SDK-Level Agent-Adapter Discovery

Summary

Add an optional discovery layer to the SDK that detects adapters availability and queries installed agents, supported models, and capabilities. This data feeds into process creation context.

API Design

`discoverExternalAgents(): ExternalAgentDiscovery`

// packages/babysitter-sdk/src/harness/externalAgentDiscovery.ts

interface ExternalAgentInfo {
  name: string;              // adapter name (e.g., "claude", "codex", "gemini")
  displayName: string;       // human-readable (e.g., "Claude Code")
  installed: boolean;        // detected on PATH
  authenticated: boolean;    // has valid credentials
  capabilities: string[];    // e.g., ["file-edit", "bash", "browser"]
}

interface ExternalAgentDiscovery {
  available: boolean;                 // is adapters installed?
  agents: ExternalAgentInfo[];        // discovered agents
  defaultProvider: string | null;     // ADAPTER_PROVIDER env
  defaultModel: string | null;        // ADAPTER_MODEL env
}

export async function discoverExternalAgents(
  options?: { timeout?: number; cwd?: string }
): Promise<ExternalAgentDiscovery>;

Detection Strategy

1. **Check if @a5c-ai/adapters is importable** — same pattern as isAmuxAvailable() in agent-platform 2. **If importable:** call adapterRegistry.list() and adapterRegistry.installed() to get agent info 3. **If not importable:** try adapters doctor --json via execFileSync (CLI fallback) 4. **If neither works:** return { available: false, agents: [], ... }

Caching

Discovery is expensive (spawns processes, checks file system). Cache for 60s. Invalidate on explicit discoverExternalAgents({ force: true }).

Files to Create/Modify

New Files

• packages/babysitter-sdk/src/harness/externalAgentDiscovery.ts — discovery API
• packages/babysitter-sdk/src/harness/__tests__/externalAgentDiscovery.test.ts — unit tests

Modified Files

• packages/babysitter-sdk/src/index.ts — export discoverExternalAgents
• packages/babysitter-sdk/src/harness/discovery.ts — integrate external agent discovery into harness discovery

Integration with Process Creation

The discovery result is passed into the process creation context:

// In agent-platform/src/harness/internal/createRun/planProcess/phase.ts
const externalAgents = await discoverExternalAgents({ timeout: 5000 });

// Inject into prompt context
const promptContext = buildPromptContext({
  workspace,
  selectedHarnessName,
  discovered,
  compressionConfig,
  externalAgents,  // NEW
});

The process creation prompt then includes:

Available external agents (via adapters):
- claude-code (installed, authenticated) — file-edit, bash, browser
- codex (installed, authenticated) — file-edit, bash
- gemini-cli (installed, not authenticated)

You may use `external: true` agent tasks to delegate work to these agents.

Dependencies

• SDK has **no hard dependency** on adapters. Discovery uses dynamic import with fallback.
• At runtime, adapters is only needed if external tasks are actually dispatched.