fix(tools): refuse an empty write_file body that would truncate a file

The handler opened the target in "w" mode without looking at the body, so a
call whose content section was lost by a parser cut the file to 0 bytes and
still answered exit_code=0 (#6414). Gate the truncating open on the size the
file has on disk — the read just above it answers "" for bytes it cannot
decode, so an undecodable target would otherwise look empty — and let only an
inline-JSON content key that is literally an empty string declare the clear.

Also stop turning a null content into the four characters "None", which could
be neither refused as a lost body nor honoured as an empty write.

tests/test_write_file_empty_body.py

@@ -0,0 +1,205 @@
+"""write_file: an empty body must not truncate a file that holds data (#6414).
+
+The reporter's shape: a model call whose arguments lost their content section
+(a #6013-class parser failure) reaches WriteFileTool with an empty body, the
+existing file is opened in "w" mode, and the tool answers exit_code=0 with
+"Wrote 0 bytes". Each "is refused" test below measures that the bytes at the
+path are still there afterwards; each "still works" test guards the write path
+this change must not narrow.
+"""
+import json
+import os
+import re
+import tempfile
+
+import pytest
+
+from src import tool_execution as te
+from src.agent_tools import ToolBlock
+from src.agent_tools.filesystem_tools import EditFileTool, WriteFileTool
+
+RECIPE = "# Classic banana cake\n\nMash 3 bananas. Bake 180C for 1 hour.\n"
+
+
+@pytest.fixture
+def target():
+    """A fresh directory under the system temp root, which _tool_path_roots allows."""
+    with tempfile.TemporaryDirectory(prefix="odysseus-6414-") as directory:
+        yield os.path.join(directory, "classic-banana-cake.md")
+
+
+def _seed(path, text=RECIPE):
+    with open(path, "w", encoding="utf-8") as handle:
+        handle.write(text)
+    return text
+
+
+def _read(path):
+    with open(path, encoding="utf-8") as handle:
+        return handle.read()
+
+
+def _text_call(path, body=None):
+    """The documented text form: first line is the path, the rest is the content."""
+    return path if body is None else f"{path}\n{body}"
+
+
+def _json_call(path, **content):
+    """The fenced inline-JSON form, which the handler decodes itself."""
+    return json.dumps({"path": path, **content})
+
+
+# ── The truncation the issue reports ──────────────────────────────────────
+@pytest.mark.asyncio
+async def test_empty_body_after_the_path_line_is_refused_and_the_file_survives(target):
+    _seed(target)
+    res = await WriteFileTool().execute(_text_call(target, ""), {})
+    assert res["exit_code"] == 1, res
+    assert _read(target) == RECIPE
+
+
+@pytest.mark.asyncio
+async def test_path_only_call_with_no_content_section_is_refused(target):
+    """`lines[1] if len(lines) > 1 else ""` has two producers; this is the no-newline one."""
+    _seed(target)
+    res = await WriteFileTool().execute(_text_call(target), {})
+    assert res["exit_code"] == 1, res
+    assert _read(target) == RECIPE
+
+
+@pytest.mark.asyncio
+async def test_inline_json_without_a_content_key_is_refused(target):
+    """A parser that keeps `path` and drops `content` is the reported failure."""
+    _seed(target)
+    res = await WriteFileTool().execute(json.dumps({"path": target}), {})
+    assert res["exit_code"] == 1, res
+    assert _read(target) == RECIPE
+
+
+@pytest.mark.asyncio
+async def test_inline_json_null_content_is_refused_and_never_written_as_the_word_none(target):
+    """`str(_a.get("content", ""))` on a null turns a lost body into the 4 bytes "None"."""
+    _seed(target)
+    res = await WriteFileTool().execute(_json_call(target, content=None), {})
+    assert res["exit_code"] == 1, res
+    assert _read(target) == RECIPE
+
+
+@pytest.mark.asyncio
+async def test_whitespace_only_body_is_refused(target):
+    """A body that carries no characters is the same failure with padding left in."""
+    _seed(target)
+    res = await WriteFileTool().execute(_text_call(target, "   \n  "), {})
+    assert res["exit_code"] == 1, res
+    assert _read(target) == RECIPE
+
+
+@pytest.mark.asyncio
+async def test_non_utf8_target_is_refused_on_its_size_not_on_the_decoded_read(target):
+    """The existing read swallows UnicodeDecodeError and answers "", which would let a
+    binary or latin-1 file look empty to the guard while holding real bytes."""
+    with open(target, "wb") as handle:
+        handle.write(b"\xc3\xa9\xe8\xaf\xad\xff\xfe\x00binary-ish payload")
+    before = os.path.getsize(target)
+    assert before > 0
+    res = await WriteFileTool().execute(_text_call(target, ""), {})
+    assert res["exit_code"] == 1, res
+    assert os.path.getsize(target) == before
+
+
+@pytest.mark.asyncio
+async def test_refusal_creates_no_extra_files_next_to_the_target(target):
+    _seed(target)
+    directory = os.path.dirname(target)
+    res = await WriteFileTool().execute(_text_call(target, ""), {})
+    assert res["exit_code"] == 1, res
+    assert os.listdir(directory) == [os.path.basename(target)]
+
+
+@pytest.mark.asyncio
+async def test_refusal_names_the_byte_count_and_the_explicit_form(target):
+    _seed(target)
+    res = await WriteFileTool().execute(_text_call(target, ""), {})
+    error = res.get("error", "")
+    assert str(len(RECIPE)) in error, error
+    # The caller in a loop has to be able to correct itself in one round.
+    assert '"content": ""' in error, error
+    assert "output" not in res, res
+
+
+@pytest.mark.asyncio
+async def test_the_resend_the_refusal_prints_actually_clears_the_file(target):
+    """The guidance is only useful if a caller can paste it back verbatim. This reads
+    the JSON object out of the refusal and runs it as the next call."""
+    _seed(target)
+    refused = await WriteFileTool().execute(_text_call(target, ""), {})
+    resend = re.search(r"\{.*\}", refused["error"], re.S)
+    assert resend, refused
+    res = await WriteFileTool().execute(resend.group(0), {})
+    assert res["exit_code"] == 0, res
+    assert os.path.getsize(target) == 0
+
+
+# ── Deliberate writes this change must keep working ───────────────────────
+@pytest.mark.asyncio
+async def test_explicit_empty_content_in_the_json_form_clears_the_file(target):
+    """"A deliberate empty-file creation can be made explicit" (the issue's own words):
+    a `content` key that is literally an empty string is a declaration, not a loss."""
+    _seed(target)
+    res = await WriteFileTool().execute(_json_call(target, content=""), {})
+    assert res["exit_code"] == 0, res
+    assert os.path.getsize(target) == 0
+
+
+@pytest.mark.asyncio
+async def test_empty_body_on_a_new_path_still_creates_an_empty_file(target):
+    res = await WriteFileTool().execute(_text_call(target, ""), {})
+    assert res["exit_code"] == 0, res
+    assert os.path.isfile(target) and os.path.getsize(target) == 0
+
+
+@pytest.mark.asyncio
+async def test_empty_body_over_an_already_empty_file_succeeds(target):
+    """Nothing is at risk, so the guard has nothing to refuse."""
+    _seed(target, "")
+    res = await WriteFileTool().execute(_text_call(target, ""), {})
+    assert res["exit_code"] == 0, res
+    assert os.path.getsize(target) == 0
+
+
+@pytest.mark.asyncio
+async def test_a_real_body_still_writes_and_reports_a_diff(target):
+    _seed(target)
+    replacement = "# Classic banana cake\n\nMash 4 bananas.\n"
+    res = await WriteFileTool().execute(_text_call(target, replacement), {})
+    assert res["exit_code"] == 0, res
+    assert _read(target) == replacement
+    assert res["diff"]["added"] == 1 and res["diff"]["removed"] == 1
+
+
+@pytest.mark.asyncio
+async def test_edit_file_remains_an_explicit_way_to_clear_a_file(target):
+    """The route this change leaves open for a caller that cannot reach the fenced
+    inline-JSON form: replace the whole content with nothing."""
+    _seed(target)
+    res = await EditFileTool().execute(
+        json.dumps({"path": target, "old_string": RECIPE, "new_string": ""}), {}
+    )
+    assert res["exit_code"] == 0, res
+    assert os.path.getsize(target) == 0
+
+
+# ── The live dispatch path, not just the handler ──────────────────────────
+@pytest.mark.asyncio
+async def test_execute_tool_block_refuses_a_lost_body_without_touching_the_file(target, monkeypatch):
+    """#6414 reached the reporter through a parsed model call, so the refusal has to
+    survive execute_tool_block's wrapping and still report failure upstream."""
+    _seed(target)
+    monkeypatch.setattr(te, "_owner_is_admin", lambda owner: True)
+    _desc, result = await te.execute_tool_block(
+        ToolBlock("write_file", _text_call(target, "")),
+        owner="admin",
+        security_context=te.NO_TOOL_SECURITY_CONTEXT,
+    )
+    assert result.get("exit_code") == 1, result
+    assert _read(target) == RECIPE