Overview

When sending a POST request from a Java RestClient (Spring Boot 3.2+, Java 21) to a FastAPI backend running on Uvicorn + httptools, we encountered a strange issue where the request body was missing.

The request looked like this:

POST /endpoint HTTP/1.1
Host: my-api.com
Upgrade: h2c
Connection: Upgrade, HTTP2-Settings
Transfer-Encoding: chunked
Content-Type: application/json

3\r\nabc\r\n0\r\n\r\n

On the server side, Uvicorn logs showed:

• Unsupported upgrade request
• No request body
• Invalid HTTP request received

But when we routed the same request through ngrok or used RestTemplate instead of RestClient, it worked fine.

🔍 Root Cause

After analyzing Uvicorn’s httptools_impl.py and httptools parser behavior, we found this:

• Upgrade: h2c is ignored by Uvicorn (as expected).
• But internally, httptools still enters the upgrade state.
• Since the upgrade is ignored and the parser is not reset, no body is parsed.
• This violates RFC 7230 §6.7, which allows the server to ignore upgrades and proceed normally.

Proposed Fix

Patch parser.pyx to resume HTTP/1.1 parsing after upgrade is ignored:

cdef int cb_on_headers_complete(cparser.llhttp_t* parser) except -1:
    cdef HttpParser pyparser = <HttpParser>parser.data
    try:
        if parser.upgrade and not pyparser._should_upgrade():
            cparser.llhttp_resume_after_upgrade(parser)
        pyparser._on_headers_complete()
    except BaseException as ex:
        pyparser._last_error = ex
        return -1
    return 0

Also expose this from Python:

def resume_after_upgrade(self):
    httptools.llhttp_resume_after_upgrade(self.cparser)

Then frameworks like Uvicorn can call it in:

def on_headers_complete(self):
    if self.upgrade and self.upgrade.lower() != b"websocket":
        self.parser.resume_after_upgrade()

Reproducible Test

def test_chunked_body_with_ignored_upgrade():
    headers = {
        "Upgrade": "h2c",
        "Connection": "Upgrade",
        "Transfer-Encoding": "chunked"
    }
    body = b"4\r\ntest\r\n0\r\n\r\n"
    request = b"POST / HTTP/1.1\r\n" + headers_to_bytes(headers) + b"\r\n" + body

    parser = HttpRequestParser(TestProtocol())
    parser.feed_data(request)

    assert protocol.body == b"test"

Why it matters

This is RFC-compliant behavior that should be supported.

RestClient in Java 21+ sends Upgrade: h2c by default.

Any server not resetting its parser state will lose the body.

This breaks many interop scenarios between Spring Boot and Python ASGI apps.

I'm happy to submit a PR if maintainers are open to it. Thanks for your time and for maintaining this great project!