fix(core): support LanguageModelV3 in MastraModelGateway.resolveLanguageModel (#11489)

## Summary

This PR enables custom gateways to return either `LanguageModelV2` (AI
SDK v5) or `LanguageModelV3` (AI SDK v6) from the `resolveLanguageModel`
method.

## Changes

- **`packages/core/src/llm/model/gateways/base.ts`**
- Added `GatewayLanguageModel` union type (`LanguageModelV2 |
LanguageModelV3`)
- Updated `MastraModelGateway.resolveLanguageModel` abstract method to
return `GatewayLanguageModel`

- **`packages/core/src/llm/model/router.ts`**
  - Added `isLanguageModelV3` type guard function
- Updated `doGenerate` and `doStream` methods to detect V3 models and
wrap them with `AISDKV6LanguageModel`
- Updated internal `resolveLanguageModel` return type and
`modelInstances` Map to use `GatewayLanguageModel`

- **`packages/core/src/llm/model/gateways/index.ts`** &
**`packages/core/src/llm/index.ts`**
  - Exported `GatewayLanguageModel` type for external use

## Example Usage

```typescript
import { MastraModelGateway, type GatewayLanguageModel } from '@mastra/core/llm';
import { createOpenAICompatible } from '@ai-sdk/openai-compatible'; // v6

class CustomGateway extends MastraModelGateway {
  async resolveLanguageModel({ modelId, providerId, apiKey }): Promise<GatewayLanguageModel> {
    // Now works with both V2 and V3!
    return createOpenAICompatible({
      name: providerId,
      apiKey,
      baseURL: 'https://...',
    }).chatModel(modelId);
  }
}
```

## Testing

- All 115 gateway and router tests pass ✅
- Type check passes ✅

Fixes #11443

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
  * Added support for AI SDK v6 language models in the core runtime.
* Routing and model resolution updated to handle mixed v5/v6 models
while preserving full backward compatibility with existing v5 workflows.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Abhi Aiyer <abhiaiyer91@gmail.com>

Author: TheIsrael1

.changeset/giant-maps-kiss.md

@@ -0,0 +1,5 @@
+---
+"@mastra/core": patch
+---
+
+fix(core): support LanguageModelV3 in MastraModelGateway.resolveLanguageModel

packages/core/src/llm/index.ts

@@ -148,7 +148,7 @@ export type LLMStreamObjectOptions<Z extends ZodSchema | JSONSchema7 | undefined
 } & LLMInnerStreamOptions<Z> &
   DefaultLLMStreamObjectOptions;
 
-export type { ProviderConfig } from './model/gateways/base';
+export type { ProviderConfig, GatewayLanguageModel } from './model/gateways/base';
 export { MastraModelGateway, NetlifyGateway, ModelsDevGateway, AzureOpenAIGateway } from './model/gateways';
 export type { AzureOpenAIGatewayConfig } from './model/gateways';
 

packages/core/src/llm/model/gateways/base.ts

@@ -4,6 +4,7 @@
  */
 
 import type { LanguageModelV2 } from '@ai-sdk/provider-v5';
+import type { LanguageModelV3 } from '@ai-sdk/provider-v6';
 
 export interface ProviderConfig {
   url?: string;
@@ -15,6 +16,12 @@ export interface ProviderConfig {
   gateway: string;
 }
 
+/**
+ * Union type for language models that can be returned by gateways.
+ * Supports both AI SDK v5 (LanguageModelV2) and v6 (LanguageModelV3).
+ */
+export type GatewayLanguageModel = LanguageModelV2 | LanguageModelV3;
+
 export abstract class MastraModelGateway {
   /**
    * Unique identifier for the gateway
@@ -51,10 +58,14 @@ export abstract class MastraModelGateway {
 
   abstract getApiKey(modelId: string): Promise<string>;
 
+  /**
+   * Resolve a language model from the gateway.
+   * Supports returning either LanguageModelV2 (AI SDK v5) or LanguageModelV3 (AI SDK v6).
+   */
   abstract resolveLanguageModel(args: {
     modelId: string;
     providerId: string;
     apiKey: string;
     headers?: Record<string, string>;
-  }): Promise<LanguageModelV2> | LanguageModelV2;
+  }): Promise<GatewayLanguageModel> | GatewayLanguageModel;
 }

packages/core/src/llm/model/gateways/index.ts

@@ -1,6 +1,6 @@
 import { MastraError } from '../../../error/index.js';
 import type { MastraModelGateway } from './base.js';
-export { MastraModelGateway, type ProviderConfig } from './base.js';
+export { MastraModelGateway, type ProviderConfig, type GatewayLanguageModel } from './base.js';
 export { AzureOpenAIGateway, type AzureOpenAIGatewayConfig } from './azure.js';
 export { ModelsDevGateway } from './models-dev.js';
 export { NetlifyGateway } from './netlify.js';

packages/core/src/llm/model/router.ts

@@ -1,9 +1,11 @@
 import { createHash } from 'node:crypto';
 import { createOpenAICompatible } from '@ai-sdk/openai-compatible-v5';
 import type { LanguageModelV2, LanguageModelV2CallOptions, LanguageModelV2StreamPart } from '@ai-sdk/provider-v5';
+import type { LanguageModelV3 } from '@ai-sdk/provider-v6';
 import { AISDKV5LanguageModel } from './aisdk/v5/model';
+import { AISDKV6LanguageModel } from './aisdk/v6/model';
 import { parseModelRouterId } from './gateway-resolver.js';
-import type { MastraModelGateway } from './gateways/base.js';
+import type { GatewayLanguageModel, MastraModelGateway } from './gateways/base.js';
 import { findGatewayForModel } from './gateways/index.js';
 
 import { ModelsDevGateway } from './gateways/models-dev.js';
@@ -12,6 +14,13 @@ import type { ModelRouterModelId } from './provider-registry.js';
 import { PROVIDER_REGISTRY } from './provider-registry.js';
 import type { MastraLanguageModelV2, OpenAICompatibleConfig } from './shared.types';
 
+/**
+ * Type guard to check if a model is a LanguageModelV3 (AI SDK v6)
+ */
+function isLanguageModelV3(model: GatewayLanguageModel): model is LanguageModelV3 {
+  return model.specificationVersion === 'v3';
+}
+
 type StreamResult = Awaited<ReturnType<LanguageModelV2['doStream']>>;
 
 function getStaticProvidersByGateway(name: string) {
@@ -127,6 +136,12 @@ export class ModelRouterLanguageModel implements MastraLanguageModelV2 {
       ...parseModelRouterId(this.config.routerId, gatewayPrefix),
     });
 
+    // Handle both V2 and V3 models
+    if (isLanguageModelV3(model)) {
+      const aiSDKV6Model = new AISDKV6LanguageModel(model);
+      // Cast V3 stream result to V2 format - the stream contents are compatible at runtime
+      return aiSDKV6Model.doGenerate(options as any) as unknown as Promise<StreamResult>;
+    }
     const aiSDKV5Model = new AISDKV5LanguageModel(model);
     return aiSDKV5Model.doGenerate(options);
   }
@@ -164,6 +179,12 @@ export class ModelRouterLanguageModel implements MastraLanguageModelV2 {
       ...parseModelRouterId(this.config.routerId, gatewayPrefix),
     });
 
+    // Handle both V2 and V3 models
+    if (isLanguageModelV3(model)) {
+      const aiSDKV6Model = new AISDKV6LanguageModel(model);
+      // Cast V3 stream result to V2 format - the stream contents are compatible at runtime
+      return aiSDKV6Model.doStream(options as any) as unknown as Promise<StreamResult>;
+    }
     const aiSDKV5Model = new AISDKV5LanguageModel(model);
     return aiSDKV5Model.doStream(options);
   }
@@ -178,7 +199,7 @@ export class ModelRouterLanguageModel implements MastraLanguageModelV2 {
     providerId: string;
     apiKey: string;
     headers?: Record<string, string>;
-  }): Promise<LanguageModelV2> {
+  }): Promise<GatewayLanguageModel> {
     const key = createHash('sha256')
       .update(
         this.gateway.id +
@@ -208,5 +229,5 @@ export class ModelRouterLanguageModel implements MastraLanguageModelV2 {
     ModelRouterLanguageModel.modelInstances.set(key, modelInstance);
     return modelInstance;
   }
-  private static modelInstances = new Map<string, LanguageModelV2>();
+  private static modelInstances = new Map<string, GatewayLanguageModel>();
 }