Prefix a path with @ to inject it directly into the conversation. @src/utils/auth.js includes the full file contents. @src/components/ shows a directory listing without reading every file inside. The difference matters for large directories where loading everything would fill the context window.
@github:repos/owner/repo/issues fetches from an MCP server rather than the local filesystem. The path after @ becomes a resource identifier that the connected MCP server resolves. I'll show you a useful combination: "Review @src/api/users.js against the spec at @docs/api-spec.md" gives Claude both files at once.