Postmaster

Devvit App Page
GitHub README
App Settings
Real User Modmail Message
Postmaster Internal Note

Inspiration

Moderators spend a lot of time reading Modmail, figuring out what kind of message came in, deciding whether it is urgent, and writing similar first responses again and again. For small or busy mod teams, this creates delay and inconsistency.

Postmaster was inspired by the idea that moderation tools should reduce repetitive coordination work without replacing human judgment. I wanted a free, Devvit-native tool that helps moderators triage Modmail while keeping the final decision completely in the hands of the mod team.

What it does

Postmaster is a Modmail triage assistant for Reddit moderators.

When a new Modmail message arrives, Postmaster:

reads the Modmail message inside Reddit/Devvit
classifies it into one of five categories:
- ban appeal
- rule question
- harassment report
- spam
- general
assigns a priority:
- urgent
- normal
- low
posts an internal moderator-only note in the same Modmail conversation
includes a reason for the classification
suggests a reply template
provides a moderator checklist

Postmaster is not an auto-reply bot. It never sends user-visible replies. The note is only for moderators, so the team stays human-in-the-loop.

How we built it

Postmaster was built with Reddit Devvit and TypeScript.

The app uses a Devvit Modmail trigger to detect new Modmail events. Because Modmail trigger events provide message and conversation IDs, the app fetches the Modmail conversation through the Reddit API, extracts the relevant message body, and passes it into a deterministic local classifier.

The classifier uses configurable keyword and phrase matching. Moderators can adjust keyword sets for ban appeals, rule questions, harassment reports, and spam through Devvit settings. The app also supports a configurable reply tone.

After classification, Postmaster formats a moderator-only internal note and posts it back into the Modmail conversation. Redis is used only for lightweight metadata logging, such as timestamp, category, priority, and conversation ID. It does not store full Modmail bodies.

Challenges we ran into

The biggest challenge was working with real Modmail behavior in Devvit.

Some important issues came up during testing:

Modmail trigger events include IDs, but not always the full message body, so the app needed to fetch the full conversation.
Internal Modmail messages cannot use hidden-author mode, so the app had to post internal notes without isAuthorHidden.
Postmaster's own internal note triggers another Modmail event, so the app needed loop prevention for moderator-originated messages.
Reddit Modmail formatting can collapse Markdown, so the note format had to be adjusted to remain readable in the Modmail UI.
Uploading a Devvit version is not the same as installing it on the test subreddit, so the app needed explicit installation of the latest version during testing.

Accomplishments that we're proud of

I am proud that Postmaster works as a practical, privacy-conscious moderation tool.

It successfully:

handles real Modmail events
posts internal moderator-only triage notes
avoids user-visible auto-replies
prevents loops from its own notes
supports configurable categories and tone
uses no external AI APIs
uses no paid backend or hosted service
keeps moderators in control

The app is simple by design, but it solves a real moderation workflow.

What we learned

I learned a lot about how Devvit Modmail events work in practice, especially the difference between trigger payloads and full conversation data.

I also learned that moderation tools need careful safety boundaries. A tool that drafts suggestions is much safer than a tool that automatically replies or takes action. Postmaster reinforced the value of human-in-the-loop moderation design.

Technically, I learned how to work with Devvit triggers, settings, Redis, Modmail replies, and real playtest installs.