Agent skill

Behavior Preservation Checker

Compare runtime behavior between original and migrated repositories to detect behavioral differences, regressions, and semantic changes.

Apache-2.0Auto-check passedDevelopment

Install Behavior Preservation Checker

skills CLI

$ npx skills add ArabelaTso/Skills-4-SE --skill behavior-preservation-checker -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI

$ gh skill install ArabelaTso/Skills-4-SE behavior-preservation-checker --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy

$ git clone --depth 1 https://github.com/ArabelaTso/Skills-4-SE.git skills-src && mkdir -p .claude/skills && cp -r skills-src/skills/behavior-preservation-checker .claude/skills/behavior-preservation-checker && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Paste into Claude Code

Install the "behavior-preservation-checker" agent skill from https://github.com/ArabelaTso/Skills-4-SE/tree/main/skills/behavior-preservation-checker into .claude/skills/behavior-preservation-checker/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "behavior-preservation-checker", then confirm the skill loads.

Claude Code copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

$skill-installer

$skill-installer install https://github.com/ArabelaTso/Skills-4-SE/tree/main/skills/behavior-preservation-checker

Type this inside Codex. $skill-installer <name> installs a curated skill from openai/skills. The installer writes to $CODEX_HOME/skills (default ~/.codex/skills). Restart Codex if the skill does not show up.

skills CLI

$ npx skills add ArabelaTso/Skills-4-SE --skill behavior-preservation-checker -a codex

Project install goes to .agents/skills/; add -g for ~/.codex/skills/.

GitHub CLI

$ gh skill install ArabelaTso/Skills-4-SE behavior-preservation-checker --agent codex

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/ArabelaTso/Skills-4-SE.git skills-src && mkdir -p .agents/skills && cp -r skills-src/skills/behavior-preservation-checker .agents/skills/behavior-preservation-checker && rm -rf skills-src

Use ~/.agents/skills/ instead of .agents/skills for a personal install.

Codex skills documentation · loads skills from .agents/skills/

Paste into Codex

Install the "behavior-preservation-checker" agent skill from https://github.com/ArabelaTso/Skills-4-SE/tree/main/skills/behavior-preservation-checker into .agents/skills/behavior-preservation-checker/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "behavior-preservation-checker", then confirm the skill loads.

Codex copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

skills CLI

$ npx skills add ArabelaTso/Skills-4-SE --skill behavior-preservation-checker -a cursor

Project install goes to .agents/skills/; add -g for ~/.cursor/skills/.

GitHub CLI

$ gh skill install ArabelaTso/Skills-4-SE behavior-preservation-checker --agent cursor

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/ArabelaTso/Skills-4-SE.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/skills/behavior-preservation-checker .cursor/skills/behavior-preservation-checker && rm -rf skills-src

Use ~/.cursor/skills/ instead of .cursor/skills for a personal install.

Cursor skills documentation · loads skills from .cursor/skills/, .agents/skills/, .claude/skills/, .codex/skills/

Paste into Cursor

Install the "behavior-preservation-checker" agent skill from https://github.com/ArabelaTso/Skills-4-SE/tree/main/skills/behavior-preservation-checker into .cursor/skills/behavior-preservation-checker/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "behavior-preservation-checker", then confirm the skill loads.

Cursor copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

gemini skills install

$ gemini skills install https://github.com/ArabelaTso/Skills-4-SE.git --path skills/behavior-preservation-checker

--scope user (default) or --scope workspace; --path is the subfolder of the repo that holds the skill; --consent skips the security confirmation prompt.

skills CLI

$ npx skills add ArabelaTso/Skills-4-SE --skill behavior-preservation-checker -a gemini-cli

Project install goes to .agents/skills/; add -g for ~/.gemini/skills/.

GitHub CLI

$ gh skill install ArabelaTso/Skills-4-SE behavior-preservation-checker --agent gemini-cli

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/ArabelaTso/Skills-4-SE.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/skills/behavior-preservation-checker .gemini/skills/behavior-preservation-checker && rm -rf skills-src

Use ~/.gemini/skills/ instead of .gemini/skills for a personal install, then run /skills reload.

Gemini CLI skills documentation · loads skills from .gemini/skills/, .agents/skills/

Paste into Gemini CLI

Install the "behavior-preservation-checker" agent skill from https://github.com/ArabelaTso/Skills-4-SE/tree/main/skills/behavior-preservation-checker into .gemini/skills/behavior-preservation-checker/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "behavior-preservation-checker", then confirm the skill loads.

Gemini CLI copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

GitHub CLI

$ gh skill install ArabelaTso/Skills-4-SE behavior-preservation-checker

Installs for Copilot at project scope by default; add --scope user for a personal install. Preview a skill first with gh skill preview. Needs GitHub CLI 2.90.0 or later (public preview).

skills CLI

$ npx skills add ArabelaTso/Skills-4-SE --skill behavior-preservation-checker -a github-copilot

Project install goes to .agents/skills/; add -g for ~/.copilot/skills/.

Manual copy

$ git clone --depth 1 https://github.com/ArabelaTso/Skills-4-SE.git skills-src && mkdir -p .github/skills && cp -r skills-src/skills/behavior-preservation-checker .github/skills/behavior-preservation-checker && rm -rf skills-src

Use ~/.copilot/skills/ instead of .github/skills for a personal install. Commit .github/skills so cloud agent and code review can use it.

GitHub Copilot skills documentation · loads skills from .github/skills/, .claude/skills/, .agents/skills/

Paste into GitHub Copilot

Install the "behavior-preservation-checker" agent skill from https://github.com/ArabelaTso/Skills-4-SE/tree/main/skills/behavior-preservation-checker into .github/skills/behavior-preservation-checker/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "behavior-preservation-checker", then confirm the skill loads.

GitHub Copilot copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

skills CLI

$ npx skills add ArabelaTso/Skills-4-SE --skill behavior-preservation-checker -a opencode

OpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.

GitHub CLI

$ gh skill install ArabelaTso/Skills-4-SE behavior-preservation-checker --agent opencode

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/ArabelaTso/Skills-4-SE.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/skills/behavior-preservation-checker .opencode/skills/behavior-preservation-checker && rm -rf skills-src

Use ~/.config/opencode/skills/ instead of .opencode/skills for a personal install.

OpenCode skills documentation · loads skills from .opencode/skills/, .claude/skills/, .agents/skills/

Paste into OpenCode

Install the "behavior-preservation-checker" agent skill from https://github.com/ArabelaTso/Skills-4-SE/tree/main/skills/behavior-preservation-checker into .opencode/skills/behavior-preservation-checker/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "behavior-preservation-checker", then confirm the skill loads.

OpenCode copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

Facts

Skill name: behavior-preservation-checker
GitHub stars: 253
Token cost: ~2.3k tokens
SKILL.md length: 618 words
Files: 7 (incl. scripts, references)
Skills in repo: 170
Repo updated: Aug 21, 2026
First seen: Oct 7, 2026
Licence: Apache-2.0

At a glance

Compare runtime behavior between original and migrated repositories to detect behavioral differences, regressions, and semantic changes.

Works in 4 steps: Setup Repositories → Run Behavior Comparison → Review Results → …
Validating code migrations
SKILL.md covers Overview, Core Workflow, Comparison Methods and Difference Detection, plus 4 more sections
Runs Python scripts from its folder; calls python, git and pytest

What it does

Behavior Preservation Checker is an agent skill from ArabelaTso/Skills-4-SE. Compare runtime behavior between original and migrated repositories to detect behavioral differences, regressions, and semantic changes. Use when validating code migrations, refactorings, language ports, framework upgrades, or any transformation that should preserve behavior. Automatically compares test results, execution traces, API responses, and observable outputs between two repository versions. Provides actionable guidance for fixing deviations and ensuring behavioral equivalence.

Its SKILL.md is about 2.3k tokens, which your agent loads only when the skill is triggered. The skill folder holds 8 other files, including scripts and reference files (for example `references/comparison_techniques.md`, `references/difference_patterns.md` and `scripts/behavior_checker.py`).

It sits in Development, covering Refactoring and Code migrations. The repository describes itself as: A curated list of 180+ useful Claude Skills for Software Engineering and resources for customizing AI for SE workflows. The licence is Apache-2.0.

When your agent uses it

Validating code migrations
Framework upgrades
Any transformation that should preserve behavior

Example prompts

“/behavior-preservation-checker”

Requirements

Python 3

Workflow steps

4 steps, taken from the step headings in SKILL.md.

Setup Repositories
Run Behavior Comparison
Review Results
Fix Deviations

What it can do on your machine

Read from SKILL.md and the folder at commit 4f38503. It shows what the files ask for, not the result of running them.

Tool permissions
Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.
From allowed-tools in the SKILL.md frontmatter.
Runs code
Ships 4 files in scripts/ (Python), which the agent can run.
Shell commands in SKILL.md call:
- python
- git
- pytest
From the folder's file list and the shell code blocks in SKILL.md.
Network
No URLs in SKILL.md. Its commands use git, which can reach the network depending on how they are called.
From URLs in SKILL.md, links to its own repository left out.
Credentials
Names no API keys, tokens, secrets or passwords.
From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Behavior Preservation Checker loads about 2.3k tokens when it runs, and up to ~7.7k if it reads all its reference files. Until then it costs about 130 tokens; SKILL.md has 618 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it: ~130
When it runs · the whole SKILL.md, loaded when a task matches: ~2.3k
With references · SKILL.md plus every file in references/, read only if the agent opens them: ~7.7k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); the scripts in this folder are not scanned.

SKILL.md

The full file from ArabelaTso/Skills-4-SE at commit 4f38503, republished under its Apache-2.0 licence (© ArabelaTso). 618 words, ~2,304 tokens.

Download SKILL.mdSave it as .claude/skills/behavior-preservation-checker/SKILL.md (or your agent's skills folder). This skill also uses 6 other files; get the full folder from GitHub.

name: behavior-preservation-checker
description: Compare runtime behavior between original and migrated repositories to detect behavioral differences, regressions, and semantic changes. Use when validating code migrations, refactorings, language ports, framework upgrades, or any transformation that should preserve behavior. Automatically compares test results, execution traces, API responses, and observable outputs between two repository versions. Provides actionable guidance for fixing deviations and ensuring behavioral equivalence.

Behavior Preservation Checker

Overview

Validate that a migrated or refactored codebase preserves the original behavior by automatically comparing runtime behavior, test results, execution traces, and observable outputs between two repository versions.

Core Workflow

1. Setup Repositories

Prepare both repositories for comparison:

bash

# Clone or locate repositories
ORIGINAL_REPO=/path/to/original
MIGRATED_REPO=/path/to/migrated

# Ensure both are at comparable states
cd $ORIGINAL_REPO && git checkout main
cd $MIGRATED_REPO && git checkout main

2. Run Behavior Comparison

Use the comparison script to analyze behavioral differences:

bash

python scripts/behavior_checker.py \
    --original $ORIGINAL_REPO \
    --migrated $MIGRATED_REPO \
    --output behavior_report.json

3. Review Results

Examine the generated report for:

Test result differences
Execution trace divergences
Output mismatches
Performance regressions
API contract violations

4. Fix Deviations

Follow actionable guidance to resolve behavioral differences.

Comparison Methods

Method 1: Test-Based Comparison

Run the same test suite on both repositories and compare results:

Workflow:

Identify common test suite (or create equivalent tests)
Run tests on original repository
Run tests on migrated repository
Compare pass/fail status, assertions, and outputs

Example:

bash

# Run on original
cd $ORIGINAL_REPO
pytest tests/ --json-report --json-report-file=original_results.json

# Run on migrated
cd $MIGRATED_REPO
pytest tests/ --json-report --json-report-file=migrated_results.json

# Compare
python scripts/compare_test_results.py \
    original_results.json \
    migrated_results.json

Method 2: Execution Trace Comparison

Capture and compare execution traces:

Workflow:

Instrument code to capture function calls, arguments, and return values
Run identical inputs through both versions
Compare execution traces for divergences

Example:

python

# Trace original
python scripts/trace_execution.py \
    --repo $ORIGINAL_REPO \
    --input test_inputs.json \
    --output original_trace.json

# Trace migrated
python scripts/trace_execution.py \
    --repo $MIGRATED_REPO \
    --input test_inputs.json \
    --output migrated_trace.json

# Compare traces
python scripts/compare_traces.py \
    original_trace.json \
    migrated_trace.json

Method 3: Observable Output Comparison

Compare program outputs for identical inputs:

Workflow:

Define test inputs (API requests, CLI commands, function calls)
Capture outputs from both versions (stdout, files, API responses)
Compare outputs for differences

Example:

bash

# Test API endpoints
python scripts/compare_api_outputs.py \
    --original-url http://localhost:8000 \
    --migrated-url http://localhost:8001 \
    --test-cases api_test_cases.json

Method 4: Property-Based Testing

Use property-based testing to find behavioral differences:

Workflow:

Define behavioral properties (invariants, contracts)
Generate random inputs
Verify properties hold for both versions
Report any property violations

Example:

python

# Property: sorting should produce same result
from hypothesis import given, strategies as st

@given(st.lists(st.integers()))
def test_sort_equivalence(input_list):
    original_result = original_sort(input_list)
    migrated_result = migrated_sort(input_list)
    assert original_result == migrated_result

Difference Detection

Test Result Differences

What to check:

Tests that pass in original but fail in migrated
Tests that fail in original but pass in migrated
New test failures
Changed assertion messages

Severity levels:

Critical: Core functionality tests fail
High: Integration tests fail
Medium: Edge case tests fail
Low: Flaky tests or timing-dependent failures

Execution Trace Differences

What to check:

Different function call sequences
Different argument values
Different return values
Missing or extra function calls

Example divergence:

Original trace:
  calculate(x=10) -> 20
  validate(20) -> True
  save(20) -> Success

Migrated trace:
  calculate(x=10) -> 21  # ← Difference!
  validate(21) -> True
  save(21) -> Success

Output Differences

What to check:

Different stdout/stderr
Different file contents
Different API response bodies
Different status codes
Different error messages

Tolerance levels:

python

# Exact match required
assert original_output == migrated_output

# Numerical tolerance
assert abs(original_value - migrated_value) < 0.001

# Structural equivalence (ignore formatting)
assert json.loads(original) == json.loads(migrated)

Actionable Guidance

Pattern 1: Logic Error

Symptom: Different outputs for same inputs

Diagnosis:

bash

python scripts/isolate_difference.py \
    --original $ORIGINAL_REPO \
    --migrated $MIGRATED_REPO \
    --failing-test test_calculation

Guidance:

Identify the diverging function
Compare implementations side-by-side
Check for off-by-one errors, operator changes, or logic inversions
Add unit test for the specific case

Show full SKILL.md (238 more words)Show less

Pattern 2: Missing Functionality

Symptom: Tests pass in original but fail in migrated with "not implemented" or "attribute error"

Diagnosis:

bash

python scripts/find_missing_functions.py \
    --original $ORIGINAL_REPO \
    --migrated $MIGRATED_REPO

Guidance:

List all missing functions/methods
Implement missing functionality
Verify with targeted tests

Pattern 3: API Contract Violation

Symptom: Different response structure or status codes

Diagnosis:

bash

python scripts/compare_api_contracts.py \
    --original-spec openapi_original.yaml \
    --migrated-spec openapi_migrated.yaml

Guidance:

Document API contract differences
Update migrated API to match original contract
Add contract tests to prevent future violations

Pattern 4: Performance Regression

Symptom: Migrated version is significantly slower

Diagnosis:

bash

python scripts/benchmark_comparison.py \
    --original $ORIGINAL_REPO \
    --migrated $MIGRATED_REPO \
    --iterations 100

Guidance:

Profile both versions to identify bottlenecks
Check for algorithmic changes (O(n) → O(n²))
Look for missing optimizations or caching
Verify database query efficiency

Pattern 5: State Management Issues

Symptom: Tests fail intermittently or depend on execution order

Diagnosis:

bash

python scripts/detect_state_issues.py \
    --repo $MIGRATED_REPO \
    --test-suite tests/

Guidance:

Identify shared state between tests
Add proper setup/teardown
Ensure test isolation
Check for global variable usage

Report Format

The behavior checker generates a comprehensive JSON report:

json

{
  "summary": {
    "total_tests": 150,
    "passed_both": 140,
    "failed_both": 2,
    "passed_original_failed_migrated": 5,
    "failed_original_passed_migrated": 3,
    "behavioral_equivalence": "92.7%"
  },
  "differences": [
    {
      "type": "test_failure",
      "test_name": "test_user_authentication",
      "severity": "critical",
      "original_result": "passed",
      "migrated_result": "failed",
      "error_message": "AssertionError: Expected 200, got 401",
      "guidance": "Check authentication logic in migrated version",
      "affected_files": ["auth/login.py"]
    }
  ],
  "recommendations": [
    "Fix 5 critical test failures before deployment",
    "Review 3 output differences for correctness"
  ]
}

Best Practices

Start with tests: Ensure comprehensive test coverage before migration
Incremental validation: Check behavior after each migration step
Document intentional changes: Mark expected behavioral differences
Use multiple comparison methods: Combine tests, traces, and outputs
Automate the process: Integrate into CI/CD pipeline
Set tolerance thresholds: Define acceptable differences (e.g., timing, formatting)

Resources

references/comparison_techniques.md: Detailed comparison methodologies
references/difference_patterns.md: Common behavioral difference patterns
scripts/behavior_checker.py: Main comparison orchestrator
scripts/compare_test_results.py: Test result comparison
scripts/trace_execution.py: Execution trace capture
scripts/compare_traces.py: Trace comparison and analysis

Files

SKILL.md and 6 other files (scripts, references) in skills/behavior-preservation-checker of ArabelaTso/Skills-4-SE.

SKILL.md
references/comparison_techniques.md
references/difference_patterns.md
scripts/behavior_checker.py
scripts/compare_test_results.py
scripts/compare_traces.py
scripts/trace_execution.py

Open the folder on GitHubat commit 4f38503

Compare with similar skills

Behavior Preservation Checker next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

Behavior Preservation Checker compared with similar skills
Skill	Stars	Used in	Tokens	Auto-check	Licence	Repo updated
Behavior Preservation Checker this skillArabelaTso/Skills-4-SE	253	—	~2.3k	Automated safety check: Pass	Apache-2.0	1 mo ago
Migrate Core Code to Submodulestinyhumansai/openhuman	42k	—	~2.6k	Automated safety check: Pass	GPL-3.0	today
ast-grep Structural Searchcode-yeongyu/oh-my-openagent	70k	—	~3.3k	Automated safety check: Pass	MIT	today
ast-grep Codemod Referencewarp-drive-data/warp-drive	3.2k	—	~2.6k	Automated safety check: Pass	MIT	yesterday
Hai Ast Grephylarucoder/hai-stack	386	—	~1.5k	Automated safety check: Pass	Custom licence	6 days ago
Java 21 Developer Guide for Grailsapache/grails-core	2.9k	—	~1.9k	Automated safety check: Pass	Apache-2.0	today

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Questions about Behavior Preservation Checker

What does Behavior Preservation Checker do?

Compare runtime behavior between original and migrated repositories to detect behavioral differences, regressions, and semantic changes. Behavior Preservation Checker is an agent skill from ArabelaTso/Skills-4-SE. Compare runtime behavior between original and migrated repositories to detect behavioral differences, regressions, and semantic changes.

When should I use Behavior Preservation Checker?

Behavior Preservation Checker fits situations like: validating code migrations; framework upgrades; any transformation that should preserve behavior.

How do I install Behavior Preservation Checker in Claude Code?

Run `npx skills add ArabelaTso/Skills-4-SE --skill behavior-preservation-checker -a claude-code`. Or copy the skill folder (skills/behavior-preservation-checker in ArabelaTso/Skills-4-SE) into .claude/skills/behavior-preservation-checker in your project. Claude Code loads it when a task matches its description.

How do I install Behavior Preservation Checker in Codex?

Run `npx skills add ArabelaTso/Skills-4-SE --skill behavior-preservation-checker -a codex`. Or copy the skill folder (skills/behavior-preservation-checker in ArabelaTso/Skills-4-SE) into .agents/skills/behavior-preservation-checker in your project. Codex loads it when a task matches its description.

Can I use Behavior Preservation Checker in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add ArabelaTso/Skills-4-SE --skill behavior-preservation-checker -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/behavior-preservation-checker, .gemini/skills/behavior-preservation-checker, .github/skills/behavior-preservation-checker and .opencode/skills/behavior-preservation-checker in your project.

What does Behavior Preservation Checker need to run?

Going by SKILL.md and its folder, Behavior Preservation Checker needs Python for the scripts in its folder and the command-line tools its instructions call (python, git and pytest). Our summary lists: Python 3.

Does Behavior Preservation Checker access the network?

SKILL.md contains no URLs. Its commands use git, which can reach the network depending on how they are called. This is read from the text; nothing was executed.

Is Behavior Preservation Checker safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. The check reads SKILL.md only: the scripts in the folder are not scanned, so read them before running anything.

What licence does Behavior Preservation Checker use?

Behavior Preservation Checker is published under the Apache-2.0 licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Behavior Preservation Checker use?

About 2.3k tokens (SKILL.md is roughly 9.2k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full. Its references folder adds about 5.4k tokens, read only when the agent opens those files.

What are the alternatives to Behavior Preservation Checker?

Skills that share tags, products or a category with Behavior Preservation Checker: Migrate Core Code to Submodules (tinyhumansai/openhuman, 42k stars), ast-grep Structural Search (code-yeongyu/oh-my-openagent, 70k stars), ast-grep Codemod Reference (warp-drive-data/warp-drive, 3.2k stars) and Hai Ast Grep (hylarucoder/hai-stack, 386 stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Behavior Preservation Checker?

ArabelaTso (a GitHub user) maintains it in ArabelaTso/Skills-4-SE, which has 253 GitHub stars. The repository holds 170 skills in this directory. The repository was last updated on August 21, 2026.

Source: ArabelaTso/Skills-4-SE on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.

Install Behavior Preservation Checker

Facts

At a glance

What it does

When your agent uses it

Example prompts

Requirements

Workflow steps

What it can do on your machine

Tool permissions

Runs code

Network

Credentials

Context cost

Safety

SKILL.md

Behavior Preservation Checker

Overview

Core Workflow

1. Setup Repositories

2. Run Behavior Comparison

3. Review Results

4. Fix Deviations

Comparison Methods

Method 1: Test-Based Comparison

Method 2: Execution Trace Comparison

Method 3: Observable Output Comparison

Method 4: Property-Based Testing

Difference Detection

Test Result Differences

Execution Trace Differences

Output Differences

Actionable Guidance

Pattern 1: Logic Error

Pattern 2: Missing Functionality

Pattern 3: API Contract Violation

Pattern 4: Performance Regression

Pattern 5: State Management Issues

Report Format

Best Practices

Resources

Files

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