Composer "composer.json does not contain valid JSON" in CI

Composer could not parse composer.json because it is not valid JSON. JSON forbids trailing commas and comments, so a stray comma or // note breaks the manifest before anything installs.

What this error means

Any Composer command fails immediately with "composer.json does not contain valid JSON" and a parse error pointing at a line/character. Nothing resolves because Composer cannot read the manifest.

composer output

[Seld\JsonLint\ParsingException]
"./composer.json" does not contain valid JSON
Parse error on line 8:
...    "phpunit/phpunit": "^11.0",  }
---------------------^
Expected one of: 'STRING', '}'

Common causes

Trailing comma or comment

JSON does not allow a comma after the last item in an object/array, nor // or /* */ comments. Either breaks parsing.

A missing brace, bracket, or quote

An unbalanced {}/[] or an unquoted key produces a parse error at the point Composer’s linter gives up.

How to fix it

Validate and locate the bad token

Composer’s validator pinpoints the offending line and character.

Terminal

composer validate --no-check-publish
# or lint the JSON directly
php -r "json_decode(file_get_contents('composer.json')); echo json_last_error_msg();"

Fix the JSON syntax

Remove any trailing comma before a closing } or ].
Delete // or /* */ comments - JSON has none.
Balance braces/brackets and quote every key and string.

How to prevent it

Run composer validate in CI before install.
Use an editor/JSON linter that flags trailing commas and comments.
Edit dependencies with composer require/remove rather than hand-editing JSON.