.yml File Extension

What kind of file is a .yml?

A .yml file normally contains YAML, a human-friendly data serialization language used for configuration, automation, and structured data. YAML represents mappings, sequences, and scalar values with readable text. Indentation carries structure, while colons separate mapping keys from values and hyphens introduce sequence entries.¹

Poly recognizes .yml as text/plain. It has no magic number, so the filename and a successful parse are more useful than checking a few opening bytes. The YAML media-type registration lists .yaml as the preferred extension and .yml as a still-used alternative.²

How do I open a .yml on my computer?

Any text editor can open .yml on Windows, macOS, or Linux. A code editor such as Visual Studio Code is more practical for editing because it recognizes YAML language mode and can add validation, formatting, and schema support through extensions.³ Mobile text and code editors can display these files too.

Use the validator or parser required by the application that consumes the file. YAML syntax alone does not prove that a GitHub Actions workflow, deployment manifest, or application configuration contains the expected keys. GitHub Actions, for example, accepts both .yml and .yaml workflow files, but requires them to be placed in .github/workflows.⁴

Poly support for .yml

Poly opens .yml files in a code-friendly editor with YAML syntax highlighting and in-file search. Their readable content is indexed for exact and semantic search, and the Poly agent can explain settings or reason about the data without executing commands described in the file.

Poly treats the file as text rather than running a YAML parser on your behalf. Validate important configuration with the target application's tools. Keep related schemas and documentation nearby when field meanings depend on a particular system.

History of .yml

YAML's first implementation appeared in Perl in 2001. Version 1.0 followed in 2004, version 1.1 in 2005, and version 1.2 in 2009. YAML 1.2 focused on making JSON a strict subset and reducing surprising implicit typing rules. Revision 1.2.2, published in 2021, clarified the specification without normative language changes.¹

The .yml and .yaml spellings identify the same language. RFC 9512 registered application/yaml in February 2024 and recommends .yaml. It warns that using both spellings in one context can create ambiguity, such as having both config.yml and config.yaml.²

Working with .yml and alternatives

Choose one filename convention within a project and follow the consuming tool's documented expectations.

YAML can contain aliases, multiple documents, and implementation-defined tags. Parsers may also differ across YAML language versions. RFC 9512 warns that unsafe deserialization can allow arbitrary code execution and that aliases or deeply nested structures can exhaust resources.² Use a maintained parser in its safe mode, limit untrusted input, and review changes before a configuration file reaches deployment or automation systems.

Conversion should preserve the data model, not merely the visible text. Comments, anchors, aliases, tags, and scalar styles may be lost when converting YAML to JSON because JSON has no direct equivalents.²