user/outputs/report.txt": b"hello"})
+    sb = _make_sandbox(FakeClient(files=files))
+
+    sb.write_file("/mnt/user-data/outputs/report.txt", " world", append=True)
+
+    assert files.write_calls == [("/home/user/outputs/report.txt", "hello world")]
+
+
+def test_append_decodes_bytes_preimage():
+    # FakeFilesAPI.read() returns str for valid utf-8. A bytes pre-image is
+    # what hits the decode branch before concatenation.
+    class BytesFilesAPI(FakeFilesAPI):
+        def read(self, path: str, *, format: str | None = None):
+            self.read_calls.append((path, format))
+            return self.store[path]
+
+    files = BytesFilesAPI(store={"/home/user/outputs/report.txt": b"hello"})
+    sb = _make_sandbox(FakeClient(files=files))
+
+    sb.write_file("/mnt/user-data/outputs/report.txt", " world", append=True)
+
+    assert files.write_calls == [("/home/user/outputs/report.txt", "hello world")]