Newsletter sponsorship is the one channel where a devtool marketer can see the audience number before writing the email. Substack prints each publication's free subscriber count. Independent newsletters print theirs on an advertise page. Neither is an open rate, and I will come back to that, but both are real numbers a buyer can plan around.
I keep devsponsors, a directory of developer creators. This is the newsletter slice of it as of September 2026: 29 newsletters, 25 on Substack and 4 on their own domain.
The largest, by the number their platform or page prints
The Rundown AI, 1,700,000 self-reported subscribers; Superhuman AI, 1,500,000 self-reported subscribers; TLDR AI, 1,100,000 self-reported subscribers; SemiAnalysis, 315,000 free subscribers; The System Design Newsletter, 251,000 free subscribers; Product Growth, 240,000 free subscribers; Deep Learning Weekly, 224,000 free subscribers; Latent.Space, 198,000 free subscribers.
Two kinds of number are mixed in that list, and the directory keeps them apart. A Substack count is stated by Substack, not by the author. A count on a newsletter's own advertise page is the publication's own claim, so it is labelled self-reported on every row, it never ranks against a platform count, and the page links the exact place it was read from. The full list, with the label on each, is at developer and AI newsletters you can sponsor.
Who already sponsors them
9 of the 29 name their sponsors in the body of their issues, and between them they name 34 brands. The most disclosed: Product Growth (10), The System Design Newsletter (9), AI Agents Simplified (6), Engineering Leadership (5), Ben's Bites (3), AlgoMaster Newsletter (1), ByteByteGo Newsletter (1), Jam with AI (1), SeattleDataGuy’s Newsletter (1).
The disclosures come in a few shapes, and I read them as they are written: "This issue is brought to you by HubSpot", "This newsletter is sponsored by Unblocked", "(Thanks to AWS for partnering on this post.)", or a "Brought to you by:" header followed by one brand per line. Each brand links to the creators who named it, with the sentence, so a buyer can see which rivals already bought a given list before asking for the rate.
Across all developer creators, 10 brands sponsor two or more, and most of those repeat buys are in newsletters: Brilliant (3), CodeRabbit (3), Unblocked (3), AWS (2), Endor Labs (2), HubSpot (2), MongoDB (2), SerpApi (2), Viktor (2), Zapier (2).
What nobody can tell you
An open rate. Substack does not publish one, and an advertise page's open rate is the publication's own claim. The subscriber count is the size of the list, not the size of the audience that reads a given issue. Some newsletters also send some issues only to paying subscribers; the directory notes the share of recent issues that went to the whole list, because a sponsored slot in a paid-only issue reaches far fewer people than the count suggests.
Prices are the other gap. In the whole directory, only a handful of creators print a rate. Advertise pages mostly take an enquiry. The directory never estimates an audience figure, and where it shows a price estimate for a placement it is labelled as arithmetic on a public benchmark, with the arithmetic shown.
How I would use this
- Start from the topic page and sort by the figure you trust: Substack counts for the platform-stated list, then the self-reported ones as a separate conversation.
- Open the brands column before you email. If three rivals sponsored a newsletter last month, you have your answer about whether that audience buys.
- Ask for the recent send count and the free-list share, not the open rate. A publisher who answers with a screenshot is the one to work with.
- Give every placement its own link so you can compare newsletters against each other, not against a benchmark.
The method behind every figure is at devsponsors.com/method, and the brands seen sponsoring developer creators, newsletter or not, are at devsponsors.com/sponsors.
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