Kyle Nazario

The time I went stir-crazy during Covid and reverse-engineered Substack’s private API

The time I went stir-crazy during Covid and reverse-engineered Substack’s private API

Via Stable Diffusion

I try to spend my time on this blog showcasing things I’m proud of - cool side projects, AI tricks, clever reactive programing, that kind of thing.

But I also wanted to once, permanently, write down what happened to my least successful side project. It’s the time I poured hundreds of hours into a terrible app no one used.

It’s time to write about the time I went stir-crazy during COVID and reverse-engineered Substack’s private API.

The setting

I was in a bad headspace in June 2020. I was living across the country from my friends and family in a small one-bedroom apartment. COVID lockdowns were in effect. I’d barely seen anyone in person except my partner for months.

My job was frustrating. I was working as a frontend developer at a local SaaS company that was, well, chaotic. To give a sense of things, our DevOps guy caught a company VIP browsing 4Chan at work. With one year of experience and no computer science degree, I wanted a new job but could not get one.

I couldn’t change my resume, so I fixated on gaining notoriety with a side project. Make something amazing and maybe somebody at a good company would notice and offer a golden ticket out of my dreary day job (this did not work).

The idea

What to make? Well, in another life, I actually worked as a writer and journalist, so I decided to make an app for writers.

At this point, Substack didn’t have any kind of native app. I thought that was strange. Why should Substack writers have to compose newsletters in a web app, even on an iPad? Also, why should they have to use a simple rich text web editor? Why not let them write in Markdown?

Thus, Compose for Substack was born.

The app

It would be a native iOS, iPadOS and macOS app for newsletter authors. You’d compose Substack newsletter drafts using Markdown, then save and publish, all from a well-designed, native interface.

A screenshot of Compose for Substack. No draft is selected

I didn’t have any iOS or macOS development experience, but Apple had just announced version 2 of SwiftUI. SwiftUI is their declarative framework for writing apps. It felt similar to React, which I knew somewhat well, and so I decided Compose should be written in SwiftUI. This plan seemed like the best way to create a high-quality, native iOS and macOS interfaces without learning the intricacies of UIKit and AppKit.

If you are an Apple ecosystem developer, you may already see some problems with this idea.


Compose for Substack had an ambitious goal - total interoperability with the Substack web app. My goal was to let the user start a draft in Compose, then view it and have it be exactly the same.

For this reason, Compose only lightly used CloudKit and Core Data. The most important data, the blog posts, was actually stored on Substack’s servers as a normal draft post - as if the user had written it on

This (way too ambitious) goal required reverse-engineering Substack’s private, undocumented drafts API. I played around with the web editor, keeping an eye on the network tab of the Chrome dev tools. I traced every request, building a mental model of how Substack’s frontend communicated with the backend.

I could see why they hadn’t opened it to third-parties. One of the properties on the draft response object actually used the ❤ character as a property key, I think to indicate how many favorites it had.

If you tried to load data for a user’s publication, it just returned a whole kitchen sink. Look at all this.

// SubstackPublication.swift
import Foundation

struct SubstackPublication: Codable {
    let id: Int?
    let name: String? = nil
    let type: String? = nil
    let homepageType: String? = nil
    let logoURL, logoURLWide: String?
    let subdomain: String?
    let authorID: Int?
    let copyright: String? = nil
    let customDomain: String?
    let customDomainOptional: Bool? = nil
    let emailBannerURL: String? = nil
    let emailFrom: String? = nil
    let trialEndOverride: String? = nil
    let emailFromName: String? = nil
    let supportEmail: String? = nil
    let firstEnabledPaymentsAt: String? = nil
    let heroImage: String? = nil
    let heroText: String? = nil
    let requireClickthrough: Bool? = nil
    let themeVarBackgroundPop: String? = nil
    let defaultCoupon: String? = nil
    let communityEnabled: Bool? = nil
    let themeVarCoverBgColor: String? = nil
    let coverPhotoURL: String? = nil
    let themeVarColorLinks: Bool? = nil
    let defaultGroupCoupon: String? = nil
    let paymentsEnabled: Bool? = nil
    let createdAt: String? = nil
    let podcastEnabled: Bool? = nil
    var pageEnabled: Bool? = nil
    var applePayDisabled: Bool? = nil
    let fbPixelID: Int? = nil
    let gaPixelID: Int? = nil
    let twitterPixelID: Int? = nil
    let podcastTitle: String? = nil
    let podcastFeedURL: String? = nil
    let hidePodcastFeedLink: Bool? = nil
    let paymentsSurveyStatus: Int? = nil
    let minimumGroupSize: Int? = nil
    let parentPublicationID: Int? = nil
    let bylinesEnabled: Bool? = nil
    let bylineImagesEnabled: Bool? = nil
    let postPreviewLimit: Int? = nil
    let defaultWriteCommentPermissions: String?
    let defaultPostPublishSendEmail: Bool?
    let googleSiteVerificationToken: String? = nil
    let pauseState: String? = nil
    let language: String? = nil
    let paidSubscriptionBenefits: String? = nil
    let freeSubscriptionBenefits: String? = nil
    let foundingSubscriptionBenefits: String? = nil
    let parentAboutPageEnabled: Bool? = nil
    let inviteOnly: Bool? = nil
    let subscriberInvites: Int? = nil
    let defaultCommentSort: String?
    let plans: String? = nil
    let stripeCountry: String? = nil
    let authorName: String? = nil
    let authorPhotoURL: String? = nil
    let authorBio: String? = nil
    let hasChildPublications: Bool? = nil
    let hasPublicUsers: Bool? = nil
    let hasPosts: Bool? = nil
    let hasPodcast: Bool? = nil
    let hasSubscriberOnlyPodcast: Bool? = nil
    let hasCommunityContent: Bool? = nil
    let twitterScreenName: String? = nil
    let draftPlans: String? = nil
    let baseURL: String?
    let hostname: String?
    let isOnSubstack: Bool? = nil
    let parentPublication: String? = nil
    let childPublications: Array<String>? = nil
    let siblingPublications: [String]? = nil

    enum CodingKeys: String, CodingKey {
        case id
        case logoURL = "logo_url"
        case logoURLWide = "logo_url_wide"
        case subdomain
        case authorID = "author_id"
        case customDomain = "custom_domain"
        case coverPhotoURL = "cover_photo_url"
        case defaultWriteCommentPermissions = "default_write_comment_permissions"
        case defaultPostPublishSendEmail = "default_post_publish_send_email"
        case defaultCommentSort = "default_comment_sort"
        case baseURL = "base_url"
        case hostname

The API wasn’t bad, just messy and not built for outside use. It took a lot of experimentation to decipher which properties to send in PUT requests to save updated newsletter drafts.

One of the hardest parts was reverse-engineering how Substack saved post contents. It looked like they were using Prosemirror and saving the text as one big JSON object. I had to write what was, as far as I knew, the world’s only Swift-language Markdown-Prosemirror converter. I’m proud of that one.

The editor

I also faced the small issue that no one had written an open-source text editor that was exactly what I wanted. Lots of editors had Markdown shortcuts. If you typed **hello**, editors with Markdown shortcuts removed the asterisks and left hello in bold.

I wanted an editor like Byword (which I am using to write this post), which uses Markdown patterns to apply formatting as you type, without removing the formatting characters. If you type **hello** in Byword, it leaves the asterisks and makes the “hello” between them bold.

Compose for Substack showing a list of drafts next to the editor

Nothing fit, so I learned just enough UIKit and AppKit to write a text editor that formats as you type. It took a long time and a lot of effort, and it still slows down if you write too much, but it worked. The editor took Markdown, and, after the user had stopped tpyping for a few seconds, converted it to Prosemirror and synced it to

That was about the extend of my SwiftUI success.

The framework

A screenshot of an iPad displaying a login screen in Compose

I like SwiftUI. Interfaces are easier to build declaratively, one reason React has taken over frontend web dev. It feels more intuitive than making subclasses and overriding parent functions, like in UIKit.


SwiftUI has enough rough edges, some indie devs have sworn off it entirely. Every year, though, Apple makes it more powerful and less buggy.

In 2020, the rough edges were twice as bad. AppKit-flavored SwiftUI, which of course I picked, is twice that. I picked SwiftUI to avoid learning AppKit and UIKit, but it lacked so much I still ended up learning both.

This was my mistake. To create a high-quality, boutique iOS & Mac app, avoid brand-new frameworks with tons of rough edges. 2020-era SwiftUI was a bad choice for a junior trying to make something nice.

The price

The app would be free to download. You would be able to publish three posts before I’d require a $5 / month subscription.

The subscription was necessary because this app would require ongoing updates to keep it compatible with the Substack API. “Lifetime purchase” was also a bad option, since Substack could cut me off at any time. Better to charge month to month.

The settings screen of Compose for Substack

I investigated in-app payments. On one hand, RevenueCat seemed like a fast way to get out the door. StoreKit seemed complicated.

I still picked StoreKit. If the app made money, I didn’t want to give RevenueCat a cut. Plus, using a third-party StoreKit library somehow seemed like cheating.

The launch

You are probably starting to sense some red flags. Reader, I sensed none of them.

For example, one small red flag was the audience. At no point during development did I:

  • Test the app with Substack writers
  • Ask writers if they wanted a native editing app
  • Ask writers if they wanted to write in Markdown
  • Ask writers if they would pay for an app enabling them to do these things

I plowed on ahead and launched the iOS and iPadOS versions of the app in the fall of 2020.

After several weeks, my total subscribers were…


I can’t remember how many people downloaded it, but it wasn’t more than 40. It is the worst-performing side project I’ve ever made.

I was so embarrassed I pulled it off the App Store after just a few weeks. The product-market mismatch was so obvious, more development seemed like a waste.

The lessons

If nothing else, the whole fiasco was a good learning experience.

  • Product-market fit is everything. If you don’t have something people will pay for, nothing else matters.
  • Technical competence loses to product-market fit every time.
  • Get your product in front of users as soon as possible.
  • Use proven, established tools you know well so you can build something good quickly.
  • Reverse-engineering private APIs is fun but a risky business idea.
  • Either build something for yourself, or something you’re extremely confident other people will use.
  • Use third-party tools like RevenueCat or Firebase if they help you build faster. It doesn’t matter if they take a bigger cut of your profits. Your top priority is to get out the door and figure out if your app is something people actually want.

The project wasn’t a total loss. I spent some time breaking out my text editor into an open source project, HighlightedTextEditor. It’s been decently successful - 643 stars, with a fair number of issues and PRs on GitHub. I’m glad people are using it, and happy to give back to the iOS dev community.

It’s the least I can do, since I can’t make a good Substack app 😉.