I've found that a good sequence diagram can solve almost any communication problem in engineering. The ones I kept drawing for API work stopped at "App calls Auth Server", and the reader still had to dig through docs to see what was actually sent. Volley fixes that: every arrow can carry its real request and response payloads, shown inline or in a tray.
Write flows in plain text, or skip the typing: point Stripe, GitHub or any webhook at an inbox and turn the deliveries into a diagram; send customers a recording link and get back a diagram with the failed requests marked, with cookies and tokens stripped in their browser; or drop in a HAR file. You can also diff a recording against the sequence you expected.
It's easy for AI tools to write too, with line-and-column errors, a CLI, an HTTP API and an MCP server. The editor is free with no account; Pro is $9/month for saved diagrams, history and higher capture limits.
