fix(read): correct off-by-one for negative offset with trailing newline

The Read tool documents that a negative offset reads from the end (-N = last N lines). Content is split on newline, so a file ending in a newline produces a trailing empty element and total_file_lines is one too high. Every negative offset was therefore off by one: offset=-1 returned the empty string after the final newline instead of the last line, and -N returned N-1 real lines.

Exclude the trailing empty element when computing the start line for negative offsets. Adds regression tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

agent/tools/read/read.py

@@ -297,8 +297,15 @@ class Read(BaseTool):
             if offset is not None:
                 if offset < 0:
                     # Negative offset: read from end
-                    # -20 means "last 20 lines" → start from (total - 20)
-                    start_line = max(0, total_file_lines + offset)
+                    # -20 means "last 20 lines" → start from (total - 20).
+                    # A file ending in "\n" produces a trailing empty element
+                    # from split('\n'); exclude it so offset=-1 returns the
+                    # real last line instead of the empty string after the
+                    # final newline (and -N returns N real lines).
+                    effective_lines = total_file_lines
+                    if all_lines and all_lines[-1] == '':
+                        effective_lines -= 1
+                    start_line = max(0, effective_lines + offset)
                 else:
                     # Positive offset: read from start (1-indexed)
                     start_line = max(0, offset - 1)  # Convert to 0-indexed