7 files changed, 67 insertions, 18 deletions
diff --git a/.changeset/large-colts-jump.md b/.changeset/large-colts-jump.md
new file mode 100644
index 000000000..faa265319
--- /dev/null
+++ b/.changeset/large-colts-jump.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Fixed an issue where attempting to assign a variable onto locals threw an error.
diff --git a/packages/astro/src/core/endpoint/index.ts b/packages/astro/src/core/endpoint/index.ts
index b62ba8bed..7e6128c37 100644
--- a/packages/astro/src/core/endpoint/index.ts
+++ b/packages/astro/src/core/endpoint/index.ts
@@ -57,7 +57,9 @@ export function createAPIContext({
 		ResponseWithEncoding,
 		url: new URL(request.url),
 		get clientAddress() {
-			if (!(clientAddressSymbol in request)) {
+			if (clientAddressSymbol in request) {
+				return Reflect.get(request, clientAddressSymbol) as string;
+			}
 				if (adapterName) {
 					throw new AstroError({
 						...AstroErrorData.ClientAddressNotAvailable,
@@ -66,26 +68,31 @@ export function createAPIContext({
 				} else {
 					throw new AstroError(AstroErrorData.StaticClientAddressNotAvailable);
 				}
-			}
-
-			return Reflect.get(request, clientAddressSymbol);
 		},
-	} as APIContext;
-
-	// We define a custom property, so we can check the value passed to locals
-	Object.defineProperty(context, 'locals', {
-		enumerable: true,
-		get() {
-			return Reflect.get(request, clientLocalsSymbol);
+		get locals() {
+			let locals = Reflect.get(request, clientLocalsSymbol)
+			
+			if (locals === undefined) {
+				locals = {}
+				Reflect.set(request, clientLocalsSymbol, locals)
+			}
+			
+			if (typeof locals !== 'object') {
+				throw new AstroError(AstroErrorData.LocalsNotAnObject);
+			}
+			
+			return locals;
 		},
-		set(val) {
+		// We define a custom property, so we can check the value passed to locals
+		set locals(val) {
 			if (typeof val !== 'object') {
 				throw new AstroError(AstroErrorData.LocalsNotAnObject);
 			} else {
 				Reflect.set(request, clientLocalsSymbol, val);
 			}
 		},
-	});
+	} satisfies APIContext;
+
 	return context;
 }
 
diff --git a/packages/integrations/node/test/fixtures/locals/src/middleware.ts b/packages/integrations/node/test/fixtures/locals/src/middleware.ts
new file mode 100644
index 000000000..e349ca41d
--- /dev/null
+++ b/packages/integrations/node/test/fixtures/locals/src/middleware.ts
@@ -0,0 +1,6 @@
+import { defineMiddleware } from 'astro:middleware';
+
+export const onRequest = defineMiddleware(({ url, locals }, next) => {
+    if (url.pathname === "/from-astro-middleware") locals.foo = "baz";
+    return next();
+})
diff --git a/packages/integrations/node/test/fixtures/locals/src/pages/api.js b/packages/integrations/node/test/fixtures/locals/src/pages/api.js
index 8b209c582..3c279e37b 100644
--- a/packages/integrations/node/test/fixtures/locals/src/pages/api.js
+++ b/packages/integrations/node/test/fixtures/locals/src/pages/api.js
@@ -1,6 +1,6 @@
 
-export async function post({ locals }) {
-	let out = { ...locals };
+export async function POST({ locals }) {
+	const out = { ...locals };
 
 	return new Response(JSON.stringify(out), {
 		headers: {
diff --git a/packages/integrations/node/test/fixtures/locals/src/pages/foo.astro b/packages/integrations/node/test/fixtures/locals/src/pages/from-astro-middleware.astro
index 224a875ec..224a875ec 100644
--- a/packages/integrations/node/test/fixtures/locals/src/pages/foo.astro
+++ b/packages/integrations/node/test/fixtures/locals/src/pages/from-astro-middleware.astro
diff --git a/packages/integrations/node/test/fixtures/locals/src/pages/from-node-middleware.astro b/packages/integrations/node/test/fixtures/locals/src/pages/from-node-middleware.astro
new file mode 100644
index 000000000..224a875ec
--- /dev/null
+++ b/packages/integrations/node/test/fixtures/locals/src/pages/from-node-middleware.astro
@@ -0,0 +1,4 @@
+---
+const { foo } = Astro.locals;
+---
+<h1>{foo}</h1>
diff --git a/packages/integrations/node/test/locals.test.js b/packages/integrations/node/test/locals.test.js
index f7fc6b73f..b593f3eb9 100644
--- a/packages/integrations/node/test/locals.test.js
+++ b/packages/integrations/node/test/locals.test.js
@@ -15,11 +15,10 @@ describe('API routes', () => {
 		await fixture.build();
 	});
 
-	it('Can render locals in page', async () => {
+	it('Can use locals added by node middleware', async () => {
 		const { handler } = await import('./fixtures/locals/dist/server/entry.mjs');
 		let { req, res, text } = createRequestAndResponse({
-			method: 'POST',
-			url: '/foo',
+			url: '/from-node-middleware',
 		});
 
 		let locals = { foo: 'bar' };
@@ -32,6 +31,34 @@ describe('API routes', () => {
 		expect(html).to.contain('<h1>bar</h1>');
 	});
 
+	it('Throws an error when provided non-objects as locals', async () => {
+		const { handler } = await import('./fixtures/locals/dist/server/entry.mjs');
+		let { req, res, done } = createRequestAndResponse({
+			url: '/from-node-middleware',
+		});
+
+		handler(req, res, undefined, "locals");
+		req.send();
+
+		await done;
+		expect(res).to.deep.include({ statusCode: 500 });
+	});
+
+	it('Can use locals added by astro middleware', async () => {
+		const { handler } = await import('./fixtures/locals/dist/server/entry.mjs');
+		
+		const { req, res, text } = createRequestAndResponse({
+			url: '/from-astro-middleware',
+		});
+
+		handler(req, res, () => {});
+		req.send();
+
+		const html = await text();
+
+		expect(html).to.contain('<h1>baz</h1>');
+	});
+
 	it('Can access locals in API', async () => {
 		const { handler } = await import('./fixtures/locals/dist/server/entry.mjs');
 		let { req, res, done } = createRequestAndResponse({