How to edit a Mermaid Preview diagram in Slack

Direct answer: Mermaid Preview's documented Slack workflow posts a PNG; its README lists editing an existing diagram as a wishlist item, not an in-place edit command. To change the picture, update the Mermaid source you saved separately, run /mermaid again and post the newly rendered image. If the team wants edits and history to stay with the Slack discussion, use a workflow that keeps an editable source and revision record there.

Why a Mermaid Preview image cannot be edited as Mermaid code

Mermaid Preview takes Mermaid text submitted through /mermaid, previews it in a Slack dialog and uploads a PNG to the channel. Its Slack Marketplace listing describes that output. The project's README describes the render-and-upload steps and lists “edit already posted diagram” as a wishlist item, rather than a shipped edit command.

A PNG is a raster image of the rendered diagram. It does not contain the original Mermaid statements that named the nodes and connected them. Mermaid Preview's Marketplace privacy statement says it does not retain submitted charts or PNG renderings on its servers, so you should keep the .mmd source in a place your team controls. Its app creates a new PNG from source; it is not a source archive or version history.

Slack messages can contain formatted code, but Slack's message formatting guide documents code blocks as formatted text. It does not describe native Mermaid rendering. To see why the source file matters, compare Slack's display options in Does Slack support Mermaid diagrams?.

A reliable update workflow with Mermaid Preview and Git

For diagrams owned by a code repository, use the repository as the source of truth and Slack as the place to share rendered snapshots. A teammate can reproduce the picture even if the original author is unavailable.

1. Save the diagram as a .mmd file, such as diagrams/checkout-flow.mmd, and commit it with the code or documentation it describes.
2. Edit the source when the design changes. Review the text diff so the team can see which nodes, labels or connections changed.
3. Render a new image. The official Mermaid CLI accepts a Mermaid file and can create SVG, PNG or PDF. For example, mmdc -i diagrams/checkout-flow.mmd -o checkout-flow.png renders a PNG.
4. Post the new image in Slack. Add a link to the relevant commit or pull request so readers can compare it with the source change.
5. Keep the older Slack post if it records an earlier decision. Make the new message's revision and source link clear instead of leaving two unlabeled images that look equally current.

The CLI does not post to Slack for you. You can upload the file manually or connect rendering and posting to a workflow your team maintains. That adds setup and ownership, but it keeps the .mmd history in the same review process as other repository files.

Example: add a retry path

Suppose a checkout diagram first shows a successful payment flowing to order creation:

flowchart LR
  Checkout --> Payment
  Payment --> Orders[(Orders database)]

A later design adds a retry queue for failed payments. Change the source, render it again and post the new snapshot:

flowchart LR
  Checkout --> Payment
  Payment --> Orders[(Orders database)]
  Payment -- failure --> Retry[Retry queue]
  Retry --> Payment

In this example, the source diff records the arrow and label change; the Slack image lets people who do not read Mermaid see the revised flow. If your team needs the code visible in Slack too, include a code block or link to the .mmd file alongside the image.

What if the only copy left is a PNG?

A posted image is not a reliable way to recover its exact Mermaid source. Check the original author's local files, repository, pull request or documentation before recreating the diagram. If no source remains, use the image as a visual reference and rebuild the text definition; then render and review it against the original before treating it as authoritative.

Do not overwrite the only image before checking its history and source. In Slack, post the corrected render with a link to the recovered .mmd file or commit. That gives the next editor a clear place to start.

When to keep revisions in Slack instead of rerendering each time

Git and Mermaid CLI work well when engineers own the diagram as documentation in a repository. A different workflow makes sense when the change is proposed in a Slack thread and the team wants the source and revision context beside that conversation.

Arialine's documented Slack workflow accepts pasted valid Mermaid without an AI call, posts a PNG and interactive HTML file, and retains board source and versions for later edits. Addressed changes can be reviewed as proposals before they are applied; the product links an applied version to the Slack request. See the detailed Arialine vs Mermaid Preview comparison for a feature-by-feature decision and the current Arialine plan limits.

The difference is history: Arialine keeps board source, versions and decision context, while Mermaid Preview says it does not retain submitted charts or PNGs. The PNG posted by either workflow is still a Slack file; Slack's file-retention settings determine how long the workspace keeps it. Review app permissions and your organization's policies before installing or sharing sensitive diagrams; Slack explains how workspace administrators manage app approval.

FAQ

Does /mermaid update the old Slack post?

No. Mermaid Preview's documented command renders Mermaid and uploads a PNG. To show a revision, submit the changed source and post the new image. Keep the original source and identify which Slack message is current.

Does Mermaid Preview save my Mermaid source?

Its Marketplace listing says submitted Mermaid charts and PNG renderings are not stored on Mermaid Preview's servers. Keep the .mmd source separately if you need to reproduce or revise the diagram.

Can I turn a Mermaid Preview PNG back into an editable diagram?

A PNG does not contain the original Mermaid code. Recover the .mmd file or recreate the diagram from the image, then review the new render against the source image.

Should I use Mermaid CLI or an app to update diagrams?

Use Mermaid CLI when source control and repository-based review should own the diagram; your team must also handle posting the output to Slack. Use a Slack app when creating and revising the diagram in its Slack conversation is more important than keeping all source operations in your own repository. Compare the source, storage and revision workflows before moving existing diagrams.

Next step: reproduce one update

For one current diagram, recover and commit the .mmd source, render a revised PNG, and post it with a link to the source change. If keeping each edit with its Slack request is a better fit, test that workflow on one low-risk diagram using the side-by-side comparison before moving the rest.

Update note

Updated September 29, 2026: Reworked this page around editing a posted Mermaid Preview diagram from saved source. For tool selection and a side-by-side comparison, see Arialine vs Mermaid Preview.