feat(coding-agent): add model_select extension hook

Fires when model changes via setModel(), cycleModel(), or session restore.
Includes source field ("set" | "cycle" | "restore") and previous model.

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

@@ -947,6 +947,21 @@ export class AgentSession {
 	// Model Management
 	// =========================================================================
 
+	private async _emitModelSelect(
+		nextModel: Model<any>,
+		previousModel: Model<any> | undefined,
+		source: "set" | "cycle" | "restore",
+	): Promise<void> {
+		if (!this._extensionRunner) return;
+		if (modelsAreEqual(previousModel, nextModel)) return;
+		await this._extensionRunner.emit({
+			type: "model_select",
+			model: nextModel,
+			previousModel,
+			source,
+		});
+	}
+
 	/**
 	 * Set model directly.
 	 * Validates API key, saves to session and settings.
@@ -958,12 +973,15 @@ export class AgentSession {
 			throw new Error(`No API key for ${model.provider}/${model.id}`);
 		}
 
+		const previousModel = this.model;
 		this.agent.setModel(model);
 		this.sessionManager.appendModelChange(model.provider, model.id);
 		this.settingsManager.setDefaultModelAndProvider(model.provider, model.id);
 
 		// Re-clamp thinking level for new model's capabilities
 		this.setThinkingLevel(this.thinkingLevel);
+
+		await this._emitModelSelect(model, previousModel, "set");
 	}
 
 	/**
@@ -1004,6 +1022,8 @@ export class AgentSession {
 		// Apply thinking level (setThinkingLevel clamps to model capabilities)
 		this.setThinkingLevel(next.thinkingLevel);
 
+		await this._emitModelSelect(next.model, currentModel, "cycle");
+
 		return { model: next.model, thinkingLevel: this.thinkingLevel, isScoped: true };
 	}
 
@@ -1031,6 +1051,8 @@ export class AgentSession {
 		// Re-clamp thinking level for new model's capabilities
 		this.setThinkingLevel(this.thinkingLevel);
 
+		await this._emitModelSelect(nextModel, currentModel, "cycle");
+
 		return { model: nextModel, thinkingLevel: this.thinkingLevel, isScoped: false };
 	}
 
@@ -1783,12 +1805,14 @@ export class AgentSession {
 
 		// Restore model if saved
 		if (sessionContext.model) {
+			const previousModel = this.model;
 			const availableModels = await this._modelRegistry.getAvailable();
 			const match = availableModels.find(
 				(m) => m.provider === sessionContext.model!.provider && m.id === sessionContext.model!.modelId,
 			);
 			if (match) {
 				this.agent.setModel(match);
+				await this._emitModelSelect(match, previousModel, "restore");
 			}
 		}
 

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

@@ -67,6 +67,8 @@ export type {
 	// Message Rendering
 	MessageRenderer,
 	MessageRenderOptions,
+	ModelSelectEvent,
+	ModelSelectSource,
 	ReadToolResultEvent,
 	// Commands
 	RegisteredCommand,

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

@@ -403,6 +403,20 @@ export interface TurnEndEvent {
 	toolResults: ToolResultMessage[];
 }
 
+// ============================================================================
+// Model Events
+// ============================================================================
+
+export type ModelSelectSource = "set" | "cycle" | "restore";
+
+/** Fired when a new model is selected */
+export interface ModelSelectEvent {
+	type: "model_select";
+	model: Model<any>;
+	previousModel: Model<any> | undefined;
+	source: ModelSelectSource;
+}
+
 // ============================================================================
 // User Bash Events
 // ============================================================================
@@ -521,6 +535,7 @@ export type ExtensionEvent =
 	| AgentEndEvent
 	| TurnStartEvent
 	| TurnEndEvent
+	| ModelSelectEvent
 	| UserBashEvent
 	| ToolCallEvent
 	| ToolResultEvent;
@@ -645,6 +660,7 @@ 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: "model_select", handler: ExtensionHandler<ModelSelectEvent>): void;
 	on(event: "tool_call", handler: ExtensionHandler<ToolCallEvent, ToolCallEventResult>): void;
 	on(event: "tool_result", handler: ExtensionHandler<ToolResultEvent, ToolResultEventResult>): void;
 	on(event: "user_bash", handler: ExtensionHandler<UserBashEvent, UserBashEventResult>): void;