Playwright in Docker: it runs, and still gets blocked

Almost every page about Playwright and Docker is about making it start: install the system dependencies, add the fonts, use the official image. Those pages are correct and they solve a different problem from this one.

This is about the container that starts perfectly, renders perfectly, and gets a different page than your laptop does. Nothing is broken. The container is simply describing a machine that is not a person’s, in about six places at once.

What the container says about itself

Work through what a page can read, and a stock container answers every one of these the same way:

No GPU. WebGL reports a software renderer, llvmpipe or SwiftShader on Linux, a basic render driver on Windows. It is the single loudest item and no page-level patch touches it, because it is not a lie you told, it is the truth about where you are. There is a longer page on what those strings mean.

A tiny font set. A stripped image has DejaVu and Liberation and very little else. Why that is a comparison and not a count matters here, because the usual advice is to install more fonts, and more is not the fix. Matching the platform you claim is.

No audio device. Sample rate, output latency and channel count fall back to defaults that say there is no sound card. What the seven audio values are.

No speech voices. speechSynthesis.getVoices() returns an empty array, because nothing in a base image installs a speech engine. Which is a statement about the operating system.

A screen nobody has. Default resolutions, an available height equal to the full height because there is no taskbar, a device pixel ratio of 1. The relationships that have to hold.

Core and memory counts that come in pairs nobody buys. A container limited to two cores reporting eight gigabytes, or the reverse. Which the machine reports directly.

Six answers, all consistent with each other and all consistent with “datacenter”. None of them is about automation. This is why fixing navigator.webdriver changes nothing here.

The official image is a cohort, not a disguise

Worth stating because the advice is universal and the consequence is never mentioned.

Using the maintained Playwright Docker image is the right call for reliability. It is also true that everyone else following that advice has the same font set, the same library versions and the same defaults. A font list that exactly matches a widely used CI image is not anonymous, it is a label.

That is not an argument against using it. It is an argument for knowing what it does and does not buy: it buys you a browser that starts and renders consistently, and it does not buy you a machine that looks like somebody’s desk.

/dev/shm, and why it is a different problem

The other thing everyone hits in Docker is the browser crashing or hanging under load, which is usually shared memory. Containers default to a 64 MB /dev/shm, and a browser will use more than that.

docker run --shm-size=1gb ...

Fix it that way rather than by disabling the shared memory usage, because the flags that disable it change how the browser behaves rather than giving it room.

This is a stability problem, not a detection one. It is in this page because it is always in the same search, and because the two get conflated: a browser that crashes under load looks like a browser that is being blocked, and the fix is completely different. Establish which one you have before you start changing your fingerprint.

What actually helps

In the order that gives the most back per unit of effort:

1. Decide what platform you are claiming, and make the font set, the screen values, the audio and the voice list all agree with it. Consistency is the thing being checked, not any single value.
2. Deal with the GPU honestly. Either give the machine one, or accept that the render is software and do not claim a discrete card on top of it, which is a contradiction that is worse than the original problem.
3. Do not install fonts until you know which ones. A larger set that belongs to the wrong platform is a stronger signal than a small set.
4. Separate the layers before debugging. Run the checklist in order. Most container problems are step three, and most people start at step seven and buy a proxy.

What this project does about it

The engine carries its own font set rather than reading the machine’s, so a container and a desktop produce the same font list, and it is the list that belongs to the claimed platform. The screen values, the audio values and the voice list come from the same seeded profile, so they describe one machine rather than six unrelated defaults.

What it does not fix, and this is the honest limit: the GPU. A container with no graphics hardware still renders in software, and while the strings can say otherwise, the pixels are drawn by the rasteriser that is actually there. That gap is written up in its own page because it is the one thing on this list that a browser cannot solve.

Short answers to the questions that lead here

Why does Playwright work locally but get blocked in Docker? Because the container answers differently about the machine: no GPU, few fonts, no audio device, a default screen. None of those change when the automation flags do.

Does the official Playwright Docker image avoid detection? It avoids the startup problems. Its font set and defaults are shared by everyone who uses it, which is the opposite of blending in.

Should I install more fonts in my container? Only the ones belonging to the platform you claim. Adding a large mixed set makes the mismatch clearer.

What does --shm-size fix? Crashes and hangs under load. It has nothing to do with detection, and conflating the two sends people to rewrite a fingerprint when they needed a flag.

Can I get a real GPU in a container? Sometimes, with hardware passthrough on a host that has one. It is the only real fix for the software renderer, and it is not available everywhere.

Is headless the problem? Rarely by itself. What gets detected is what usually accompanies it, and every item on that list is also true of a headful browser in the same container.

See also: the checklist for being detected on one site, which is the order to work in, and three ways to make Playwright undetected, which explains why none of the page-level tools reach any of this.

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From the notes of invisible_playwright, a Firefox patched at the C++ level. The font set travels with the engine here, which is the only reason a container and a desktop can agree.