placeOrder returns HTTP 400 with a completely empty body (Content-Length: 0) — no status/message/errorcode JSON at all, so there's no error detail to act on. Reproducible with a real, valid instrument, not just a malformed request.

Environment

• smartapi-python v1.5.5 (latest on PyPI)
• App registered with a static primary IP (via SmartAPI dashboard → Add App)
• Login (generateSession via client code + PIN + TOTP) succeeds every time

What works fine with the same session

• getProfile
• rmsLimit
• orderBook
• position
• getMarketData

Headers sent (verified against the official Orders doc example, exact match)

Authorization: Bearer <token>
Content-Type: application/json
Accept: application/json
X-UserType: USER
X-SourceID: WEB
X-ClientLocalIP: <local ip>
X-ClientPublicIP: <public ip>
X-MACAddress: <mac>
X-PrivateKey: <api key>

Steps to reproduce

1. placeOrder with a deliberately invalid tradingsymbol / symboltoken — expected a normal validation-error JSON response, got empty 400 instead.
2. placeOrder with a real, valid NSE equity symbol/token (e.g. TCS-EQ) and a realistic limit price — same empty 400.
3. Tried both via the SDK's placeOrder() / placeOrderFullResponse() and via a raw requests.post() directly to https://apiconnect.angelone.in/rest/secure/angelbroking/order/v1/placeOrder with the same headers/payload — identical result either way.

What I've ruled out

• Not a header problem — verified the exact header set against the official Orders documentation page, line by line.
• Not insufficient funds/margin — the docs' own example of a real rejection (insufficient funds) returns a full JSON body with a human-readable text field. An empty body doesn't match that pattern.
• Possibly not market hours — tests were run outside market hours (~10:30 PM IST). Per the docs, off-hours orders should become AMO orders with a normal success response (order_id returned), not fail outright — so I'm not fully confident this explains it either, though I haven't yet confirmed the same request during live market hours.

Question

Has anyone else hit a zero-byte 400 specifically (not a JSON error) on placeOrder? Trying to determine whether this is account-specific (some provisioning step I'm missing) or a broader gateway issue.