Part of Mechanisms of Vibe Coding

Mechanism: PR-Sized Vibes

One-sentence definition

PR-sized vibes means sizing each agent session so its output fits a reviewable pull request — diff a human can read in one sitting — by combining scope fence, vertical slice, and split & conquer.

The problem

One agent session produces a 4,000-line diff across forty files. CI might pass. Nobody will review it. It merges with a shrug or sits forever. Large vibes optimize for agent completion, not team throughput.

Your future self is also the reviewer — unreadable diffs hide bugs and scope violations.

Symptoms:

• PR description says “various fixes”
• Review skipped because “AI did it”
• Single commit mixing feature, refactor, format
• Cannot bisect regressions

How it works

Target one PR per session (or per slice):

  Session goal
       │
       ▼
  Diff budget (files · lines · time to review)
       │
       ▼
  Agent work + verify
       │
       ▼
  PR / merge unit ≤ budget

Heuristics (tune per team):

• ~200–400 lines changed or ≤5–10 files for routine work
• One vertical slice or one numbered slice from split plan
• PR title = goal one-liner

Over budget → stop, handoff, next PR.

When to use it

• Team repos with code review culture
• Solo devs who want audit trails
• Before merge after any multi-file agent run
• Checkpoint prep — if you cannot review diff, split

When not to use it

• Mechanical codegen with generated files excluded from human review — still separate PR, note generator
• Line limits as excuse to hide unrelated drive-by edits — budget is not a license to skip constraints

Failure modes

Line counting only — 300 lines of obfuscated one-liners. Fix: Reviewability, not just size.

Micro-PR spam — 20 PRs for one feature without plan. Fix: Split & conquer with ordered slices.

One mega-PR “because CI green” — Fix: Split by slice; use rollback on excess.

PR size creep mid-session — Fix: Scope fence; new PR for new goal.

Minimal example

Context: Verbose health — three slices, three PRs.

• PR1: Response shape + tests (3 files)
• PR2: Bootstrap uptime source (2 files)
• PR3: README query param (1 file)

Each PR: goal in description, verify command, under file budget.

Done when: Each merged PR was reviewable in under 15 minutes.

Tool instances (optional deep-dive)

Portable idea above; this section is tool-specific. Date: June 2026.

Cursor

• Before accept-all: git diff --stat against budget in rules.
• Agent: “Stop when diff exceeds N files; summarize for next PR.”
• One branch per PR; branch-per-experiment for alternatives.

Other tools

GitHub PR size bots, danger.js — automate warnings on huge agent PRs.

Related mechanisms

• Scope fence — one job per PR
• Vertical slice — PR content shape
• Split & conquer — sequence of PRs

Try it yourself

Exercise: Set a diff budget for your next agent task (max files + max lines). Stop the session when hit; open PR or handoff.

Done when: You stopped or split before exceeding budget — not after.