diff --git a/src/commands/models/auth.test.ts b/src/commands/models/auth.test.ts
index 7ee02a1411fe..c8c6f5e34084 100644
--- a/src/commands/models/auth.test.ts
+++ b/src/commands/models/auth.test.ts
@@ -1473,6 +1473,21 @@ describe("modelsAuthLoginCommand", () => {
     expect(mocks.updateConfig).not.toHaveBeenCalled();
   });
 
+  it("rejects oversized piped auth input before buffering it", async () => {
+    const runtime = createRuntime();
+    restoreStdin?.();
+    const oversized = "x".repeat(1024 * 1024 + 1);
+    restoreStdin = withPipedStdin(oversized);
+
+    await expect(modelsAuthPasteApiKeyCommand({ provider: "openai" }, runtime)).rejects.toThrow(
+      "Piped auth input exceeds 1048576 bytes.",
+    );
+
+    expect(mocks.clackPassword).not.toHaveBeenCalled();
+    expect(mocks.upsertAuthProfileWithLock).not.toHaveBeenCalled();
+    expect(mocks.updateConfig).not.toHaveBeenCalled();
+  });
+
   it("writes pasted API keys to the requested agent store", async () => {
     const runtime = createRuntime();
     useCoderAgentConfig();
diff --git a/src/commands/models/auth.ts b/src/commands/models/auth.ts
index 298aa2ae313e..5e66f8d61b96 100644
--- a/src/commands/models/auth.ts
+++ b/src/commands/models/auth.ts
@@ -130,13 +130,20 @@ const password = async (params: Parameters<typeof clackPassword>[0]) =>
 const select = async <T>(params: Parameters<typeof clackSelect<T>>[0]) =>
   guardCancel(await clackSelect(styleSelectParams(params)));
 
+const MODELS_AUTH_STDIN_MAX_BYTES = 1024 * 1024;
+
 async function readPipedStdin(): Promise<string> {
-  process.stdin.setEncoding("utf8");
-  let input = "";
+  const chunks: Buffer[] = [];
+  let totalBytes = 0;
   for await (const chunk of process.stdin) {
-    input += String(chunk);
+    const buffer = Buffer.isBuffer(chunk) ? chunk : Buffer.from(String(chunk));
+    totalBytes += buffer.byteLength;
+    if (totalBytes > MODELS_AUTH_STDIN_MAX_BYTES) {
+      throw new Error(`Piped auth input exceeds ${MODELS_AUTH_STDIN_MAX_BYTES} bytes.`);
+    }
+    chunks.push(buffer);
   }
-  return input;
+  return Buffer.concat(chunks, totalBytes).toString("utf8");
 }
 
 async function readPastedSecret(params: {