Build a Custom Adapter

import {
  defineAdapter,
  planSingleFileInstall,
  planSingleFileRemoval,
  planSkillBundleInstall,
  planSkillBundleRemoval,
  type AdapterMetadata,
  type AssetCapability,
  type AssetRequestContext,
  type SkillBundleTarget,
  type Validated,
} from '@agent-facets/adapter'
import { homedir } from 'node:os'
import { join } from 'node:path'

// The project root arrives on every request. Never read process.cwd().
function baseDir(context: AssetRequestContext): string | null {
  switch (context.scope) {
    case 'user':
      return join(homedir(), '.my-tool')
    case 'project':
      return join(context.projectRoot, '.my-tool')
    case 'system':
      return null
  }
}

function skillTarget(base: string, name: string): SkillBundleTarget {
  const root = join(base, 'skills', name)
  return { root, primaryFile: join(root, 'SKILL.md'), boundary: base }
}

const assets: AssetCapability = {
  async planInstall(request) {
    const base = baseDir(request)
    if (base === null) return { ok: false, failure: { code: 'unsupported-scope', scope: request.scope } }
    const metadata = request.metadata as Record<string, unknown>
    switch (request.assetType) {
      case 'skill':
        return planSkillBundleInstall(skillTarget(base, request.name), {
          content: request.content,
          metadata,
          companions: request.companions,
          ownedCompanionPaths: request.ownedCompanionPaths,
        })
      case 'agent':
        return planSingleFileInstall(
          { file: join(base, 'agents', `${request.name}.md`), boundary: base },
          request.content,
          metadata,
        )
      case 'command':
        return planSingleFileInstall(
          { file: join(base, 'commands', `${request.name}.md`), boundary: base },
          request.content,
          metadata,
        )
    }
  },

  async planRemoval(request) {
    const base = baseDir(request)
    if (base === null) return { ok: false, failure: { code: 'unsupported-scope', scope: request.scope } }
    switch (request.assetType) {
      case 'skill':
        return planSkillBundleRemoval(skillTarget(base, request.name), request.ownedCompanionPaths)
      case 'agent':
        return planSingleFileRemoval({ file: join(base, 'agents', `${request.name}.md`), boundary: base })
      case 'command':
        return planSingleFileRemoval({ file: join(base, 'commands', `${request.name}.md`), boundary: base })
    }
  },
}

export default defineAdapter({
  name: 'my-tool',

  assets,

  // Required. `false` means this adapter does not configure MCP servers.
  mcpServers: false,

  // Validate the author's `adapters.my-tool` block at build time.
  buildAssetMetadata(data: unknown): Validated<AdapterMetadata> {
    const meta = (data ?? {}) as AdapterMetadata
    return { ok: true, data: meta }
  },
})