diff --git a/src/config/schema.test.ts b/src/config/schema.test.ts
index 7415888..fa7b703 100644
--- a/src/config/schema.test.ts
+++ b/src/config/schema.test.ts
@@ -89,6 +89,57 @@ describe('configSchema — routing', () => {
   });
 });
 
+describe('configSchema — per-tier fallback', () => {
+  const minimalConfig = {
+    telegram: { bot_token: 'test', allowed_chat_ids: [1] },
+    models: { default: { provider: 'anthropic', model: 'claude-3' } },
+  };
+
+  it('accepts per-tier fallback config', () => {
+    const result = configSchema.parse({
+      ...minimalConfig,
+      models: {
+        default: {
+          provider: 'anthropic',
+          model: 'claude-sonnet-4-5-20250929',
+          fallback: { provider: 'github', model: 'claude-sonnet-4-5-20250929' },
+        },
+        fast: {
+          provider: 'anthropic',
+          model: 'claude-haiku-4-5-20251001',
+          fallback: { provider: 'github', model: 'claude-haiku-4-5-20251001' },
+        },
+      },
+    });
+    expect(result.models.default.fallback?.provider).toBe('github');
+    expect(result.models.fast?.fallback?.provider).toBe('github');
+  });
+
+  it('works without fallback field (backward compat)', () => {
+    const result = configSchema.parse(minimalConfig);
+    expect(result.models.default.fallback).toBeUndefined();
+  });
+
+  it('fallback does not itself accept a nested fallback', () => {
+    const result = configSchema.parse({
+      ...minimalConfig,
+      models: {
+        default: {
+          provider: 'anthropic',
+          model: 'claude-3',
+          fallback: {
+            provider: 'github',
+            model: 'claude-3',
+            fallback: { provider: 'ollama', model: 'llama' },
+          },
+        },
+      },
+    });
+    // Zod strips unknown keys from the base schema, so nested fallback is dropped
+    expect((result.models.default.fallback as Record<string, unknown>)?.fallback).toBeUndefined();
+  });
+});
+
 describe('configSchema automation', () => {
   const baseConfig = {
     telegram: { bot_token: 'test-token', allowed_chat_ids: [123] },
diff --git a/src/config/schema.ts b/src/config/schema.ts
index 642c842..26e0f6d 100644
--- a/src/config/schema.ts
+++ b/src/config/schema.ts
@@ -18,7 +18,7 @@ const serverSchema = z.object({
   auth_http: z.boolean().default(true),
 });
 
-const modelConfigSchema = z.object({
+const modelConfigBaseSchema = z.object({
   provider: z.enum(['anthropic', 'openai', 'gemini', 'ollama', 'llamacpp', 'openrouter', 'bedrock', 'github']),
   model: z.string(),
   endpoint: z.string().optional(),
@@ -29,6 +29,10 @@ const modelConfigSchema = z.object({
   context_window: z.number().optional(),
 });
 
+const modelConfigSchema = modelConfigBaseSchema.extend({
+  fallback: modelConfigBaseSchema.optional(),
+});
+
 const modelsSchema = z.object({
   local: modelConfigSchema.optional(),
   fast: modelConfigSchema.optional(),