How are AI Gateway model strings converted to language models?

how does the ai gateway string, "anthorpic/claude-sonnet-4.5" get converted into a usable langauge model? does it happen inside streamText?

why isn't it the same behavior for other providers? like could it work with "bedrock:anthoric/..." potentially?

Now I have a complete picture. Let me summarize what I've found:

---

##How "anthropic/claude-sonnet-4.5" Gets Converted to a Usable Model

###The Flow

1. In streamText (or generateText, etc.): When you pass a string like "anthropic/claude-sonnet-4.5" to streamText, it calls resolveLanguageModel():

   // packages/ai/src/generate-text/stream-text.ts:465
   model: resolveLanguageModel(model),
   

2. In resolveLanguageModel (packages/ai/src/model/resolve-model.ts):

   export function resolveLanguageModel(model: LanguageModel): LanguageModelV3 {
     if (typeof model !== 'string') {
       // If it's already a model object, use it directly
       return asLanguageModelV3(model);
     }
     // If it's a string, use the global provider
     return getGlobalProvider().languageModel(model);
   }
   
   function getGlobalProvider(): ProviderV3 {
     return globalThis.AI_SDK_DEFAULT_PROVIDER ?? gateway;  // defaults to gateway!
   }
   

3. The gateway is the default global provider (packages/gateway/src/gateway-provider.ts):

   export const gateway = createGatewayProvider();
   

4. The GatewayLanguageModel doesn't parse the string locally — it just sends it to Vercel's AI Gateway API as a header:

   // packages/gateway/src/gateway-language-model.ts:200-205
   private getModelConfigHeaders(modelId: string, streaming: boolean) {
     return {
       'ai-language-model-specification-version': '3',
       'ai-language-model-id': modelId,  // <-- sent as a header!
       'ai-language-model-streaming': String(streaming),
     };
   }
   

   The actual model resolution (routing to Anthropic, OpenAI, etc.) happens server-side at https://ai-gateway.vercel.sh/v3/ai.

---

##Why Doesn't "bedrock:anthropic/..." Work The Same Way?

It's a design difference:

1. AI Gateway strings (like "anthropic/claude-sonnet-4.5") are interpreted by Vercel's Gateway server. The SDK just forwards them as-is.

2. Provider registry strings (like "bedrock:anthropic.claude-v2") use a local separator-based routing mechanism via createProviderRegistry():

   // packages/ai/src/registry/provider-registry.ts:204-217
   private splitId(id: string, modelType): [string, string] {
     const index = id.indexOf(this.separator);  // default separator is ':'
     // ...
     return [id.slice(0, index), id.slice(index + this.separator.length)];
   }
   

   This requires you to explicitly register providers in a registry first:

   const registry = createProviderRegistry({
     bedrock: amazonBedrock,
     openai: openai,
   });
   
   registry.languageModel('bedrock:anthropic.claude-v2');
   

###Could "bedrock:anthropic/..." Work?

Not automatically with the current design. You have two options:

1. Use a provider registry:

   const registry = createProviderRegistry({
     bedrock: amazonBedrock,
   });
   
   streamText({
     model: registry.languageModel('bedrock:anthropic.claude-v2'),
     // ...
   });
   

2. Set a custom global provider:

   import { amazonBedrock } from '@ai-sdk/amazon-bedrock';
   
   globalThis.AI_SDK_DEFAULT_PROVIDER = amazonBedrock;
   
   // Now string model IDs go to Bedrock
   streamText({
     model: 'anthropic.claude-v2',  // interpreted by Bedrock
   });
   

3. Or wait for the AI Gateway to add Bedrock support — if Vercel's Gateway supported Bedrock, you could potentially use "bedrock/anthropic.claude-v2" the same way you use "anthropic/claude-sonnet-4.5".

---

###Key Takeaway

The magic of "anthropic/claude-sonnet-4.5" isn't in streamText — it's that the Vercel AI Gateway is the default global provider, and the Gateway server handles the actual provider/model resolution. Other providers require explicit instantiation or registry setup because there's no centralized server doing the routing for them.