HTTP/2 POST to Anthropics API endpoint hangs once the body size reaches 1024 bytes

[erikarn]

hi! I'm writing a test lua client to talk to the anthropic API endpoint and it hangs when I set a request body / POST 1024 bytes or larger. If I force HTTP/1.1 then everything works fine.

Are there any known issues with HTTP/2, POST and flow-control handling? I'm still getting my head around the code and what HTTP/2 is doing here. It looks like the initial advertised window size is 64k.

[daurnimator]

Possibly 47225d0 ?
Are you running v0.4?

[trescenzi]

I'm running into a similar issue trying to stream from a local llm api. Stepped through the code with debugger and it's hanging here while polling for, I believe, response headers. It goes into cqueues and then hangs at the second of the calls to step. This is a compact reproducer showing it works at 1024 and fails at 1025. Run with lua reproducer.lua break for it to break by sending a body of 1025 bytes otherwise it'll work.

local request = require "http.request"

local should_break = arg[1] == "break"
local req = request.new_from_uri("https://www.example.com")
local body = 'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Etiam lorem mauris, venenatis ac nulla in, posuere faucibus nisl. Nullam id elit vel magna elementum varius. Curabitur placerat ipsum orci. Integer tincidunt, est ac pharetra vestibulum, sapien justo mollis nunc, vel faucibus nulla nulla at diam. Nullam vulputate molestie orci. Praesent vel nisi et elit efficitur eleifend. In hac habitasse platea dictumst. Etiam sed urna non orci luctus semper. Phasellus tempor pharetra hendrerit. Nullam neque tortor, laoreet quis eleifend at, iaculis eget ipsum. Etiam imperdiet orci sed lacus scelerisque, nec porta massa semper. Proin venenatis urna purus, non ultrices metus rhoncus et. Vivamus non sem nec dolor malesuada blandit. Pellentesque ultrices orci quis convallis auctor. Proin hendrerit, augue vitae sagittis rutrum, nisi risus pharetra risus, nec aliquet felis ante in magna. Donec tristique nec dolor quis ultrices. In quis tincidunt eros. Praesent ac mattis ante, non pharetra orci. In ut eleifend diam, eu nisi'

if should_break then
  body = body .. "."
end

print("Body size:", #body)

req.headers:upsert(":method", "POST")
req:set_body(body)

local headers, stream = req:go()
if not headers then
  error("Request failed: " .. (stream or "unknown error"))
end

for chunk in stream:each_chunk() do
  print(chunk)
end

Will keep digging but I'm about at my limit here in terms of digging deeper into why exactly might be hanging. Hope this helps.

Edit: Oh I'm using 0.4.

[erikarn]

hi! sorry I missed this yup, it's 0.4.0: http 0.4-0 (installed) - /usr/local/lib/luarocks/rocks-5.4

…

On Tue, 10 Mar 2026 at 04:02, daurnimator ***@***.***> wrote: *daurnimator* left a comment (daurnimator/lua-http#239) <#239 (comment)> Possibly 47225d0 <47225d0> ? Are you running v0.4? — Reply to this email directly, view it on GitHub <#239 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAI2L3KPTQDYZGQLMOO4JQ34P7Y3RAVCNFSM6AAAAACWLN5RLKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHM2DAMZQGU2TMMJTHA> . You are receiving this because you authored the thread.Message ID: ***@***.***>

[daurnimator]

@trescenzi thanks for the script to replicate.

The request size of 1024 is special because it triggers expect: 100-continue.

lua-http/http/request.lua

Lines 342 to 344 in ee3cf4b

	if not length or length > 1024 then
	self.headers:append("expect", "100-continue")
	end

This then waits for the 1 second before sending the body.

Which it does, and then.... no response is ever received?

Still trying to figure out what's in the wrong here