Docker Compose Validator

What this tool does

You paste the contents of your compose.yaml and it tells you whether it matches the official Compose schema, flagging every problem with its line, the path inside the file and a plain-English explanation. No Docker installation required, and the file never has to be on your disk.

The practical difference from docker compose config is context. That command resolves environment variables, build paths and .env files, so it fails for reasons that have nothing to do with the document’s structure: it will tell you a directory is missing before it tells you that you misspelled a key. Here the document is validated exactly as written, which is what you actually need when someone hands you a file, when you are halfway through writing one, or when you want to understand what is wrong before spinning anything up.

How to use it

1. Paste the file into the text box.
2. Read the list of problems. Each one carries a line number, the path (services › web › ports) and what was expected.
3. Click a line number to jump to that part of the file.

If the file is correct you will see how many services it defines. There is no validate button — it checks as you type.

The mistakes that show up most in a compose.yaml

A typo in a key name

This is by far the most common one, and the one command-line tools explain worst. You write imagee instead of image, or enviroment instead of environment, and what you get back is a terse additional property is not allowed — technically true, but it does not tell you where to look.

The schema declares additionalProperties: false on almost every block, so any key it does not recognise is an error. This tool takes advantage of that: when it finds an unknown key, it compares it against the valid keys at that exact point in the schema and suggests the closest match. It is not a generic list of Compose words — it is what is genuinely allowed inside that service, network or volume.

A value where a list belongs

ports, volumes, depends_on and networks take list form. Writing ports: "8080:80" instead of

ports:
  - "8080:80"

is an easy slip when there is only one entry. The error you will see is that it must be a list, with the line pointed out.

The mirror case exists too: environment accepts both forms, a map (KEY: value) and a list (- KEY=value), but you cannot mix them inside the same block.

Indentation

If the YAML cannot even be parsed, no schema will help: the tool reports it as a syntax error rather than a structural one. The two culprits are almost always an indentation level that does not line up with its siblings, and a colon with no space after it. YAML does not allow tabs for indentation; if your editor inserts them, convert them to spaces.

version: at the top of the file

It still shows up in tutorials and in old answers, but it is deprecated. The schema itself marks it as declared for backward compatibility and ignored. It will not raise an error, but you can delete it: Compose v2 does not use it for anything.

Unquoted ports

ports: - 8080:80 without quotes is valid as far as the schema is concerned, but YAML reads some pairs as sexagesimal numbers and silently changes the value. That is why the documentation recommends always quoting ports. It will not appear as an error here, but it is worth knowing.

What this tool does not validate

It is worth being precise about scope, because a validator that promises too much is worse than none:

• It does not check that images exist or that they can be pulled.
• It does not resolve environment variables or .env files, so a ${MY_VARIABLE} is accepted as text.
• It does not verify build paths or that referenced files are on your disk.
• It does not catch logical problems such as a depends_on pointing at a service that does not exist, two services fighting over the same port, or a volume that is declared and never used.

What it does guarantee is that the file’s structure is the one Compose expects: recognised keys, correct types and valid nesting. That is the filter that catches most broken files before they ever reach docker compose up.

Why it runs in your browser

The official schema is precompiled into the page itself and the entire validation happens on your machine. No network request fires when you validate, and you can confirm that yourself by opening the network tab of your developer tools.

This matters more than it might seem. A real compose.yaml describes a project’s internal topology: service names, ports, volume paths and — more often than anyone likes to admit — development database passwords. Pasting it into a tool that uploads it to someone else’s server means leaking a map of your infrastructure in exchange for a syntax check.

About the schema

Validation runs against compose-spec, the open specification maintained by Docker and the other implementations, published under the Apache 2.0 licence. It is the same one Docker Compose v2 consumes, so what passes here passes on your machine too.