Use proper HookAppMessage type instead of _hookData marker

Following the same pattern as BashExecutionMessage:
- HookAppMessage has role: 'hookMessage' with customType, content, display, details
- isHookAppMessage() type guard for checking message type
- messageTransformer converts to user message for LLM context
- TUI checks isHookAppMessage() for rendering as CustomMessageComponent

This makes the API clean for anyone building on AgentSession - they can
use the type guard instead of knowing about internal marker fields.

packages/coding-agent/src/core/agent-session.ts

@@ -37,7 +37,7 @@ import {
 	type TurnEndEvent,
 	type TurnStartEvent,
 } from "./hooks/index.js";
-import type { BashExecutionMessage } from "./messages.js";
+import { type BashExecutionMessage, type HookAppMessage, isHookAppMessage } from "./messages.js";
 import type { ModelRegistry } from "./model-registry.js";
 import type { CompactionEntry, SessionManager } from "./session-manager.js";
 import type { SettingsManager, SkillsSettings } from "./settings-manager.js";
@@ -111,12 +111,6 @@ export interface SessionStats {
 }
 
 /** Internal marker for hook messages queued through the agent loop */
-interface HookMessageData {
-	customType: string;
-	display: boolean;
-	details?: unknown;
-}
-
 // ============================================================================
 // Constants
 // ============================================================================
@@ -228,15 +222,13 @@ export class AgentSession {
 		// Handle session persistence
 		if (event.type === "message_end") {
 			// Check if this is a hook message (has _hookData marker)
-			type HookAppMessage = AppMessage & { _hookData?: HookMessageData; content: (TextContent | ImageContent)[] };
-			const hookMessage = event.message as HookAppMessage;
-			if (hookMessage._hookData) {
+			if (isHookAppMessage(event.message)) {
 				// Persist as CustomMessageEntry
 				this.sessionManager.appendCustomMessageEntry(
-					hookMessage._hookData.customType,
-					hookMessage.content,
-					hookMessage._hookData.display,
-					hookMessage._hookData.details,
+					event.message.customType,
+					event.message.content,
+					event.message.display,
+					event.message.details,
 				);
 			} else {
 				// Regular message - persist as SessionMessageEntry
@@ -578,16 +570,14 @@ export class AgentSession {
 		const content: (TextContent | ImageContent)[] =
 			typeof message.content === "string" ? [{ type: "text", text: message.content }] : message.content;
 
-		// Create AppMessage with _hookData marker for routing in message_end handler
-		const appMessage: AppMessage & { _hookData: HookMessageData } = {
-			role: "user",
+		// Create HookAppMessage with proper role for type-safe handling
+		const appMessage: HookAppMessage = {
+			role: "hookMessage",
+			customType: message.customType,
 			content,
+			display: message.display,
+			details: message.details,
 			timestamp: Date.now(),
-			_hookData: {
-				customType: message.customType,
-				display: message.display,
-				details: message.details,
-			},
 		};
 
 		if (this.isStreaming) {
@@ -596,12 +586,6 @@ export class AgentSession {
 		} else if (triggerTurn) {
 			// Append to agent state and session, then trigger a turn
 			this.agent.appendMessage(appMessage);
-			this.sessionManager.appendCustomMessageEntry(
-				message.customType,
-				message.content,
-				message.display,
-				message.details,
-			);
 			// Start a new turn - emit message events for the hook message so TUI can render it
 			await this.agent.continue(true);
 		} else {

packages/coding-agent/src/core/messages.ts

@@ -26,10 +26,26 @@ export interface BashExecutionMessage {
 	timestamp: number;
 }
 
+import type { ImageContent, TextContent } from "@mariozechner/pi-ai";
+
+/**
+ * Message type for hook-injected messages via sendMessage().
+ * These are custom messages that hooks can inject into the conversation.
+ */
+export interface HookAppMessage<T = unknown> {
+	role: "hookMessage";
+	customType: string;
+	content: (TextContent | ImageContent)[];
+	display: boolean;
+	details?: T;
+	timestamp: number;
+}
+
 // Extend CustomMessages via declaration merging
 declare module "@mariozechner/pi-agent-core" {
 	interface CustomMessages {
 		bashExecution: BashExecutionMessage;
+		hookMessage: HookAppMessage;
 	}
 }
 
@@ -44,6 +60,13 @@ export function isBashExecutionMessage(msg: AppMessage | Message): msg is BashEx
 	return (msg as BashExecutionMessage).role === "bashExecution";
 }
 
+/**
+ * Type guard for HookAppMessage.
+ */
+export function isHookAppMessage(msg: AppMessage | Message): msg is HookAppMessage {
+	return (msg as HookAppMessage).role === "hookMessage";
+}
+
 // ============================================================================
 // Message Formatting
 // ============================================================================
@@ -91,6 +114,14 @@ export function messageTransformer(messages: AppMessage[]): Message[] {
 					timestamp: m.timestamp,
 				};
 			}
+			if (isHookAppMessage(m)) {
+				// Convert hook message to user message
+				return {
+					role: "user",
+					content: m.content,
+					timestamp: m.timestamp,
+				};
+			}
 			// Pass through standard LLM roles
 			if (m.role === "user" || m.role === "assistant" || m.role === "toolResult") {
 				return m as Message;

packages/coding-agent/src/modes/interactive/interactive-mode.ts

@@ -28,7 +28,7 @@ import { APP_NAME, getAuthPath, getDebugLogPath } from "../../config.js";
 import type { AgentSession, AgentSessionEvent } from "../../core/agent-session.js";
 import type { LoadedCustomTool, SessionEvent as ToolSessionEvent } from "../../core/custom-tools/index.js";
 import type { HookUIContext } from "../../core/hooks/index.js";
-import { isBashExecutionMessage } from "../../core/messages.js";
+import { isBashExecutionMessage, isHookAppMessage } from "../../core/messages.js";
 import {
 	getLatestCompactionEntry,
 	type SessionContext,
@@ -1016,7 +1016,22 @@ export class InteractiveMode {
 			return;
 		}
 
-		if (message.role === "user") {
+		if (isHookAppMessage(message)) {
+			// Render as custom message if display is true
+			if (message.display) {
+				const entry = {
+					type: "custom_message" as const,
+					customType: message.customType,
+					content: message.content,
+					display: true,
+					id: "",
+					parentId: null,
+					timestamp: new Date().toISOString(),
+				};
+				const renderer = this.session.hookRunner?.getCustomMessageRenderer(message.customType);
+				this.chatContainer.addChild(new CustomMessageComponent(entry, renderer));
+			}
+		} else if (message.role === "user") {
 			const textContent = this.getUserMessageText(message);
 			if (textContent) {
 				const userComponent = new UserMessageComponent(textContent, this.isFirstUserMessage);