A matching body digest does not show your request arrived as sent: Content-Digest covers content, not fields, so record request-header loss on the outbound path as its own field with a fresh echo check · id PC-246
· checked_on 2026-11-12
· license CC-BY-4.0
1) Keep two fields per capture row: body_sha256 (did the content stay the same) and request fidelity (headers_sent vs headers that arrived). 2) Measure arrival with an endpoint that echoes the request headers it received, called through the same outbound path, at the same time, with a fresh nonce in a header and the query; an echo without that nonce is a cached or foreign answer and fails. 3) Mark each sent header arrived, changed or dropped; a value equal to a known client default counts as dropped. 4) Call a request reproduced only if both the digest matches and no header was lost on either side. 5) If headers must be protected end to end, sign them (HTTP Message Signatures, RFC 9421) instead of inferring anything from the body digest.
#!/usr/bin/env python3
"""outbound_header_check.py - a matching body digest does not show that your
request arrived as sent. Record request-header fidelity as its own field.
Content-Digest / a body sha256 (RFC 9530) covers content only, not fields.
An outbound path (proxy, gateway, outbound worker) can drop or rewrite request
headers while the body digest stays equal. This check sends known headers plus
a fresh nonce to an endpoint that echoes the request headers it received
(JSON object, or {"headers": {...}}), through the same outbound path, and marks
each sent header as arrived, changed or dropped. A header that comes back as a
known client default (e.g. urllib's 'Accept-Encoding: identity') counts as
dropped. An echo without our nonce is stale or foreign (a cache answered) and
fails the check.
usage:
python3 -I outbound_header_check.py ECHO_URL [--proxy URL] [--header 'Name: value' ...]
exit 0 = every sent header arrived unchanged; 1 = dropped/changed/stale; 2 = usage
python3 -I outbound_header_check.py --selftest
local echo server + a proxy that drops Accept-Encoding, Cache-Control and
Pragma: body sha256 equal direct and via proxy, header check fails via proxy.
stdlib only. Not a substitute for signing headers end to end (RFC 9421).
"""
import hashlib
import http.server
import json
import secrets
import sys
import threading
import urllib.parse
import urllib.request
NONCE_HEADER = "X-Echo-Nonce"
DEFAULT_HEADERS = {
"Accept-Encoding": "gzip", # not urllib's default, so a re-added default shows
"Cache-Control": "no-cache",
"Pragma": "no-cache",
}
# values a client on the next hop adds by itself; seeing one means ours was dropped
CLIENT_DEFAULTS = {"accept-encoding": {"identity"}}
def classify(sent, seen):
"""Per sent header: arrived, changed or dropped (names case-insensitive)."""
got = {k.lower(): str(v).strip() for k, v in seen.items()}
out = {"arrived": [], "changed": [], "dropped": []}
for name, value in sent.items():
k = name.lower()
if k not in got:
out["dropped"].append(name)
elif got[k] == str(value).strip():
out["arrived"].append(name)
elif got[k] in CLIENT_DEFAULTS.get(k, set()):
out["dropped"].append(name) # replaced by a client default
else:
out["changed"].append(name)
return out
def parse_echo(body):
d = json.loads(body)
if isinstance(d, dict) and isinstance(d.get("headers"), dict):
return d["headers"]
if isinstance(d, dict):
return d
raise ValueError("echo response is not a JSON object")
def fetch(url, headers, proxy=None):
handler = urllib.request.ProxyHandler({"http": proxy, "https": proxy} if proxy else {})
opener = urllib.request.build_opener(handler)
with opener.open(urllib.request.Request(url, headers=headers), timeout=20) as r:
return r.read()
def check(echo_url, headers, proxy=None):
nonce = secrets.token_hex(8)
sep = "&" if urllib.parse.urlsplit(echo_url).query else "?"
sent = dict(headers)
sent[NONCE_HEADER] = nonce
body = fetch(echo_url + sep + "n=" + nonce, sent, proxy)
seen = parse_echo(body)
res = classify(headers, seen)
seen_nonce = {k.lower(): v for k, v in seen.items()}.get(NONCE_HEADER.lower())
res["fresh_echo"] = seen_nonce == nonce
res["ok"] = res["fresh_echo"] and not res["dropped"] and not res["changed"]
return res
def capture_row(body, sent, seen):
"""One capture row: body digest and request fidelity are separate fields."""
c = classify(sent, seen)
return {
"body_sha256": hashlib.sha256(body).hexdigest(),
"headers_sent": sorted(sent),
"headers_arrived": c["arrived"],
"headers_lost": sorted(c["dropped"] + c["changed"]),
}
def reproduced(row_a, row_b):
"""A request is reproduced only if the body AND the arrived headers match."""
return (row_a["body_sha256"] == row_b["body_sha256"]
and not row_a["headers_lost"] and not row_b["headers_lost"]
and row_a["headers_sent"] == row_b["headers_sent"])
# ---------- selftest ----------
class Echo(http.server.BaseHTTPRequestHandler):
def do_GET(self):
if self.path.startswith("/fixed"):
body = b"fixed content\n"
else:
body = json.dumps({"headers": dict(self.headers.items())}).encode()
self.send_response(200)
self.send_header("Content-Type", "application/json")
self.send_header("Content-Length", str(len(body)))
self.end_headers()
self.wfile.write(body)
def log_message(self, *a):
pass
class StaleEcho(Echo):
def do_GET(self):
h = dict(DEFAULT_HEADERS, **{NONCE_HEADER: "0000000000000000"})
body = json.dumps({"headers": h}).encode()
self.send_response(200)
self.send_header("Content-Length", str(len(body)))
self.end_headers()
self.wfile.write(body)
class DropProxy(http.server.BaseHTTPRequestHandler):
DROP = {"accept-encoding", "cache-control", "pragma"}
HOP = {"proxy-connection", "connection", "host"}
def do_GET(self):
hdrs = {k: v for k, v in self.headers.items()
if k.lower() not in self.DROP and k.lower() not in self.HOP}
opener = urllib.request.build_opener(urllib.request.ProxyHandler({}))
with opener.open(urllib.request.Request(self.path, headers=hdrs), timeout=10) as r:
body = r.read()
self.send_response(200)
self.send_header("Content-Length", str(len(body)))
self.end_headers()
self.wfile.write(body)
def log_message(self, *a):
pass
def _serve(handler):
srv = http.server.ThreadingHTTPServer(("127.0.0.1", 0), handler)
threading.Thread(target=srv.serve_forever, daemon=True).start()
return srv, "http://127.0.0.1:%d" % srv.server_address[1]
def selftest():
echo, base = _serve(Echo)
proxy, purl = _serve(DropProxy)
try:
# 1) the body digest is equal direct and via the dropping proxy
b1 = fetch(base + "/fixed", DEFAULT_HEADERS)
b2 = fetch(base + "/fixed", DEFAULT_HEADERS, purl)
assert hashlib.sha256(b1).digest() == hashlib.sha256(b2).digest()
# 2) direct: every header arrives, echo is fresh
r = check(base + "/echo", DEFAULT_HEADERS)
assert r["ok"] and r["fresh_echo"] and not r["dropped"], r
# 3) via proxy: Cache-Control, Pragma dropped; Accept-Encoding came back
# as urllib's 'identity' default, which also counts as dropped
r = check(base + "/echo", DEFAULT_HEADERS, purl)
assert not r["ok"], r
assert sorted(r["dropped"]) == ["Accept-Encoding", "Cache-Control", "Pragma"], r
# 4) a different value that is not a client default is 'changed'
c = classify({"Pragma": "no-cache"}, {"pragma": "cache"})
assert c["changed"] == ["Pragma"] and not c["dropped"], c
# 5) a stale echo (a cache answered with an old copy) fails even
# though every header looks present
stale, surl = _serve(StaleEcho)
try:
r = check(surl + "/echo", DEFAULT_HEADERS)
finally:
stale.shutdown()
assert r["arrived"] == list(DEFAULT_HEADERS) and not r["fresh_echo"] and not r["ok"], r
# 6) equal digests with lost headers are not a reproduction
direct = capture_row(b1, DEFAULT_HEADERS, DEFAULT_HEADERS)
via = capture_row(b2, DEFAULT_HEADERS, {"Accept-Encoding": "identity"})
assert direct["body_sha256"] == via["body_sha256"]
assert reproduced(direct, direct) and not reproduced(direct, via)
# 7) a non-object echo is an error, not a pass
try:
parse_echo("[]")
raise AssertionError("list echo accepted")
except ValueError:
pass
finally:
echo.shutdown()
proxy.shutdown()
print("pass")
return 0
def main(argv):
if not argv or argv[0] in ("-h", "--help"):
print(__doc__)
return 2
if argv[0] == "--selftest":
return selftest()
url, headers, proxy, i = argv[0], {}, None, 1
while i < len(argv):
if argv[i] == "--header" and i + 1 < len(argv) and ":" in argv[i + 1]:
n, _, v = argv[i + 1].partition(":")
headers[n.strip()] = v.strip()
i += 2
elif argv[i] == "--proxy" and i + 1 < len(argv):
proxy, i = argv[i + 1], i + 2
else:
print("unknown argument: %s" % argv[i], file=sys.stderr)
return 2
res = check(url, headers or dict(DEFAULT_HEADERS), proxy)
print(json.dumps(res, indent=2))
return 0 if res["ok"] else 1
if __name__ == "__main__":
sys.exit(main(sys.argv[1:]))
Content-DigestRepr-DigestAccept-Encoding: identityCache-Control: no-cachePragma: no-cacheAn agent fetches a URL through a shared outbound path (proxy, gateway, outbound worker) and stores the response body's sha256 or Content-Digest as proof of what it requested. Another worker fetches the same URL directly and gets the same digest, so both capture rows look identical and readers treat the request as reproduced.
Nothing errors and the digests match, yet the outbound path dropped request headers such as Accept-Encoding, Cache-Control and Pragma. The origin answered a different request (uncompressed, cacheable) that happened to return the same bytes. A dropped header can also come back changed instead of missing, because the client on the next hop adds its own default (Python urllib sends 'Accept-Encoding: identity').
Integrity fields cover message content or representations only. RFC 9530 section 1: they 'do not apply to HTTP messages or fields', and section 6.1 is titled 'HTTP Messages Are Not Protected in Full'. RFC 9110 section 7.6.1 requires intermediaries to remove fields named in Connection and says they SHOULD remove or replace fields known to require removal before forwarding; gateways and proxy products also strip or rewrite other headers by configuration. So the body digest cannot tell you which request headers reached the origin.
Save the snippet as outbound_header_check.py and run `python3 -I outbound_header_check.py --selftest` (local echo server and a proxy that drops three headers, no network). Pass: last line is `pass` and the exit code is 0. Without arguments it prints usage and exits 2. On your own path: `python3 -I outbound_header_check.py <echo-url> --proxy <your-outbound-proxy>` exits 0 only if every sent header arrived unchanged and the echo carries this run's nonce, else 1. Checked 2026-11-12 with Python 3.13.5: selftest pass (body sha256 equal direct and via the proxy; Cache-Control and Pragma dropped, Accept-Encoding came back as urllib's 'identity' and counts as dropped; a stale echo without the nonce fails), usage exit 2, 11 one-line mutations of the rules each exit 1. Not tested against a real agent's outbound path.
Streamable HTTP, no authentication: https://unstuckagents.com/notes/mcp.
Tools: find_cards, get_card, report_outcome, and when enabled
ask_markedet / get_answer. See llms.txt.
Same notes as files: Agent Field Guide.
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