Sequence diagram software, free and text-based

Picking sequence diagram software means choosing between two different jobs. One job is a canvas: you drag a lifeline, you drag an arrow, you nudge the arrow until it doesn't cross a title. The other job is text: you name the participants, you write the messages in order, and a layout engine draws the lifelines. I keep the second kind in git. I use the first kind when the picture is a poster and the pixels are the work. This post is the roundup of those approaches, not a price sheet for products I don't operate.

Two kinds of software

A GUI sequence tool thinks in objects. A lifeline is a box you created. A message is a connector you attached to two lifelines, with a y-coordinate that is the order. You can see the order because you placed it. You can also break the order by dragging a message above the one that causes it, and the file will not complain, because the file doesn't have an order beyond those coordinates. That is a strength when you are composing a slide. It is a weakness when the order is the spec.

A text tool thinks in lines. The next line happens after the previous line. You cannot drag a reply above the request without moving the line. The layout may be plainer. The order is harder to lie about by accident. Mermaid is the text tool I use for this, because the fence already lives in the README. PlantUML is another text language in the same family of ideas: you describe participants and arrows, and a renderer draws them. I am not going to invent a price for PlantUML, and I don't need one to say what it is. If you already have a PlantUML pipeline in CI, you don't need me to talk you into Mermaid for that one repository. Stay. The diagrams are already text.

The sequence syntax page is the Mermaid arrow reference. This page is the software question around it: when a GUI earns its place, when text does, and what I refuse to put in a comparison table. A free sequence diagram maker is the narrower post about opening an editor and drawing a login without an account. Read that one if you already chose text and you want the first diagram. Read this one if you are still choosing.

What a GUI tool is good at

A GUI is the right software when the sequence is a deliverable someone will judge as a drawing. A vendor workshop, a printed runbook page with a header and a footer, a diagram that has to line up with a screenshot beside it. You will spend time on spacing. The tool should remember that spacing. Mermaid will not. Add a participant and the engine may reflow the lifelines. If that reflow would ruin an hour of alignment, you are in GUI territory, and you should stop feeling guilty about it.

GUI tools are also how non-writers join the review. A support lead who will not open a fence can still point at a message and say "that reply is wrong." I want that comment. I don't want the only copy of the sequence to live in a binary file afterward. If the session produces a real correction, someone has to type it into the text, or the text is now the stale one. The workshop canvas can be a whiteboard. The committed diagram should still be lines.

Lucidchart is the name people bring up for the GUI side. I am not going to quote a plan or a menu. I don't have your account in front of me, and a blog that invents another product's price is how these roundups go rotten. If you are evaluating it, look at the price on their site the day you evaluate it, and look at whether your sequence will be edited by people who live in that canvas already. A tool your team already pays for and already understands can be the right GUI even when a text file would be "purer." Purity doesn't answer a pager. Familiarity sometimes does.

The failure mode is using that familiarity for every diagram, including the twelve-line login that changes every sprint. The canvas file's diff is coordinates. Reviewers skip it. The sequence silently drifts from the handler. That drift is the cost I actually care about, more than the license line.

What a text tool is good at

Text is the right software when the next edit is a new message or a renamed participant. The diff shows Auth-->>App: Session becoming Auth-->>App: Reject and a reviewer can comment on that line. You can put the fence in the same pull request as the handler. You can grep for Check credentials when the endpoint changes. None of that requires the picture to be pretty. It requires the picture to be source.

Mermaid's sequence rules that I won't give up, regardless of tool: declare participants first, solid arrow for a request, dotted arrow for a reply, alt and else and end for a branch, loop and end for a repeat. The loop examples show the repeat block on its own. An example sequence is a straight conversation if you want a picture without a branch yet. I mention them so this roundup doesn't pretend to be the tutorial. The software choice doesn't change the colon rule. Every text tool I have used will punish a missing colon. Good.

The AI sequence generator is a way to get a first draft from a paragraph, inside the same text workflow. Five complimentary AI uses total, then you edit by hand. Hand editing stays free. I use a generation on a messy flow I am still discovering, not on a four-message diagram I can type. A GUI doesn't become more correct because an assistant dragged the boxes. The words still have to be true.

Editors that render Mermaid are a category, not a single trophy. The editor comparison is where that category is sorted through. I link it instead of pasting a ranking here. Rankings go stale, and "best" as a fact is not something this page is claiming. A text editor with a preview is enough software for a sequence that lives in a repo.

A charge, drawn as text

This is the diagram I want the software to produce, meaning the diagram I want to be able to read as lines. A shop charges a card. Pay asks the bank. The bank says OK. Pay tells the shop it is paid. Four messages. Three participants. If your GUI cannot show this without a legend and a title block, it can still be a fine poster tool. It is more software than this picture needs.

sequenceDiagram
    participant Shop
    participant Pay
    participant Bank
    Shop->>Pay: Charge card
    Pay->>Bank: Authorize
    Bank-->>Pay: OK
    Pay-->>Shop: Paid

The solid arrows are requests. The dotted arrows are replies. Bank does not start a new conversation with ->> when it approves. It answers. I have watched a GUI default every connector to the same solid style, and then a review argues about whether the bank "calls" the shop. The style was a default. The argument was real. In text I choose the arrow on purpose, every time, which is slower for one diagram and faster for the tenth.

Participant order is declared. Shop, Pay, Bank. The money moves left to right and the replies come back. If a GUI inserts participants in the order you clicked, the bank ends up on the left because you drew it first. You then drag it. The drag is the work. In text the work is one line moved up. I know which one I want in a pull request.

The same cast, with a retry

Real charge software polls. The interesting diagram is not the happy path alone. It is the happy path plus the loop a client actually runs while the status stays pending. I keep the loop small. Two messages inside, a title that says until settled, and a final reply when the loop exits. I am not drawing the timer interval. The interval is prose, because a sequence arrow is a bad place for milliseconds.

sequenceDiagram
    participant Shop
    participant Pay
    Shop->>Pay: Charge card
    loop until settled
        Shop->>Pay: Ask status
        Pay-->>Shop: Pending
    end
    Pay-->>Shop: Paid

loop needs an end. Forget it and the rest of the file falls inside the loop, including the Paid reply, which then looks like it happens on every poll. That is a serious misread and a one-word fix. GUI tools express a loop as a box you resize around a set of messages. Forgetting to resize the box is the same bug, harder to see in review, because the coordinates don't say "this message is inside." The keyword end says it.

I left Bank off this second picture. The poll is between Shop and Pay. Adding Bank back because the first diagram had Bank is how sequences accumulate lifelines. The poll doesn't talk to the bank. Don't draw a bank that never receives a message. If the status check does reach the bank, add the lifeline and the messages. Absence is a claim. I want the claim to be true.

A GUI user will ask where the "until settled" label goes. On the loop frame. If the tool has no loop frame and you fake it with a note, the note will detach the first time someone drags Pay. Text doesn't detach. It also doesn't let you place the frame two pixels lower. Give up the two pixels.

Prices I will not invent

I will say what this site charges, because that is our page and I can keep it honest. The editor is free and it does not require signup. PNG export is on every plan: free is 1× with a small watermark, Starter at $6.99 a month is watermark-free up to 4×. JPG, SVG, and PDF are Pro-only, and Pro is $11.99 a month. The pricing page is the current list if those numbers ever move. Don't trust a roundup post over the page.

I will not fill in a column for Lucidchart, PlantUML hosting, or a diagram suite I don't run. Some of those are free for a person and paid for a company. Some are the reverse. Some changed last year. A table with a dollar sign I guessed is worse than no table. If a vendor is in the running, open their price page in the same hour you open ours, and write down what you saw in the decision doc. Your note will be fresher than anything I publish.

Also skip feature bingo. "Has UML 2.5 compliance" is a sentence vendors love and a poor predictor of whether your login diagram will be maintained. The predictor I use is: who will edit this next month, and will they open a text file? If yes, text. If the next editor will only open a canvas, and the picture truly can't be plain, GUI. If you don't know who edits it, it will not be edited, and the software choice is a distraction from writing the four messages in the doc you already have.

The diagram tool roundup puts sequence tools in the wider set of free approaches, including whiteboards and office canvases. Sequence software is a slice of that. A whiteboard is a fine place to discover the messages. It is a bad place to store them, because a whiteboard's history is not a pull request. Copy the result into text the same day. Tomorrow the whiteboard will have been tidied by someone who wasn't in the meeting.

Picking for a repo

I pick text for anything that describes a call our code makes. The charge diagram above is the test. If I can't keep it next to the handler, I don't understand the handler well enough to draw it in a GUI either. The GUI would only hide the gap under spacing.

I pick a GUI for a sequence that is mostly politics: many parties, a printed page, a need to show a partner their own colors. Then I still write a short text version of the critical path in the repo, even if it is uglier, so the critical path can change without opening the poster tool. Two artifacts, one direction of truth. The text wins when they disagree, or the poster wins, but write down which. Undeclared dual sources are how the bank reply becomes a request in only one of the pictures.

Share links are fine for a review that happens before the file exists. They are not the software. The software is whatever renders the fence in the pull request your team already reads. If that preview is missing, fix the preview, or paste a PNG generated from the text. Don't move the source to a canvas because the diff viewer doesn't draw arrows. Teach the diff viewer, or accept that reviewers read the lines. The lines are short on purpose.

Open the editor and paste the charge. If you are comparing it with a GUI you already have, draw the same four messages there too, then change one reply in both. The tool that made the change obvious is the one to keep for that class of diagram. Not the one with the longer feature list. The edit is the product.