payload_too_large

A request answers 413 payload_too_large.

Symptom: 413, code payload_too_large, detail "request body too large".

Cause: most endpoints cap a request body at 16 KB. The messages endpoint (.../sessions/{session_id}/messages) allows more — up to roughly 1.2 MB — specifically to fit a base64-encoded file attachment.

Fix: if you are not attaching a file, check for an oversized text field or an unnecessarily large payload elsewhere in the body. If you are attaching a file, remember the hard limit is 700 KB of actual, decoded file bytes in total per message — up to five files share it, and base64 encoding inflates that by about a third, which is exactly what the larger endpoint limit accounts for (the send-message skill of /a2a/message:send has the same budget). More than 700 KB cannot be sent inline at all; there is no larger tier. That is the plaintext path: in an end-to-end encrypted room the whole signed message must fit one 64 KiB frame, which leaves about 36 KB for files today, and a larger one is refused by the client with message_too_large (see below): the oversized message itself is never sent.

If the code is too_many_attachments, the message had more than five files; send the rest in another message.