feat(extensions): forward message and tool execution events to extensions (#1375)

The extension system currently only forwards agent_start, agent_end,
turn_start, and turn_end events. This means extensions cannot access
streaming text (token-by-token), message lifecycle, or tool execution
progress — all of which are available to internal subscribers.

This adds forwarding for the remaining 6 agent event types:
- message_start, message_update, message_end
- tool_execution_start, tool_execution_update, tool_execution_end

These follow the exact same pattern as the existing forwarded events:
new interfaces in types.ts, exports in index.ts, and else-if blocks
in _emitExtensionEvent(). The new types are included in ExtensionEvent
and automatically flow through RunnerEmitEvent (they're not in the
exclusion list).

This enables extensions to build real-time UIs, streaming WebSocket
bridges, and other integrations that need fine-grained event access.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Mario Zechner <badlogicgames@gmail.com>

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

@@ -50,12 +50,18 @@ import {
 	ExtensionRunner,
 	type ExtensionUIContext,
 	type InputSource,
+	type MessageEndEvent,
+	type MessageStartEvent,
+	type MessageUpdateEvent,
 	type SessionBeforeCompactResult,
 	type SessionBeforeForkResult,
 	type SessionBeforeSwitchResult,
 	type SessionBeforeTreeResult,
 	type ShutdownHandler,
 	type ToolDefinition,
+	type ToolExecutionEndEvent,
+	type ToolExecutionStartEvent,
+	type ToolExecutionUpdateEvent,
 	type ToolInfo,
 	type TreePreparation,
 	type TurnEndEvent,
@@ -444,6 +450,51 @@ export class AgentSession {
 			};
 			await this._extensionRunner.emit(extensionEvent);
 			this._turnIndex++;
+		} else if (event.type === "message_start") {
+			const extensionEvent: MessageStartEvent = {
+				type: "message_start",
+				message: event.message,
+			};
+			await this._extensionRunner.emit(extensionEvent);
+		} else if (event.type === "message_update") {
+			const extensionEvent: MessageUpdateEvent = {
+				type: "message_update",
+				message: event.message,
+				assistantMessageEvent: event.assistantMessageEvent,
+			};
+			await this._extensionRunner.emit(extensionEvent);
+		} else if (event.type === "message_end") {
+			const extensionEvent: MessageEndEvent = {
+				type: "message_end",
+				message: event.message,
+			};
+			await this._extensionRunner.emit(extensionEvent);
+		} else if (event.type === "tool_execution_start") {
+			const extensionEvent: ToolExecutionStartEvent = {
+				type: "tool_execution_start",
+				toolCallId: event.toolCallId,
+				toolName: event.toolName,
+				args: event.args,
+			};
+			await this._extensionRunner.emit(extensionEvent);
+		} else if (event.type === "tool_execution_update") {
+			const extensionEvent: ToolExecutionUpdateEvent = {
+				type: "tool_execution_update",
+				toolCallId: event.toolCallId,
+				toolName: event.toolName,
+				args: event.args,
+				partialResult: event.partialResult,
+			};
+			await this._extensionRunner.emit(extensionEvent);
+		} else if (event.type === "tool_execution_end") {
+			const extensionEvent: ToolExecutionEndEvent = {
+				type: "tool_execution_end",
+				toolCallId: event.toolCallId,
+				toolName: event.toolName,
+				result: event.result,
+				isError: event.isError,
+			};
+			await this._extensionRunner.emit(extensionEvent);
 		}
 	}
 

packages/coding-agent/src/core/extensions/index.ts

@@ -81,9 +81,13 @@ export type {
 	LoadExtensionsResult,
 	LsToolCallEvent,
 	LsToolResultEvent,
+	// Events - Message
+	MessageEndEvent,
 	// Message Rendering
 	MessageRenderer,
 	MessageRenderOptions,
+	MessageStartEvent,
+	MessageUpdateEvent,
 	ModelSelectEvent,
 	ModelSelectSource,
 	// Provider Registration
@@ -124,6 +128,10 @@ export type {
 	ToolCallEventResult,
 	// Tools
 	ToolDefinition,
+	// Events - Tool Execution
+	ToolExecutionEndEvent,
+	ToolExecutionStartEvent,
+	ToolExecutionUpdateEvent,
 	ToolInfo,
 	ToolRenderResultOptions,
 	ToolResultEvent,

packages/coding-agent/src/core/extensions/types.ts

@@ -16,6 +16,7 @@ import type {
 } from "@mariozechner/pi-agent-core";
 import type {
 	Api,
+	AssistantMessageEvent,
 	AssistantMessageEventStream,
 	Context,
 	ImageContent,
@@ -512,6 +513,51 @@ export interface TurnEndEvent {
 	toolResults: ToolResultMessage[];
 }
 
+/** Fired when a message starts (user, assistant, or toolResult) */
+export interface MessageStartEvent {
+	type: "message_start";
+	message: AgentMessage;
+}
+
+/** Fired during assistant message streaming with token-by-token updates */
+export interface MessageUpdateEvent {
+	type: "message_update";
+	message: AgentMessage;
+	assistantMessageEvent: AssistantMessageEvent;
+}
+
+/** Fired when a message ends */
+export interface MessageEndEvent {
+	type: "message_end";
+	message: AgentMessage;
+}
+
+/** Fired when a tool starts executing */
+export interface ToolExecutionStartEvent {
+	type: "tool_execution_start";
+	toolCallId: string;
+	toolName: string;
+	args: any;
+}
+
+/** Fired during tool execution with partial/streaming output */
+export interface ToolExecutionUpdateEvent {
+	type: "tool_execution_update";
+	toolCallId: string;
+	toolName: string;
+	args: any;
+	partialResult: any;
+}
+
+/** Fired when a tool finishes executing */
+export interface ToolExecutionEndEvent {
+	type: "tool_execution_end";
+	toolCallId: string;
+	toolName: string;
+	result: any;
+	isError: boolean;
+}
+
 // ============================================================================
 // Model Events
 // ============================================================================
@@ -752,6 +798,12 @@ export type ExtensionEvent =
 	| AgentEndEvent
 	| TurnStartEvent
 	| TurnEndEvent
+	| MessageStartEvent
+	| MessageUpdateEvent
+	| MessageEndEvent
+	| ToolExecutionStartEvent
+	| ToolExecutionUpdateEvent
+	| ToolExecutionEndEvent
 	| ModelSelectEvent
 	| UserBashEvent
 	| InputEvent
@@ -883,6 +935,12 @@ export interface ExtensionAPI {
 	on(event: "agent_end", handler: ExtensionHandler<AgentEndEvent>): void;
 	on(event: "turn_start", handler: ExtensionHandler<TurnStartEvent>): void;
 	on(event: "turn_end", handler: ExtensionHandler<TurnEndEvent>): void;
+	on(event: "message_start", handler: ExtensionHandler<MessageStartEvent>): void;
+	on(event: "message_update", handler: ExtensionHandler<MessageUpdateEvent>): void;
+	on(event: "message_end", handler: ExtensionHandler<MessageEndEvent>): void;
+	on(event: "tool_execution_start", handler: ExtensionHandler<ToolExecutionStartEvent>): void;
+	on(event: "tool_execution_update", handler: ExtensionHandler<ToolExecutionUpdateEvent>): void;
+	on(event: "tool_execution_end", handler: ExtensionHandler<ToolExecutionEndEvent>): void;
 	on(event: "model_select", handler: ExtensionHandler<ModelSelectEvent>): void;
 	on(event: "tool_call", handler: ExtensionHandler<ToolCallEvent, ToolCallEventResult>): void;
 	on(event: "tool_result", handler: ExtensionHandler<ToolResultEvent, ToolResultEventResult>): void;