PromptMill

PromptMill

PromptMill

Batch-generate images on Google Flow.
Paste your prompts, press Start, walk away. Every image lands in your Downloads folder.

Chrome MV3 License MIT No account No telemetry No build step Buy me a coffee

English · Tiếng Việt

PromptMill running a 105-prompt queue against Google Flow
A 105-prompt queue mid-run. Finished prompts are skipped on the next pass.

[!IMPORTANT] The extension’s interface is currently in Vietnamese only. The code, this README, and all configuration are documented in English, but every button and label you see in the side panel is Vietnamese. If that’s a blocker for you, issue #1 tracks internationalisation — the strings are not extracted yet, so it’s a good first contribution.

What it does

You have 200 prompts and one Google Flow tab. Doing it by hand means 200 rounds of paste → Enter → wait → right-click → Save As. PromptMill does that loop for you.

  1. Paste a list of prompts — one per line — or drag a .txt file onto the panel.
  2. Press Start. PromptMill types each prompt into Flow, submits it, waits for the images, and downloads every one of them into Downloads/<your folder>/.
  3. Close the laptop. It pauses between prompts on a human-shaped rhythm so the run doesn’t look like a script hammering the page.

Prompts that fail get a Retry button. Prompts that already succeeded are skipped when you run again, so you never pay for the same image twice.

Why this one

There are a lot of Google Flow automators. Most of them chase feature count — parallel jobs, video, 4K. PromptMill deliberately goes the other way and optimises for the thing those tools’ reviews complain about most: it should just work, and you should be able to tell what it’s doing.

   
Nothing leaves your machine No account, no server, no analytics, no remote config. The extension talks to exactly two hosts: labs.google and flow.google.com. Read background.js and sidepanel.js — that’s the whole network surface, and the privacy policy shows you the one-line command to verify it yourself.
It tells you when it’s confused Before pressing Enter it reads the prompt box back and compares it to what it meant to type. If they don’t match, it stops and shows you what’s actually in the box, instead of waiting four minutes for images that were never coming.
It reads Flow’s own error messages Out of quota, content policy, service overloaded — PromptMill catches the on-page alert, stops the queue, and shows you the message verbatim.
It survives its own restarts Chrome kills MV3 service workers after ~30 seconds idle, but a single image can take four minutes. PromptMill detects the restart and reclaims its debugger session instead of dying.
No build step Plain JS, HTML and CSS. Clone it, load it, edit it. No npm, no bundler, no transpiler.

And what it deliberately does not do

Being honest about this up front, because the alternatives often aren’t:

If you need video or parallel batches, use one of the other extensions — genuinely, they’re better at that. Come back if reliability turns out to matter more.

Install (about 30 seconds)

No developer account and no $5 fee needed.

  1. Download this repository and unzip it.
  2. Open chrome://extensions.
  3. Turn on Developer mode (top right).
  4. Click Load unpacked → pick the PromptMill folder.
  5. The PromptMill icon appears in your toolbar. Done.

After editing any code, hit the reload (↻) button on the extension card and press F5 on the Flow tab. Skipping the F5 leaves the old content script alive in the page.

Using it

  1. Open a project on Google Flow (labs.google/fx/tools/flow or flow.google.com/project/…).
  2. Click the PromptMill icon to open the side panel.
  3. Wait for the status line to go green.
  4. Load your prompts — one per line — by pasting, by dragging text files onto the panel, or with the Tải file .txt (Load .txt file) button.
  5. Pick a download subfolder, toggle numbering, set the per-image timeout.
  6. Press Bắt đầu (Start).
  7. Tạm dừng (Pause) holds the queue after the current image finishes — press it again to resume. Dừng (Stop) ends the run.
  8. Drag queue rows to reorder them while stopped.

Two ways to start over, both next to what they affect:

Both need two clicks: the first turns the button into Chắc chưa? (“sure?”), the second acts. It disarms itself after 4 seconds. No modal dialog — one destroys your list, the other spends quota, and a blocking dialog in a side panel is worse than a button that asks twice.

Dragging vs. the button: dragging appends to your list; the button replaces it. Drop something by mistake and you lose nothing.

Dropping a text file onto the side panel

Accepted files: .txt, .md, .csv, .log, .json, or anything with a text/* MIME type, up to 2 MB each. Windows CRLF line endings and UTF-8 BOMs are handled — no cleanup needed.

JSON is read in the three shapes people actually have: ["a","b"], [{"prompt":"a"}], and {"prompts":[…]} ({"items":[…]} works too). Prompts containing newlines are folded onto one line, since the model here is one prompt per line.

Images are saved to Downloads / <your folder> / 001_your-prompt.jpg. Chrome creates the subfolder for you — nothing to set up.

The number’s width follows the size of your list, so filenames always sort correctly: 210 prompts give 001 … 210, while 1,500 prompts give 0001 … 1500. Turn numbering off and the prompt text alone is used. When one prompt returns several images they’re all saved, suffixed _1, _2.

The extension matches the real image format rather than trusting the URL, so a JPEG is saved as .jpg even though Google’s CDN links carry no extension.

How it actually works

Worth understanding, because it explains the one scary-looking thing.

Flow’s prompt box is a Slate.js editor, and Slate ignores synthetic keyboard events — the usual dispatchEvent tricks silently do nothing. So PromptMill types through the Chrome DevTools Protocol instead, using the chrome.debugger API. Real keystrokes at the browser layer, indistinguishable from your fingers.

Two consequences you need to know about:

  1. You must close DevTools (F12) on the Flow tab. Only one debugger client can attach at a time, and DevTools wins.
  2. Chrome shows a yellow “PromptMill is debugging this browser” bar while it runs. That bar is Chrome telling you the truth. Leave it alone — dismissing it cuts the connection mid-run.

This is also why sending prompts is the most robust part of the tool rather than the most fragile: it doesn’t hunt for a Send button in the DOM, so Google redesigning that button changes nothing.

The pause between prompts

Not a fixed number, and deliberately not a uniform random one. Gaps between real human actions are right-skewed — mostly short, occasionally long — while uniform randomness spreads flatly between its bounds, and that flatness is itself a machine fingerprint. PromptMill draws from a log-normal distribution and resamples out-of-range draws rather than clamping them (clamping would pile ~4% of all gaps onto one exact value, which is the very thing it’s trying to avoid).

Defaults: median 6 s, half of all gaps between 4.9 s and 7.6 s, 5% of runs take a 45–100 s break. About 10 s per prompt on average — roughly 17 minutes of waiting across 100 prompts.

Tune it in the DELAY block at the top of sidepanel.js.

The queue counting down between prompts
The countdown between prompts, and per-prompt status in the queue.

Known issues

Real bugs, listed here rather than discovered by you at 2am.

Issue What you’ll see Workaround
Very large lists get slow Above ~2,000 prompts, each keystroke in the prompt box costs 100 ms or more, because the queue is re-rendered on every input event Work in batches of a few hundred
“Done” means “download started” PromptMill doesn’t wait for the file to finish writing, so an expired CDN link can still show as successful Check the Downloads folder count against the queue

Fixing it when Google changes Flow

PromptMill drives Flow’s UI, not an API, so a redesign can break it. It touches Flow in three places — everything else is browser-level and immune.

# Job Code Symptom when it breaks
1 Recognise the Flow tab getFlowTab() in sidepanel.js Status stays red: “Hãy mở một project Google Flow”
2 Find the prompt box findPromptInput() / wakePromptBox() in content.js “Không tìm thấy ô prompt”
3 Know when images are done getCompletedImages() / waitForNewImages() in content.js Long wait, then Quá giờ (timeout)

1. Flow tab not recognised

Google changed the URL. Fix three places — missing any one breaks it:

// sidepanel.js → getFlowTab()
if (url.hostname === "flow.google.com") {
  return /^\/project(?:\/|$)/.test(url.pathname);   // ← here
}

…plus host_permissions and content_scripts[0].matches in manifest.json. Copy the URL straight out of your address bar and compare.

2. “Không tìm thấy ô prompt” (prompt box not found)

PromptMill auto-detects using a list of selectors ([data-slate-editor="true"], [role="textbox"], textarea…). If that misses, name it explicitly in the CONFIG block at the top of content.js:

promptSelector: '[data-slate-editor="true"]',

Open Flow, press F12, use the element picker on the prompt box, and look for a stable attribute — data-slate-editor, role, aria-label.

Don’t use DevTools’ Copy → Copy selector. It produces things like #root > div:nth-child(3) > div > div:nth-child(2), which breaks the next time Google touches the layout.

If Flow’s prompt box only materialises after a click, wakePromptBox() handles that — it finds the button carrying the arrow_forward icon and clicks to the left of it. If Google renames that icon, change the arrow_forward string in content.js.

3. “Chữ không vào được ô prompt” (text didn’t reach the box)

PromptMill clicks the centre of the prompt box. If something covers that point, the click lands elsewhere and the text goes nowhere.

Before pressing Enter it reads the box back and compares. On a mismatch it stops immediately and shows what it actually found, rather than waiting out the full four minutes. When you hit this:

The whole queue stops on this error rather than continuing — a click that misses will miss for every prompt, so continuing just burns time.

4. Long wait, then “Quá giờ” (timeout)

Adjust the CONFIG block at the top of content.js:

Setting Default When to change it
maxWaitMs 240000 (4 min) Slow model or congested Flow → raise it
minImageSize 256 Results smaller than this are skipped as icons → lower it
settleMs 1800 Images arriving broken or partial → raise it
pollMs 2000 How often it checks for new images. Rarely needs touching
maxSettleRounds 6 Flow spacing its images far apart → raise it

If it has never worked on your machine, check these two before touching any selector — they cause more failures than everything else combined: DevTools must be closed on the Flow tab, and the yellow debugging bar must be left alone.

5. “Google Flow báo lỗi: …” (Flow reported an error)

PromptMill read an error Flow put on the page — out of quota, content policy, service overloaded — and stopped the queue, quoting it verbatim. Without this, every doomed prompt would still eat the full four-minute timeout.

It stops the whole queue rather than continuing because the usual cause is a quota limit, and that fails identically for every remaining prompt.

False positives are rare (it only looks at role="alert" elements that appeared after the prompt was sent, and only when the text matches a keyword list), but if you hit one:

detectPageError: false,

…or keep it on and edit the errorHint pattern just below it.

6. Two settings that do nothing

generateSelector and submitWithEnter are still in CONFIG but are dead legacy — changing them has no effect. Since the move to chrome.debugger, the code path that read them is never called. (Along with them, roughly 230 lines of content.js are unreachable — see CONTRIBUTING if you’d like to delete them.)

General debugging tip: open the Console on the Flow tab before starting, and filter for [PromptMill]. It logs which prompt box it found, how long it’s been waiting, and when images appeared. Then close DevTools before pressing Start — it holds the debugger channel.

Known limitations

Make it yours

The interface is dark neumorphism. Shape comes entirely from two opposing shadows — light from the top-left (--hi), dark from the bottom-right (--lo) — with no borders. Raised means clickable, inset means editable, flat means disabled.

Three rules you’ll break if nobody tells you:

--bg must be a mid-tone, never pure black or pure white. It needs room for a shadow on both sides. On the current dark base, --hi is lighter than --bg — the opposite of what a light theme does.

Never give body a gradient. The whole style rests on the illusion of one evenly-lit surface.

Colour pairs invert with the base. On a dark base each state is a dark tinted fill plus a light -ink for the text on it. Copying a light theme’s pairs across leaves the text invisible.

All 54 text elements in the default palette meet WCAG AA (≥ 4.5:1); if you change the palette, re-check contrast.

Contributing

Issues and PRs welcome. Good places to start:

No build step, no test framework, no CI yet. Adding eslint would be a genuine improvement.

FAQ

Is this against Google’s Terms? Automating a web UI may conflict with Google’s terms. Use it in moderation and keep the delays reasonable. That’s your call to make, not this README’s.

Can I put it on the Chrome Web Store? Yes, with a caveat. The debugger permission is on Chrome’s sensitive list, which triggers mandatory manual review and a request to justify it — not an automatic rejection. Several comparable extensions ship on the store today. Expect a longer review and be ready to explain why Slate.js forces the CDP approach.

Where’s the data going? Nowhere. Your prompts live in chrome.storage.local on your own machine, and images go straight to your Downloads folder. The full privacy policy lists every stored value, every DevTools Protocol command the extension issues, and how to check all of it yourself.

Can I pick a different drive for downloads? No — Chrome extensions can only write inside the browser’s download directory. Set that in chrome://settings/downloads.

Support the project

PromptMill is free, has no ads, no account, and collects nothing — and it will stay that way. If it saved you an afternoon of clicking Save As, you can buy me a coffee:

Buy me a coffee via PayPal

Entirely optional. Reporting a bug or sending a pull request helps just as much.

License

MIT © Bui Van Thuong. Do what you like with it — fork it, rebrand it, ship it to your own community.

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