fix: prevent isr routes from handling remote function calls (#15085) (#15098)

* fix: prevent isr routes from handling remote function calls (#15085)

* add changeset for observability changes

Author: arichardsmith

.changeset/brown-baboons-drop.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': patch
+---
+
+feat: show remote function calls under the /_app/remote route in observability

.changeset/full-points-battle.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': patch
+---
+
+fix: prevent isr routes from handling remote function calls

packages/adapter-vercel/index.js

@@ -379,6 +379,31 @@ const plugin = function (defaults = {}) {
 				);
 			}
 
+			if (builder.config.kit.experimental.remoteFunctions) {
+				// Ensure remote functions are always handled by the catchall route, which will be symlinked to /_app/remote.
+				// This stops them from being affected by ISR config from other routes that match /[...rest] (ref: #15085)
+				// and also makes them show as handled by `/_app/remote` in Vercel's observability.
+
+				const app_path = builder.getAppPath();
+				const remote_dir = path.join(dirs.functions, app_path, 'remote'); // Usually .vercel/output/functions/_app/remote
+				const remote_symlink_path = `${remote_dir}.func`;
+
+				// Handle remote functions with the catchall route as it won't have any ISR settings
+				const target = path.join(dirs.functions, INTERNAL, 'catchall.func');
+
+				// Ensure the parent directory exists before symlinking
+				builder.mkdirp(path.join(dirs.functions, app_path));
+
+				const relative = path.relative(path.dirname(remote_symlink_path), target);
+
+				fs.symlinkSync(relative, remote_symlink_path);
+
+				static_config.routes.push({
+					src: `/${app_path}/remote/.+`,
+					dest: `/${app_path}/remote` // Maps to /![-]/catchall via the symlink
+				});
+			}
+
 			for (const route of builder.routes) {
 				if (is_prerendered(route)) continue;