DiffStory — behavior-level sequence diagrams for PRs

Turn code diffs into readable sequence diagrams and behavior changes, not just red and green lines.

Problem

During PR review, normal diffs only show text changes and make it hard to see what behavior actually changed. One developer built Mermaidiff to show PR behavior changes as Mermaid sequence diagrams, proving reviewers want behavior-level visualization.

Solution

A CLI / GitHub App that analyzes a PR's call paths and critical-path changes, auto-generates Mermaid sequence or behavior-diff diagrams, and posts them into the PR comment. It supports before/after comparison and highlighting key paths.

Users

Backend engineers writing complex business logic, tech leads doing code review, open-source maintainers.

MVP

GitHub App install; generate Mermaid behavior sequence diagrams for selected PRs; detect added/removed call paths; post back to the PR; start with Python and TypeScript.

Revenue

Free open-source CLI plus a hosted GitHub App with monthly per-repo pricing; self-hosted team tier.

Why now

AI-generated code makes PRs larger and harder to review, so behavior-level summaries become essential; the Mermaid ecosystem is mature and cheap to render.