Publish Substack Post

Publish a Substack Post

You receive: article text (markdown or structured blocks) + title + subtitle. You insert content, add images, and publish or schedule.

This skill automates the long-form Substack editor with Playwright driving a logged-in browser profile (a persistent Chromium profile already signed into your Substack account). The selectors and timing below are the load-bearing part.

Pre-requisites

• A persistent Playwright/Chromium profile already logged into Substack
• Python venv with Playwright installed
• Your publication's publish URL: https://<your-publication>.substack.com/publish/post
• Image-generation tooling of your choice (any model that outputs PNG/JPEG)

Copy guidelines

Match your own publication's voice, language, and house style. Verify spelling/accents and that every numeric claim is sourced. Verify the final word count against the original draft.

Step 1 — Assemble article content

Parse your source into a list of (type, html_content) tuples. Preserve bold (<strong>), italic (<em>), headings (h2), dividers (<hr>), paragraphs (<p>). Save as JSON for the publish script.

Verify word count against the original.

Step 2 — Plan image placement

Identify 3 image insertion points between major sections. Images go BETWEEN content chunks, not at the end. Split content into 4 chunks:

• Chunk A: Intro + Section 1
• → Image 1
• Chunk B: Section 2 + Section 3
• → Image 2
• Chunk C: Section 4 + Section 5
• → Image 3
• Chunk D: Remaining sections + conclusion + CTA

Step 3 — Generate images (4 total)

Generate with whatever image model you use, applying a consistent style prefix so all images share a look:

1. Hero (cover/thumbnail): landscape 16:9, relates to the article thesis
2. Inline 1: square 1:1, relates to the first section
3. Inline 2: square 1:1, relates to the middle section
4. Inline 3: square 1:1, relates to a later section

Tip: some image models output PNG regardless of the requested extension. Convert to a real JPEG before upload:

from PIL import Image
img = Image.open(tmp_png_path).convert('RGB')
img.save(output_path, 'JPEG', quality=92)

Step 4 — Create the post and insert content

4a. Navigate and fill title/subtitle

await page.goto('https://<your-publication>.substack.com/publish/post', timeout=30000)
await asyncio.sleep(5)

# Title
await page.click('[placeholder*="Title"]')
await page.keyboard.type(TITLE, delay=15)
await asyncio.sleep(0.5)

# Subtitle
await page.keyboard.press('Tab')
await asyncio.sleep(0.3)
await page.keyboard.type(SUBTITLE, delay=10)
await asyncio.sleep(0.5)

# Move to body editor
await page.keyboard.press('Tab')
await asyncio.sleep(1)

4b. Insert content with images (THE CRITICAL PART)

NEVER use keyboard.type() for article body. It drops characters on long text.

Use insertHTML in chunks. The cursor stays at the end after each insertion, so images inserted between chunks land at the correct position.

async def insert_html(page, html):
    return await page.evaluate("""(html) => {
        const e = document.querySelector('.ProseMirror');
        e.focus();
        const s = window.getSelection();
        s.selectAllChildren(e);
        s.collapseToEnd();
        document.execCommand('insertHTML', false, html);
        return e.innerText.length;
    }""", html)

async def insert_image(page, path):
    img_btn = page.locator('[aria-label*="image"], [aria-label*="Image"]')
    if await img_btn.count() > 0:
        await img_btn.first.click()
        await asyncio.sleep(1)
        menu = page.locator('[role="menuitem"]:has-text("Image")')
        if await menu.count() > 0:
            await menu.first.click()
            await asyncio.sleep(1)
        fi = page.locator('input[type="file"]')
        if await fi.count() > 0:
            await fi.last.set_input_files(path)  # .last — Substack has multiple file inputs
            await asyncio.sleep(4)
            return True
    return False

Insertion sequence:

for ctype, content in CHUNKS:
    if ctype == 'html':
        length = await insert_html(page, content)
    elif ctype == 'image':
        await insert_image(page, content)

4c. Verify word count

wc = len((await page.evaluate("document.querySelector('.ProseMirror').innerText")).split())
print(f"Word count: {wc} (target: XXXX)")
# Must match the original. If not, stop and investigate.

4d. Take screenshots to verify layout

await page.evaluate("window.scrollTo(0, 0)")
await page.screenshot(path='/tmp/sub_top.png')
await page.evaluate("window.scrollTo(0, document.body.scrollHeight/2)")
await page.screenshot(path='/tmp/sub_mid.png')
await page.evaluate("window.scrollTo(0, document.body.scrollHeight)")
await page.screenshot(path='/tmp/sub_bot.png')

Step 5 — Add a subscribe button

MUST be done BEFORE clicking Continue/Publish. Otherwise Substack shows a popup that disrupts the scheduling flow.

await page.evaluate("""() => {
    const e = document.querySelector('.ProseMirror');
    e.focus();
    const s = window.getSelection();
    s.selectAllChildren(e);
    s.collapseToEnd();
}""")
await page.keyboard.press('Enter')

# Toolbar > Button dropdown > Subscribe
await page.locator('button:has-text("Button")').first.click()
await asyncio.sleep(1)
# nth(1) because the first "Subscribe" match is the toolbar button itself
subs = page.locator('button:has-text("Subscribe")')
if await subs.count() > 1:
    await subs.nth(1).click()
await asyncio.sleep(2)

Step 6 — Upload the hero image

# Use data-testid — there are 2 "Settings" buttons; this targets the right one
await page.locator('[data-testid="settings-button"]').click()
await asyncio.sleep(2)

fi = page.locator('input[type="file"]')
if await fi.count() > 0:
    await fi.first.set_input_files(HERO_PATH)
    await asyncio.sleep(5)

await page.locator('button:has-text("Done")').first.click()
await asyncio.sleep(2)

⚠️ Do NOT use button:has-text("Settings") — it resolves to 2 elements (strict-mode violation).

Step 7 — Schedule (or publish immediately)

# Open publish dialog
await page.locator('button:has-text("Continue")').first.click()
await asyncio.sleep(3)

# Set audience to Everyone (click the LABEL, not the hidden radio input)
await page.locator('label:has-text("Everyone")').first.click()
await asyncio.sleep(1)

# Enable scheduling (click the LABEL, not the aria-hidden checkbox)
sched = page.locator('label:has-text("Schedule time to email and publish")')
await sched.scroll_into_view_if_needed()  # often below the viewport
await sched.click()
await asyncio.sleep(2)

# Set datetime via JS (native .fill() doesn't work on this input)
await page.evaluate("""(dt_value) => {
    const dt = document.querySelector('input[type="datetime-local"]');
    const setter = Object.getOwnPropertyDescriptor(
        window.HTMLInputElement.prototype, 'value'
    ).set;
    setter.call(dt, dt_value);
    dt.dispatchEvent(new Event('input', { bubbles: true }));
    dt.dispatchEvent(new Event('change', { bubbles: true }));
}""", '2026-04-16T10:00')  # Format: YYYY-MM-DDTHH:MM
await asyncio.sleep(1)

# Click send (button text is dynamic: "Send to everyone in X hours/days")
send = page.locator('button:has-text("Send to everyone")')
await send.last.click()
await asyncio.sleep(5)

# Handle the subscribe-button popup IF it still appears (shouldn't if Step 5 worked)
for sel in ['button:has-text("Publish without buttons")', 'button:has-text("Add subscribe buttons")']:
    b = page.locator(sel)
    if await b.count() > 0:
        await b.first.click()
        await asyncio.sleep(3)
        break

For immediate publish (no scheduling): skip the schedule-label click and datetime, just click "Send to everyone now".

Step 8 — Verify

Screenshot the confirmation page. It should say "Your post is scheduled!" (or "Your post is live!").

Step 9 — Delete a scheduled post (if needed)

The ... menu buttons are SVG elements, not <button>. Find by position:

results = await page.evaluate("""() => {
    const all = document.querySelectorAll('button, svg');
    return [...all].filter(el => {
        const r = el.getBoundingClientRect();
        return r.x > 550 && r.width < 40 && r.width > 10 && !el.textContent.trim();
    }).map(el => ({
        tag: el.tagName,
        x: el.getBoundingClientRect().x + el.getBoundingClientRect().width/2,
        y: el.getBoundingClientRect().y + el.getBoundingClientRect().height/2
    }));
}""")
# Click the SVG at the y-position of the target post row
await page.mouse.click(target_x, target_y)
# Then: [role="menuitem"]:has-text("Delete") → confirm button

Known issues

• Excessive spacing between paragraphs: insertHTML with <p> tags sometimes adds extra margin in the Substack ProseMirror editor. Monitor visually. Potential fix: test <br> between paragraphs instead of separate <p> blocks.
• Audience radio visual state: The "Everyone" label click may not visually update the radio in the Settings panel, but the publish-button text ("Send to everyone") confirms the correct audience is selected.
• Image-model timeouts: Hero-image generation can time out. Retry once on failure.