> pi can help you use the SDK. Ask it to build an integration for your use case.

# SDK

The SDK provides programmatic access to pi's agent capabilities. Use it to embed pi in other applications, build custom interfaces, or integrate with automated workflows.

**Example use cases:**
- Build a custom UI (web, desktop, mobile)
- Integrate agent capabilities into existing applications
- Create automated pipelines with agent reasoning
- Build custom tools that spawn sub-agents
- Test agent behavior programmatically

See [examples/sdk/](../examples/sdk/) for working examples from minimal to full control.

## Quick Start

```typescript
import { AuthStorage, createAgentSession, ModelRegistry, SessionManager } from "@bastani/atomic";

// Set up credential storage and model registry
const authStorage = AuthStorage.create();
const modelRegistry = ModelRegistry.create(authStorage);

const { session } = await createAgentSession({
  sessionManager: SessionManager.inMemory(),
  authStorage,
  modelRegistry,
});

session.subscribe((event) => {
  if (event.type === "message_update" && event.assistantMessageEvent.type === "text_delta") {
    process.stdout.write(event.assistantMessageEvent.delta);
  }
});

await session.prompt("What files are in the current directory?");
```

## Installation

```bash
npm install @bastani/atomic
```

The SDK is included in the main package. No separate installation needed.

## Core Concepts

### createAgentSession()

The main factory function for a single `AgentSession`.

`createAgentSession()` uses a `ResourceLoader` to supply extensions, skills, prompt templates, themes, and context files. If you do not provide one, it uses `DefaultResourceLoader` with standard discovery.

```typescript
import { createAgentSession, SessionManager } from "@bastani/atomic";

// Minimal: defaults with DefaultResourceLoader
const { session } = await createAgentSession();

// Custom: override specific options
const { session } = await createAgentSession({
  model: myModel,
  tools: ["read", "bash"],
  sessionManager: SessionManager.inMemory(),
});
```

### AgentSession

The session manages agent lifecycle, message history, model state, compaction, and event streaming.

```typescript
interface AgentSession {
  // Send a prompt and wait for completion
  prompt(text: string, options?: PromptOptions): Promise<void>;

  // Queue messages during streaming
  steer(text: string): Promise<void>;
  followUp(text: string): Promise<void>;

  // Subscribe to events (returns unsubscribe function)
  subscribe(listener: (event: AgentSessionEvent) => void): () => void;

  // Session info
  sessionFile: string | undefined;
  sessionId: string;

  // Model control
  setModel(model: Model): Promise<void>;
  setThinkingLevel(level: ThinkingLevel): void;
  cycleModel(): Promise<ModelCycleResult | undefined>;
  cycleThinkingLevel(): ThinkingLevel | undefined;

  // State access
  agent: Agent;
  model: Model | undefined;
  thinkingLevel: ThinkingLevel;
  messages: AgentMessage[];
  isStreaming: boolean;

  // In-place tree navigation within the current session file
  navigateTree(targetId: string, options?: { summarize?: boolean; customInstructions?: string; replaceInstructions?: boolean; label?: string }): Promise<{ editorText?: string; cancelled: boolean }>;

  // Compaction
  compact(customInstructions?: string): Promise<CompactionResult>;
  abortCompaction(): void;

  // Abort current operation
  abort(): Promise<void>;

  // Cleanup
  dispose(): void;
}
```

Session replacement APIs such as new-session, resume, fork, and import live on `AgentSessionRuntime`, not on `AgentSession`.

### createAgentSessionRuntime() and AgentSessionRuntime

Use the runtime API when you need to replace the active session and rebuild cwd-bound runtime state.
This is the same layer used by the built-in interactive, print, and RPC modes.

`createAgentSessionRuntime()` takes a runtime factory plus the initial cwd/session target. The factory closes over process-global fixed inputs, recreates cwd-bound services for the effective cwd, resolves session options against those services, and returns a full runtime result.

```typescript
import {
  type CreateAgentSessionRuntimeFactory,
  createAgentSessionFromServices,
  createAgentSessionRuntime,
  createAgentSessionServices,
  getAgentDir,
  SessionManager,
} from "@bastani/atomic";

const createRuntime: CreateAgentSessionRuntimeFactory = async ({ cwd, sessionManager, sessionStartEvent }) => {
  const services = await createAgentSessionServices({ cwd });
  return {
    ...(await createAgentSessionFromServices({
      services,
      sessionManager,
      sessionStartEvent,
    })),
    services,
    diagnostics: services.diagnostics,
  };
};

const runtime = await createAgentSessionRuntime(createRuntime, {
  cwd: process.cwd(),
  agentDir: getAgentDir(),
  sessionManager: SessionManager.create(process.cwd()),
});
```

`AgentSessionRuntime` owns replacement of the active runtime across:

- `newSession()`
- `switchSession()`
- `fork()`
- clone flows via `fork(entryId, { position: "at" })`
- `importFromJsonl()`

Important behavior:

- `runtime.session` changes after those operations
- event subscriptions are attached to a specific `AgentSession`, so re-subscribe after replacement
- if you use extensions, call `runtime.session.bindExtensions(...)` again for the new session
- creation returns diagnostics on `runtime.diagnostics`
- if runtime creation or replacement fails, the method throws and the caller decides how to handle it

```typescript
let session = runtime.session;
let unsubscribe = session.subscribe(() => {});

await runtime.newSession();

unsubscribe();
session = runtime.session;
unsubscribe = session.subscribe(() => {});
```

### Prompting and Message Queueing

`PromptOptions` controls prompt expansion, queueing behavior while streaming, and prompt preflight notifications:

```typescript
interface PromptOptions {
  expandPromptTemplates?: boolean;
  images?: ImageContent[];
  streamingBehavior?: "steer" | "followUp";
  source?: InputSource;
  preflightResult?: (success: boolean) => void;
}
```

`preflightResult` is called once per `prompt()` invocation:

- `true` when the prompt was accepted, queued, or handled immediately
- `false` when prompt preflight rejected before acceptance

It fires before `prompt()` resolves. `prompt()` still resolves only after the full accepted run finishes, including retries. Failures after acceptance are reported through the normal event and message stream, not through `preflightResult(false)`.

The `prompt()` method handles prompt templates, extension commands, and message sending:

```typescript
// Basic prompt (when not streaming)
await session.prompt("What files are here?");

// With images
await session.prompt("What's in this image?", {
  images: [{ type: "image", source: { type: "base64", mediaType: "image/png", data: "..." } }]
});

// During streaming: must specify how to queue the message
await session.prompt("Stop and do this instead", { streamingBehavior: "steer" });
await session.prompt("After you're done, also check X", { streamingBehavior: "followUp" });
```

**Behavior:**
- **Extension commands** (e.g., `/mycommand`): Execute immediately, even during streaming. They manage their own LLM interaction via `pi.sendMessage()`.
- **File-based prompt templates** (from `.md` files): Expanded to their content before sending or queueing.
- **During streaming without `streamingBehavior`**: Throws an error. Use `steer()` or `followUp()` directly, or specify the option.
- **`preflightResult(true)`**: Means the prompt was accepted, queued, or handled immediately.
- **`preflightResult(false)`**: Means preflight rejected before acceptance.

For explicit queueing during streaming:

```typescript
// Queue a steering message for delivery after the current assistant turn finishes its tool calls
await session.steer("New instruction");

// Wait for agent to finish (delivered only when agent stops)
await session.followUp("After you're done, also do this");
```

Both `steer()` and `followUp()` expand file-based prompt templates but error on extension commands (extension commands cannot be queued).

### Agent and AgentState

The `Agent` class (from `@earendil-works/pi-agent-core`) handles the core LLM interaction. Access it via `session.agent`.

```typescript
// Access current state
const state = session.agent.state;

// state.messages: AgentMessage[] - conversation history
// state.model: Model - current model
// state.thinkingLevel: ThinkingLevel - current thinking level
// state.systemPrompt: string - system prompt
// state.tools: AgentTool[] - available tools
// state.streamingMessage?: AgentMessage - current partial assistant message
// state.errorMessage?: string - latest assistant error

// Replace messages (useful for branching or restoration)
session.agent.state.messages = messages; // copies the top-level array

// Replace tools
session.agent.state.tools = tools; // copies the top-level array

// Wait for agent to finish processing
await session.agent.waitForIdle();
```

### Events

Subscribe to events to receive streaming output and lifecycle notifications.

```typescript
session.subscribe((event) => {
  switch (event.type) {
    // Streaming text from assistant
    case "message_update":
      if (event.assistantMessageEvent.type === "text_delta") {
        process.stdout.write(event.assistantMessageEvent.delta);
      }
      if (event.assistantMessageEvent.type === "thinking_delta") {
        // Thinking output (if thinking enabled)
      }
      break;
    
    // Tool execution
    case "tool_execution_start":
      console.log(`Tool: ${event.toolName}`);
      break;
    case "tool_execution_update":
      // Streaming tool output
      break;
    case "tool_execution_end":
      console.log(`Result: ${event.isError ? "error" : "success"}`);
      break;
    
    // Message lifecycle
    case "message_start":
      // New message starting
      break;
    case "message_end":
      // Message complete
      break;
    
    // Agent lifecycle
    case "agent_start":
      // Agent started processing prompt
      break;
    case "agent_end":
      // Agent finished (event.messages contains new messages)
      break;
    
    // Turn lifecycle (one LLM response + tool calls)
    case "turn_start":
      break;
    case "turn_end":
      // event.message: assistant response
      // event.toolResults: tool results from this turn
      break;
    
    // Session events (queue, compaction, retry)
    case "queue_update":
      console.log(event.steering, event.followUp);
      break;
    case "compaction_start":
    case "compaction_end":
    case "auto_retry_start":
    case "auto_retry_end":
      break;
  }
});
```

## Options Reference

### Directories

```typescript
const { session } = await createAgentSession({
  // Working directory for DefaultResourceLoader discovery
  cwd: process.cwd(), // default
  
  // Global config directory
  agentDir: "~/.atomic/agent", // default (expands ~)
});
```

Atomic reads primary `.atomic` locations first and legacy `.pi` locations for compatibility when multiple config directories are supported. Passing an explicit `agentDir` makes that directory the user override.

`cwd` is used by `DefaultResourceLoader` for:
- Project extensions (`.atomic/extensions/`, then legacy `.pi/extensions/`)
- Project skills:
  - `.atomic/skills/`, then legacy `.pi/skills/`
  - `.agents/skills/` in `cwd` and ancestor directories (up to git repo root, or filesystem root when not in a repo)
- Project prompts (`.atomic/prompts/`, then legacy `.pi/prompts/`)
- Context files (`AGENTS.md` walking up from cwd)
- Session directory naming

`agentDir` is used by `DefaultResourceLoader` for:
- Global extensions (`extensions/`)
- Global skills:
  - `skills/` under `agentDir` (for example `~/.atomic/agent/skills/`; legacy `~/.pi/agent/skills/` is also considered by default)
  - `~/.agents/skills/`
- Global prompts (`prompts/`)
- Global context file (`AGENTS.md`)
- Settings (`settings.json`)
- Custom models (`models.json`)
- Credentials (`auth.json`)
- Sessions (`sessions/`)

When you pass a custom `ResourceLoader`, `cwd` and `agentDir` no longer control resource discovery. They still influence session naming and tool path resolution.

### Model

```typescript
import { getModel } from "@earendil-works/pi-ai";
import { AuthStorage, ModelRegistry } from "@bastani/atomic";

const authStorage = AuthStorage.create();
const modelRegistry = ModelRegistry.create(authStorage);

// Find specific built-in model (doesn't check if API key exists)
const opus = getModel("anthropic", "claude-opus-4-5");
if (!opus) throw new Error("Model not found");

// Find any model by provider/id, including custom models from models.json
// (doesn't check if API key exists)
const customModel = modelRegistry.find("my-provider", "my-model");

// Get only models that have valid API keys configured
const available = await modelRegistry.getAvailable();

const { session } = await createAgentSession({
  model: opus,
  thinkingLevel: "medium", // off, minimal, low, medium, high, xhigh
  
  // Models for cycling (CTRL+P in interactive mode)
  scopedModels: [
    { model: opus, thinkingLevel: "high" },
    { model: haiku, thinkingLevel: "off" },
  ],
  
  authStorage,
  modelRegistry,
});
```

If no model is provided:
1. Tries to restore from session (if continuing)
2. Uses default from settings
3. Falls back to first available model

> See [examples/sdk/02-custom-model.ts](../examples/sdk/02-custom-model.ts)

### API Keys and OAuth

API key resolution priority (handled by AuthStorage):
1. Runtime overrides (via `setRuntimeApiKey`, not persisted)
2. Stored credentials in `auth.json` (API keys or OAuth tokens)
3. Environment variables (`ANTHROPIC_API_KEY`, `OPENAI_API_KEY`, etc.)
4. Fallback resolver (for custom provider keys from `models.json`)

```typescript
import { AuthStorage, ModelRegistry } from "@bastani/atomic";

// Default: uses ~/.atomic/agent/auth.json and ~/.atomic/agent/models.json,
// with legacy ~/.pi/agent/* compatibility reads when available.
const authStorage = AuthStorage.create();
const modelRegistry = ModelRegistry.create(authStorage);

const { session } = await createAgentSession({
  sessionManager: SessionManager.inMemory(),
  authStorage,
  modelRegistry,
});

// Runtime API key override (not persisted to disk)
authStorage.setRuntimeApiKey("anthropic", "sk-my-temp-key");

// Custom auth storage location
const customAuth = AuthStorage.create("/my/app/auth.json");
const customRegistry = ModelRegistry.create(customAuth, "/my/app/models.json");

const { session } = await createAgentSession({
  sessionManager: SessionManager.inMemory(),
  authStorage: customAuth,
  modelRegistry: customRegistry,
});

// No custom models.json (built-in models only)
const simpleRegistry = ModelRegistry.inMemory(authStorage);
```

> See [examples/sdk/09-api-keys-and-oauth.ts](../examples/sdk/09-api-keys-and-oauth.ts)

### System Prompt

Use a `ResourceLoader` to override the system prompt:

```typescript
import { createAgentSession, DefaultResourceLoader } from "@bastani/atomic";

const loader = new DefaultResourceLoader({
  systemPromptOverride: () => "You are a helpful assistant.",
});
await loader.reload();

const { session } = await createAgentSession({ resourceLoader: loader });
```

> See [examples/sdk/03-custom-prompt.ts](../examples/sdk/03-custom-prompt.ts)

### Tools

Specify which built-in tools to enable:

- Built-in tool names: `read`, `bash`, `edit`, `write`, `grep`, `find`, `ls`
- Default built-ins: `read`, `bash`, `edit`, `write`
- `noTools: "all"` disables all tools
- `noTools: "builtin"` disables default built-ins while keeping extension and custom tools enabled

```typescript
import { createAgentSession } from "@bastani/atomic";

// Read-only mode
const { session } = await createAgentSession({
  tools: ["read", "grep", "find", "ls"],
});

// Pick specific tools
const { session } = await createAgentSession({
  tools: ["read", "bash", "grep"],
});
```

#### Tools with Custom cwd

When you pass a custom `cwd`, `createAgentSession()` builds selected built-in tools for that cwd.

```typescript
import { createAgentSession, SessionManager } from "@bastani/atomic";

const cwd = "/path/to/project";

// Use default tools for custom cwd
const { session } = await createAgentSession({
  cwd,
  sessionManager: SessionManager.inMemory(cwd),
});

// Or pick specific tools for custom cwd
const { session } = await createAgentSession({
  cwd,
  tools: ["read", "bash", "grep"],
  sessionManager: SessionManager.inMemory(cwd),
});
```

> See [examples/sdk/05-tools.ts](../examples/sdk/05-tools.ts)

### Custom Tools

```typescript
import { Type } from "typebox";
import { createAgentSession, defineTool } from "@bastani/atomic";

// Inline custom tool
const myTool = defineTool({
  name: "my_tool",
  label: "My Tool",
  description: "Does something useful",
  parameters: Type.Object({
    input: Type.String({ description: "Input value" }),
  }),
  execute: async (_toolCallId, params) => ({
    content: [{ type: "text", text: `Result: ${params.input}` }],
    details: {},
  }),
});

// Pass custom tools directly
const { session } = await createAgentSession({
  customTools: [myTool],
});
```

Use `defineTool()` for standalone definitions and arrays like `customTools: [myTool]`. Inline `pi.registerTool({ ... })` already infers parameter types correctly.

Custom tools passed via `customTools` are combined with extension-registered tools. Extensions loaded by the ResourceLoader can also register tools via `pi.registerTool()`.

If you pass `tools`, include each custom or extension tool name you want enabled, for example `tools: ["read", "bash", "my_tool"]`.

> See [examples/sdk/05-tools.ts](../examples/sdk/05-tools.ts)

### Extensions

Extensions are loaded by the `ResourceLoader`. `DefaultResourceLoader` discovers extensions from `~/.atomic/agent/extensions/` and `.atomic/extensions/` first, then legacy `~/.pi/agent/extensions/` and `.pi/extensions/`, plus settings.json extension sources.

```typescript
import { createAgentSession, DefaultResourceLoader } from "@bastani/atomic";

const loader = new DefaultResourceLoader({
  additionalExtensionPaths: ["/path/to/my-extension.ts"],
  extensionFactories: [
    (pi) => {
      pi.on("agent_start", () => {
        console.log("[Inline Extension] Agent starting");
      });
    },
  ],
});
await loader.reload();

const { session } = await createAgentSession({ resourceLoader: loader });
```

Extensions can register tools, subscribe to events, add commands, and more. See [extensions.md](extensions.md) for the full API.

**Event Bus:** Extensions can communicate via `pi.events`. Pass a shared `eventBus` to `DefaultResourceLoader` if you need to emit or listen from outside:

```typescript
import { createEventBus, DefaultResourceLoader } from "@bastani/atomic";

const eventBus = createEventBus();
const loader = new DefaultResourceLoader({
  eventBus,
});
await loader.reload();

eventBus.on("my-extension:status", (data) => console.log(data));
```

> See [examples/sdk/06-extensions.ts](../examples/sdk/06-extensions.ts) and [docs/extensions.md](extensions.md)

### Skills

```typescript
import {
  createAgentSession,
  DefaultResourceLoader,
  type Skill,
} from "@bastani/atomic";

const customSkill: Skill = {
  name: "my-skill",
  description: "Custom instructions",
  filePath: "/path/to/SKILL.md",
  baseDir: "/path/to",
  source: "custom",
};

const loader = new DefaultResourceLoader({
  skillsOverride: (current) => ({
    skills: [...current.skills, customSkill],
    diagnostics: current.diagnostics,
  }),
});
await loader.reload();

const { session } = await createAgentSession({ resourceLoader: loader });
```

> See [examples/sdk/04-skills.ts](../examples/sdk/04-skills.ts)

### Context Files

```typescript
import { createAgentSession, DefaultResourceLoader } from "@bastani/atomic";

const loader = new DefaultResourceLoader({
  agentsFilesOverride: (current) => ({
    agentsFiles: [
      ...current.agentsFiles,
      { path: "/virtual/AGENTS.md", content: "# Guidelines\n\n- Be concise" },
    ],
  }),
});
await loader.reload();

const { session } = await createAgentSession({ resourceLoader: loader });
```

> See [examples/sdk/07-context-files.ts](../examples/sdk/07-context-files.ts)

### Slash Commands

```typescript
import {
  createAgentSession,
  DefaultResourceLoader,
  type PromptTemplate,
} from "@bastani/atomic";

const customCommand: PromptTemplate = {
  name: "deploy",
  description: "Deploy the application",
  source: "(custom)",
  content: "# Deploy\n\n1. Build\n2. Test\n3. Deploy",
};

const loader = new DefaultResourceLoader({
  promptsOverride: (current) => ({
    prompts: [...current.prompts, customCommand],
    diagnostics: current.diagnostics,
  }),
});
await loader.reload();

const { session } = await createAgentSession({ resourceLoader: loader });
```

> See [examples/sdk/08-prompt-templates.ts](../examples/sdk/08-prompt-templates.ts)

### Session Management

Sessions use a tree structure with `id`/`parentId` linking, enabling in-place branching.

```typescript
import {
  type CreateAgentSessionRuntimeFactory,
  createAgentSession,
  createAgentSessionFromServices,
  createAgentSessionRuntime,
  createAgentSessionServices,
  getAgentDir,
  SessionManager,
} from "@bastani/atomic";

// In-memory (no persistence)
const { session } = await createAgentSession({
  sessionManager: SessionManager.inMemory(),
});

// New persistent session
const { session: persisted } = await createAgentSession({
  sessionManager: SessionManager.create(process.cwd()),
});

// Continue most recent
const { session: continued, modelFallbackMessage } = await createAgentSession({
  sessionManager: SessionManager.continueRecent(process.cwd()),
});
if (modelFallbackMessage) {
  console.log("Note:", modelFallbackMessage);
}

// Open specific file
const { session: opened } = await createAgentSession({
  sessionManager: SessionManager.open("/path/to/session.jsonl"),
});

// List sessions
const currentProjectSessions = await SessionManager.list(process.cwd());
const allSessions = await SessionManager.listAll(process.cwd());

// Session replacement API for /new, /resume, /fork, /clone, and import flows.
const createRuntime: CreateAgentSessionRuntimeFactory = async ({ cwd, sessionManager, sessionStartEvent }) => {
  const services = await createAgentSessionServices({ cwd });
  return {
    ...(await createAgentSessionFromServices({
      services,
      sessionManager,
      sessionStartEvent,
    })),
    services,
    diagnostics: services.diagnostics,
  };
};

const runtime = await createAgentSessionRuntime(createRuntime, {
  cwd: process.cwd(),
  agentDir: getAgentDir(),
  sessionManager: SessionManager.create(process.cwd()),
});

// Replace the active session with a fresh one
await runtime.newSession();

// Replace the active session with another saved session
await runtime.switchSession("/path/to/session.jsonl");

// Replace the active session with a fork from a specific user entry
await runtime.fork("entry-id");

// Clone the active path through a specific entry
await runtime.fork("entry-id", { position: "at" });
```

**SessionManager tree API:**

```typescript
const sm = SessionManager.open("/path/to/session.jsonl");

// Session listing
const currentProjectSessions = await SessionManager.list(process.cwd());
const allSessions = await SessionManager.listAll(process.cwd());

// Tree traversal
const entries = sm.getEntries();        // All entries (excludes header)
const tree = sm.getTree();              // Full tree structure
const path = sm.getPath();              // Path from root to current leaf
const leaf = sm.getLeafEntry();         // Current leaf entry
const entry = sm.getEntry(id);          // Get entry by ID
const children = sm.getChildren(id);    // Direct children of entry

// Labels
const label = sm.getLabel(id);          // Get label for entry
sm.appendLabelChange(id, "checkpoint"); // Set label

// Branching
sm.branch(entryId);                     // Move leaf to earlier entry
sm.branchWithSummary(id, "Summary...");  // Branch with context summary
sm.createBranchedSession(leafId);       // Extract path to new file
```

> See [examples/sdk/11-sessions.ts](../examples/sdk/11-sessions.ts) and [Session Format](session-format.md)

### Settings Management

```typescript
import { createAgentSession, SettingsManager, SessionManager } from "@bastani/atomic";

// Default: loads from files (global + project merged)
const { session } = await createAgentSession({
  settingsManager: SettingsManager.create(),
});

// With overrides
const settingsManager = SettingsManager.create();
settingsManager.applyOverrides({
  compaction: { enabled: false },
  retry: { enabled: true, maxRetries: 5 },
});
const { session } = await createAgentSession({ settingsManager });

// In-memory (no file I/O, for testing)
const { session } = await createAgentSession({
  settingsManager: SettingsManager.inMemory({ compaction: { enabled: false } }),
  sessionManager: SessionManager.inMemory(),
});

// Custom directories
const { session } = await createAgentSession({
  settingsManager: SettingsManager.create("/custom/cwd", "/custom/agent"),
});
```

**Static factories:**
- `SettingsManager.create(cwd?, agentDir?)` - Load from files
- `SettingsManager.inMemory(settings?)` - No file I/O

**Project-specific settings:**

Settings load from Atomic-first locations and merge:
1. Global: `~/.atomic/agent/settings.json`, then legacy `~/.pi/agent/settings.json`
2. Project: `<cwd>/.atomic/settings.json`, then legacy `<cwd>/.pi/settings.json`

Project overrides global. Nested objects merge keys. Setters modify global settings by default.

**Persistence and error handling semantics:**

- Settings getters/setters are synchronous for in-memory state.
- Setters enqueue persistence writes asynchronously.
- Call `await settingsManager.flush()` when you need a durability boundary (for example, before process exit or before asserting file contents in tests).
- `SettingsManager` does not print settings I/O errors. Use `settingsManager.drainErrors()` and report them in your app layer.

> See [examples/sdk/10-settings.ts](../examples/sdk/10-settings.ts)

## ResourceLoader

Use `DefaultResourceLoader` to discover extensions, skills, prompts, themes, and context files.

```typescript
import {
  DefaultResourceLoader,
  getAgentDir,
} from "@bastani/atomic";

const loader = new DefaultResourceLoader({
  cwd,
  agentDir: getAgentDir(),
});
await loader.reload();

const extensions = loader.getExtensions();
const skills = loader.getSkills();
const prompts = loader.getPrompts();
const themes = loader.getThemes();
const contextFiles = loader.getAgentsFiles().agentsFiles;
```

## Return Value

`createAgentSession()` returns:

```typescript
interface CreateAgentSessionResult {
  // The session
  session: AgentSession;
  
  // Extensions result (for runner setup)
  extensionsResult: LoadExtensionsResult;
  
  // Warning if session model couldn't be restored
  modelFallbackMessage?: string;
}

interface LoadExtensionsResult {
  extensions: Extension[];
  errors: Array<{ path: string; error: string }>;
  runtime: ExtensionRuntime;
}
```

## Complete Example

```typescript
import { getModel } from "@earendil-works/pi-ai";
import { Type } from "typebox";
import {
  AuthStorage,
  createAgentSession,
  DefaultResourceLoader,
  defineTool,
  ModelRegistry,
  SessionManager,
  SettingsManager,
} from "@bastani/atomic";

// Set up auth storage (custom location)
const authStorage = AuthStorage.create("/custom/agent/auth.json");

// Runtime API key override (not persisted)
if (process.env.MY_KEY) {
  authStorage.setRuntimeApiKey("anthropic", process.env.MY_KEY);
}

// Model registry (no custom models.json)
const modelRegistry = ModelRegistry.create(authStorage);

// Inline tool
const statusTool = defineTool({
  name: "status",
  label: "Status",
  description: "Get system status",
  parameters: Type.Object({}),
  execute: async () => ({
    content: [{ type: "text", text: `Uptime: ${process.uptime()}s` }],
    details: {},
  }),
});

const model = getModel("anthropic", "claude-opus-4-5");
if (!model) throw new Error("Model not found");

// In-memory settings with overrides
const settingsManager = SettingsManager.inMemory({
  compaction: { enabled: false },
  retry: { enabled: true, maxRetries: 2 },
});

const loader = new DefaultResourceLoader({
  cwd: process.cwd(),
  agentDir: "/custom/agent",
  settingsManager,
  systemPromptOverride: () => "You are a minimal assistant. Be concise.",
});
await loader.reload();

const { session } = await createAgentSession({
  cwd: process.cwd(),
  agentDir: "/custom/agent",

  model,
  thinkingLevel: "off",
  authStorage,
  modelRegistry,

  tools: ["read", "bash", "status"],
  customTools: [statusTool],
  resourceLoader: loader,

  sessionManager: SessionManager.inMemory(),
  settingsManager,
});

session.subscribe((event) => {
  if (event.type === "message_update" && event.assistantMessageEvent.type === "text_delta") {
    process.stdout.write(event.assistantMessageEvent.delta);
  }
});

await session.prompt("Get status and list files.");
```

## Run Modes

The SDK exports run mode utilities for building custom interfaces on top of `createAgentSession()`:

### InteractiveMode

Full TUI interactive mode with editor, chat history, and all built-in commands:

```typescript
import {
  type CreateAgentSessionRuntimeFactory,
  createAgentSessionFromServices,
  createAgentSessionRuntime,
  createAgentSessionServices,
  getAgentDir,
  InteractiveMode,
  SessionManager,
} from "@bastani/atomic";

const createRuntime: CreateAgentSessionRuntimeFactory = async ({ cwd, sessionManager, sessionStartEvent }) => {
  const services = await createAgentSessionServices({ cwd });
  return {
    ...(await createAgentSessionFromServices({ services, sessionManager, sessionStartEvent })),
    services,
    diagnostics: services.diagnostics,
  };
};
const runtime = await createAgentSessionRuntime(createRuntime, {
  cwd: process.cwd(),
  agentDir: getAgentDir(),
  sessionManager: SessionManager.create(process.cwd()),
});

const mode = new InteractiveMode(runtime, {
  migratedProviders: [],
  modelFallbackMessage: undefined,
  initialMessage: "Hello",
  initialImages: [],
  initialMessages: [],
});

await mode.run();
```

### runPrintMode

Single-shot mode: send prompts, output result, exit:

```typescript
import {
  type CreateAgentSessionRuntimeFactory,
  createAgentSessionFromServices,
  createAgentSessionRuntime,
  createAgentSessionServices,
  getAgentDir,
  runPrintMode,
  SessionManager,
} from "@bastani/atomic";

const createRuntime: CreateAgentSessionRuntimeFactory = async ({ cwd, sessionManager, sessionStartEvent }) => {
  const services = await createAgentSessionServices({ cwd });
  return {
    ...(await createAgentSessionFromServices({ services, sessionManager, sessionStartEvent })),
    services,
    diagnostics: services.diagnostics,
  };
};
const runtime = await createAgentSessionRuntime(createRuntime, {
  cwd: process.cwd(),
  agentDir: getAgentDir(),
  sessionManager: SessionManager.create(process.cwd()),
});

await runPrintMode(runtime, {
  mode: "text",
  initialMessage: "Hello",
  initialImages: [],
  messages: ["Follow up"],
});
```

### runRpcMode

JSON-RPC mode for subprocess integration:

```typescript
import {
  type CreateAgentSessionRuntimeFactory,
  createAgentSessionFromServices,
  createAgentSessionRuntime,
  createAgentSessionServices,
  getAgentDir,
  runRpcMode,
  SessionManager,
} from "@bastani/atomic";

const createRuntime: CreateAgentSessionRuntimeFactory = async ({ cwd, sessionManager, sessionStartEvent }) => {
  const services = await createAgentSessionServices({ cwd });
  return {
    ...(await createAgentSessionFromServices({ services, sessionManager, sessionStartEvent })),
    services,
    diagnostics: services.diagnostics,
  };
};
const runtime = await createAgentSessionRuntime(createRuntime, {
  cwd: process.cwd(),
  agentDir: getAgentDir(),
  sessionManager: SessionManager.create(process.cwd()),
});

await runRpcMode(runtime);
```

See [RPC documentation](rpc.md) for the JSON protocol.

## RPC Mode Alternative

For subprocess-based integration without building with the SDK, use the CLI directly:

```bash
pi --mode rpc --no-session
```

See [RPC documentation](rpc.md) for the JSON protocol.

The SDK is preferred when:
- You want type safety
- You're in the same Node.js process
- You need direct access to agent state
- You want to customize tools/extensions programmatically

RPC mode is preferred when:
- You're integrating from another language
- You want process isolation
- You're building a language-agnostic client

## Exports

The main entry point exports:

```typescript
// Factory
createAgentSession
createAgentSessionRuntime
AgentSessionRuntime

// Auth and Models
AuthStorage
ModelRegistry

// Resource loading
DefaultResourceLoader
type ResourceLoader
createEventBus

// Helpers
defineTool

// Session management
SessionManager
SettingsManager

// Tool factories
createCodingTools
createReadOnlyTools
createReadTool, createBashTool, createEditTool, createWriteTool
createGrepTool, createFindTool, createLsTool

// Types
type CreateAgentSessionOptions
type CreateAgentSessionResult
type ExtensionFactory
type ExtensionAPI
type ToolDefinition
type Skill
type PromptTemplate
type Tool
```

For extension types, see [extensions.md](extensions.md) for the full API.
