A Substack publication exposes its posts through an RSS feed at https://your.substack.com/feed, where your is the publication’s subdomain. Polling that feed and checking each item against a list of items you have already seen is enough to detect new posts. The difficult part is keeping that list, the alerts, and the identity of each post consistent across restarts, deploys, crashes, and edits. Most of the time spent on a monitor like this goes into those problems, not into fetching the feed.
What the feed gives you, and what it does not
Substack’s support documentation says that a publication’s RSS feed is at https://your.substack.com/feed, with your replaced by the publication name. The help page was last updated November 8, 2023. It confirms the address pattern. It does not describe rate limits, how far back the feed reaches, or any delivery guarantee, so treat those as unknown and test them against the publication you are watching.
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The author of the prototype describes a public-feed approach that covers public and free posts. Paywalled material, in that author’s stated scope, requires an authenticated session. If your monitor must catch paid posts, plan for that session as a separate piece of work rather than assuming the public feed covers them.
A reasonable question is why not simply subscribe by email. An email arrives in an inbox and stops there. A monitor produces a machine-readable event that you can route into a chat channel, a log, or another automation. That routing is the reason to build one.
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The core loop is short. In the prototype described by its author, it runs in this order:
- Fetch the publication’s feed.
- Parse the entries.
- Extract a stable identifier for each entry.
- Compare each identifier with the set of identifiers already recorded.
- Send an alert for each identifier that is not in the set.
- Record the identifier as seen.
The author wires the output to a Slack webhook and runs the job on a 15-minute cron schedule. That interval is the author’s example. It is not a recommended or guaranteed polling frequency, and the feed’s own update timing is not described in the source. Pick an interval that suits how quickly you need alerts and how often you are willing to fetch the feed.
Steps 4 through 6 are where the real design work sits. Each of the failures below comes from the order and durability of those steps.
Failure one: state that does not travel with the process
The simplest version stores the seen identifiers in a local file such as seen_ids.json beside the script. That works until the process moves. The author reports that a machine move, or a deploy that left the state file out, caused previously seen posts to be announced again.
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Keep state in durable storage that belongs to the scheduled process, and back it up. If you use a managed scheduler, confirm that its state store survives between scheduled runs before you rely on it. If you watch several publications from one monitor, key the state by publication, so that one feed’s identifiers do not affect another. The author’s packaged monitor reports multi-publication state isolation as one of the behaviors it was tested for.
Failure two: the crash window between sending and saving
Alerting and state persistence are two separate operations. The order you choose decides which failure you get.
Send first, record later
If the monitor sends the alert and then crashes before saving the identifier, the next run sees the post as new and sends it again. You get a duplicate.
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Record first, send later
If the monitor saves the identifier and then crashes before sending, the next run sees the post as already seen and never sends it. You get a lost alert, which is worse because nothing signals the loss.
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A pending-alert queue
The author’s proposed fix is to persist both the seen identifiers and a queue of pending alerts in one atomic write, then drain the pending queue at startup before polling the feed again. Recovery then works in both directions: an alert that was recorded but not sent is still in the queue, and an alert that was sent but not yet marked is handled by the delivery step described below.
The queue needs retry handling, a limit on attempts, and an acknowledgement step that removes an item only after the destination confirms receipt. Without those, the queue only moves the problem.
What the queue does not guarantee
A pending queue does not give you exactly-once delivery. The remaining case is this: the destination accepts a message, the sender crashes before recording the acknowledgement, and the retry sends the message again. Where the destination supports it, store the destination’s message identifier or use an idempotency key. Where it does not, describe your delivery as at-least-once, and make each message recognizable as a repeat, for example by including the post’s stable identifier in the message text. Do not assume that any particular chat or webhook service behaves a certain way on this point without checking its current documentation.
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The identifier you choose decides what counts as “new.” Two candidates look reasonable and both break.
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- URL. A post’s URL can change, so a changed URL produces a false new post.
- Title. Authors edit titles, so an edited title produces a false new post.
The author suggests using the RSS GUID as the primary key, with the URL as a fallback when a GUID is missing. The packaged monitor described in the same write-up uses Substack post IDs instead. Either approach avoids the two failures above, provided the identifier is taken from the feed and never generated from the title.
New is not the same as changed
The author’s actor keys on the post ID and does not hash content. It stays silent when a post with the same ID is edited, or republished with a new date. That makes it a detector of “what is new,” not “what changed.”
If readers need edit detection, add a separate mechanism. A reasonable starting point is to store, for each post ID, a content fingerprint and the time it was last seen, then send an update alert only when the fingerprint changes. Keep that path separate from new-post alerts so that an edit never looks like a fresh publication.
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There are three realistic ways to get this behavior. The table compares them on the axes that matter for reliability. A value marked “not stated” means the source material does not establish it, and you should check the provider’s current documentation.
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| Approach | Publications covered | Durable state across runs | Retries and acknowledgements | Detects edits to existing posts | Paid posts | Pricing |
|---|---|---|---|---|---|---|
| DIY RSS poller (the author’s prototype) | One or more, with per-publication state keyed separately | Only if you provide durable storage and backups | Only if you build the queue and acknowledgement logic | No, by default; add a fingerprint store | Requires an authenticated session, per the author’s stated scope | Your own hosting cost; not stated for the prototype |
| Automation trigger service (IFTTT documents a trigger for new posts by a followed user) | Not stated beyond followed users | Not stated | Not stated | Not stated | Not stated | Not stated |
| Scraping platform with a watchlist mode (Apify documents a watchlist mode with persistent key-value state) | Watchlist-based; scope per actor not stated | Persistent key-value state documented | Not stated | Not stated | Not stated | Not stated |
The automation and scraping rows show that alternatives exist. They do not show that those services handle the three failures above. Test any of them with the same scenarios you would use on your own build: a repeated run, an interrupted run, and an edited post.
Where to run it
The author mentions a Raspberry Pi as one always-on host. That is an option, not a requirement. An existing machine or a hosted scheduler can do the same job. Whichever you choose, the requirement is the same: durable state, backups, and a way to confirm that the state survived the last deploy.
Testing the monitor yourself
The author reports that the actor passed 7/7 stress tests. The author’s exact wording was: “It passed 7/7 of my stress tests: duplicate suppression across runs, crash recovery, multi-publication state isolation, feed hiccups.” That result comes from the author’s own actor and test setup, and it is not an independent verification. Run your own versions of the same scenarios before you trust the monitor:
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- Run the job twice against an unchanged feed and confirm that no alert repeats.
- Kill the process between sending and recording, then restart, and confirm that the alert is neither lost nor duplicated beyond what your delivery model allows.
- Move or redeploy the job without its state, and check whether old posts are replayed. If they are, your state storage is not durable.
- Watch two publications from one monitor and confirm that each keeps its own state.
- Edit a post’s title and URL in a test publication and confirm that no false new-post alert fires.
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