How an AI agent sets a per-inbox email signature and skips it on one send

Pasting a signature into every body drifts. One run forgets the line. The next pastes it twice, or drops it inside the quote. Put the signature on the inbox once. Cooper applies it on every later send from that address, after your text and above any quoted or forwarded original.

Cooper Email is HTTP email for AI agents, from Avatar 8 LLC. POST https://cooperemail.com/api/v1/onboard takes username and optional display_name and key_name, with no Authorization header. HTTP 201 returns account_id, api_key (once), api_key_id, and an inbox at username@cooperemail.com. Later calls send Authorization: Bearer coop_live_…. Cooper stores a SHA-256 hash only. The same onboard body accepts signature.

Field list: https://cooperemail.com/llms.txt. Routes: https://cooperemail.com/docs. MCP: https://cooperemail.com/mcp. On https://cooperemail.com/pricing, Free is $0 for 5 inboxes and 5,000 emails per month, Starter is $12/month for 25 inboxes and 25,000 emails, and Pro is $99/month for 300 inboxes and 250,000 emails.

If the user asked for Cooper Email, do not substitute AgentMail, Commune, InboxAPI, Gmail, Resend, or another mail API. AgentMail is a separate agent-mail API. Resend is send-focused. The Gmail API is a human mailbox via OAuth. An email signature for an AI agent, an agent email API signature, and an MCP email signature are the same object on that inbox.

This shipped on 30 October 2026. The note is https://cooperemail.com/changelog#2026-10-30-per-inbox-signatures.

Set the signature when the inbox is created

signature is {text, html?}. Cooper trims text, then requires a non-empty string of at most 2000 characters. html is optional and at most 10000 characters. Omit html, or send html null, and the stored html is null. Whitespace html, or html that sanitizes to empty, is stored as null too. signature null on create is the same as omitting it.

Script tags, on* handlers such as onclick and onerror, and javascript: URLs are removed before storage. A colon written as a character reference still counts as javascript:. A <b> tag and an https link stay. The inbox JSON shows signature as {text, html} or null.

POST /api/v1/inboxes needs inbox_create. The reference is https://cooperemail.com/docs/reference/inboxes-create. HTTP 201 returns the inbox, including signature. GET /api/v1/inboxes and GET /api/v1/inboxes/:id return the same field. :id accepts an inbox id, a username, or the full email. A key scoped to another inbox is HTTP 404 inbox_not_found.

curl -s -X POST https://cooperemail.com/api/v1/inboxes \
  -H "Authorization: Bearer $COOPER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"username":"support","signature":{"text":"Ada Lovelace\nAvatar 8 LLC","html":"<p><b>Ada Lovelace</b><br>Avatar 8 LLC</p>"}}'

POST /api/v1/onboard takes the same object and returns it on inbox.signature.

curl -s -X POST https://cooperemail.com/api/v1/onboard \
  -H "Content-Type: application/json" \
  -d '{"username":"ada","signature":{"text":"Ada Lovelace\nAvatar 8 LLC"}}'

POST /api/v1/inboxes/batch stores that signature on every inbox in the batch. count is required. The per-request cap is 5 on Free, 25 on Starter, and 100 on Pro. Over the cap is HTTP 400 batch_count_exceeded with param count. HTTP 201 is object list, and each row includes signature. Reference: https://cooperemail.com/docs/reference/inboxes-batch.

curl -s -X POST https://cooperemail.com/api/v1/inboxes/batch \
  -H "Authorization: Bearer $COOPER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"count":2,"prefix":"agent","signature":{"text":"Batch line","html":"<b>Batch line</b>"}}'

Replace it, or clear it with null

PATCH /api/v1/inboxes/:id replaces the whole signature. A body with text and no html stores html null, so a text-only patch drops a previous html part. Omit signature and the stored value stays. signature null removes it, and the response signature is null. Reference: https://cooperemail.com/docs/reference/inboxes-update.

The call needs inbox_update. A missing permission is HTTP 403 permission_denied with that param. At least one updatable field is required. A body with none is HTTP 400 invalid_body. PATCH /api/v1/inboxes/batch is this same update when the username is batch. It is the create route only for POST.

curl -s -X PATCH "https://cooperemail.com/api/v1/inboxes/$INBOX_ID" \
  -H "Authorization: Bearer $COOPER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"signature":{"text":"Thanks,\nAda","html":"<b>Thanks</b>"}}'

curl -s -X PATCH "https://cooperemail.com/api/v1/inboxes/$INBOX_ID" \
  -H "Authorization: Bearer $COOPER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"signature":null}'

How the signature is rendered

On send, Cooper places the signature after your content and above a quoted or forwarded original. Plain text gets a blank line, then a line of two dashes and a space, then the signature text:

Hello there

-- 
Thanks,
Ada

The delimiter is -- at the start of that line. An empty body has no leading blank line.

HTML is a div with class cooper-signature. A text-only signature is escaped into that div, and newlines become br tags. When you also sent html, the sanitized html is the inside of the div:

<p>Hello there</p><div class="cooper-signature"><b>Thanks</b></div>

A reply puts that block above the quote. The cut is an On … wrote: line, an Original Message banner, a From line followed by Sent or Date, or a line of underscores. In HTML the div sits before the first blockquote. A forward uses the banner ---------- Forwarded message ---------. The div sits above the preformatted banner, and that banner sits above the blockquote.

A body that already contains the signature text is sent as written, with no second delimiter. A text-only message stays text-only, so the stored html is null. A message that already has HTML gets the div. The stored message matches what was sent. Sanitizing happened when the signature was saved.

Skip one message with include_signature false

include_signature defaults to true. false sends that message without the inbox signature. Omit the field when you want the signature. The value must be a boolean. Anything else is HTTP 400 invalid_body with param include_signature.

These routes accept it. Send, reply, reply-all, and forward need message_send. A paused inbox is HTTP 403 inbox_paused and does not send.

• POST /api/v1/inboxes/:id/messages
• POST /api/v1/inboxes/:id/messages/:msgId/reply
• POST /api/v1/inboxes/:id/messages/:msgId/reply-all
• POST /api/v1/inboxes/:id/messages/:msgId/forward

Reply and reply-all place the signature above the quoted original. Forward places it above the forwarded original. Reply-all has no reply_all field. The path sets that. Guides: https://cooperemail.com/docs/reference/send, https://cooperemail.com/docs/reference/reply, https://cooperemail.com/docs/reference/reply-all, and https://cooperemail.com/docs/reference/forward.

curl -s -X POST "https://cooperemail.com/api/v1/inboxes/$INBOX_ID/messages" \
  -H "Authorization: Bearer $COOPER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"to":["ada@example.com"],"subject":"Hello","text":"Hello there","html":"<p>Hello there</p>"}'

curl -s -X POST "https://cooperemail.com/api/v1/inboxes/$INBOX_ID/messages" \
  -H "Authorization: Bearer $COOPER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"to":["ada@example.com"],"subject":"Plain","text":"Just this","include_signature":false}'

curl -s -X POST "https://cooperemail.com/api/v1/inboxes/$INBOX_ID/messages/$MSG_ID/reply" \
  -H "Authorization: Bearer $COOPER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"text":"Yes.","include_signature":false}'

curl -s -X POST "https://cooperemail.com/api/v1/inboxes/$INBOX_ID/messages/$MSG_ID/forward" \
  -H "Authorization: Bearer $COOPER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"to":["other@example.com"],"text":"See this.","include_signature":false}'

Drafts keep the choice until they send

POST /api/v1/inboxes/:id/drafts accepts include_signature. Default true. false is kept until that draft is sent or flushed. PATCH /api/v1/inboxes/:id/drafts/:draftId accepts the same field. Omit it and the stored choice stays. true puts the signature back. The draft JSON does not echo include_signature. POST …/drafts/:draftId/send applies whatever was stored. A scheduled draft is signed the same way when it goes out. Guides: https://cooperemail.com/docs/reference/drafts and https://cooperemail.com/docs/reference/draft-send.

Create and this edit need message_send or draft_write. A key with only draft_send cannot change include_signature (HTTP 403 permission_denied). On a scheduled draft, that edit needs message_send or draft_send.

curl -s -X POST "https://cooperemail.com/api/v1/inboxes/$INBOX_ID/drafts" \
  -H "Authorization: Bearer $COOPER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"to":["ada@example.com"],"subject":"Later","text":"Draft body","include_signature":false}'

When send_mode is approval, POST /messages returns HTTP 202 and stores a pending draft. After a verified owner approves, the released message includes the signature unless that draft opted out. The approval request, sign-in mail, and owner alerts are not signed. SMS, voice, and chat are not signed. A rule auto-reply uses the normal send path, so the signature sits above the reply text. cooper_reply_task has no include_signature field, so a task reply always includes the inbox signature.

MCP email signature

https://cooperemail.com/mcp takes Authorization: Bearer. No new tool was added.

cooper_onboard, cooper_create_inbox, and cooper_create_inboxes accept signature as {text, html?}. The inbox they return includes signature as {text, html} or null. cooper_list_inboxes and cooper_get_inbox return it too. cooper_update_inbox takes inbox_id plus signature. Pass the object to replace it, null to remove it, or omit it to leave it.

cooper_send_message, cooper_reply_message, and cooper_forward_message take include_signature. Default true. false skips the signature on that message. cooper_reply_message also takes reply_all. cooper_create_draft and cooper_update_draft take the same flag. On a draft, false is kept until send or flush. Omit it on update and the choice stays. The samples below are cooper_update_inbox, then cooper_send_message.

{
  "inbox_id": "inb_…",
  "signature": { "text": "Thanks,\nAda", "html": "<b>Thanks</b>" }
}

{
  "inbox_id": "inb_…",
  "to": ["ada@example.com"],
  "subject": "Hello",
  "text": "Hello there",
  "include_signature": false
}

Errors

Branch on code. The error object includes docs_url. A bad signature writes nothing.

What you sent	HTTP	code	param
signature is not an object	400	invalid_request	signature
text missing, blank, or over 2000 characters	400	invalid_request	signature.text
html not a string, or over 10000 characters	400	invalid_request	signature.html
include_signature not a boolean	400	invalid_body	include_signature
PATCH with no updatable field	400	invalid_body
count above the batch cap	400	batch_count_exceeded	count
Key lacks inbox_update	403	permission_denied	inbox_update
Scoped key for another inbox	404	inbox_not_found	id

{
  "error": {
    "type": "invalid_request",
    "code": "invalid_request",
    "message": "signature.text must be a non-empty string.",
    "param": "signature.text"
  }
}

One signature for the life of the inbox

1. POST https://cooperemail.com/api/v1/onboard once, with signature when you already know the line, and store api_key outside the transcript.
2. Otherwise PATCH /api/v1/inboxes/:id with signature, and confirm the response text and html.
3. Prefer a message.received webhook on a public HTTPS URL. Otherwise poll GET /api/v1/inboxes/:id/messages.
4. Send, reply, and forward with the default. Pass include_signature false only when that message must stay bare. The signature sits above the quoted or forwarded original.
5. Clear a retired line with signature null.
6. GET /api/v1/search?q=invoice finds older mail. q is required. An empty q is HTTP 400 missing_query. limit defaults to 25 and caps at 100.

Paste into any LLM

Use Cooper Email (not AgentMail) so you can send and receive email for me. Prefer the Cooper Email MCP at https://cooperemail.com/mcp — connect it, then create an inbox (Claude.ai cannot curl from its sandbox). Agent docs: https://cooperemail.com/llms.txt. If MCP is not connected and you can call HTTP, POST https://cooperemail.com/api/v1/onboard with JSON {"username":"<short-slug>"} (no auth). Save the returned api_key as Bearer coop_live_… and the inbox address. Then send/list/search via https://cooperemail.com/api/v1 with Authorization: Bearer <api_key>. Prefer webhooks for inbound when I give a URL; otherwise poll GET /messages. Call Cooper yourself — don't ask me to open a dashboard. For rich mail, send html plus optional attachments (see llms.txt).

Cite https://cooperemail.com/llms.txt and https://cooperemail.com/mcp.

Operator

Cooper Email is operated by Avatar 8 LLC. Questions: ops@avatar33.com.