How to create and edit Mermaid diagrams in Slack

Direct answer: Slack formats Mermaid as a code block, but does not turn that code into a diagram. To show a rendered diagram in a channel, use a renderer. With Arialine, type /arialine and either paste valid Mermaid or describe the diagram in plain language. Valid pasted source renders without an AI call; a plain-language description uses AI. To revise the saved board, mention @Arialine in its thread. In proposal mode, review the suggestion and choose Apply; in direct mode, an addressed edit is applied immediately. See the proposal and direct mode guide.

Choose the route that matches what you have

Starting point	What to do	What happens
Mermaid source already exists	Open the New diagram form, paste the source and create the board	Valid source is rendered as supplied, without AI
You know the system but have no Mermaid	Describe the diagram and its intended paths	Arialine drafts Mermaid with AI; inspect the source and result before relying on it
You need a separate preview or export	Use Mermaid’s Live Editor	The editor previews source and exports PNG, SVG or Markdown; its edit history is stored in that browser, not shared as a Slack board

Install the app and open the form

1. Install Arialine in Slack. Slack workspace settings can require an owner or admin to approve an app; Slack’s install guide explains the approval path.
2. Invite Arialine to the channel where you want the board, using /invite @Arialine.
3. In the channel message composer, submit /arialine. The New diagram form opens with that channel selected.
4. Choose source-first or plain-language input below.

Arialine’s Slack commands work on every Slack plan. Slack’s slash-command documentation notes that app slash commands run from the message composer, not inside message threads. Use a mention when you are asking for a change in an existing board thread.

Paste Mermaid you already have

Use this route when the source is in a repository, README, or document:

1. Submit /arialine in the channel.
2. Paste a complete Mermaid diagram in the New diagram form.
3. Select Create and inspect the rendered diagram.

For example, this flow distinguishes a retryable payment failure from one that needs review. The retry condition and business rule are placeholders; replace them with the policy your system actually uses.

flowchart LR
    Request[Checkout request] --> Authorized{Payment accepted?}
    Authorized -->|yes| Order[Create order]
    Authorized -->|no| Retry{Retry allowed?}
    Retry -->|yes| Authorized
    Retry -->|no| Review[Review payment failure]

Arialine validates Mermaid and renders valid pasted source as written; it does not send that valid source to an AI model or count it against the daily AI-generation allowance. See the Mermaid syntax reference when you need to check diagram syntax, and the current diagram-type gallery for the types Arialine supports.

When saving pasted Mermaid is enabled, a parse error follows the channel’s validation setting. Autovalidation is on by default and can ask AI to repair invalid Mermaid; that repair may use an AI generation. Submit /arialine validate off to disable automatic repair; a user can still choose the fix card for a deliberate repair. Use /arialine validate on to restore automatic repair, as described in the slash command reference. When saving is disabled, invalid pasted or attached Mermaid is reported without AI repair or retention, regardless of the channel setting; see the workspace privacy guide. Valid pasted source remains model-free either way.

If you already have a .mmd or .mermaid file, share it in the channel to create a board automatically. Use /arialine mmd auto off to disable automatic board creation from shared Mermaid files; you can still explicitly address Arialine or use the message shortcut. The automatic rendering guide covers these shortcuts.

Describe a diagram when you do not have source

Use the form’s description field or mention Arialine with a concrete request. Name the diagram type, the participants or components, the normal path, and the exception paths. State where the behavior is unknown instead of asking Arialine to guess.

Example prompt:

Create a sequence diagram for a sign-in request.
Participants: mobile app, authentication API, session store.
Show a valid-session response and a missing-session path.
On a miss, show the API checking the user database before returning a signed token.
Do not invent retry counts or token lifetimes.

Arialine creates Mermaid source and renders it into a board. Open Export → Show source or export the .mmd file to review what the model produced. A diagram can be valid Mermaid and still contain an incorrect system relationship, omitted failure path, or unsupported assumption, so compare it with the source requirements or implementation before sharing it as current architecture.

Check the board and its files

By default, the board includes a PNG preview and an interactive HTML file. A channel can choose which files new boards and versions post with /arialine files both, /arialine files png, or /arialine files html; this setting does not change exports. The HTML viewer supports pan and zoom and exposes the source and history. A PNG is easier to scan in the channel; open the HTML when you need to inspect details.

The board’s root message shows the current version; earlier versions appear in its thread. Export the Mermaid source when you want to keep a local copy or commit it to Git.

Edit the board from its thread

Reply in the board thread with one specific change, such as:

@Arialine add a fraud check between Checkout and Payment

In the default proposal mode, Arialine shows the suggested edit with Apply, Edit proposal, and Dismiss. Review the Mermaid and preview, then choose Apply to create the next board version. Until it is applied, the current version remains the board’s head. A channel configured for direct mode applies an addressed edit immediately; it still creates a version.

Applied changes are linked to the Slack message that requested them. Use the version history to compare revisions and see the change summary. Rewind makes an earlier source version current by creating a new version; it does not erase the intervening history. Branch starts a separate board from a version so the team can test an alternative without changing the original. The diagram review guide explains how to review these changes and keep the source understandable.

Know the Free-plan limits

Arialine’s current Free plan includes up to 3 boards per workspace, 8 versions per board, and 10 AI generations per workspace each day. It includes unlimited renders of valid pasted Mermaid. See current plan limits and pricing.

At the Free board cap, valid Mermaid pasted or attached solely for rendering can still be rendered once, but no editable board or source is saved. A paste combined with an edit request still needs board capacity; invalid Mermaid that needs AI repair does too. If the workspace disables saving pasted Mermaid, valid source-only renders are posted once without an editable board, as explained in the privacy guide. That setting does not make edit requests unsaved: an edit request still needs board capacity and is saved, while invalid pasted or attached source is reported without repair or retention. At the version cap, new edits and proposals on that board require Team. See current plan limits and pricing.

Troubleshoot the first attempt

• Nothing happens after /arialine: Confirm the app is installed in this workspace and that you submitted the command in the channel composer. Ask a workspace admin if app approval is required.
• The diagram looks like code in Slack: A Mermaid code fence is still text formatting. Use the Arialine form, an addressed mention, or the Render as diagram shortcut.
• The Mermaid is rejected: Check the first parser error and the diagram header. When saving is enabled and automatic repair is off, use the fix card for a deliberate repair or correct the source yourself. When saving is disabled, correct and paste it again; invalid source is neither repaired nor retained.
• The edit has not changed the board: In proposal mode, the suggestion is separate until someone selects Apply.
• The diagram appears as only one file: Check the channel’s /arialine files setting. Exports are separate from the files attached to new board versions.

For other Slack Mermaid workflows and trade-offs, see does Slack support Mermaid? and the Mermaid integration comparison.

Next step: create one small diagram from real source, request a single thread edit, apply it, and export the resulting .mmd. Confirm that the version and source match what your team meant before treating the board as current.

FAQ

Does Slack render Mermaid code by itself?

No. Slack’s message-formatting guide lists code blocks as a formatting option; it does not render Mermaid syntax inside the block. Use a Slack app or a separate Mermaid renderer.

Does valid pasted Mermaid use an AI generation?

No. Arialine renders valid pasted Mermaid without a model call. When saving is enabled, invalid Mermaid can use AI if channel autovalidation is on. With saving disabled, invalid pasted or attached source is reported without repair or retention, regardless of that setting; see the workspace privacy guide.

Which Mermaid diagram types can I use?

Check the diagram-type gallery for the current supported types and examples; the renderer’s supported types can change as Mermaid versions change.

Can I create a diagram without installing a Slack app?

Yes. Mermaid’s Live Editor lets you write or paste Mermaid, preview it, and export PNG, SVG, or Markdown. Its edit history is stored in browser storage, so it does not provide the shared Slack thread and board history described above.