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Reading a Substack publication’s feed is the easy part of watching for new posts. The hard part is remembering which posts you have already announced, and keeping that memory intact across restarts, deploys, crashes, and edits. A monitor that polls a feed, compares entries with a saved list, and posts to a chat channel will work on day one. Whether it keeps working depends on three things: where its state lives, what identifies a post, and how alerts are delivered.
What you are actually polling
Every Substack publication exposes an RSS feed at https://your.substack.com/feed, where your is replaced with the publication name. Substack’s official support article answering whether a publication has an RSS feed gives this address pattern and was last updated November 8, 2023. Use the pattern as your starting point, then confirm it against your own publication before building anything on it.
The feed is the signal, not the whole story. The approach described in the source write-up covers public, free posts. Paywalled material requires an authenticated session, which a plain feed request does not provide. The support article does not establish coverage by post type, archive depth, rate limits, or delivery guarantees, so do not assume a feed poll will show every post a paying subscriber can see.
Why not just use email subscriptions?
An email subscription delivers a post to an inbox for a person to read. It does not hand your code a structured entry with a stable identifier that you can log, deduplicate, or route to a channel. If the goal is an automated alert into a team workspace, the feed gives you the structured data you need. Email is a reasonable fallback for a single reader, not a building block for a monitor.
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The minimal loop
A basic poller is a short cycle. The version that survives crashes adds a durable pending queue, but the core steps look like this:
- Request the publication feed with a timeout and handle HTTP errors without clearing state.
- Parse each entry and extract a stable identifier (see the identity section below).
- Load the set of identifiers you have already processed from durable storage.
- Compute the entries whose identifiers are not in that set.
- Write those entries to a pending-alert queue in durable storage.
- Send each pending alert. Mark it sent only after the destination confirms receipt.
- Persist the updated set of seen identifiers together with the queue state.
The author ran the poller on a 15-minute cron schedule. That interval is an example from one implementation, not a recommended or guaranteed polling frequency. Choose an interval based on how quickly you need alerts and how often the publication posts. A Raspberry Pi is one optional always-on host; an existing computer or a hosted scheduler can run the same job.
Failure one: state that does not travel with the process
The author’s first version kept seen identifiers in a seen_ids.json file beside the script. After a machine move, and after a deploy that left out the state file, previously announced posts were announced again. The code was correct; the memory was not where the process could find it.
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Self-hosted or DIY deployments
- Store state in durable storage that is attached to the scheduled job, not in a working directory that a deploy can replace.
- Back up the state store on the same schedule you back up anything else you cannot easily recreate.
- Test the recovery path: stop the job, move it to a clean environment, restore only the backup, and confirm that no old posts are announced.
Managed or hosted deployments
- Confirm in the platform’s documentation that key-value or storage state persists between scheduled runs.
- Check whether state is isolated per publication if you watch more than one.
- Run a test: trigger two consecutive runs and verify that the second run announces nothing new.
Failure two: the crash window between sending and saving
Sending an alert and recording that you sent it are two separate operations. A crash between them creates one of two failures, and the order you choose decides which one you get.
| Order of operations | Where a crash lands | Result on restart |
|---|---|---|
| Send the alert, then save the seen identifier | After the send, before the save | The same post is announced again (duplicate) |
| Save the seen identifier, then send the alert | After the save, before the send | The post is never announced (lost alert) |
| Atomically save seen identifiers and a pending-alert queue, then drain the queue at startup | Anywhere in the cycle | Unsent work is recovered; delivery becomes at-least-once rather than lost |
The third option is the design the author proposes. It requires a pending queue with retry and acknowledgement handling: each queued item stays in place until the destination confirms it, and the process checks the queue before polling the feed. The queue turns a silent loss into a delayed alert, which is usually the trade you want, but it introduces the delivery question covered below.
Failure three: the identity key is mutable
Using a post’s URL or title as the only key for deduplication is a common shortcut, and the author shows why it fails. A URL can change, and authors can edit titles. Either change can make an already announced post look new, which produces a false alert.
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- Preferred key: the RSS GUID from each feed entry, which the author uses in the prototype.
- Fallback: the URL, used only when a GUID is absent.
- Packaged monitor: the author’s packaged version uses Substack post IDs instead.
Whichever key you choose, write it down, store it with the post’s first-seen timestamp, and test it by changing a title and a URL on a test post and confirming that no second alert fires.
New post versus changed post
The author’s monitor keys on the post ID and does not hash content. It stays silent when a post with the same ID is edited, or when it is republished with a new date. In other words, it answers the question “what is new?” and does not answer “what changed?”
If you need edit detection, treat it as a separate mechanism. Store a content fingerprint or a version history for each post, compare it on each poll, and send edit events through a different message type so readers can tell them apart from new-post alerts. Do not overload the new-post channel with edit notices.
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Delivery: at-least-once, not exactly-once
A pending queue recovers unsent work, but it does not by itself guarantee that each message arrives once. The remaining case is this: the destination accepts a message, then the sender crashes before recording the acknowledgement. On restart, the sender retries and the message is sent twice.
- Where the destination returns a message identifier, store it before marking the alert complete, and check for it before retrying.
- Where it does not, or where you cannot confirm the destination’s behavior, describe the delivery as at-least-once and make duplicates recognizable. Including the post ID and publication name in every message lets a reader spot a repeat.
- Do not claim exactly-once behavior unless the destination’s own API documents an idempotency mechanism you are using.
The author’s comments describe this trade-off in general terms. They do not establish how any specific chat service handles retries or duplicate detection, so check that before you choose a destination.
Options compared
There are three realistic ways to build this: a self-built RSS poller, a hosted monitor, or an automation trigger service. The table compares them on the axes that determine whether the monitor works over time.
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| Approach | Publications covered | Seen-state durability | Retries and acknowledgements | New posts versus edits | Setup and ownership |
|---|---|---|---|---|---|
| Self-built RSS poller | Any publication whose feed you configure | Your responsibility; must be attached to the scheduled job and backed up | Your responsibility; a pending queue is needed for recovery | New-post detection only unless you add content fingerprinting | Highest effort; you own uptime, storage, and updates |
| Hosted monitor | Depends on the vendor; not stated in the source material | Not stated; confirm with the vendor | Not stated; confirm with the vendor | Not stated; confirm with the vendor | Lowest effort; ownership depends on the vendor’s terms |
| Automation trigger service (for example, IFTTT’s new-post trigger for followed users, or Apify’s watchlist mode with persistent key-value state) | IFTTT documents a trigger for new posts by a followed user; Apify documents watchlist monitoring; per-publication coverage not stated | Apify documents persistent key-value state; IFTTT’s storage behavior not stated | Not stated in the documentation reviewed (checked October 7, 2026) | Not stated in the documentation reviewed (checked October 7, 2026) | Low effort to start; you depend on the service’s current features and pricing |
Service capabilities and pricing change, and the table reflects what the documentation said when it was checked on October 7, 2026. Confirm current behavior before you commit to a service.
How the author tested the prototype
The author reports that the actor passed 7 of 7 stress tests covering duplicate suppression across runs, crash recovery, multi-publication state isolation, and feed hiccups. That result comes from the author’s own test set, described in a DEV article published October 4, 2026 and edited October 5. It is a self-reported result for one implementation, not an independent verification, and it does not show how the monitor would behave against other publications, other feed conditions, or other destinations.
Checks before you rely on the monitor
- Delete the local working directory and confirm that a restored state file prevents old posts from being announced.
- Stop the process between sending and saving, then restart it and count the duplicates or losses.
- Change the title and URL of a test post and confirm that no second new-post alert fires.
- Run a second publication through the same job and confirm that its state does not overwrite or mix with the first.
- Simulate a feed outage and confirm that the job keeps its saved state and resumes without re-alerting.
- Check that every outgoing message carries the post ID and publication name, so a duplicate can be recognized.
A monitor that passes these checks answers the new-post question reliably. It will still not tell you what changed in an existing post unless you build that separately.
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