feat: add support for auto-router plugin (#403)

## Description

Closes #392

Adds the `auto-router` plugin type to the `plugins` configuration, enabling users to configure allowed models when using `openrouter/auto`.

**Before:** Using `{ id: 'auto-router' }` in the plugins array was not accepted by the TypeScript types.

**After:**
```ts
const model = openrouter.chat('openrouter/auto', {
  plugins: [
    { id: 'auto-router', allowed_models: ['anthropic/*', 'openai/gpt-5.1'] },
  ],
});
```

### Changes
- Added `IdAutoRouter` type literal (`'auto-router'`) in `openrouter-api-types.ts`
- Added auto-router variant to the `plugins` union type in `openrouter-chat-settings.ts` with optional `allowed_models: string[]`
- Added three unit tests covering: with `allowed_models`, without `allowed_models`, and combined with other plugins
- Added e2e regression test (`e2e/issues/issue-392-auto-router-plugin.test.ts`) — verified against live API with `generateText` and `streamText`

No runtime behavior changes — plugins are passed through directly to the API via `getArgs`.

### Cross-referenced with `openrouter-web`
Schema verified against `packages/llm-interfaces/plugins/auto-router/schemas.ts` in `openrouter-web`. The `id` and `allowed_models` fields match. The `enabled?: boolean` field present in `openrouter-web` is intentionally omitted here for consistency — no other plugin type in this SDK exposes `enabled`.

### Reviewer checklist
- [ ] Confirm `allowed_models` property name matches the [OpenRouter auto-router API](https://openrouter.ai/docs/guides/routing/routers/auto-router)
- [ ] Decide whether `enabled?: boolean` should be added (exists in `openrouter-web` for auto-router, web, file-parser, response-healing — but is absent from all plugin types in this SDK today)
- [ ] Check if the auto-router plugin has any additional properties beyond `allowed_models` that should be included

## Checklist

- [x] I have run `pnpm stylecheck` and `pnpm typecheck`
- [x] I have run `pnpm test` and all tests pass
- [x] I have added tests for my changes (if applicable)
- [ ] I have updated documentation (if applicable) — N/A, types are self-documenting with JSDoc

### Changeset

- [x] I have run `pnpm changeset` to create a changeset file

---

[Link to Devin run](https://app.devin.ai/sessions/59f40b83460544bbbf789dabbc3ab1d8) | Requested by: @robert-j-y

Author: robert-j-y

.changeset/fluffy-buses-attend.md

@@ -0,0 +1,5 @@
+---
+'@openrouter/ai-sdk-provider': patch
+---
+
+Add support for `auto-router` plugin to configure allowed models when using `openrouter/auto`

e2e/issues/issue-392-auto-router-plugin.test.ts

@@ -0,0 +1,84 @@
+/**
+ * Regression test for GitHub issue #392
+ * https://github.com/OpenRouterTeam/ai-sdk-provider/issues/392
+ *
+ * Issue: "Add support for auto-router plugin"
+ *
+ * Issue thread timeline:
+ * - Feb 4, 2026: User requests support for the `auto-router` plugin
+ *   to configure allowed models when using `openrouter/auto`.
+ */
+import { generateText, streamText } from 'ai';
+import { describe, expect, it, vi } from 'vitest';
+import { createOpenRouter } from '@/src';
+
+vi.setConfig({
+  testTimeout: 60_000,
+});
+
+describe('Issue #392: auto-router plugin support', () => {
+  const openrouter = createOpenRouter({
+    apiKey: process.env.OPENROUTER_API_KEY,
+    baseUrl: `${process.env.OPENROUTER_API_BASE}/api/v1`,
+  });
+
+  it('should generate text with openrouter/auto and auto-router plugin with allowed_models', async () => {
+    const model = openrouter('openrouter/auto', {
+      plugins: [
+        {
+          id: 'auto-router',
+          allowed_models: ['anthropic/*', 'openai/*'],
+        },
+      ],
+      usage: { include: true },
+    });
+
+    const response = await generateText({
+      model,
+      prompt: 'What is 2+2? Reply with just the number.',
+    });
+
+    expect(response.text).toBeDefined();
+    expect(response.text.length).toBeGreaterThan(0);
+    expect(response.usage.totalTokens).toBeGreaterThan(0);
+    expect(response.finishReason).toBeDefined();
+  });
+
+  it('should generate text with openrouter/auto and auto-router plugin without allowed_models', async () => {
+    const model = openrouter('openrouter/auto', {
+      plugins: [{ id: 'auto-router' }],
+      usage: { include: true },
+    });
+
+    const response = await generateText({
+      model,
+      prompt: 'What is 2+2? Reply with just the number.',
+    });
+
+    expect(response.text).toBeDefined();
+    expect(response.text.length).toBeGreaterThan(0);
+    expect(response.usage.totalTokens).toBeGreaterThan(0);
+    expect(response.finishReason).toBeDefined();
+  });
+
+  it('should stream text with openrouter/auto and auto-router plugin with allowed_models', async () => {
+    const model = openrouter('openrouter/auto', {
+      plugins: [
+        {
+          id: 'auto-router',
+          allowed_models: ['anthropic/*', 'openai/*'],
+        },
+      ],
+      usage: { include: true },
+    });
+
+    const response = streamText({
+      model,
+      prompt: 'What is 2+2? Reply with just the number.',
+    });
+
+    const result = await response.text;
+    expect(result).toBeDefined();
+    expect(result.length).toBeGreaterThan(0);
+  });
+});

src/chat/index.test.ts

@@ -818,6 +818,76 @@ describe('doGenerate', () => {
     });
   });
 
+  it('should pass auto-router plugin with allowed_models in request payload', async () => {
+    prepareJsonResponse({ content: 'Hello from auto-selected model' });
+
+    const autoModel = provider.chat('openrouter/auto', {
+      plugins: [
+        {
+          id: 'auto-router',
+          allowed_models: ['anthropic/*', 'openai/gpt-5.1'],
+        },
+      ],
+    });
+
+    await autoModel.doGenerate({
+      prompt: TEST_PROMPT,
+    });
+
+    expect(await server.calls[0]!.requestBodyJson).toStrictEqual({
+      model: 'openrouter/auto',
+      messages: [{ role: 'user', content: 'Hello' }],
+      plugins: [
+        {
+          id: 'auto-router',
+          allowed_models: ['anthropic/*', 'openai/gpt-5.1'],
+        },
+      ],
+    });
+  });
+
+  it('should pass auto-router plugin without allowed_models in request payload', async () => {
+    prepareJsonResponse({ content: 'Hello from auto-selected model' });
+
+    const autoModel = provider.chat('openrouter/auto', {
+      plugins: [{ id: 'auto-router' }],
+    });
+
+    await autoModel.doGenerate({
+      prompt: TEST_PROMPT,
+    });
+
+    expect(await server.calls[0]!.requestBodyJson).toStrictEqual({
+      model: 'openrouter/auto',
+      messages: [{ role: 'user', content: 'Hello' }],
+      plugins: [{ id: 'auto-router' }],
+    });
+  });
+
+  it('should pass auto-router plugin combined with other plugins', async () => {
+    prepareJsonResponse({ content: 'Hello from auto-selected model' });
+
+    const autoModel = provider.chat('openrouter/auto', {
+      plugins: [
+        { id: 'auto-router', allowed_models: ['anthropic/*'] },
+        { id: 'web' },
+      ],
+    });
+
+    await autoModel.doGenerate({
+      prompt: TEST_PROMPT,
+    });
+
+    expect(await server.calls[0]!.requestBodyJson).toStrictEqual({
+      model: 'openrouter/auto',
+      messages: [{ role: 'user', content: 'Hello' }],
+      plugins: [
+        { id: 'auto-router', allowed_models: ['anthropic/*'] },
+        { id: 'web' },
+      ],
+    });
+  });
+
   it('should pass images', async () => {
     prepareJsonResponse({
       content: '',

src/types/openrouter-api-types.ts

@@ -35,6 +35,13 @@ export type IdModeration = 'moderation';
  */
 export type IdResponseHealing = 'response-healing';
 
+/**
+ * Plugin identifier for auto-router model selection.
+ * Configures allowed models when using the openrouter/auto model.
+ * @see https://openrouter.ai/docs/guides/routing/routers/auto-router
+ */
+export type IdAutoRouter = 'auto-router';
+
 /**
  * Search engine options for web search.
  * Open enum - accepts known values or any string for forward compatibility.

src/types/openrouter-chat-settings.ts

@@ -2,6 +2,7 @@ import type { OpenRouterSharedSettings } from '..';
 import type {
   DataCollection,
   Engine,
+  IdAutoRouter,
   IdFileParser,
   IdModeration,
   IdResponseHealing,
@@ -87,6 +88,18 @@ monitor and detect abuse. Learn more.
          */
         id: IdResponseHealing;
       }
+    | {
+        /**
+         * Auto-router plugin - configures allowed models when using `openrouter/auto`.
+         *
+         * Use wildcard patterns to restrict which models the auto router can select from.
+         * When no `allowed_models` are specified, the auto router uses all supported models.
+         *
+         * @see https://openrouter.ai/docs/guides/routing/routers/auto-router
+         */
+        id: IdAutoRouter;
+        allowed_models?: string[];
+      }
   >;
 
   /**