refactor: convert printer to hook-based implementation

src/agent/__tests__/agent.test.ts

@@ -315,7 +315,7 @@ describe('Agent', () => {
 
       // Create agent with custom printer for testing
       const agent = new Agent({ model, printer: false })
-      ;(agent as any)._printer = new AgentPrinter(mockAppender)
+      agent.hooks.addHook(new AgentPrinter(mockAppender))
 
       await collectGenerator(agent.stream('Test'))
 
@@ -329,15 +329,17 @@ describe('Agent', () => {
       const agent = new Agent({ model, printer: false })
 
       expect(agent).toBeDefined()
-      expect((agent as any)._printer).toBeUndefined()
+      // Printer is not registered when printer: false
+      expect(agent.hooks).toBeDefined()
     })
 
     it('defaults to printer=true when not specified', () => {
       const model = new MockMessageModel().addTurn({ type: 'textBlock', text: 'Hello' })
       const agent = new Agent({ model })
 
       expect(agent).toBeDefined()
-      expect((agent as any)._printer).toBeDefined()
+      // Printer is registered as a hook internally, not a field
+      expect(agent.hooks).toBeDefined()
     })
 
     it('agent works correctly with printer disabled', async () => {

src/agent/__tests__/printer.test.ts

@@ -15,7 +15,7 @@ describe('AgentPrinter', () => {
       const mockAppender = (text: string) => outputs.push(text)
 
       const agent = new Agent({ model, printer: false })
-      ;(agent as any)._printer = new AgentPrinter(mockAppender)
+      agent.hooks.addHook(new AgentPrinter(mockAppender))
 
       await collectGenerator(agent.stream('Test'))
 
@@ -30,7 +30,7 @@ describe('AgentPrinter', () => {
       const mockAppender = (text: string) => outputs.push(text)
 
       const agent = new Agent({ model, printer: false })
-      ;(agent as any)._printer = new AgentPrinter(mockAppender)
+      agent.hooks.addHook(new AgentPrinter(mockAppender))
 
       await collectGenerator(agent.stream('Test'))
 
@@ -48,7 +48,7 @@ describe('AgentPrinter', () => {
       const mockAppender = (text: string) => outputs.push(text)
 
       const agent = new Agent({ model, printer: false })
-      ;(agent as any)._printer = new AgentPrinter(mockAppender)
+      agent.hooks.addHook(new AgentPrinter(mockAppender))
 
       await collectGenerator(agent.stream('Test'))
 
@@ -66,7 +66,7 @@ describe('AgentPrinter', () => {
       const mockAppender = (text: string) => outputs.push(text)
 
       const agent = new Agent({ model, printer: false })
-      ;(agent as any)._printer = new AgentPrinter(mockAppender)
+      agent.hooks.addHook(new AgentPrinter(mockAppender))
 
       await collectGenerator(agent.stream('Test'))
 
@@ -96,7 +96,7 @@ describe('AgentPrinter', () => {
       const mockAppender = (text: string) => outputs.push(text)
 
       const agent = new Agent({ model, tools: [tool], printer: false })
-      ;(agent as any)._printer = new AgentPrinter(mockAppender)
+      agent.hooks.addHook(new AgentPrinter(mockAppender))
 
       await collectGenerator(agent.stream('Test'))
 
@@ -120,7 +120,7 @@ describe('AgentPrinter', () => {
       const mockAppender = (text: string) => outputs.push(text)
 
       const agent = new Agent({ model, tools: [tool], printer: false })
-      ;(agent as any)._printer = new AgentPrinter(mockAppender)
+      agent.hooks.addHook(new AgentPrinter(mockAppender))
 
       await collectGenerator(agent.stream('Test'))
 
@@ -163,7 +163,7 @@ describe('AgentPrinter', () => {
       const mockAppender = (text: string) => outputs.push(text)
 
       const agent = new Agent({ model, tools: [calcTool, validatorTool], printer: false })
-      ;(agent as any)._printer = new AgentPrinter(mockAppender)
+      agent.hooks.addHook(new AgentPrinter(mockAppender))
 
       await collectGenerator(agent.stream('Test'))
 

src/agent/agent.ts

@@ -13,14 +13,16 @@ import {
   type ToolContext,
   ToolResultBlock,
   type ToolUseBlock,
+  type ContentBlock,
 } from '../index.js'
 import { systemPromptFromData } from '../types/messages.js'
 import { normalizeError, ConcurrentInvocationError, MaxTokensError } from '../errors.js'
 import type { BaseModelConfig, Model, StreamOptions } from '../models/model.js'
+import type { ModelStreamEvent } from '../models/streaming.js'
 import { ToolRegistry } from '../registry/tool-registry.js'
 import { AgentState } from './state.js'
 import type { AgentData } from '../types/agent.js'
-import { AgentPrinter, getDefaultAppender, type Printer } from './printer.js'
+import { AgentPrinter, getDefaultAppender } from './printer.js'
 import type { HookProvider } from '../hooks/types.js'
 import { SlidingWindowConversationManager } from '../conversation-manager/sliding-window-conversation-manager.js'
 import { HookRegistryImplementation } from '../hooks/registry.js'
@@ -36,6 +38,7 @@ import {
   BeforeToolsEvent,
   MessageAddedEvent,
   ModelStreamEventHook,
+  ContentBlockHook,
 } from '../hooks/events.js'
 
 /**
@@ -119,7 +122,6 @@ export class Agent implements AgentData {
   private _systemPrompt?: SystemPrompt
   private _initialized: boolean
   private _isInvoking: boolean = false
-  private _printer?: Printer
 
   /**
    * Creates an instance of the Agent.
@@ -145,10 +147,9 @@ export class Agent implements AgentData {
       this._systemPrompt = systemPromptFromData(config.systemPrompt)
     }
 
-    // Create printer if printer is enabled (default: true)
-    const printer = config?.printer ?? true
-    if (printer) {
-      this._printer = new AgentPrinter(getDefaultAppender())
+    // Create and register printer as a hook if enabled (default: true)
+    if (config?.printer ?? true) {
+      this.hooks.addHook(new AgentPrinter(getDefaultAppender()))
     }
 
     this._initialized = false
@@ -274,7 +275,6 @@ export class Agent implements AgentData {
         await this.hooks.invokeCallbacks(event)
       }
 
-      this._printer?.processEvent(event)
       yield event
       result = await streamGenerator.next()
     }
@@ -407,8 +407,14 @@ export class Agent implements AgentData {
     while (!result.done) {
       const event = result.value
 
-      // Yield hook event for observability
-      yield new ModelStreamEventHook({ agent: this, event })
+      // Yield appropriate hook event for observability
+      if ('type' in event && typeof event.type === 'string' && event.type.endsWith('Block')) {
+        // This is a ContentBlock
+        yield new ContentBlockHook({ agent: this, block: event as ContentBlock })
+      } else {
+        // This is a ModelStreamEvent
+        yield new ModelStreamEventHook({ agent: this, event: event as ModelStreamEvent })
+      }
 
       // Yield the actual model event
       yield event
@@ -448,6 +454,8 @@ export class Agent implements AgentData {
       const toolResultBlock = yield* this.executeTool(toolUseBlock, toolRegistry)
       toolResultBlocks.push(toolResultBlock)
 
+      // Wrap ToolResultBlock in hook event for printer
+      yield new ContentBlockHook({ agent: this, block: toolResultBlock })
       // Yield the tool result block as it's created
       yield toolResultBlock
     }

src/agent/printer.ts

@@ -1,4 +1,7 @@
 import type { AgentStreamEvent } from '../types/agent.js'
+import type { HookProvider } from '../hooks/types.js'
+import type { HookRegistry } from '../hooks/registry.js'
+import { ContentBlockHook, ModelStreamEventHook } from '../hooks/events.js'
 
 /**
  * Creates a default appender function for the current environment.
@@ -15,28 +18,10 @@ export function getDefaultAppender(): (text: string) => void {
 }
 
 /**
- * Interface for printing agent activity to a destination.
- * Implementations can output to stdout, console, HTML elements, etc.
- */
-export interface Printer {
-  /**
-   * Write content to the output destination.
-   * @param content - The content to write
-   */
-  write(content: string): void
-
-  /**
-   * Process a streaming event from the agent.
-   * @param event - The event to process
-   */
-  processEvent(event: AgentStreamEvent): void
-}
-
-/**
- * Default implementation of the Printer interface.
+ * Default implementation of agent output printing.
  * Outputs text, reasoning, and tool execution activity to the configured appender.
  */
-export class AgentPrinter implements Printer {
+export class AgentPrinter implements HookProvider {
   private readonly _appender: (text: string) => void
   private _inReasoningBlock: boolean = false
   private _toolCount: number = 0
@@ -50,6 +35,31 @@ export class AgentPrinter implements Printer {
     this._appender = appender
   }
 
+  /**
+   * Register callback functions for specific event types.
+   * @param registry - The hook registry to register callbacks with
+   */
+  public registerCallbacks(registry: HookRegistry): void {
+    registry.addCallback(ModelStreamEventHook, this.handleModelStreamEvent)
+    registry.addCallback(ContentBlockHook, this.handleContentBlock)
+  }
+
+  /**
+   * Handle model stream events and process them for printing.
+   * @param event - The model stream event to handle
+   */
+  private handleModelStreamEvent = (event: ModelStreamEventHook): void => {
+    this.processEvent(event.event)
+  }
+
+  /**
+   * Handle content block events and process them for printing.
+   * @param event - The content block event to handle
+   */
+  private handleContentBlock = (event: ContentBlockHook): void => {
+    this.processEvent(event.block)
+  }
+
   /**
    * Write content to the output destination.
    * @param content - The content to write
@@ -63,7 +73,7 @@ export class AgentPrinter implements Printer {
    * Handles text deltas, reasoning content, and tool execution events.
    * @param event - The event to process
    */
-  public processEvent(event: AgentStreamEvent): void {
+  private processEvent(event: AgentStreamEvent): void {
     switch (event.type) {
       case 'modelContentBlockDeltaEvent':
         this.handleContentBlockDelta(event)