feat(routing): Initialize model routing architecture (#8153)

packages/core/src/routing/modelRouterService.ts

@@ -0,0 +1,54 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type { Config } from '../config/config.js';
+import type {
+  RoutingContext,
+  RoutingDecision,
+  TerminalStrategy,
+} from './routingStrategy.js';
+import { DefaultStrategy } from './strategies/defaultStrategy.js';
+import { CompositeStrategy } from './strategies/compositeStrategy.js';
+import { FallbackStrategy } from './strategies/fallbackStrategy.js';
+import { OverrideStrategy } from './strategies/overrideStrategy.js';
+
+/**
+ * A centralized service for making model routing decisions.
+ */
+export class ModelRouterService {
+  private config: Config;
+  private strategy: TerminalStrategy;
+
+  constructor(config: Config) {
+    this.config = config;
+    this.strategy = this.initializeDefaultStrategy();
+  }
+
+  private initializeDefaultStrategy(): TerminalStrategy {
+    // Initialize the composite strategy with the desired priority order.
+    // The strategies are ordered in order of highest priority.
+    return new CompositeStrategy(
+      [new FallbackStrategy(), new OverrideStrategy(), new DefaultStrategy()],
+      'agent-router',
+    );
+  }
+
+  /**
+   * Determines which model to use for a given request context.
+   *
+   * @param context The full context of the request.
+   * @returns A promise that resolves to a RoutingDecision.
+   */
+  async route(context: RoutingContext): Promise<RoutingDecision> {
+    const decision = await this.strategy.route(
+      context,
+      this.config,
+      this.config.getBaseLlmClient(),
+    );
+
+    return decision;
+  }
+}