--- title: "Substack API: the official MCP can't publish, an agent can" description: "Substack's official MCP server only reads publication data. What an AI agent can do on Substack in your signed-in browser, and what we measured logged out." canonical: "https://scalebrowser.net/blog/substack-api" last_modified: "2026-10-25" published: "2026-10-25" author: "elias" category: "use-cases" --- > ## Documentation Index > Fetch the complete documentation index at: https://scalebrowser.net/llms.txt > Use this file to discover all available pages before exploring further. # Substack API: the official MCP can't publish, an agent can > Substack's official MCP server only reads publication data. What an AI agent can do on Substack in your signed-in browser, and what we measured logged out. Picture a writer who finishes the week's newsletter in Google Docs on Sunday evening and then pastes it into Substack piece by piece: the title, the subtitle, each heading, the image, the send time. Every part is a small click, and none of it needs the writer's judgement. It's the kind of job people hope an API will take over. ## Does Substack have an API? Substack has one official interface for AI assistants, a read-only MCP server, and we found no interface for publishing. MCP is the connector standard that lets an assistant such as Claude or ChatGPT read data from a service. We searched Substack's help center for ["API"](https://support.substack.com/hc/en-us/search?query=API) on 11 October 2026 and got 1 result, the article on [how to connect Substack to your AI assistant](https://support.substack.com/hc/en-us/articles/50834026608916-How-to-connect-Substack-to-your-AI-Assistant). Only an admin of a Bestseller publication can connect the server, at `mcp.substack.com/api/v1/mcp`. Anyone else who tries sees an error saying the publication isn't eligible. ### What can the official Substack MCP do? The official Substack MCP reads publication data and changes nothing. Substack's own examples of what you can ask once it's connected are these four: - What's my current subscriber count and revenue? - How is my paid subscriber retention trending after 6 months? - Which traffic source drove the most views on my last post? - How many free subscribers have I gained since January? The help center names the three things it can't do: It cannot publish posts, send Notes, or modify your account. It also can't read profile data or Notes activity. Connecting it in Claude takes 7 steps, from opening Customize to returning to a new chat, and you have to be signed in to Substack as the admin. ## What can an agent do on Substack through a browser? An agent can do on Substack's website what you can do there, as soon as it is signed in as you, and the MCP server can't do any of it. Once [signing an agent into Substack](/blog/ai-agent-login) is done, these are the prompts the website side opens up: - Turn this Google Doc into a draft with the right headings. - Schedule it for Sunday at 8:00. - Answer the new comments on last week's post. - Post a Note about it. ![A Google Doc window leads through an agent box in your browser to a blue scheduled post window; above them an official MCP box reads subscribers, traffic and revenue, and its arrow to the post is crossed out and labelled cannot publish](/blog/substack-api/reads-versus-publishes.png "The official MCP server reads subscribers, traffic and revenue and cannot publish; the agent in your signed-in browser takes the draft to a scheduled post.") You choose how far it goes. The agent drafts and schedules, and you press publish if you want to be the last click. Replies to comments are website work too, because the MCP server is read-only; the two On Substack posts we opened logged out showed 0 comments, so we have no measurement for reading them yet. ## How does the agent use Substack's sign-in link? Substack's sign-in starts from an email address: its sign-in page asks for the address first and keeps the password as a second option. The link then goes to the agent's own inbox, and Scalebrowser's `read_inbox` waits up to 60 seconds for it and returns up to 25 candidates. The agent opens the link in the same profile that asked for it, the case Substack's emailed sign-in link describes in full. We didn't submit an address for this article, so no email was sent and the step is described from the inbox documentation, not measured here. ## What does a publishing run look like? A publishing run is the sequence you'd do by hand, in your own signed-in browser: open the draft, check each part, set the time and stop where you said to stop. We measured the half that needs no account, on 11 October 2026, 3 runs each, without signing in or sending anything. | What we did, logged out | Steps | Seconds, 3 runs | What came back | | --- | --- | --- | --- | | Searched the help center for "API" and opened the result | 3 | 9.6 to 11.8 | 1 result, the MCP article: 7 connect steps, read-only | | Opened a public post on [On Substack](https://on.substack.com/p/a-new-vision-for-sponsorships) | 1 | 3.4 to 4.0 | the whole post, 8,546 characters of page text, no sign-in needed to read it | | Opened the sign-in page (1 run) | 1 | not timed | an email field first, a password as the second option | A step is one page opened, one search typed or one click. The numbers say what the read-only half costs: a public post is readable in a single step, and the help center answer took 3. A publish needs the account, so we have no time for it, and that run isn't part of this article yet. #### Where does the browser run? The browser runs on your own machine, in a profile that keeps its Substack sign-in between runs, which makes it [the browser that publishes the draft](/blog/ai-agent-browser). More jobs where a service has no interface for the job, only a website, are collected in the [overview of use cases](/use-cases) and on the page for [tools with no API](/use-cases/tools-with-no-api).