If you want to create diagrams from textual descriptions, https://kroki.io/ https://kroki.io/ is very versatile and supports several different markup languages

If you want to create diagrams from textual descriptions, https://kroki.io/ https://kroki.io/ is very versatile and supports several different markup languages including C4. Alternatively, if you want to stick to ASCII output, https://arthursonzogni.com/Diagon/#Sequence https://arthursonzogni.com/Diagon/#Sequence supports several formats (see the dropdown) which can pipe into something like Typogram (https://google.github.io/typograms/ https://google.github.io/typograms/) for more beautiful output. For example, input: Renderer -> Browser: BeginNavigation() Browser -> Network: URLRequest() Browser <- Network: URLResponse() Renderer <- Browser: CommitNavigation() Renderer -> Browser: DidCommitNavigation() will output the following sequence diagram: .--------. .-------. .-------. |Renderer| |Browser| |Network| '--------' '-------' '-------' | | | | BeginNavigation() | | |-------------------->| | | | | | |URLRequest() | | |------------>| | | | | |URLResponse()| | |<------------| | | | | CommitNavigation() | | |<--------------------| | | | | |DidCommitNavigation()| | |-------------------->| | .--------. .-------. .-------. |Renderer| |Browser| |Network| '--------' '-------' '-------' and then you can perform further edits using something like https://asciiflow.com/ https://asciiflow.com/ (web, free) or https://ivanceras.github.io/bob-editor/ https://ivanceras.github.io/bob-editor/ (web, free) or https://monodraw.helftone.com/ https://monodraw.helftone.com/ (Mac only, proprietary) as mentioned in other comments from https://news.ycombinator.com/item?id=37040883 https://news.ycombinator.com/item?id=37040883.