fix(http): decide reject-before-read closes from the framing, not the method

Two framing gaps the re-gate reproduced on the production handlers, both the
same class: something other than the wire was used to decide whether unread
body bytes are queued.

1. api/routes.py — the sidecar provenance 403 gated its close on
   `read_request_body`, a static per-method flag. That is wrong in both
   directions: a GET carrying `Content-Length`/`Transfer-Encoding` was answered
   with keep-alive and its body then parsed as the next request line
   (`501 Unsupported method ('{}GET')`), while a body-less DELETE/PUT/PATCH had
   a healthy pooled connection closed for nothing.

2. api/upload.py — all four multipart handlers read framing from
   `Content-Length` alone, so a chunked upload (which has none) was treated as
   length zero, matched no parts and was rejected 400 "No file field in
   request" with keep-alive intact. Wire trace before the fix:

       HTTP/1.1 400 Bad Request                  <- the upload rejection
       HTTP/1.1 400 Bad request syntax ('54')    <- our own chunk-size line

New primitive in api/helpers.py: `request_declares_body()` (Content-Length
present and non-zero, or any Transfer-Encoding other than identity; an
unparseable length counts as present, since it cannot prove absence) plus
`arm_connection_close_if_body_pending()` and `unsupported_transfer_encoding()`.

api/upload.py now routes all four handlers through one preflight,
`_read_multipart_or_reject()`, which refuses an undecodable transfer coding
with 411 before touching rfile — http.server never decodes chunked framing, so
that request can only be rejected — and keeps every previous status and message
(413 oversize, 400 invalid Content-Length/boundary) with its close.

Regressions: chunked reject that closes and a body-less reject that does not,
at all four upload handlers and at the sidecar 403, plus two pipelined
real-socket tests against the running server proving the follow-up request is
never answered off a poisoned socket.

api/helpers.py

@@ -220,6 +220,94 @@ def arm_connection_close(handler) -> None:
         pass
 
 
+_UNFRAMED_TRANSFER_CODINGS = frozenset({'', 'identity'})
+
+
+def _framing_header(handler, name: str) -> str | None:
+    """Read one framing header, tolerating handler stubs and plain dicts.
+
+    A real handler carries an ``email.message.Message``, whose ``get()`` is
+    case-insensitive like the wire; the suite's ``SimpleNamespace(headers={...})``
+    doubles are plain dicts, which are not, so fall back to a case-insensitive
+    scan for those. A missing or odd ``headers`` must never raise: every caller
+    is on a rejection path where an exception becomes a spurious 500.
+    """
+    headers = getattr(handler, 'headers', None)
+    if headers is None:
+        return None
+    try:
+        value = headers.get(name)
+        if value is None and isinstance(headers, dict):
+            lowered = name.lower()
+            for key, candidate in headers.items():
+                if str(key).lower() == lowered:
+                    value = candidate
+                    break
+    except Exception:
+        return None
+    return None if value is None else str(value)
+
+
+def request_declares_body(handler) -> bool:
+    """True when the request's framing declares a body that is still queued.
+
+    Keyed on the wire, not on the method: ``Transfer-Encoding`` present (chunked
+    framing hides the length until the terminating chunk is read) or
+    ``Content-Length`` present and non-zero. The method proves nothing in either
+    direction -- a GET may carry a declared body and a POST/PUT/DELETE may carry
+    none -- and a static per-method flag therefore both misses body-bearing
+    "read" requests (whose unread bytes poison the next pooled request) and
+    kills healthy keep-alive on body-less writes.
+
+    An unparseable ``Content-Length`` counts as a declared body: a value we
+    cannot interpret does not prove the body's absence, and the safe assumption
+    for connection reuse is that bytes are pending.
+    """
+    coding = _framing_header(handler, 'Transfer-Encoding')
+    if coding is not None and coding.strip().lower() not in _UNFRAMED_TRANSFER_CODINGS:
+        return True
+    raw_length = _framing_header(handler, 'Content-Length')
+    if raw_length is None or not raw_length.strip():
+        return False
+    try:
+        return int(raw_length.strip()) != 0
+    except ValueError:
+        return True
+
+
+def unsupported_transfer_encoding(handler) -> str | None:
+    """Return the transfer coding this server cannot decode, or ``None``.
+
+    ``BaseHTTPRequestHandler`` never decodes ``Transfer-Encoding``: it hands
+    every reader a raw ``rfile``, so a chunked body reads back as its literal
+    chunk framing while a ``Content-Length``-based reader sees length 0 and
+    consumes nothing at all. Such a request can only be rejected -- and only
+    with the connection armed for close, because its chunk bytes are still
+    queued on the socket.
+    """
+    coding = _framing_header(handler, 'Transfer-Encoding')
+    if coding is None:
+        return None
+    normalized = coding.strip()
+    if normalized.lower() in _UNFRAMED_TRANSFER_CODINGS:
+        return None
+    return normalized
+
+
+def arm_connection_close_if_body_pending(handler) -> bool:
+    """Arm close for a reject-before-read, but only if a body was really declared.
+
+    Returns whether the connection was armed. Use this instead of
+    ``arm_connection_close()`` wherever the rejection can also fire on a request
+    with no body: closing then would drop a pooled client's healthy connection
+    for no framing reason.
+    """
+    if not request_declares_body(handler):
+        return False
+    arm_connection_close(handler)
+    return True
+
+
 def advertise_connection_close(handler) -> None:
     """Serialize ``close_connection`` into exactly one ``Connection: close`` header.
 

api/routes.py

@@ -2975,6 +2975,7 @@ from api.helpers import (
     bad,
     safe_resolve,
     arm_connection_close,
+    arm_connection_close_if_body_pending,
     j,
     t,
     read_body,
@@ -6067,12 +6068,14 @@ def _handle_extension_sidecar_proxy(
     # DELETE fell through the CSRF compatibility path that intentionally admits
     # non-browser clients, giving unsafe methods weaker provenance than GET.
     if not _check_same_origin_browser_request(handler, require_provenance=True):
-        # Provenance rejection runs before read_body(); when a body is pending,
-        # close-and-advertise rather than reusing an HTTP/1.1 connection whose
-        # unread body would corrupt the next request (same class as _check_csrf).
-        # GET (read_request_body=False) has nothing unread — keep-alive survives.
-        if read_request_body:
-            arm_connection_close(handler)
+        # Provenance rejection runs before read_body(), so close-and-advertise
+        # whenever the request DECLARED a body (Content-Length non-zero, or any
+        # Transfer-Encoding): those bytes are still queued in rfile and a reused
+        # HTTP/1.1 connection would parse them as the next request line (same
+        # class as _check_csrf). Gating on read_request_body instead was wrong in
+        # both directions — it missed a GET that carries a declared body, and it
+        # closed a healthy connection on a body-less DELETE/PUT/PATCH.
+        arm_connection_close_if_body_pending(handler)
         return j(handler, {"error": _csrf_rejection_error(handler)}, status=403)
     try:
         request_body = _read_body_bytes(handler) if read_request_body else None

api/upload.py

@@ -8,7 +8,7 @@ import tempfile
 from pathlib import Path
 
 from api.config import MAX_UPLOAD_BYTES, STATE_DIR
-from api.helpers import arm_connection_close, j
+from api.helpers import arm_connection_close, j, unsupported_transfer_encoding
 from api.models import get_session
 from api.profiles import _profiles_match, get_active_profile_name as _get_active_profile_name
 from api.workspace import (
@@ -51,6 +51,62 @@ def _reject_before_read(handler, message, status):
     return j(handler, {'error': message}, status=status)
 
 
+class _RejectBeforeRead(ValueError):
+    """A rejection raised before any byte of the request body was consumed.
+
+    Carries the status the caller must answer with. Subclasses ``ValueError`` so
+    that if one ever escapes an unusual call path, each handler's existing
+    ``except ValueError`` still answers 400 with the same message instead of
+    turning it into a 500.
+    """
+
+    def __init__(self, message: str, status: int):
+        super().__init__(message)
+        self.message = message
+        self.status = status
+
+
+def _read_multipart_or_reject(handler):
+    """Framing preflight + body read, shared by all four multipart handlers.
+
+    Returns ``(fields, files)``. Raises ``_RejectBeforeRead`` when the request
+    must be refused before its body is read -- the caller answers through
+    ``_reject_before_read()``, which arms ``Connection: close`` first, because the
+    declared body is still queued in ``rfile`` and a reused HTTP/1.1 connection
+    would parse those bytes as the next request line.
+
+    Framing is judged from the headers, not from a present ``Content-Length``:
+
+    * ``Transfer-Encoding: chunked`` carries no Content-Length, so the old code
+      read it as length 0, found no parts, and answered "No file field in
+      request" with keep-alive intact -- leaving every chunk byte on the socket
+      to poison the next pooled request. ``http.server`` cannot decode chunked
+      framing at all (``rfile`` is the raw socket), so 411 is the only honest
+      answer.
+    * a non-numeric or oversized Content-Length cannot be read either, and
+      ``parse_multipart()`` validates the boundary and the length before its
+      first ``rfile.read()`` — so every raise below is still pre-read.
+    """
+    content_type = handler.headers.get('Content-Type', '')
+    coding = unsupported_transfer_encoding(handler)
+    if coding is not None:
+        raise _RejectBeforeRead(
+            f'Unsupported Transfer-Encoding: {coding} (uploads require a Content-Length)',
+            411,
+        )
+    try:
+        content_length = int(handler.headers.get('Content-Length', 0) or 0)
+    except (TypeError, ValueError):
+        raise _RejectBeforeRead('Invalid Content-Length', 400) from None
+    if content_length > MAX_UPLOAD_BYTES:
+        raise _RejectBeforeRead(
+            f'File too large (max {MAX_UPLOAD_BYTES//1024//1024}MB)', 413)
+    try:
+        return parse_multipart(handler.rfile, content_type, content_length)
+    except ValueError as exc:
+        raise _RejectBeforeRead(str(exc), 400) from None
+
+
 def parse_multipart(rfile, content_type, content_length) -> tuple:
     import re as _re, email.parser as _ep
     # Imported locally (not just module-level) so the function stays
@@ -212,22 +268,10 @@ def _write_office_upload_sidecar(workspace: Path, dest: Path, file_bytes: bytes)
 def handle_upload(handler):
     import traceback as _tb
     try:
-        content_type = handler.headers.get('Content-Type', '')
         try:
-            content_length = int(handler.headers.get('Content-Length', 0) or 0)
-        except (TypeError, ValueError):
-            # A non-numeric Content-Length is rejected before the body is read.
-            arm_connection_close(handler)
-            raise ValueError('Invalid Content-Length') from None
-        if content_length > MAX_UPLOAD_BYTES:
-            return _reject_before_read(handler, f'File too large (max {MAX_UPLOAD_BYTES//1024//1024}MB)', 413)
-        try:
-            fields, files = parse_multipart(handler.rfile, content_type, content_length)
-        except ValueError:
-            # parse_multipart validates Content-Length/boundary before reading;
-            # a ValueError here means the body was never consumed.
-            arm_connection_close(handler)
-            raise
+            fields, files = _read_multipart_or_reject(handler)
+        except _RejectBeforeRead as rejection:
+            return _reject_before_read(handler, rejection.message, rejection.status)
         session_id = fields.get('session_id', '')
         if 'file' not in files:
             return j(handler, {'error': 'No file field in request'}, status=400)
@@ -413,22 +457,10 @@ def handle_upload_extract(handler):
     """Handle archive upload and extraction."""
     import traceback as _tb
     try:
-        content_type = handler.headers.get('Content-Type', '')
         try:
-            content_length = int(handler.headers.get('Content-Length', 0) or 0)
-        except (TypeError, ValueError):
-            # A non-numeric Content-Length is rejected before the body is read.
-            arm_connection_close(handler)
-            raise ValueError('Invalid Content-Length') from None
-        if content_length > MAX_UPLOAD_BYTES:
-            return _reject_before_read(handler, f'File too large (max {MAX_UPLOAD_BYTES//1024//1024}MB)', 413)
-        try:
-            fields, files = parse_multipart(handler.rfile, content_type, content_length)
-        except ValueError:
-            # parse_multipart validates Content-Length/boundary before reading;
-            # a ValueError here means the body was never consumed.
-            arm_connection_close(handler)
-            raise
+            fields, files = _read_multipart_or_reject(handler)
+        except _RejectBeforeRead as rejection:
+            return _reject_before_read(handler, rejection.message, rejection.status)
         session_id = fields.get('session_id', '')
         if 'file' not in files:
             return j(handler, {'error': 'No file field in request'}, status=400)
@@ -456,22 +488,10 @@ def handle_transcribe(handler):
     import traceback as _tb
     temp_path = None
     try:
-        content_type = handler.headers.get('Content-Type', '')
         try:
-            content_length = int(handler.headers.get('Content-Length', 0) or 0)
-        except (TypeError, ValueError):
-            # A non-numeric Content-Length is rejected before the body is read.
-            arm_connection_close(handler)
-            raise ValueError('Invalid Content-Length') from None
-        if content_length > MAX_UPLOAD_BYTES:
-            return _reject_before_read(handler, f'File too large (max {MAX_UPLOAD_BYTES//1024//1024}MB)', 413)
-        try:
-            fields, files = parse_multipart(handler.rfile, content_type, content_length)
-        except ValueError:
-            # parse_multipart validates Content-Length/boundary before reading;
-            # a ValueError here means the body was never consumed.
-            arm_connection_close(handler)
-            raise
+            fields, files = _read_multipart_or_reject(handler)
+        except _RejectBeforeRead as rejection:
+            return _reject_before_read(handler, rejection.message, rejection.status)
         if 'file' not in files:
             return j(handler, {'error': 'No file field in request'}, status=400)
         filename, file_bytes = files['file']
@@ -651,23 +671,10 @@ def handle_workspace_upload(handler):
     """
     import traceback as _tb
     try:
-        content_type = handler.headers.get('Content-Type', '')
         try:
-            content_length = int(handler.headers.get('Content-Length', 0) or 0)
-        except (TypeError, ValueError):
-            # A non-numeric Content-Length is rejected before the body is read.
-            arm_connection_close(handler)
-            raise ValueError('Invalid Content-Length') from None
-        if content_length > MAX_UPLOAD_BYTES:
-            return _reject_before_read(handler, f'File too large (max {MAX_UPLOAD_BYTES//1024//1024}MB)', 413)
-
-        try:
-            fields, files = parse_multipart(handler.rfile, content_type, content_length)
-        except ValueError:
-            # parse_multipart validates Content-Length/boundary before reading;
-            # a ValueError here means the body was never consumed.
-            arm_connection_close(handler)
-            raise
+            fields, files = _read_multipart_or_reject(handler)
+        except _RejectBeforeRead as rejection:
+            return _reject_before_read(handler, rejection.message, rejection.status)
         session_id = fields.get('session_id', '')
         subpath = fields.get('path', '')
 

tests/test_rejected_write_connection_close.py

@@ -8,6 +8,8 @@ The connection must close after those early rejections.
 from types import SimpleNamespace
 from typing import Any, cast
 
+import pytest
+
 
 def test_auth_rejected_write_closes_connection(monkeypatch):
     import api.auth as auth
@@ -63,6 +65,7 @@ def test_sidecar_provenance_rejected_closes_connection(monkeypatch):
     handler = SimpleNamespace(
         path="/api/extensions/ext1/sidecar/proxy",
         command="POST",
+        headers={"Content-Length": "15"},
         close_connection=False,
     )
     monkeypatch.setattr(routes, "_match_extension_sidecar_proxy_path", lambda _path: ("ext1", "/proxy"))
@@ -87,6 +90,7 @@ def test_sidecar_get_provenance_rejected_keeps_connection(monkeypatch):
     handler = SimpleNamespace(
         path="/api/extensions/ext1/sidecar/proxy",
         command="GET",
+        headers={},
         close_connection=False,
     )
     monkeypatch.setattr(routes, "_match_extension_sidecar_proxy_path", lambda _path: ("ext1", "/proxy"))
@@ -98,6 +102,116 @@ def test_sidecar_get_provenance_rejected_keeps_connection(monkeypatch):
     assert handler.close_connection is False
 
 
+# ── Framing, not the method, decides whether a rejection must close ──────────
+#
+# The re-gate found both halves of the same mistake: `read_request_body` is a
+# static per-method flag, so it neither proves a body is pending (a body-less
+# DELETE closed a healthy connection) nor proves one is absent (a GET carrying a
+# declared body kept the connection and poisoned it). Both directions below run
+# against the production handler.
+
+
+def _rejected_sidecar_call(monkeypatch, method, headers, **kwargs):
+    """Drive a provenance-rejected sidecar proxy call; return the handler."""
+    import api.routes as routes
+
+    handler = SimpleNamespace(
+        path="/api/extensions/ext1/sidecar/proxy",
+        command=method,
+        headers=headers,
+        close_connection=False,
+    )
+    monkeypatch.setattr(routes, "_match_extension_sidecar_proxy_path", lambda _path: ("ext1", "/proxy"))
+    monkeypatch.setattr(routes, "_check_same_origin_browser_request", lambda _handler, **_: False)
+    monkeypatch.setattr(routes, "j", lambda _handler, _payload, status=200: None)
+
+    routes._handle_extension_sidecar_proxy(
+        cast(Any, handler), SimpleNamespace(path=handler.path, query=""), method, **kwargs
+    )
+    return handler
+
+
+@pytest.mark.parametrize(
+    "headers",
+    [
+        {"Content-Length": "12"},
+        {"Transfer-Encoding": "chunked"},
+        {"Content-Length": "banana"},  # unreadable framing proves nothing is absent
+    ],
+    ids=["content-length", "chunked", "garbage-content-length"],
+)
+def test_sidecar_get_with_a_declared_body_closes_connection(headers, monkeypatch):
+    """A GET may still declare a body — those bytes are unread, so close.
+
+    Reproduced by the gate as a 403 WITHOUT `Connection: close`, followed by
+    `501 Unsupported method ('{}GET')` on the same socket.
+    """
+    handler = _rejected_sidecar_call(monkeypatch, "GET", headers)
+
+    assert handler.close_connection is True
+
+
+@pytest.mark.parametrize("method", ["POST", "PUT", "PATCH", "DELETE"])
+@pytest.mark.parametrize(
+    "headers", [{}, {"Content-Length": "0"}], ids=["no-content-length", "zero-content-length"]
+)
+def test_bodyless_write_method_rejection_keeps_connection(method, headers, monkeypatch):
+    """A write method with no declared body has nothing unread — keep-alive.
+
+    `read_request_body=True` is passed for every one of these (that is what the
+    real unsafe-method call site does), so this is exactly the case the old
+    per-method gate got wrong in the over-close direction.
+    """
+    handler = _rejected_sidecar_call(monkeypatch, method, headers, read_request_body=True)
+
+    assert handler.close_connection is False
+
+
+@pytest.mark.parametrize(
+    ("headers", "expected"),
+    [
+        ({}, False),
+        ({"Content-Length": "0"}, False),
+        ({"Content-Length": " 0 "}, False),
+        ({"Content-Length": ""}, False),
+        ({"Content-Length": "12"}, True),
+        ({"Content-Length": "-1"}, True),
+        ({"Content-Length": "banana"}, True),
+        ({"content-length": "7"}, True),
+        ({"Transfer-Encoding": "chunked"}, True),
+        ({"transfer-encoding": "Chunked"}, True),
+        ({"Transfer-Encoding": "identity"}, False),
+        ({"Transfer-Encoding": ""}, False),
+    ],
+)
+def test_request_declares_body_reads_the_framing_headers(headers, expected):
+    from api.helpers import request_declares_body
+
+    handler = SimpleNamespace(headers=headers)
+
+    assert request_declares_body(cast(Any, handler)) is expected
+
+
+def test_request_declares_body_is_case_insensitive_on_a_real_message():
+    """Production handlers carry an email.message.Message, not a dict."""
+    from email.parser import Parser
+
+    from api.helpers import request_declares_body
+
+    message = Parser().parsestr("Host: x\r\ntransfer-encoding: Chunked\r\n\r\n")
+
+    assert request_declares_body(cast(Any, SimpleNamespace(headers=message))) is True
+
+
+def test_request_declares_body_tolerates_a_handler_without_headers():
+    """Never raise on a rejection path: an exception there becomes a 500."""
+    from api.helpers import request_declares_body
+
+    assert request_declares_body(cast(Any, SimpleNamespace())) is False
+    assert request_declares_body(cast(Any, SimpleNamespace(headers=None))) is False
+    assert request_declares_body(cast(Any, SimpleNamespace(headers=object()))) is False
+
+
 def test_csp_report_rate_limited_closes_connection(monkeypatch):
     """A rate-limited CSP report is dropped before its body is read."""
     import api.routes as routes
@@ -137,6 +251,90 @@ def test_upload_oversize_rejects_before_read(monkeypatch):
     assert handler.close_connection is True
 
 
+# ── Multipart framing preflight: chunked uploads must close, empty ones must not ─
+#
+# All four multipart handlers keyed on Content-Length alone, so a chunked upload
+# (which has none) was read as length 0, matched no parts, and was rejected with
+# "No file field in request" and keep-alive intact — leaving every chunk byte on
+# the socket, which is the exact poisoning this PR exists to prevent.
+# http.server never decodes chunked framing, so the only safe answer is 411 with
+# the connection armed for close.
+
+_MULTIPART_HANDLERS = [
+    "handle_upload",
+    "handle_upload_extract",
+    "handle_transcribe",
+    "handle_workspace_upload",
+]
+
+
+class _NeverRead:
+    """An rfile that fails the test if a reject-before-read path reads the body."""
+
+    def read(self, *_args):
+        raise AssertionError("the body must not be read on a reject-before-read path")
+
+
+def _run_multipart_handler(monkeypatch, handler_name, headers, rfile):
+    import api.upload as upload
+
+    answered: dict[str, Any] = {}
+
+    def _record(_handler, payload, status=200):
+        answered["payload"] = payload
+        answered["status"] = status
+        return True
+
+    monkeypatch.setattr(upload, "j", _record)
+    handler = SimpleNamespace(headers=headers, rfile=rfile, close_connection=False)
+
+    getattr(upload, handler_name)(cast(Any, handler))
+    return handler, answered
+
+
+@pytest.mark.parametrize("handler_name", _MULTIPART_HANDLERS)
+def test_chunked_multipart_rejected_before_read_closes_connection(handler_name, monkeypatch):
+    handler, answered = _run_multipart_handler(
+        monkeypatch,
+        handler_name,
+        {"Content-Type": "multipart/form-data; boundary=x", "Transfer-Encoding": "chunked"},
+        _NeverRead(),
+    )
+
+    assert answered["status"] == 411, answered
+    assert "Transfer-Encoding" in answered["payload"]["error"], answered
+    assert handler.close_connection is True
+
+
+@pytest.mark.parametrize("handler_name", _MULTIPART_HANDLERS)
+def test_bodyless_multipart_rejection_keeps_connection(handler_name, monkeypatch):
+    """A well-framed upload with no body is rejected with nothing left unread."""
+    handler, answered = _run_multipart_handler(
+        monkeypatch,
+        handler_name,
+        {"Content-Type": "multipart/form-data; boundary=x", "Content-Length": "0"},
+        SimpleNamespace(read=lambda *_args: b""),
+    )
+
+    assert answered["status"] == 400, answered
+    assert handler.close_connection is False
+
+
+@pytest.mark.parametrize("handler_name", _MULTIPART_HANDLERS)
+def test_non_numeric_content_length_still_closes(handler_name, monkeypatch):
+    """The pre-existing invalid-Content-Length reject keeps its 400 and its close."""
+    handler, answered = _run_multipart_handler(
+        monkeypatch,
+        handler_name,
+        {"Content-Type": "multipart/form-data; boundary=x", "Content-Length": "banana"},
+        _NeverRead(),
+    )
+
+    assert answered["status"] == 400, answered
+    assert answered["payload"]["error"] == "Invalid Content-Length", answered
+    assert handler.close_connection is True
+
+
 # ── Header serialization: exactly one Connection: close, never a crash ────────
 #
 # The advertisement lives in Handler.end_headers() via

tests/test_rejected_write_pooled_client.py

@@ -19,8 +19,11 @@ stale tab gets 410, not 403).
 """
 
 import http.client
+import socket
 import urllib.parse
 
+import pytest
+
 from tests._pytest_port import BASE
 
 _PORT = urllib.parse.urlparse(BASE).port
@@ -61,3 +64,96 @@ def test_rejected_write_advertises_close_and_pooled_followup_succeeds():
     assert follow_status == 200, (
         f"pooled follow-up after a reject-before-read must succeed; got {follow_status}"
     )
+
+
+# ── Framing the Content-Length gate could not see ─────────────────────────────
+#
+# Both cases below are pipelined down ONE socket against the real server: the
+# reject-before-read request first, an ordinary GET immediately after it in the
+# same write. If the rejection closes, the server answers once and EOFs and the
+# GET is never seen. If it does not, the leftover body/chunk bytes are parsed as
+# the next request line and a second response comes back — a 400 bad-syntax or
+# `501 Unsupported method` naming bytes the client never sent as a request.
+
+_FOLLOWING_GET = b"GET /api/health/agent HTTP/1.1\r\nHost: 127.0.0.1\r\nAccept: */*\r\n\r\n"
+_MULTIPART_UPLOAD_PATHS = (
+    "/api/upload",
+    "/api/upload/extract",
+    "/api/workspace/upload",
+    "/api/transcribe",
+)
+_MULTIPART_PAYLOAD = (
+    b'--x\r\nContent-Disposition: form-data; name="file"; filename="a.txt"\r\n'
+    b"\r\nhello\r\n--x--\r\n"
+)
+_CHUNKED_MULTIPART_BODY = (
+    f"{len(_MULTIPART_PAYLOAD):X}\r\n".encode() + _MULTIPART_PAYLOAD + b"\r\n0\r\n\r\n"
+)
+
+
+def _pipelined_after(request: bytes) -> bytes:
+    """Send *request* and a plain GET in one write; return every byte answered."""
+    sock = socket.create_connection(("127.0.0.1", _PORT), timeout=10)
+    try:
+        sock.sendall(request + _FOLLOWING_GET)
+        received = b""
+        while True:
+            try:
+                chunk = sock.recv(65536)
+            except (TimeoutError, socket.timeout, ConnectionError):
+                break
+            if not chunk:
+                break
+            received += chunk
+        return received
+    finally:
+        sock.close()
+
+
+def _assert_single_closed_response(answered: bytes, status: bytes, leftover: bytes) -> None:
+    text = answered.decode("latin-1", errors="replace")
+    assert answered.startswith(b"HTTP/1.1 " + status), text
+    assert b"Connection: close" in answered, text
+    assert answered.count(b"HTTP/1.1 ") == 1, (
+        f"the pipelined GET was answered, so the socket stayed open: {text}"
+    )
+    assert leftover not in answered, f"the unread body reached the request parser: {text}"
+    assert b"Bad request syntax" not in answered, text
+    assert b"Unsupported method" not in answered, text
+
+
+@pytest.mark.parametrize("path", _MULTIPART_UPLOAD_PATHS)
+def test_chunked_upload_rejection_closes_and_cannot_poison_the_socket(path):
+    """A chunked upload has no Content-Length — the old gate read it as empty.
+
+    All four multipart handlers then answered 400 "No file field in request"
+    with keep-alive intact, leaving the chunk bytes on the socket.
+    """
+    answered = _pipelined_after(
+        f"POST {path} HTTP/1.1\r\n"
+        "Host: 127.0.0.1\r\n"
+        "Content-Type: multipart/form-data; boundary=x\r\n"
+        "Transfer-Encoding: chunked\r\n"
+        "\r\n".encode() + _CHUNKED_MULTIPART_BODY
+    )
+
+    _assert_single_closed_response(answered, b"411", _MULTIPART_PAYLOAD)
+
+
+def test_sidecar_get_with_a_body_closes_and_cannot_poison_the_socket():
+    """A provenance-rejected GET that carries a declared body.
+
+    `read_request_body=False` said "no body" and the 403 went out with
+    keep-alive; the gate's repro then got `501 Unsupported method ('{}GET')` on
+    the same socket. No Origin/Referer/Sec-Fetch-Site here, so provenance fails
+    before the body would ever be read.
+    """
+    answered = _pipelined_after(
+        "GET /api/extensions/probe/sidecar/ping HTTP/1.1\r\n"
+        "Host: 127.0.0.1\r\n"
+        "Content-Type: application/json\r\n"
+        f"Content-Length: {len(_BODY)}\r\n"
+        "\r\n".encode() + _BODY
+    )
+
+    _assert_single_closed_response(answered, b"403", _BODY)