## TL;DR
Add a `site_name` key to mkdocs.yml. It is the one required setting; every other key is optional.

## Error
```text
KeyError: 'site_name'
```

## Steps
1. Open mkdocs.yml and look for a top-level `site_name` entry. Expected: it is absent or nested under another key by mistake.
2. Add `site_name: Your Project Name` at the top level of the file, with no indentation. Expected: the key sits at column zero like other top-level settings.
3. Validate the file parses by running `mkdocs build` again or `python -c "import yaml; yaml.safe_load(open('mkdocs.yml'))"`. Expected: the config loads without errors.
4. Run the full build: `mkdocs build`. Expected: the site builds into the site directory.

## When to use
- MkDocs aborts with KeyError naming site_name
- A new mkdocs.yml was hand-written or generated by a script
- A docs agent emitted a config without the required key

## When not to use
- The error is a YAML syntax error (different message, fix the syntax)
- site_name exists but a plugin fails (the plugin is the problem)
- You are configuring a MkDocs plugin that has its own required keys

## Tool compatibility
MkDocs 1.4 through 1.6. site_name has been required in every MkDocs release.

### Variant phrasings
### KeyError site_name after splitting config with plugins
Some config-inheritance plugins merge files; ensure the merged result still carries site_name at the top level.

### mkdocs.yml exists but MkDocs says it cannot find it
You are running the command from the wrong directory. Run it where mkdocs.yml lives, or pass `-f path/to/mkdocs.yml`.

## Why it happens
MkDocs validates the config dict and reads `config['site_name']` unconditionally to title the site. A missing key raises KeyError before any building starts. Generators and agents often emit plugin-heavy configs and forget the single mandatory field.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_6a1AVT52Tnrsn1pxFi8w5w