rtk-ai/rtk

`diff` reports "Files are identical" when lines differ by only a few characters

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#781 opened on Mar 22, 2026

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bugeffort-smallfilter-qualitygood first issue

Description

Description

rtk diff incorrectly reports files as identical when modified lines share a long common prefix. The underlying /usr/bin/diff correctly detects the differences.

Version

Built from source at commit 97b6878

Minimal reproduction

# FAILS — reports identical (4+ char common prefix)
printf "abcd\n" > /tmp/a.txt
printf "abce\n" > /tmp/b.txt
rtk diff /tmp/a.txt /tmp/b.txt
# → [ok] Files are identical

# WORKS — correctly detects difference (3 char prefix)
printf "abc\n" > /tmp/a.txt
printf "abd\n" > /tmp/b.txt
rtk diff /tmp/a.txt /tmp/b.txt
# → +1 added, -1 removed

# Verification with system diff
/usr/bin/diff /tmp/a.txt /tmp/b.txt
# → 1c1 < abcd --- > abce

Additional examples

# Also fails with multi-word lines
printf "foo bar\n" > /tmp/a.txt
printf "foo baz\n" > /tmp/b.txt
rtk diff /tmp/a.txt /tmp/b.txt
# → [ok] Files are identical

# Also fails with longer content
printf "abc def\n" > /tmp/a.txt
printf "abc deg\n" > /tmp/b.txt
rtk diff /tmp/a.txt /tmp/b.txt
# → [ok] Files are identical

Expected behavior

rtk diff should report the differences, consistent with /usr/bin/diff.

Suspected cause

A similarity threshold in rtk's diff filtering appears to treat lines that are "close enough" as identical, when it should only filter output formatting — not discard actual differences.

Contributor guide

Tech stack
rust
Domain
cli
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Basic RustFamiliarity with rtk diff internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75
Research direction
The issue is in the diff functionality of rtk. The suspected cause is a similarity threshold in diff filtering that incorrectly treats lines with long common prefixes as identical. Start by examining the diff module, likely in `src/diff.rs`, to find where line comparisons and similarity checks are performed. Look for any threshold or filtering that might discard differences based on similarity. Reproduce the bug using the provided examples, then trace the logic to identify where the premature comparison occurs. Ensure that only output formatting is filtered, not the actual line differences. Consider adding debug output to compare the internal diff results with the expected system diff output. Once the root cause is found, modify the code to compare lines exactly without applying a similarity threshold, and test with the given cases to confirm the fix.