Seal vs HackMD for Markdown review and sign-off
HackMD and Seal both speak native Markdown, but they cover different halves of a document’s life. HackMD is a collaborative editor: several people type in the same .md at once and watch each other’s cursors. Seal deliberately isn’t an editor — your AI agent or your own editor writes the file, and Seal is where the finished draft gets reviewed: each reviewer reads an AI summary written for their role, comments and proposes old→new suggestions the owner accepts or rejects, and — on the hosted tier — a verified human signs off on the exact wording. Use HackMD to write together; use Seal to get a draft reviewed and approved.
Isn’t editing together the same as reviewing?
Not once approval matters. In a shared editor, a reviewer’s change lands directly in the text — there is no moment where the owner accepts or rejects it, and no record of what the document said when someone called it good. Seal keeps those roles separate: reviewers propose suggestions instead of editing, the owner decides each one, and every version is immutable. That separation is what makes an approval meaningful — the approver saw a specific frozen version, and Seal binds their sign-off to its content hash. If the text changes afterward, the approval lifts on the changed part and returns to the approver automatically.
What does Seal add for non-writers?
A collaborative editor still shows every reviewer the same raw document. Seal starts each reviewer with a short AI summary written for their role — legal sees the compliance angle, engineering sees the technical one — and lands them on the slice that’s theirs. Reviewers who will never touch Markdown read a clean web page and comment like they would in Google Docs, from a single share link with no account. The free local plugin runs the same review in your repo via your AI agent (Claude Code, Cursor, Codex, Copilot), and the hosted tier adds identity-verified approval and an audit log.
Side by side
| HackMD | Seal | |
|---|---|---|
| What it is | A collaborative Markdown editor | A review + sign-off layer — not an editor |
| Markdown stays Markdown | Yes — native .md | Yes — native .md |
| Real-time co-writing | Yes, its core strength | No — your agent or editor writes the file |
| Per-reviewer summary | No — everyone reads the whole doc | Yes — an AI summary written for each reviewer’s role |
| Suggested edits | Reviewers edit the text directly | Propose old→new suggestions the owner accepts or rejects |
| Formal sign-off | No — a comment is not a verified approval | Identity-verified approval bound to the exact wording (hosted) |
| Approval goes stale on edit | No approval concept | Automatic — approval lifts on the changed part and returns to you |
| Reviewer setup | Account to edit; share links for reading | A link; free account only for verified sign-off |
When should I use HackMD instead of Seal?
Use HackMD when the job is writing: brainstorming, live note-taking, co-authoring a doc in real time with people who are comfortable in Markdown. Reach for Seal when the draft already exists — usually because an AI agent wrote it — and what you need is structured human review: role summaries for busy reviewers, an accept/reject decision on every proposed change, and a verified sign-off you can point to later. The two compose naturally: draft in HackMD or with your agent, then run the approval through Seal.
Related: Seal vs GitHub PR review · Seal vs Google Docs · Seal vs Notion · Guides