Simon Willison’s Weblog

Agent Skills. Anthropic have turned their skills mechanism into an "open standard", which I guess means it lives in an independent agentskills/agentskills GitHub repository now? I wouldn't be surprised to see this end up in the AAIF, recently the new home of the MCP specification.

The specification itself lives at agentskills.io/specification, published from docs/specification.mdx in the repo.

It is a deliciously tiny specification - you can read the entire thing in just a few minutes. It's also quite heavily under-specified - for example, there's a metadata field described like this:

Clients can use this to store additional properties not defined by the Agent Skills spec

We recommend making your key names reasonably unique to avoid accidental conflicts

And an allowed-skills field:

Experimental. Support for this field may vary between agent implementations

Example:

allowed-tools: Bash(git:*) Bash(jq:*) Read

The Agent Skills homepage promotes adoption by OpenCode, Cursor,Amp, Letta, goose, GitHub, and VS Code. Notably absent is OpenAI, who are quietly tinkering with skills but don't appear to have formally announced their support just yet.