pywrangler installs httpx from a Pyodide-specific fork that replaces httpcore with a jsfetch.py transport. This transport filters out the User-Agent header to prevent CORS preflight failures in browser-based Pyodide (see pyodide-http#22):

# python_modules/httpx/_transports/jsfetch.py line 57
HEADERS_TO_IGNORE = ("user-agent",)

# line 157 (_do_fetch)
headers = {k: v for k, v in request.headers.items() if k not in HEADERS_TO_IGNORE}

Cloudflare Workers are not browsers — there are no CORS restrictions on outbound fetch. But the filter still applies because the fork's emscripten support PR (encode/httpx#3330) was closed without merge, so the browser-oriented workaround lives on.

Other headers (Authorization, Accept, custom headers) are transmitted normally. Only User-Agent is stripped. This causes 403s from APIs that require it — notably GitHub's API (api.github.com/user):

Request forbidden by administrative rules.
Please make sure your request has a User-Agent header.

cd 3-httpx-headers
uv sync
uv run pywrangler dev
# Visit http://localhost:8787/test

The /test endpoint sends {"User-Agent": "repro/1.0", "X-Custom": "preserved"} to httpbin.org/headers (which echoes back received headers) via both httpx and js.fetch().

{
  "headers_sent": {
    "User-Agent": "repro/1.0",
    "X-Custom": "preserved"
  },
  "httpx_received": {
    "X-Custom": "preserved"
  },
  "jsfetch_received": {
    "User-Agent": "repro/1.0",
    "X-Custom": "preserved"
  }
}

httpx_received is missing User-Agent. jsfetch_received has both headers.

The HEADERS_TO_IGNORE filter exists in the hoodmane/httpx fork's jsfetch.py transport — originally from pyodide-http to prevent CORS preflight failures in browsers. The upstream PR to merge emscripten support into encode/httpx (#3330) was closed without merge, so the fork is the only path.

workerd already solved this once: src/pyodide/internal/patches/httpx.py monkey-patches AsyncClient._send_single_request() to bypass jsfetch.py entirely, passing all headers via js.fetch() directly. But that patch is gated to Pyodide 0.26.0a2 only (python-entrypoint-helper.ts) and is not applied for Pyodide 0.27+.

Options:

1. Extend the workerd monkey-patch to Pyodide 0.27+. Re-apply httpx_patch.py for all Pyodide versions, not just 0.26.
2. Patch the fork. Set HEADERS_TO_IGNORE = () in the hoodmane/httpx fork when running in Workers (not a browser). Workers set sys.platform == "emscripten" like browsers, so a Workers-specific environment check would be needed.
3. Patch at install time. Have pywrangler post-process the installed httpx to clear HEADERS_TO_IGNORE before bundling into python_modules/.

Option 1 is the smallest change — the patch already exists and works.

Use js.fetch() directly for outbound HTTP that needs User-Agent:

from js import Object, fetch
from pyodide.ffi import to_js

opts = to_js(
    {"method": "GET", "headers": {"User-Agent": "myapp/1.0"}},
    dict_converter=Object.fromEntries,  # required: without this, dicts become JS Maps
)
response = await fetch(url, opts)

Or patch the bundled transport locally: set HEADERS_TO_IGNORE = () in python_modules/httpx/_transports/jsfetch.py line 57.