feat(vercel): enable streaming support out of the box

src/presets/vercel.ts

@@ -5,7 +5,10 @@ import { withoutLeadingSlash } from "ufo";
 import { writeFile } from "../utils";
 import { defineNitroPreset } from "../preset";
 import type { Nitro } from "../types";
-import type { VercelBuildConfigV3 } from "../types/presets";
+import type {
+  VercelBuildConfigV3,
+  VercelServerlessFunctionConfig,
+} from "../types/presets";
 
 // https://vercel.com/docs/build-output-api/v3
 
@@ -36,11 +39,12 @@ export const vercel = defineNitroPreset({
         nitro.options.output.serverDir,
         ".vc-config.json"
       );
-      const functionConfig = {
+      const functionConfig: VercelServerlessFunctionConfig = {
         runtime: runtimeVersion,
         handler: "index.mjs",
         launcherType: "Nodejs",
         shouldAddHelpers: false,
+        supportsResponseStreaming: true,
         ...nitro.options.vercel?.functions,
       };
       await writeFile(

src/types/presets.ts

@@ -52,18 +52,43 @@ export interface VercelBuildConfigV3 {
   }[];
 }
 
+/**
+ * https://vercel.com/docs/build-output-api/v3/primitives#serverless-function-configuration
+ */
+export interface VercelServerlessFunctionConfig {
+  /**
+   * Amount of memory (RAM in MB) that will be allocated to the Serverless Function.
+   */
+  memory?: number;
+
+  /**
+   * Maximum execution duration (in seconds) that will be allowed for the Serverless Function.
+   */
+  maxDuration?: number;
+
+  /**
+   * True if a custom runtime has support for Lambda runtime wrappers.
+   */
+  supportsWrapper?: boolean;
+
+  /**
+   * When true, the Serverless Function will stream the response to the client.
+   */
+  supportsResponseStreaming?: boolean;
+
+  [key: string]: unknown;
+}
+
 export interface PresetOptions {
   vercel: {
     config: VercelBuildConfigV3;
+
     /**
      * If you are using `vercel-edge`, you can specify the region(s) for your edge function.
      * @see https://vercel.com/docs/concepts/functions/edge-functions#edge-function-regions
      */
     regions?: string[];
-    functions?: {
-      memory: number;
-      maxDuration: number;
-      [key: string]: unknown;
-    };
+
+    functions?: VercelServerlessFunctionConfig;
   };
 }