diff --git a/src/scrapingbee_cli/client.py b/src/scrapingbee_cli/client.py
index ea4c25a..66e35c0 100644
--- a/src/scrapingbee_cli/client.py
+++ b/src/scrapingbee_cli/client.py
@@ -58,6 +58,13 @@ async def __aexit__(self, *args: object) -> None:
             await self._session.close()
             self._session = None
 
+    def _redact(self, body: bytes, status: int) -> bytes:
+        """The API echoes a wrong key back in its 401 body; keep it out of output and .err files."""
+        key = self.api_key.strip()
+        if status >= 400 and key:
+            return body.replace(key.encode(), b"[redacted]")
+        return body
+
     def _ensure_session(self) -> aiohttp.ClientSession:
         if self._session is None:
             raise RuntimeError("Client must be used as async with Client(...) as client")
@@ -76,7 +83,7 @@ async def _get(
         if headers:
             req_kwargs["headers"] = headers
         async with session.get(url, **req_kwargs) as resp:
-            body = await resp.read()
+            body = self._redact(await resp.read(), resp.status)
             out_headers = dict(resp.headers)
             return body, out_headers, resp.status
 
@@ -131,7 +138,7 @@ async def _request(
                     else {"Content-Type": content_type}
                 )
         async with session.request(method, url, **req_kwargs) as resp:
-            body = await resp.read()
+            body = self._redact(await resp.read(), resp.status)
             return body, dict(resp.headers), resp.status
 
     @staticmethod
diff --git a/tests/unit/test_key_redaction.py b/tests/unit/test_key_redaction.py
new file mode 100644
index 0000000..24e2dca
--- /dev/null
+++ b/tests/unit/test_key_redaction.py
@@ -0,0 +1,65 @@
+"""Regression tests: the API key must not come back out of an error body.
+
+The API answers a wrong key with 401 ``{"message": "Invalid api key: <key>"}``.
+Every command prints error bodies to stderr (and batch writes them to .err
+files), so the full key ended up in terminals, logs and agent transcripts.
+
+These tests run the real Client against a local fake API endpoint.
+"""
+
+from __future__ import annotations
+
+import asyncio
+
+from aiohttp import web
+
+from scrapingbee_cli.client import Client
+
+KEY = "FAKEKEY1234567890abcdef"
+
+
+def _call_fake_api(status, call, key=KEY):
+    """Run ``call(client)`` against an endpoint that echoes the key with ``status``."""
+
+    async def endpoint(request):
+        return web.json_response({"message": f"Invalid api key: {KEY}"}, status=status)
+
+    async def run():
+        app = web.Application()
+        app.router.add_route("*", "/{tail:.*}", endpoint)
+        runner = web.AppRunner(app)
+        await runner.setup()
+        site = web.TCPSite(runner, "127.0.0.1", 0)
+        await site.start()
+        port = runner.addresses[0][1]
+        try:
+            async with Client(key, base_url=f"http://127.0.0.1:{port}") as client:
+                return await call(client)
+        finally:
+            await runner.cleanup()
+
+    return asyncio.run(run())
+
+
+class TestKeyIsRedactedInErrorBodies:
+    def test_get_401_body_does_not_contain_the_key(self):
+        data, _, status = _call_fake_api(401, lambda c: c.usage(retries=0))
+        assert status == 401
+        assert KEY.encode() not in data
+        assert b"Invalid api key: [redacted]" in data
+
+    def test_post_401_body_does_not_contain_the_key(self):
+        data, _, status = _call_fake_api(
+            401, lambda c: c.scrape("https://example.com", method="post", body="x=1", retries=0)
+        )
+        assert status == 401
+        assert KEY.encode() not in data
+
+    def test_key_with_trailing_whitespace_is_still_redacted(self):
+        data, _, _ = _call_fake_api(401, lambda c: c.usage(retries=0), key=KEY + " ")
+        assert KEY.encode() not in data
+
+    def test_success_body_is_left_alone(self):
+        data, _, status = _call_fake_api(200, lambda c: c.usage(retries=0))
+        assert status == 200
+        assert KEY.encode() in data