git-diff-2dot-vs-3dot-merge-safety

git-diff-2dot-vs-3dot-merge-safety is a skill for Claude Code from wan-huiyan/agent-traffic-control. It costs 54 tokens per session (2,319 once invoked), scanned A, original, MIT.

A guide to understanding Git's two-dot and three-dot diff forms. A two-dot diff compares two complete snapshots, while a three-dot diff shows changes from the point where a branch split from its base.

In plain words
What is it for?
Use it when checking pull-request changes, investigating branches that seem out of sync, or deciding whether a merge is safe.
Why use it?
It prevents you from mistaking files added to the main branch after branching for deletions your pull request will make.

Skill for Claude Code

Written for Claude Code: shipped in a Claude Code plugin. Also seen: mentions Claude Code.

Part of the agent-traffic-control plugin — 105 skills shipped together

Good fit Use it when checking pull-request changes, investigating branches that seem out of sync, or deciding whether a merge is safe.

Compare 6 skills from other repositories ↓
Install with agentmods
npx agentmods add skills/wan-huiyan/agent-traffic-control/git-diff-2dot-vs-3dot-merge-safety
Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent
npx skills add wan-huiyan/agent-traffic-control --skill git-diff-2dot-vs-3dot-merge-safety
Clone the repo
git clone --depth 1 https://github.com/wan-huiyan/agent-traffic-control

Made for: Claude Code.

Or install agent-traffic-control, the plugin that ships this one along with the rest of its 105 skills.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

agentmods badge for git-diff-2dot-vs-3dot-merge-safety

README.md
[![agentmods](https://agentmods.dev/badge/skills/wan-huiyan/agent-traffic-control/git-diff-2dot-vs-3dot-merge-safety/github.svg)](https://agentmods.dev/skills/wan-huiyan/agent-traffic-control/git-diff-2dot-vs-3dot-merge-safety)
Your own site
<a href="https://agentmods.dev/skills/wan-huiyan/agent-traffic-control/git-diff-2dot-vs-3dot-merge-safety"><img src="https://agentmods.dev/badge/skills/wan-huiyan/agent-traffic-control/git-diff-2dot-vs-3dot-merge-safety/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

agentmods 80×15 button for git-diff-2dot-vs-3dot-merge-safety

Your own site · 80×15
<a href="https://agentmods.dev/skills/wan-huiyan/agent-traffic-control/git-diff-2dot-vs-3dot-merge-safety"><img src="https://agentmods.dev/badge/skills/wan-huiyan/agent-traffic-control/git-diff-2dot-vs-3dot-merge-safety.svg" alt="Reviewed on agentmods" width="80" height="20"></a>
Per session 54 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 2,319 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5.1 $0.00054 $0.02319
Opus 5 $0.00027 $0.01159
Sonnet 5 $0.00011 $0.00464
Haiku 4.5 $0.00005 $0.00232

Measured 10d ago against content hash 0f08635a3e5d, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-10, from the pricing page.

Security

Grade A, and why

git-diff-2dot-vs-3dot-merge-safety scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 10d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

plugins/agent-traffic-control/skills/git-diff-2dot-vs-3dot-merge-safety/SKILL.md · 191 lines

How it starts

The opening of the file, as written. The whole thing — 191 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Git Diff 2-dot vs 3-dot — Merge Safety Assessment

Problem

You're reviewing a PR before merging and run git diff origin/main..pr-branch --stat. The output shows files being deleted — files that just landed on main via another PR yesterday and that you absolutely don't want to lose. Looks like merging this PR will wipe them out.

It won't. The 2-dot diff is showing the symmetric difference between two trees, not "what the branch will change about main." Files added to main AFTER the PR's branch point appear in the diff as "deletions" simply because the branch's tree doesn't have them yet. GitHub's 3-way merge will preserve them.

Same trap appears when assessing a "divergent" local commit: git log main..origin/main may show commits "missing" that are actually present under a different SHA (squash merges produce new hashes; rebases rewrite history).

Context / Trigger Conditions

Any of these:

  1. PR review: git diff origin/main..pr-branch --stat shows file deletions, BUT gh pr view N --json mergeable,mergeStateStatus returns {"mergeable":"MERGEABLE","mergeStateStatus":"CLEAN"}.
  2. Divergent local branch: git status says "Your branch and 'origin/main' have diverged, and have N and M different commits each, respectively." But you don't recall making real local commits.
  3. Suspicious orphan commit: git log origin/main..local-branch shows a commit you'd expect to be on origin already (e.g., an [auto-docs] commit from a sibling session, or a cherry-pick from a since-merged PR).
  4. About to take a destructive action — demanding a rebase, force-push, or git reset --hard origin/main "to clean up" — based purely on a 2-dot diff.

Solution

For "will this PR delete files?" questions

# WRONG — shows everything different between trees, including files
# added on main since the branch point
git diff origin/main..pr-branch --stat                 # misleading

# RIGHT — shows only what THIS BRANCH changed since the merge-base
git diff origin/main...pr-branch --stat                # symmetric, since merge-base
git diff origin/main...pr-branch --diff-filter=D --name-only   # ONLY deletions

# If the second command is empty, the PR deletes nothing.

Read the full file on GitHub · 191 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 10d ago First seen · 191 lines · 54 tokens per session scan A 0f08635a3e5d

Subscribe to this mod's changes

git-diff-2dot-vs-3dot-merge-safety is a skill published in the GitHub repository wan-huiyan/agent-traffic-control (3 stars, last pushed 5d ago), licensed MIT. It adds 54 tokens to every session and 2,319 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

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