How to resolve a “Can only install one of:” conflict?

The “Can only install one [x, y]” message appears when two different packages point to the same dependency, but different major, mutually exclusive versions where only one can be installed.

Troubleshooting

For example, one version can be “locked at” due to the information present in your composer.lock file which may conflict with what you’re trying to install. In this case, when the confusion error comes with “locked at x.y.z” message, you may use the following commands to understand the existing dependencies of installed packages:

composer show -t

^{Note: -t shows as a nested tree view, drop it to see the flat list.}

To see where the problematic package is referenced from in your project, run:

composer why org/package -t

^{Note: -t shows as a nested tree view, drop it to see the flat list.}

To see the details of the package which you’re trying to install, you may run:

composer show -a org/package # Package to inspect.

^{Note: To inspect the specific version, add x.y.z, e.g.: composer show -a guzzlehttp/guzzle 6.2.0}

To further troubleshoot the problem, depending on your situation, you can try to:

• Update the existing packages with dependencies via:

  composer update --with-dependencies
  

• Upgrade or remove conflicting dependencies from your composer.json (keep it simple).

• When the confusion message shows “locked at x.y.z“, use composer why org/package to see where the package is referenced (or check the content of composer.lock manually by looking for x.y.z). If won’t help, consider removing composer.lock and re-try again;
• When asked to use composer.json from the different folder, select n.
• Re-try your plain composer.json, simplified configuration on the empty folder.
• Run composer diagnose to check for any common error.
• Use -v/-vv/-vvv to increase the verbosity of your commands.
• See also: How to explain Composer’s error log?