Fix SSE double-wrapping bug when returning pre-formatted Response (#1661)

src/adapter/utils.ts

@@ -152,7 +152,8 @@ export const createStreamHandler =
 	async (
 		generator: Generator | AsyncGenerator | ReadableStream,
 		set?: Context['set'],
-		request?: Request
+		request?: Request,
+		skipFormat?: boolean
 	) => {
 		// Since ReadableStream doesn't have next, init might be undefined
 		let init = (generator as Generator).next?.() as
@@ -171,13 +172,17 @@ export const createStreamHandler =
 			return mapCompactResponse(init.value, request)
 		}
 
+		// Check if stream is from a pre-formatted Response body
 		const isSSE =
-			// @ts-ignore First SSE result is wrapped with sse()
-			init?.value?.sse ??
-			// @ts-ignore ReadableStream is wrapped with sse()
-			generator?.sse ??
-			// User explicitly set content-type to SSE
-			set?.headers['content-type']?.startsWith('text/event-stream')
+			!skipFormat &&
+			(
+				// @ts-ignore First SSE result is wrapped with sse()
+				init?.value?.sse ??
+				// @ts-ignore ReadableStream is wrapped with sse()
+				generator?.sse ??
+				// User explicitly set content-type to SSE
+				set?.headers['content-type']?.startsWith('text/event-stream')
+			)
 
 		const format = isSSE
 			? (data: string) => `data: ${data}\n\n`
@@ -385,7 +390,8 @@ export const createResponseHandler = (handler: CreateHandlerParameter) => {
 				return handleStream(
 					streamResponse(newResponse as Response),
 					responseToSetHeaders(newResponse as Response, set),
-					request
+					request,
+					true // skipFormat: don't auto-format SSE for pre-formatted Response
 				) as any
 
 			return newResponse
@@ -399,7 +405,8 @@ export const createResponseHandler = (handler: CreateHandlerParameter) => {
 			return handleStream(
 				streamResponse(response as Response),
 				responseToSetHeaders(response as Response, set),
-				request
+				request,
+				true // skipFormat: don't auto-format SSE for pre-formatted Response
 			) as any
 
 		return response

test/response/sse-double-wrap.test.ts

@@ -0,0 +1,83 @@
+import { describe, it, expect } from 'bun:test'
+import { Elysia } from '../../src'
+
+describe('SSE - Response Double Wrapping', () => {
+	it('should not double-wrap SSE data when returning pre-formatted Response', async () => {
+		const app = new Elysia().get('/', ({ set }) => {
+			set.headers.hello = 'world'
+
+			return new Response('data: hello\n\ndata: world\n\n', {
+				headers: {
+					'content-type': 'text/event-stream',
+					'transfer-encoding': 'chunked'
+				},
+				status: 200
+			})
+		})
+
+		const response = await app.handle(new Request('http://localhost/')).then(r => r.text())
+
+		// Should NOT double-wrap with "data: data:"
+		expect(response).toBe('data: hello\n\ndata: world\n\n')
+		expect(response).not.toContain('data: data:')
+	})
+
+	it('should not double-wrap SSE when using set.headers with pre-formatted content', async () => {
+		const app = new Elysia().get('/', ({ set }) => {
+			set.headers['x-custom'] = 'test'
+			set.headers['content-type'] = 'text/event-stream'
+
+			return new Response('data: message1\n\ndata: message2\n\n', {
+				headers: {
+					'transfer-encoding': 'chunked'
+				}
+			})
+		})
+
+		const response = await app.handle(new Request('http://localhost/')).then(r => r.text())
+
+		expect(response).toBe('data: message1\n\ndata: message2\n\n')
+		expect(response).not.toContain('data: data:')
+	})
+
+	it('should properly format SSE for generator functions', async () => {
+		const app = new Elysia().get('/', function* () {
+			yield 'hello'
+			yield 'world'
+		})
+
+		const response = await app
+			.handle(new Request('http://localhost/'))
+			.then((r) => r.text())
+
+		// Generator without explicit SSE should format as plain text
+		expect(response).toContain('hello')
+		expect(response).toContain('world')
+		// Verify it's NOT SSE formatted
+		expect(response).not.toContain('data: hello')
+		expect(response).not.toContain('data: world')
+	})
+
+	it('should format SSE correctly for generators with explicit SSE configuration', async () => {
+		const { sse } = await import('../../src')
+
+		const app = new Elysia().get('/', ({ set }) => {
+			set.headers['content-type'] = 'text/event-stream'
+
+			return (async function* () {
+				yield sse({ data: 'first message' })
+				yield sse({ data: 'second message' })
+			})()
+		})
+
+		const response = await app
+			.handle(new Request('http://localhost/'))
+			.then((r) => r.text())
+
+		// Generator WITH explicit SSE markers should get properly formatted
+		expect(response).toContain('data: first message\n\n')
+		expect(response).toContain('data: second message\n\n')
+		// Should NOT double-wrap
+		expect(response).not.toContain('data: data:')
+	})
+})