fix(tools): keep apply_patch additions line-separated

nanobot/agent/tools/apply_patch.py

@@ -75,6 +75,18 @@ def _line_diff_stats(before: str, after: str) -> tuple[int, int]:
     return added, deleted
 
 
+def _append_text(content: str, addition: str) -> str:
+    """Append text without merging it into an unterminated final line."""
+    base = content.replace("\r\n", "\n")
+    extra = addition.replace("\r\n", "\n")
+    if base and extra and not base.endswith("\n") and not extra.startswith("\n"):
+        base += "\n"
+    combined = base + extra
+    if combined and not combined.endswith("\n"):
+        combined += "\n"
+    return combined
+
+
 def _format_summary(summary: _PatchSummary) -> str:
     stats = ""
     if summary.added or summary.deleted:
@@ -177,9 +189,7 @@ class ApplyPatchTool(_FsTool):
 
                     if exists:
                         uses_crlf = "\r\n" in content
-                        new_norm = content.replace("\r\n", "\n") + new_text.replace("\r\n", "\n")
+                        new_norm = _append_text(content, new_text)
-                        if new_norm and not new_norm.endswith("\n"):
-                            new_norm += "\n"
                         if uses_crlf:
                             new_norm = new_norm.replace("\n", "\r\n")
                         writes[source] = new_norm

tests/tools/test_apply_patch_tool.py

@@ -89,6 +89,69 @@ def test_apply_patch_edits_add_to_existing_file(tmp_path):
     )
 
 
+def test_apply_patch_edits_add_to_existing_file_without_final_newline(tmp_path):
+    target = tmp_path / "notes.txt"
+    target.write_text("alpha", encoding="utf-8")
+    tool = ApplyPatchTool(workspace=tmp_path)
+
+    result = asyncio.run(
+        tool.execute(
+            edits=[
+                {
+                    "path": "notes.txt",
+                    "action": "add",
+                    "new_text": "beta",
+                }
+            ]
+        )
+    )
+
+    assert "update notes.txt" in result
+    assert target.read_text(encoding="utf-8") == "alpha\nbeta\n"
+
+
+def test_apply_patch_edits_add_to_existing_crlf_file_without_final_newline(tmp_path):
+    target = tmp_path / "notes.txt"
+    target.write_bytes(b"alpha\r\nbravo")
+    tool = ApplyPatchTool(workspace=tmp_path)
+
+    result = asyncio.run(
+        tool.execute(
+            edits=[
+                {
+                    "path": "notes.txt",
+                    "action": "add",
+                    "new_text": "charlie",
+                }
+            ]
+        )
+    )
+
+    assert "update notes.txt" in result
+    assert target.read_bytes() == b"alpha\r\nbravo\r\ncharlie\r\n"
+
+
+def test_apply_patch_edits_add_to_existing_file_respects_leading_newline(tmp_path):
+    target = tmp_path / "notes.txt"
+    target.write_text("alpha", encoding="utf-8")
+    tool = ApplyPatchTool(workspace=tmp_path)
+
+    result = asyncio.run(
+        tool.execute(
+            edits=[
+                {
+                    "path": "notes.txt",
+                    "action": "add",
+                    "new_text": "\nbeta",
+                }
+            ]
+        )
+    )
+
+    assert "update notes.txt" in result
+    assert target.read_text(encoding="utf-8") == "alpha\nbeta\n"
+
+
 def test_apply_patch_rejects_delete_action(tmp_path):
     target = tmp_path / "utils.py"
     target.write_text("def unused():\n    pass\ndef used():\n    return 1\n")