feat(ai): add OpenRouter provider routing support

Allows custom models to specify which upstream providers OpenRouter
should route requests to via the `openRouterRouting` field in model
definitions.

Supported fields:
- `only`: list of provider slugs to exclusively use
- `order`: list of provider slugs to try in order

packages/ai/src/providers/openai-completions.ts

@@ -445,6 +445,11 @@ function buildParams(model: Model<"openai-completions">, context: Context, optio
 		params.reasoning_effort = options.reasoningEffort;
 	}
 
+	// OpenRouter provider routing preferences
+	if (model.baseUrl.includes("openrouter.ai") && model.compat?.openRouterRouting) {
+		(params as any).provider = model.compat.openRouterRouting;
+	}
+
 	return params;
 }
 
@@ -777,6 +782,7 @@ function detectCompat(model: Model<"openai-completions">): Required<OpenAIComple
 		requiresThinkingAsText: isMistral,
 		requiresMistralToolIds: isMistral,
 		thinkingFormat: isZai ? "zai" : "openai",
+		openRouterRouting: {},
 	};
 }
 
@@ -800,5 +806,6 @@ function getCompat(model: Model<"openai-completions">): Required<OpenAICompletio
 		requiresThinkingAsText: model.compat.requiresThinkingAsText ?? detected.requiresThinkingAsText,
 		requiresMistralToolIds: model.compat.requiresMistralToolIds ?? detected.requiresMistralToolIds,
 		thinkingFormat: model.compat.thinkingFormat ?? detected.thinkingFormat,
+		openRouterRouting: model.compat.openRouterRouting ?? {},
 	};
 }

packages/ai/src/types.ts

@@ -214,6 +214,8 @@ export interface OpenAICompletionsCompat {
 	requiresMistralToolIds?: boolean;
 	/** Format for reasoning/thinking parameter. "openai" uses reasoning_effort, "zai" uses thinking: { type: "enabled" }. Default: "openai". */
 	thinkingFormat?: "openai" | "zai";
+	/** OpenRouter-specific routing preferences. Only used when baseUrl points to OpenRouter. */
+	openRouterRouting?: OpenRouterRouting;
 }
 
 /** Compatibility settings for OpenAI Responses APIs. */
@@ -221,6 +223,18 @@ export interface OpenAIResponsesCompat {
 	// Reserved for future use
 }
 
+/**
+ * OpenRouter provider routing preferences.
+ * Controls which upstream providers OpenRouter routes requests to.
+ * @see https://openrouter.ai/docs/provider-routing
+ */
+export interface OpenRouterRouting {
+	/** List of provider slugs to exclusively use for this request (e.g., ["amazon-bedrock", "anthropic"]). */
+	only?: string[];
+	/** List of provider slugs to try in order (e.g., ["anthropic", "openai"]). */
+	order?: string[];
+}
+
 // Model interface for the unified model system
 export interface Model<TApi extends Api> {
 	id: string;