diff --git a/packages/core/test/pty/pty-session.test.ts b/packages/core/test/pty/pty-session.test.ts
index f74abf88a80..a238ffb579c 100644
--- a/packages/core/test/pty/pty-session.test.ts
+++ b/packages/core/test/pty/pty-session.test.ts
@@ -80,17 +80,19 @@ const attachCollecting = Effect.fn("PtySessionTest.attachCollecting")(function*
   return { attachment, output, ended }
 })
 
-const waitForOutput = (output: Queue.Queue<string>, text: string) =>
-  Effect.gen(function* () {
-    let received = ""
+const waitForOutput = (output: Queue.Queue<string>, text: string) => {
+  let received = ""
+  const pull = Effect.gen(function* () {
     while (!received.includes(text)) received += yield* Queue.take(output)
     return received
-  }).pipe(
+  })
+  return pull.pipe(
     Effect.timeoutOrElse({
       duration: PTY_TEST_TIMEOUT, // kilocode_change
-      orElse: () => Effect.fail(new Error(`timeout waiting for output containing ${JSON.stringify(text)}`)),
+      orElse: () => Effect.fail(new Error(`timeout waiting for output containing ${JSON.stringify(text)}, received ${JSON.stringify(received)}`)),
     }),
   )
+}
 
 describe("pty", () => {
   it.live("returns typed not found errors for missing sessions", () =>
@@ -143,11 +145,13 @@ describe("pty", () => {
   )
 
   // (script terminals forward raw output to xterm without transcoding).
+  // The child must outlive its output: an immediate exit can race bun-pty's
+  // reader thread and drop trailing bytes under load, so print then sleep.
   ptyTest("round-trips non-ASCII output byte-identically", () =>
     Effect.gen(function* () {
       const pty = yield* Pty.Service
       const marker = "café-über-北京-🚀"
-      const info = yield* createPty("sh", ["-c", "printf 'caf\\303\\251-\\303\\274ber-\\345\\214\\227\\344\\272\\254-\\360\\237\\232\\200\\n'"])
+      const info = yield* createPty("sh", ["-c", "printf 'caf\\303\\251-\\303\\274ber-\\345\\214\\227\\344\\272\\254-\\360\\237\\232\\200\\n'; sleep 5"])
       const attached = yield* attachCollecting(info.id)
       expect(yield* waitForOutput(attached.output, marker)).toContain(marker)
     }),