diff --git a/services/api/app/main.py b/services/api/app/main.py
index 9ed60e2..ceafab6 100644
--- a/services/api/app/main.py
+++ b/services/api/app/main.py
@@ -40,8 +40,10 @@
     ensure_qdrant_collection,
     qdrant_health,
 )
-from fastapi import Depends, FastAPI
+from fastapi import Depends, FastAPI, Request
+from fastapi.exceptions import RequestValidationError
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.responses import JSONResponse
 
 log = logging.getLogger("memorygate")
 
@@ -49,6 +51,21 @@
 
 app = FastAPI(title="MemoryGate", version=SERVICE_VERSION)
 
+
+@app.exception_handler(RequestValidationError)
+async def validation_error(_request: Request, error: RequestValidationError):
+    # FastAPI's default reply repeats the refused input (for a missing field, the whole body),
+    # and key changes carry the admin key itself: say where and why, never what was sent.
+    return JSONResponse(
+        {
+            "detail": [
+                {"loc": list(item["loc"]), "msg": item["msg"], "type": item["type"]}
+                for item in error.errors()
+            ]
+        },
+        status_code=422,
+    )
+
 # Only the bundled dashboard's own origin by default. `*` is available as an
 # explicit development override via MEMORYGATE_CORS_ORIGINS and is logged when
 # used, because a wildcard puts every route in reach of any page the owner has
diff --git a/services/api/tests/test_validation_no_echo.py b/services/api/tests/test_validation_no_echo.py
new file mode 100644
index 0000000..e801a7c
--- /dev/null
+++ b/services/api/tests/test_validation_no_echo.py
@@ -0,0 +1,31 @@
+"""A refused request body is never sent back: a key in a bad request stays out of the reply."""
+
+import pytest
+from memory_stack import stack as stack
+
+SECRET = "never-echo-current-key-0123456789"
+PROVIDER_KEY = "sk-never-echo-provider-0123456789"
+
+
+@pytest.mark.parametrize(
+    ("method", "path", "body"),
+    [
+        # A missing field: pydantic's error input is the whole body, the current key included.
+        ("POST", "/auth/change-key", {"current_key": SECRET}),
+        ("POST", "/auth/change-key", {"current_key": SECRET, "new_key": "short"}),
+        ("PUT", "/system/ai-runtime", {"current_key": SECRET, "api_key": PROVIDER_KEY}),
+        # A wrong type: the error input is the value itself.
+        ("POST", "/auth/rotate-key", {"current_key": [SECRET]}),
+        ("POST", "/system/memory-reset", {"current_key": {"key": SECRET}}),
+    ],
+)
+def test_refused_body_is_not_echoed(stack, method, path, body):
+    response = stack.http.request(method, path, json=body, headers=stack.admin_headers())
+
+    assert response.status_code == 422
+    assert SECRET not in response.text
+    assert PROVIDER_KEY not in response.text
+    # Callers still learn which field was wrong and why.
+    for error in response.json()["detail"]:
+        assert set(error) == {"loc", "msg", "type"}
+        assert error["loc"][0] == "body"