fix(harness): count a trailing newline as a line terminator in file tools - #5260

Human Overview

Description

The file tools treat a trailing newline as the start of a new empty line. For the canonical POSIX file "alpha\nbeta\n" (2 lines, wc -l = 2), read returns a phantom third line that offset=3 can select, the footer says of 3, and write reports Wrote 3 lines. An empty file reads as one numbered line but writes as 0 lines, and read(limit=0) returns a misleading "offset None is past the end" message. Coding agents rely on these line numbers, so the phantom line sends them to a location that does not exist.

Related Issues

None — small bug fix, no prior issue (duplicate search clean).

Documentation PR

Not needed — tool output is corrected, no API change.

Type of Change

Bug fix

Testing

Added five Python tests and five mirrored TypeScript tests: trailing-newline file has exactly two numbered lines with no phantom/footer; offset=3 reports [File has 2 lines; offset 3 is past the end.]; write("alpha\nbeta\n") reports Wrote 2 lines; empty file is 0 lines consistently ([File is empty.] / Wrote 0 lines); limit=0 raises ValueError("limit must be at least 1.").

Red on 3bd98e6 with the final tests: 5 failed in harness-py/tests/tools/test_file_tools.py, no collection errors. Green with the fix: Python 21 passed and TypeScript 21 passed (both file-tool suites).

• I ran hatch run prepare

Additional Details

Checklist

• I have read the CONTRIBUTING document
• I have reviewed and understand every line of code in this PR, including any generated by AI tools, and I can explain why it works
• My change is focused and reasonably small; I have split unrelated work into separate PRs
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly (no doc changes needed)
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

By submitting this pull request, I confirm that I can use, modify, copy, and redistribute this contribution, under the terms of your choice.