feat(01-04): add BeforeModelSelectEvent to extension API and wire emission

- Add BeforeModelSelectEvent interface and BeforeModelSelectResult type to types.ts
- Add on('before_model_select') subscription overload to ExtensionAPI interface
- Add emitBeforeModelSelect() method to ExtensionAPI interface and ExtensionRuntimeState
- Implement emitBeforeModelSelect() on ExtensionRunner using invokeHandlers (first-override-wins)
- Bind runner's emitBeforeModelSelect into shared runtime at construction time
- Wire emitBeforeModelSelect delegation through createExtensionAPI in loader.ts

packages/pi-coding-agent/src/core/extensions/loader.ts

@@ -428,6 +428,8 @@ export function createExtensionRuntime(): ExtensionRuntime {
 		unregisterProvider: (name) => {
 			runtime.pendingProviderRegistrations = runtime.pendingProviderRegistrations.filter((r) => r.name !== name);
 		},
+		// Stub replaced by ExtensionRunner at construction time via bindEmitMethods().
+		emitBeforeModelSelect: async () => undefined,
 	};
 
 	return runtime;
@@ -579,6 +581,10 @@ function createExtensionAPI(
 			runtime.unregisterProvider(name);
 		},
 
+		async emitBeforeModelSelect(event: Omit<import("./types.js").BeforeModelSelectEvent, "type">): Promise<import("./types.js").BeforeModelSelectResult | undefined> {
+			return runtime.emitBeforeModelSelect(event);
+		},
+
 		events: eventBus,
 	} as ExtensionAPI;
 

packages/pi-coding-agent/src/core/extensions/runner.ts

@@ -13,6 +13,8 @@ import type { SessionManager } from "../session-manager.js";
 import type {
 	BeforeAgentStartEvent,
 	BeforeAgentStartEventResult,
+	BeforeModelSelectEvent,
+	BeforeModelSelectResult,
 	BeforeProviderRequestEvent,
 	CompactOptions,
 	ContextEvent,
@@ -230,6 +232,8 @@ export class ExtensionRunner {
 		this.cwd = cwd;
 		this.sessionManager = sessionManager;
 		this.modelRegistry = modelRegistry;
+		// Bind emit methods into the shared runtime so createExtensionAPI can delegate to them.
+		this.runtime.emitBeforeModelSelect = (event) => this.emitBeforeModelSelect(event);
 	}
 
 	bindCore(actions: ExtensionActions, contextActions: ExtensionContextActions): void {
@@ -694,6 +698,21 @@ export class ExtensionRunner {
 		return currentPayload;
 	}
 
+	async emitBeforeModelSelect(event: Omit<BeforeModelSelectEvent, "type">): Promise<BeforeModelSelectResult | undefined> {
+		let result: BeforeModelSelectResult | undefined;
+		await this.invokeHandlers("before_model_select", () => ({
+			type: "before_model_select" as const,
+			...event,
+		} satisfies BeforeModelSelectEvent), (handlerResult) => {
+			if (handlerResult) {
+				result = handlerResult as BeforeModelSelectResult;
+				return { done: true }; // first override wins
+			}
+			return { done: false };
+		});
+		return result;
+	}
+
 	async emitBeforeAgentStart(
 		prompt: string,
 		images: ImageContent[] | undefined,

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

@@ -603,6 +603,22 @@ export interface ModelSelectEvent {
 	source: ModelSelectSource;
 }
 
+/** Fired before model selection runs capability scoring. Extensions can override the selected model. */
+export interface BeforeModelSelectEvent {
+	type: "before_model_select";
+	unitType: string;
+	unitId: string;
+	classification: { tier: string; reason: string; downgraded: boolean };
+	taskMetadata?: Record<string, unknown>;
+	eligibleModels: string[];
+	phaseConfig?: { primary: string; fallbacks: string[] };
+}
+
+/** Result from before_model_select event handler. Return { modelId } to override selection. */
+export interface BeforeModelSelectResult {
+	modelId: string;
+}
+
 // ============================================================================
 // User Bash Events
 // ============================================================================
@@ -1052,6 +1068,14 @@ export interface ExtensionAPI {
 	on(event: "tool_result", handler: ExtensionHandler<ToolResultEvent, ToolResultEventResult>): void;
 	on(event: "user_bash", handler: ExtensionHandler<UserBashEvent, UserBashEventResult>): void;
 	on(event: "input", handler: ExtensionHandler<InputEvent, InputEventResult>): void;
+	on(event: "before_model_select", handler: ExtensionHandler<BeforeModelSelectEvent, BeforeModelSelectResult>): void;
+
+	// =========================================================================
+	// Event Emission (for host extensions that orchestrate model selection)
+	// =========================================================================
+
+	/** Emit before_model_select event. Returns override model ID or undefined. */
+	emitBeforeModelSelect(event: Omit<BeforeModelSelectEvent, "type">): Promise<BeforeModelSelectResult | undefined>;
 
 	// =========================================================================
 	// Tool Registration
@@ -1367,6 +1391,8 @@ export interface ExtensionRuntimeState {
 	 */
 	registerProvider: (name: string, config: ProviderConfig) => void;
 	unregisterProvider: (name: string) => void;
+	/** Emit before_model_select event to all registered handlers. Bound by ExtensionRunner. */
+	emitBeforeModelSelect: (event: Omit<BeforeModelSelectEvent, "type">) => Promise<BeforeModelSelectResult | undefined>;
 }
 
 /**