diff --git a/src/openapi_mcp_sdk/main.py b/src/openapi_mcp_sdk/main.py
index ddff299..dcaa8ad 100644
--- a/src/openapi_mcp_sdk/main.py
+++ b/src/openapi_mcp_sdk/main.py
@@ -161,11 +161,28 @@ async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
         token: str | None = (params.pop("token", None) or [None])[0]
 
         if token:
-            # Strip any existing Authorization header and inject the new one.
-            # Also remove ?token= from the query string so it never appears in
+            # Reject conflicting auth sources — both ?token= and Authorization header present.
+            # Silent override hides client misconfiguration and makes debugging harder.
+            has_auth_header = any(
+                k.lower() == b"authorization" for k, _ in scope["headers"]
+            )
+            if has_auth_header:
+                from starlette.responses import JSONResponse
+
+                response = JSONResponse(
+                    status_code=400,
+                    content={
+                        "error": "conflicting_auth",
+                        "message": "Both Authorization header and ?token= query parameter are provided. Use only one authentication method.",
+                    },
+                )
+                await response(scope, receive, send)
+                return
+
+            # Remove ?token= from the query string so it never appears in
             # access logs — the token travels as a header from this point on.
             clean_qs = urlencode(params, doseq=True).encode()
-            headers = [(k, v) for k, v in scope["headers"] if k.lower() != b"authorization"]
+            headers = [(k, v) for k, v in scope["headers"]]
             headers.append((b"authorization", f"Bearer {token}".encode()))
             scope = {**scope, "headers": headers, "query_string": clean_qs}
         else: