apply_patch grammar accepts empty updates that the runtime parser rejects

[22373448]

The custom apply_patch grammar allows an Update File operation containing only a rename:

# src/agents/sandbox/capabilities/tools/apply_patch_tool.py
update_hunk: "*** Update File: " filename LF change_move? change?

Because change is optional, the tool grammar tells the LLM that a move-only update is valid:

*** Begin Patch
*** Update File: foo.py
*** Move to: bar.py
*** End Patch

However, _parse_update_file() rejects it because no diff lines are present:

# src/agents/sandbox/capabilities/tools/apply_patch_tool.py
if not diff_lines:
    raise ValueError(f"Update File patch for {path} must include a hunk")

This creates a grammar/runtime contract mismatch for a legitimate file operation and wastes a tool call.

Note: This issue was identified by an automated testing tool for academic research and manually verified. If you have any concerns about this type of reporting, please let me know, and I will adjust my workflow accordingly.

[samrusani]

I verified the grammar/parser mismatch on current main. I’m taking a narrow, non-exclusive pass to require a diff hunk after an optional *** Move to: line and add focused regression coverage that a move-only update is rejected consistently. I’ll leave parser behavior and broader patch semantics unchanged.