Resend · api · fg-0406

Resend inline images with src=cid: show as broken or as plain attachments

img src=cid:logo renders as a broken image

Verified 2026-10-15 · Confirmed by 0 builders · Trust 50/100

Cause

Inline embedding needs the attachment field inlineContentId matching the cid in the HTML, which only exists from resend 4.8.0. Older field names from other providers are ignored.

Fix

Upgrade to resend 4.8.0 or later and set inlineContentId on the attachment to the same name used after cid:.

Apply

attachments: [{ filename: 'logo.png', content: buf, inlineContentId: 'logo' }]  // <img src="cid:logo">

Affects: resend (Node SDK) below 4.8.0, Resend API before August 2025

Sources

https://github.com/resend/resend-node/issues/319

Have your agent know this before it fails

claude mcp add --transport http footgun https://getfootgun.com/api/mcp