## TL;DR
Install the myst-parser package into the environment running the build and spell the extension name exactly `myst_parser` in conf.py. The package name on PyPI uses a hyphen while the extension module uses an underscore.

## Error
```text
ERROR: Failed to load extension 'myst_parser'
```

## Steps
1. Check whether the package is installed in the build environment: `pip show myst-parser`. Expected: either version info or "not installed", which tells you which fix applies.
2. Install it with `pip install myst-parser`, pinning a version compatible with your Sphinx release (myst-parser 2.x pairs with Sphinx 7+, 3.x with Sphinx 8). Expected: pip reports a successful install.
3. Open conf.py and confirm the extensions list contains the string `myst_parser` exactly, not `myst-parser` or `myst`. Expected: the underscore spelling is present.
4. Rebuild with `sphinx-build -b html docs docs/_build`. Expected: the extension loads and Markdown sources parse.

## When to use
- The build fails naming myst_parser as the unloadable extension
- You added Markdown sources to a previously RST-only project
- CI or Read the Docs fails while the local build works

## When not to use
- The extension loads but Markdown renders wrong (a myst configuration issue, not a load failure)
- You meant to use recommonmark, which is a different package and extension name
- The failure names a different extension

## Tool compatibility
Sphinx 7.x/8.x with myst-parser 2.x/3.x. If you are stuck on Sphinx 5 or 6, use myst-parser 1.x.

### Variant phrasings
### Sphinx reports "No module named 'myst_parser'"
Same root cause: the package is missing from the build environment. Install it.

### extension loads locally but not on Read the Docs
Add myst-parser to the docs requirements file that Read the Docs installs; the local environment is irrelevant to the hosted build.

## Why it happens
The PyPI distribution name (myst-parser) differs from the importable module name (myst_parser), so installs and config edits get out of sync. Clean CI environments expose the missing dependency that a developer's laptop hides.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_1v9zHheJYOUkMHiuGPVeaQ