diff --git a/actions/setup/js/copilot_driver.cjs b/actions/setup/js/copilot_driver.cjs
index a66511be843..e0bf2c038c8 100644
--- a/actions/setup/js/copilot_driver.cjs
+++ b/actions/setup/js/copilot_driver.cjs
@@ -62,15 +62,7 @@ const MODEL_NOT_SUPPORTED_PATTERN = /The requested model is not supported/;
 const NO_AUTH_INFO_PATTERN = /No authentication information found/;
 
 /**
- * @typedef {(path: import("node:fs").PathOrFileDescriptor, data: string | Uint8Array, options?: import("node:fs").WriteFileOptions) => void} NodeAppendFileSync
- */
-
-/**
- * @typedef {(path: string, data: string, encoding: string) => void} StringAppendLineWriter
- */
-
-/**
- * @typedef {NodeAppendFileSync | StringAppendLineWriter} AppendFileSyncLike
+ * @typedef {(path: import("node:fs").PathOrFileDescriptor, data: string | Uint8Array, options?: import("node:fs").WriteFileOptions) => void} AppendFileSyncLike
  */
 
 /**
@@ -154,7 +146,7 @@ function buildInfrastructureIncompletePayload(details) {
  * @param {string} payload
  */
 function appendSafeOutputLine(appendFileSync, safeOutputsPath, payload) {
-  appendFileSync(safeOutputsPath, payload + "\n", "utf8");
+  appendFileSync(safeOutputsPath, payload + "\n", { encoding: "utf8" });
 }
 
 /**
diff --git a/actions/setup/js/copilot_driver.test.cjs b/actions/setup/js/copilot_driver.test.cjs
index 367c39c53e4..7ee562823be 100644
--- a/actions/setup/js/copilot_driver.test.cjs
+++ b/actions/setup/js/copilot_driver.test.cjs
@@ -151,9 +151,9 @@ describe("copilot_driver.cjs", () => {
 
     it("appends one JSONL line through appendSafeOutputLine", () => {
       const writes = [];
-      const appendStub = (file, data, encoding) => writes.push({ file, data, encoding });
+      const appendStub = (file, data, options) => writes.push({ file, data, options });
       appendSafeOutputLine(appendStub, "/tmp/safeoutputs.jsonl", '{"type":"report_incomplete"}');
-      expect(writes).toEqual([{ file: "/tmp/safeoutputs.jsonl", data: '{"type":"report_incomplete"}\n', encoding: "utf8" }]);
+      expect(writes).toEqual([{ file: "/tmp/safeoutputs.jsonl", data: '{"type":"report_incomplete"}\n', options: { encoding: "utf8" } }]);
     });
 
     it("emitInfrastructureIncomplete writes payload when path is configured", () => {
@@ -161,7 +161,7 @@ describe("copilot_driver.cjs", () => {
       const logs = [];
       emitInfrastructureIncomplete("temporary outage", {
         safeOutputsPath: "/tmp/safeoutputs.jsonl",
-        appendFileSync: (file, data, encoding) => writes.push({ file, data, encoding }),
+        appendFileSync: (file, data, options) => writes.push({ file, data, options }),
         logger: message => logs.push(message),
       });
       expect(writes).toHaveLength(1);