Suppress duplicate 100-continue response

[brasky]

I have been having an issue integrating with moto using the S3 async client. I did some digging and it appears that when the S3 client includes Expect: 100-continue in its PUT requests it gets back two responses which prevents the client from getting the actual response. It seems to be because Werkzeug and BaseHTTPRequestHandler both return a 100 continue response. The AWS SDK v2 async Java client handles a single 100 correctly but misinterprets the second one as the final response and throws S3Exception: null (Service: S3, Status Code: 100, Request ID: null), which has been super frustrating post-localstack migration debugging this.

I believe this is a reasonable fix where we are just suppressing one of the responses. My tests pass with this change at least :)

Here's a repro of the issue

"""
Reproduce: moto sends duplicate HTTP 100 Continue responses.
"""
import socket
import time

from moto.moto_server.threaded_moto_server import ThreadedMotoServer

server = ThreadedMotoServer(port=15555, verbose=False)
server.start()


def raw_request(method, path, body=None, headers=None):
    sock = socket.create_connection(("127.0.0.1", 15555))
    lines = [f"{method} {path} HTTP/1.1", "Host: localhost:15555"]
    for k, v in (headers or {}).items():
        lines.append(f"{k}: {v}")
    if body:
        lines.append(f"Content-Length: {len(body)}")
    lines.append("")
    lines.append("")
    sock.sendall("\r\n".join(lines).encode())
    if body:
        time.sleep(0.1)
        sock.sendall(body)
    sock.settimeout(2)
    chunks = []
    try:
        while True:
            data = sock.recv(4096)
            if not data:
                break
            chunks.append(data)
    except socket.timeout:
        pass
    sock.close()
    return b"".join(chunks)


# Create bucket
raw_request("PUT", "/test-bucket")

# PUT with Expect: 100-continue
resp = raw_request(
    "PUT",
    "/test-bucket/key",
    body=b'{"test": true}',
    headers={"Content-Type": "application/json", "Expect": "100-continue"},
)

server.stop()

status_lines = [
    l for l in resp.decode(errors="replace").split("\r\n") if l.startswith("HTTP/")
]
print("Response status lines:")
for line in status_lines:
    print(f"  {line}")

n_100 = sum(1 for l in status_lines if "100" in l)
assert n_100 <= 1, f"Expected at most 1 '100 Continue', got {n_100}"
print(f"\nPASS: got {n_100} '100 Continue' response(s)")

Without the patch you get:

127.0.0.1 - - [15/Apr/2026 18:33:39] "PUT /test-bucket HTTP/1.1" 200 -
127.0.0.1 - - [15/Apr/2026 18:33:39] "PUT /test-bucket/key HTTP/1.1" 200 -
Response status lines:
  HTTP/1.1 100 Continue
  HTTP/1.1 100 Continue
  HTTP/1.1 200 OK
Traceback (most recent call last):
  File "C:\Users\elliot.dematteis\repos\moto\repro.py", line 61, in <module>
    assert n_100 <= 1, f"Expected at most 1 '100 Continue', got {n_100}"
           ^^^^^^^^^^
AssertionError: Expected at most 1 '100 Continue', got 2

With the patch you get:

127.0.0.1 - - [15/Apr/2026 18:34:42] "PUT /test-bucket HTTP/1.1" 200 -
127.0.0.1 - - [15/Apr/2026 18:34:42] "PUT /test-bucket/key HTTP/1.1" 200 -
Response status lines:
  HTTP/1.1 100 Continue
  HTTP/1.1 200 OK

PASS: got 1 '100 Continue' response(s)

[bpandola]

Looks like this was a bug in Werkzeug (pallets/werkzeug/issues/3138) that has recently been fixed (but not yet released). I'd prefer this be handled upstream rather than with a change to Moto. Hopefully their next release will be out soon.