fix(ai): honor OpenAI Responses developer role compat

closes #5456

packages/ai/src/providers/openai-responses-shared.ts

@@ -124,7 +124,8 @@ export function convertResponsesMessages<TApi extends Api>(
 
 	const includeSystemPrompt = options?.includeSystemPrompt ?? true;
 	if (includeSystemPrompt && context.systemPrompt) {
-		const role = model.reasoning ? "developer" : "system";
+		const compat = model.compat as { supportsDeveloperRole?: boolean } | undefined;
+		const role = model.reasoning && compat?.supportsDeveloperRole !== false ? "developer" : "system";
 		messages.push({
 			role,
 			content: sanitizeSurrogates(context.systemPrompt),

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

@@ -39,6 +39,7 @@ function resolveCacheRetention(cacheRetention?: CacheRetention): CacheRetention
 
 function getCompat(model: Model<"openai-responses">): Required<OpenAIResponsesCompat> {
 	return {
+		supportsDeveloperRole: model.compat?.supportsDeveloperRole ?? true,
 		sendSessionIdHeader: model.compat?.sendSessionIdHeader ?? true,
 		supportsLongCacheRetention: model.compat?.supportsLongCacheRetention ?? true,
 	};

packages/ai/src/types.ts

@@ -421,6 +421,8 @@ export interface OpenAICompletionsCompat {
 
 /** Compatibility settings for OpenAI Responses APIs. */
 export interface OpenAIResponsesCompat {
+	/** Whether the provider supports the `developer` role (vs `system`). Default: true. */
+	supportsDeveloperRole?: boolean;
 	/** Whether to send the OpenAI `session_id` cache-affinity header from `options.sessionId` when caching is enabled. Default: true. */
 	sendSessionIdHeader?: boolean;
 	/** Whether the provider supports `prompt_cache_retention: "24h"`. Default: true. */