I Published 12 Newsletters via CDP Automation (Without Touching Substack UI)

TL;DR: After bashing my head against Reddit / IH / Twitter Lexical anti-bot for 2 hours, I tried the same approach on Substack and it just worked. TipTap is friendlier to programmatic input than Lexical (with anti-bot). Below: the exact selectors + the working 30-line script.

Context

I'm at Day 60 of an indie iOS experiment. Distribution requires posting to:

• Reddit (Lexical) — body input blocked
• IndieHackers (Lexical) — body input blocked
• Twitter X (Lexical) — body input blocked
• Substack (TipTap) — works
• dev.to (REST API) — works
• Apple ASC web UI (React SPA) — works with JS click

This article is about the Substack TipTap path because it's the only rich-text editor I found that actually accepts CDP-driven input.

The selectors

Substack's editor has 3 input fields per post:

// Title
document.querySelector('textarea[aria-label="title"]')

// Subtitle
Array.from(document.querySelectorAll('textarea'))
  .find(t => (t.placeholder||'').includes('サブ'))  // JP locale: サブタイトル
  // or .includes('subtitle') for EN

// Body (TipTap editor)
document.querySelector('.tiptap.ProseMirror')

The body editor is .tiptap.ProseMirror — TipTap's standard class names exposed in the DOM.

The working flow (30 lines)

import time
from playwright.sync_api import sync_playwright

with sync_playwright() as pw:
    browser = pw.chromium.connect_over_cdp("http://localhost:9222", timeout=30000)
    ctx = browser.contexts[0]
    page = ctx.new_page()
    page.goto("https://your-newsletter.substack.com/publish/post", wait_until="domcontentloaded", timeout=30000)
    time.sleep(8)  # let SPA hydrate

    # Title
    page.evaluate(f"""
        () => {{
            const t = document.querySelector('textarea[aria-label="title"]');
            t.focus();
            Object.getOwnPropertyDescriptor(t.constructor.prototype, 'value').set.call(t, 'My Title');
            t.dispatchEvent(new Event('input', {{bubbles: true}}));
        }}
    """)
    time.sleep(1)

    # Body via clipboard + Ctrl+V (TipTap accepts this)
    body_md = "## Body content\nYour markdown here..."
    page.evaluate(f"() => navigator.clipboard.writeText({body_md!r})")
    time.sleep(0.5)
    page.evaluate("() => document.querySelector('.tiptap.ProseMirror').focus()")
    time.sleep(0.5)
    page.keyboard.press("Control+V")
    time.sleep(4)

    # Click Continue (続ける in JP, Continue in EN)
    page.locator('button:has-text("続ける")').first.click(timeout=8000)
    time.sleep(8)

    # Click Send Now (今すぐ全員に送信 in JP, Send Now in EN)
    page.locator('button:has-text("今すぐ全員に送信")').first.click(timeout=8000)
    time.sleep(10)

    # Verify URL changed to /share-center
    print(f"Published: {page.url}")

Why this works (vs Lexical anti-bot)

TipTap doesn't have anti-bot guards built-in. The standard editor accepts:

• Native React setter for text inputs (title/subtitle)
• Clipboard paste via Ctrl+V (body)
• Programmatic button clicks (Continue/Send)

Lexical has the same APIs but additional event filtering:

• isTrusted: false events get rejected
• Editor reconciler reverts external DOM mutations
• Synthetic keypresses get filtered

Substack chose TipTap for their editor; Reddit/IH/Twitter chose Lexical. The choice cascades.

Edge cases

1. UTF-8 BOM in source files

Substack posts often have YAML frontmatter that starts with BOM (Windows files especially). Strip it:

text = Path(file).read_text(encoding="utf-8-sig")  # strips BOM

2. Frontmatter leakage

If you don't strip the YAML frontmatter from your markdown body, it leaks into the published post. Strip with regex:

import re
m = re.match(r"^---\s*\n(.*?)\n---\s*\n", text, re.DOTALL)
if m:
    text = text[m.end():].lstrip()

3. Locale detection

Substack switches button labels by browser locale. Japanese (JP): 続ける / 今すぐ全員に送信. English: Continue / Send Now. Match both:

page.locator('button:has-text("続ける"), button:has-text("Continue")').first.click()

4. Multiple drafts queue

If you publish multiple posts in rapid succession (e.g., 3 in 5 min), Substack's UI may queue them. Wait 30+ seconds between publishes.

5. Slug collision

If you publish a title that matches an existing post's slug, Substack auto-appends a numeric suffix (-693, etc.) to make it unique. Track via final URL, not predicted slug.

What I shipped using this flow

In one /autoiter session today:

• Substack #21: B2B funnel 15 pages → LIVE
• Substack #22: 90-min IAP setup → LIVE
• Substack #23: Day 60 milestone → LIVE

Each took ~30 seconds wall-clock from script invocation to public URL.

Source

Working script: dashboard/substack_publish_22.py in autoapp-toolkit (MIT).

For Lexical (Reddit / IH / Twitter), see my earlier article: Why Reddit Lexical Editors Block Programmatic Input.

When to use this approach

• You have a Substack newsletter
• You want to publish multiple drafts programmatically (CI/CD style)
• You don't want to manually paste-publish each post

When NOT to use this approach

• You have <1 post per week (manual is fine)
• You haven't already automated everything else (this is end-stage automation)
• Substack's TipTap version updates regularly (selectors may drift; verify each major version)

If you're indie iOS or indie SaaS and want the playbook with all the CDP automation chapters: iOS Indie Launch Playbook ($19).