feat: add model router with fallback chain support

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/models/router.ts

@@ -0,0 +1,58 @@
+import type { ChatRequest, ChatResponse, ModelClient } from './types.js';
+
+export type ModelTier = 'fast' | 'default' | 'complex' | 'local';
+
+export interface ModelRouterConfig {
+  default: ModelClient;
+  fast?: ModelClient;
+  complex?: ModelClient;
+  local?: ModelClient;
+  fallbackChain: ModelClient[];
+}
+
+export class ModelRouter implements ModelClient {
+  private clients: Map<ModelTier, ModelClient>;
+  private defaultClient: ModelClient;
+  private fallbackChain: ModelClient[];
+
+  constructor(config: ModelRouterConfig) {
+    this.clients = new Map();
+    this.defaultClient = config.default;
+    this.fallbackChain = config.fallbackChain;
+
+    this.clients.set('default', config.default);
+    if (config.fast) this.clients.set('fast', config.fast);
+    if (config.complex) this.clients.set('complex', config.complex);
+    if (config.local) this.clients.set('local', config.local);
+  }
+
+  async chat(request: ChatRequest, tier?: ModelTier): Promise<ChatResponse> {
+    const primaryClient = tier ? this.clients.get(tier) ?? this.defaultClient : this.defaultClient;
+    const errors: Error[] = [];
+
+    // Try primary client
+    try {
+      return await primaryClient.chat(request);
+    } catch (error) {
+      errors.push(error instanceof Error ? error : new Error(String(error)));
+      console.warn(`Primary model failed: ${errors[0].message}`);
+    }
+
+    // Try fallback chain
+    for (const fallbackClient of this.fallbackChain) {
+      try {
+        console.log('Trying fallback model...');
+        return await fallbackClient.chat(request);
+      } catch (error) {
+        errors.push(error instanceof Error ? error : new Error(String(error)));
+        console.warn(`Fallback model failed: ${errors[errors.length - 1].message}`);
+      }
+    }
+
+    throw new Error(`All model providers failed: ${errors.map(e => e.message).join(', ')}`);
+  }
+
+  getClient(tier: ModelTier): ModelClient | undefined {
+    return this.clients.get(tier);
+  }
+}