Merge model-select-hook: add model_select extension hook (#628)

packages/coding-agent/CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Added
 
+- `model_select` extension hook fires when model changes via `/model`, model cycling, or session restore with `source` field and `previousModel` ([#628](https://github.com/badlogic/pi-mono/pull/628) by [@marckrenn](https://github.com/marckrenn))
 - `ctx.ui.setWorkingMessage()` extension API to customize the "Working..." message during streaming ([#625](https://github.com/badlogic/pi-mono/pull/625) by [@nicobailon](https://github.com/nicobailon))
 - Skill slash commands: loaded skills are registered as `/skill:name` commands for quick access. Toggle via `/settings` or `skills.enableSkillCommands` in settings.json. ([#630](https://github.com/badlogic/pi-mono/pull/630) by [@Dwsy](https://github.com/Dwsy))
 - Slash command autocomplete now uses fuzzy matching (type `/skbra` to match `/skill:brave-search`)

packages/coding-agent/docs/extensions.md

@@ -290,6 +290,9 @@ user sends another prompt ◄─────────────────
   ├─► session_before_tree (can cancel or customize)
   └─► session_tree
 
+/model or Ctrl+P (model selection/cycling)
+  └─► model_select
+
 exit (Ctrl+C, Ctrl+D)
   └─► session_shutdown
 ```
@@ -481,6 +484,31 @@ pi.on("context", async (event, ctx) => {
 
 **Examples:** [plan-mode.ts](../examples/extensions/plan-mode.ts)
 
+### Model Events
+
+#### model_select
+
+Fired when the model changes via `/model` command, model cycling (`Ctrl+P`), or session restore.
+
+```typescript
+pi.on("model_select", async (event, ctx) => {
+  // event.model - newly selected model
+  // event.previousModel - previous model (undefined if first selection)
+  // event.source - "set" | "cycle" | "restore"
+  
+  const prev = event.previousModel 
+    ? `${event.previousModel.provider}/${event.previousModel.id}` 
+    : "none";
+  const next = `${event.model.provider}/${event.model.id}`;
+  
+  ctx.ui.notify(`Model changed (${event.source}): ${prev} -> ${next}`, "info");
+});
+```
+
+Use this to update UI elements (status bars, footers) or perform model-specific initialization when the active model changes.
+
+**Examples:** [model-status.ts](../examples/extensions/model-status.ts)
+
 ### Tool Events
 
 #### tool_call

packages/coding-agent/examples/extensions/model-status.ts

@@ -0,0 +1,31 @@
+/**
+ * Model status extension - shows model changes in the status bar.
+ *
+ * Demonstrates the `model_select` hook which fires when the model changes
+ * via /model command, Ctrl+P cycling, or session restore.
+ *
+ * Usage: pi -e ./model-status.ts
+ */
+
+import type { ExtensionAPI } from "@mariozechner/pi-coding-agent";
+
+export default function (pi: ExtensionAPI) {
+	pi.on("model_select", async (event, ctx) => {
+		const { model, previousModel, source } = event;
+
+		// Format model identifiers
+		const next = `${model.provider}/${model.id}`;
+		const prev = previousModel ? `${previousModel.provider}/${previousModel.id}` : "none";
+
+		// Show notification on change
+		if (source !== "restore") {
+			ctx.ui.notify(`Model: ${next}`, "info");
+		}
+
+		// Update status bar with current model
+		ctx.ui.setStatus("model", `🤖 ${model.id}`);
+
+		// Log change details (visible in debug output)
+		console.log(`[model_select] ${prev} → ${next} (${source})`);
+	});
+}

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;