SendGrid 413 content too large: attachments blew the request size

TL;DR

Check each attachment entry has content (base64), type, filename. Fix: host large files and link them instead of attaching.

Steps

1. Symptom: 413 content too large on /mail/send, usually right after adding attachments.
2. Remember base64 inflates files by about a third. A 20 MB file is ~27 MB on the wire before the rest of the JSON.
3. Check each attachment entry has content (base64), type, filename. A missing type or a double-encoded file bloats or breaks the payload.
4. Fix: host large files and link them instead of attaching. Attachments are for small files the recipient must have offline.
5. Compress or downscale before attaching: PDFs, images, and decks usually shrink a lot.
6. If you must send big, split across messages rather than one giant request.
7. Over SMTP the same problem surfaces as 552 (message larger than the system limit). Same fix.

When to use

You are seeing this: Symptom: 413 content too large on /mail/send, usually right after adding attachments. Use this skill when you run into "SendGrid 413 content too large: attachments blew the request size".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

Edge cases

• Remember base64 inflates files by about a third.

The errors doc lists 413 as content too large. The mail send reference documents attachments as base64 content with type, filename, and disposition. The SMTP errors doc lists 552 for messages larger than the system limit. Sources: https://docs.sendgrid.com/api-reference/how-to-use-the-sendgrid-v3-api/errors | https://docs.sendgrid.com/api-reference/mail-send/mail-send | http://sendgrid.com/docs/Glossary/emailerrormessages.html